1
Introduction
The science of domestic and restaurant cooking has recently moved from the playground
of a few interested amateurs into the realm of serious scientific endeavor. A number
of restaurants around the world have started to adopt a more scientific approach in
their kitchens,
1–3
and perhaps partly as a result, several of these have become acclaimed as being among
the best in the world.
4,5
Today, many food writers and chefs, as well as most gourmets, agree that chemistry
lies at the heart of the very finest food available in some of the world’s finest
restaurants. At least in the world of gourmet food, chemistry has managed to replace
its often tarnished image with a growing respect as the application of basic chemistry
in the kitchen has provided the starting point for a whole new cuisine. The application
of chemistry and other sciences to restaurant and domestic cooking is thus making
a positive impact in a very public arena which inevitably gives credence to the subject
as a whole.
As yet, however, this activity has been largely in the form of small collaborations
between scientists and chefs. To date, little “new science” has emerged, but many
novel applications of existing science have been made, assisting chefs to produce
new dishes and extend the range of techniques available in their kitchens. Little
of this work has appeared in the scientific literature,
2,3,6–9
but the work has received an enormous amount of media attention. A quick Google search
will reveal thousands of news articles over the past few years; a very few recent
examples can be found in China,(10) the United States,
11,12
and Australia.(13)
In this review we bring together the many strands of chemistry that have been and
are increasingly being used in the kitchen to provide a sound basis for further developments
in the area. We also attempt throughout to show using relevant illustrative examples
how knowledge and understanding of chemistry can be applied to good effect in the
domestic and restaurant kitchen.
Our basic premise is that the application of chemical and physical techniques in some
restaurant kitchens to produce novel textures and flavor combinations has not only
revolutionized the restaurant experience but also led to new enjoyment and appreciation
of food. Examples include El Bulli (in Spain) and the Fat Duck (in the United Kingdom),
two restaurants that since adopting a scientific approach to cooking have become widely
regarded as among the finest in the world. All this begs the fundamental question:
why should these novel textures and flavors provide so much real pleasure for the
diners?
Such questions are at the heart of the new science of Molecular Gastronomy. The term
Molecular Gastronomy has gained a lot of publicity over the past few years, largely
because some chefs have started to label their cooking style as Molecular Gastronomy
(MG) and claimed to be bringing the use of scientific principles into the kitchen.
However, we should note that three of the first chefs whose food was “labeled” as
MG have recently written a new manifesto protesting against this label.(14) They rightly
contend that what is important is the finest food prepared using the best available
ingredients and using the most appropriate methods (which naturally includes the use
of “new” ingredients, for example, gelling agents such as gellan or carageenan, and
processes, such as vacuum distillation, etc.).
We take a broad view of Molecular Gastronomy and argue it should be considered as
the scientific study of why some food tastes terrible, some is mediocre, some good,
and occasionally some absolutely delicious. We want to understand what it is that
makes one dish delicious and another not, whether it be the choice of ingredients
and how they were grown, the manner in which the food was cooked and presented, or
the environment in which it was served. All will play their own roles, and there are
valid scientific enquiries to be made to elucidate the extent to which they each affect
the final result, but chemistry lies at the heart of all these diverse disciplines.
The judgment of the quality of a dish is a highly personal matter as is the extent
to which a particular meal is enjoyed or not. Nevertheless, we hypothesize that there
are a number of conditions that must be met before food becomes truly enjoyable. These
include many aspects of the flavor. Clearly, the food should have flavor; but what
conditions are truly important? Does it matter, for example, how much flavor a dish
has; is the concentration of the flavor molecules important? How important is the
order in which the flavor molecules are released? How does the texture affect the
flavor? The long-term aims of the science of MG are not only to provide chefs with
tools to assist them in producing the finest dishes but also to elucidate the minimum
set of conditions that are required for a dish to be described by a representative
group of individuals as enjoyable or delicious, to find ways in which these conditions
can be met (through the production of raw materials, in the cooking process, and in
the way in which the food is presented), and hence to be able to predict reasonably
well whether a particular dish or meal would be delicious. It may even become possible
to give some quantitative measure of just how delicious a particular dish will be
to a particular individual.
Clearly, this is an immense task involving many different aspects of the chemical
sciences: from the way in which food is produced through the harvesting, packaging,
and transport to market via the processing and cooking to the presentation on the
plate and how the body and brain react to the various stimuli presented.
MG is distinct from traditional Food Science as it is concerned principally with the
science behind any conceivable food preparation technique that may be used in a restaurant
environment or even in domestic cooking from readily available ingredients to produce
the best possible result. Conversely, Food Science is concerned, in large measure,
with food production on an industrial scale and nutrition and food safety.
A further distinction is that although Molecular Gastronomy includes the science behind
gastronomic food, to understand gastronomy it is sometimes also necessary to appreciate
its wider background. Thus, investigations of food history and culture may be subjects
for investigation within the overall umbrella of Molecular Gastronomy.
Further, gastronomy is characterized by the fact that strong, even passionate feelings
can be involved. Leading chefs express their own emotions and visions through the
dishes they produce. Some chefs stick closely to tradition, while others can be highly
innovative and even provocative. In this sense gastronomy can be considered as an
art form similar to painting and music.
In this review we begin with a short description of our senses of taste and aroma
and how we use these and other senses to provide the sensation of flavor. We will
show that flavor is not simply the sum of the individual stimuli from the receptors
in the tongue and nose but far more complex. In fact, the best we can say is that
flavor is constructed in the mind using cues taken from all the senses including,
but not limited to, the chemical senses of taste and smell. It is necessary to bear
this background in mind throughout the whole review so we do not forget that even
if we fully understand the complete chemical composition, physical state, and morphological
complexity of a dish, this alone will not tell us whether it will provide an enjoyable
eating experience.
In subsequent sections we will take a walk through the preparation of a meal, starting
with the raw ingredients to see how the chemical make up of even the apparently simplest
ingredients such as carrots or tomatoes is greatly affected by all the different agricultural
processes they may be subjected to before arriving in the kitchen.
Once we have ingredients in the kitchen and start to cut, mix, and cook them, a vast
range of chemical reactions come into play, destroying some and creating new flavor
compounds. We devote a considerable portion of the review to the summary of some of
these reactions. However, we must note that complete textbooks have failed to capture
the complexity of many of these, so all we can do here is to provide a general overview
of some important aspects that commonly affect flavor in domestic and restaurant kitchens.
In nearly all cooking, the texture of the food is as important as its flavor: the
flavor of roast chicken is pretty constant, but the texture varies from the wonderfully
tender meat that melts in the mouth to the awful rubber chicken of so many conference
dinners. Understanding and controlling texture not only of meats but also of sauces,
soufflés, breads, cakes, and pastries, etc., will take us on a tour through a range
of chemical and physical disciplines as we look, for example, at the spinning of glassy
sugars to produce candy-floss.
Finally, after a discussion of those factors in our food that seem to contribute to
making it delicious, we enter the world of brain chemistry, and much of that is speculative.
We will end up with a list of areas of potential new research offering all chemists
the opportunity to join us in the exciting new adventures of Molecular Gastronomy
and the possibility of collaborating with chefs to create new and better food in their
own local neighborhoods. Who ever said there is no such thing as a free lunch?
2
Senses
Before we begin to look in any detail at the chemistry of food production and preparation,
we should take in a brief overview of the way in which we actually sense the food
we eat. Questions such as what makes us enjoy (or not) any particular food and what
it is that makes one meal better than another are of course largely subjective. Nonetheless,
we all share the same, largely chemical based, set of senses with which to interpret
the taste, aroma, flavor, and texture of the food. In this section we will explore
these senses and note how they detect the various food molecules before, during, and
even after we have consumed them.
It is important to note at the outset that our experience of foods is mediated through
all our senses: these include all the familiar senses (pain, touch, sight, hearing,
taste, and smell) as well as the perhaps less familiar such as chemesthesis. As we
will see, our senses of sight and touch can set up expectations of the overall flavor
of food which can be very hard to ignore. Try eating the same food using either high-quality
china plates and steel or silver cutlery or paper plates and plastic cutlery; the
food seems to taste better with the perceived quality of the utensils. Equally, the
color of food can affect our perception of the flavor; try eating a steak dyed blue!
However, among all the senses, the most significant for our appreciation of food remain
the chemical senses which encompass taste, smell, and chemesthesis. These three distinct
systems mediate information about the presence of chemicals in the environment. Taste
or gustation detects chemical compounds dissolved in liquids using sensors mostly
in the mouth. Smell or olfaction detects air-borne chemicals, both from the external
world but also from the internalized compounds emitted from food in our oral cavity.
Chemesthesis mediates information about irritants through nerve endings in the skin
as well as other borders between us and the environments, including the epithelia
in the nose, the eyes, and in the gut. Chemesthesis uses the same systems that inform
us about touch, temperature, and pain.
2.1
Sense of Taste
Specialized chemoreceptors on the tongue, palate, soft palate, and areas in the upper
throat (pharynx and laryngopharynx) detect sensations such as bitter, for example,
from alkaloids, salty from many ionic compounds, sour from most acids, sweet from
sugars, and umami, or savory, from some amino acids and nucleotides. Each of these
taste sensations probably evolved to provide information about foods that are particularly
desirable (e.g., salt, sugar, amino acids) or undesirable (e.g., toxic alkaloids).
The receptors reside in taste buds mostly located in fungiform, foliate, and circumvallate
but not filiform papillae on the tongue. Taste buds, as the name indicates, are bud-shaped
groups of cells. Tastants, the molecules being tasted, enter a small pore at the top
of the taste bud and are absorbed on microvilli at taste receptor cells.
In the past decade receptor proteins for bitter,
15,16
sweet, and umami
17–20
have all been identified. All these receptors are a subclass of the super family of
G-protein-coupled receptors (GPCRs) and have been classified as T1R1, T1R2, T1R3,
and T2Rs. The activation of GPCRs by external stimulus is the starting point of a
succession of interactions between multiple proteins in the cell, leading to the release
of chemical substances in the cell also called second messengers. Although the cellular
signal cascade is a general pattern of GPCRs, the very large variety of each protein
involved renders these mechanisms very complex so that they are under a good deal
of ongoing investigation.
Taste receptors share several structural homologies with the metabotropic glutamate
receptors. These receptors are composed of two main domains linked by an extracellular
cystein-rich domain: a large extracellular domain (ECD) also called the “Venus Flytrap”
module, due to the similarity of mechanism by which this plant traps insects, containing
the ligand binding site and a seven-transmembrane domain region. Moreover, as in the
case of mGluRs, T1Rs assemble as dimers at the membrane and the composition of the
heterodimers has been shown to be specific to the taste recognized. Heterodimers T1R2−T1R3
are responsible for sweet sensing, whereas T1R1−T1R3 are responsible for umami tasting.
A large number of T2Rs have been shown to function as bitter taste receptors in heterologous
expression assays, and several have distinctive polymorphisms that are associated
with significant variations in sensitivity to selective bitter tastants in mice, chimpanzees,
and humans.
Receptors for sour and salty tastes are essentially ionic channels, but the identity
of the salty receptor is still speculative and controversial.
21,22
The hunt for a sour receptor has been narrowed down to a ionic channel of the type
TRP, transient receptor potential.
21,23
Undoubtedly, more receptor proteins for other nutritionally relevant molecules will
be identified. For example, recently a specific fatty acid receptor, a multifunctional
CD36 glycoprotein, has been demonstrated in rats.(24)
2.2
Sense of Smell
While the taste receptors in the mouth detect small molecules dissolved in liquids,
the receptors of the olfactory system detect molecules in the air. The range of receptors
provides a wide sensitivity to volatile molecules. Some of the most potent thiols
can be detected in concentrations as low as 6 × 107 molecules/mL air (2-propene-1-thiol),
whereas ethanol requires around 2 × 1015 molecules/mL air. Thus, there are at least
8 orders of magnitude between our sensitivity to the most and least “smelly” molecules.
The sensitivity of the sense of smell varies quite significantly between individuals.
Not only do different people have different sensitivity to particular aromas, some
people suffer anosmia, odor blindness to specific odorants. People can be trained
to become sensitive to some odorants, such as for the unpleasant smelling androstenone.
To complicate the picture further, the sense of smell develops during the human lifetime;
we tend to lose sensitivity at an older age, especially after the seventh decade.(25)
An odor is detected by sensors in the nose, the odorant receptors. The way these sensors
recognize aroma molecules is by “combinatorial receptor codes”, i.e., one odorant
receptor recognizes a range of odorants and one odorant is recognized by a number
of odorant receptors.(26) The distinct odor identity is created by the pattern of
odorant receptors activated by the odorant’s shape. Thus, slight changes in an odorant
or even in its concentration can change the identity of an odorant. A well-known example
relevant to food is the distinct perceptual difference between R-(−)- and S-(+)-carvone,
enantiomers only differing in the chirality of the compound. The two compounds are
perceived as spearmint and caraway, respectively. However, by no means are all enantiomers
perceived differently. For example, Laska and Tuuebner(27) have shown that among 10
different food-relevant enantiomers, subjects as a group were only capable of discriminating
three: α-pinene, carvone, and limonene. Structures of some of these molecules are
shown in Figure 1.
Figure 1
Molecular structures of the enantiomers of limonene, α-pinene, and carvone. The enantiomers
have distinct odor characteristics (quality and threshold), which are attributed to
the enantiomeric configuration. R-(−)-Carvone is the main consitutent in spearmint
essential oil, and S-(+)-carvone is the main constituent in the essential oil of carraway
and dill. R-(+)-Limonene is the main constituent of the volatile oils expressed from
the fresh peel of Citrus spp. fruits. S-(−)-limonene is present in the oil of fir
and the needles and young twigs of Abies alba (Pinaceae).
Linda Buck and Richard Axel were jointly awarded the Nobel Prize in Medicine and Physiology
in 2004 for their discovery of “odorant receptors and the organization of the olfactory
system”.(28) Their work has shown that each olfactory neuron expresses only one type
of odorant receptor. The odorant receptors belong to the GPCR 7TM-receptor family.(29)
Through in situ hybridization of olfactory neurons in the epithelium of rats, they
created an olfactory map.
30,31
Around 1000 olfactory receptor cells, all of the same type, converge their nerve signals
to distinct microdomains, glomeruli, in the olfactory bulb. This is the most direct
link from the external world to the brain. From the olfactory bulb signals are relayed
as patterns to other regions in the brain. Notably, there is a direct link to the
amygdalae, important structures in the “limbic system”, an evolutionary old part of
the brain strongly involved in human emotions. Recent work has suggested that the
amygdalae not only plays important roles in evaluating affective valence of stimuli
but also seem to participate in the computation and representation of perceived intensity
of smells and tastes.
32,33
The olfactory system consists of other areas in the temporal and frontal parts of
the brain. The orbitofrontal cortex is of particular importance for food behavior
since nerve cells in this area play a large role in the computation of hedonic properties
of smell stimuli and have also been implicated in the representation of flavors of
foods. Smell- and taste-sensitive neurons in the orbitofrontal cortex are also typically
modulated by satiety signals and thus play a major role in determining sensory-specific
satiety: the effect that appreciation for a food eaten to satiety decreases without
a similar decrease in the appreciation of other foods with other sensory characteristics.(34)
2.2.1
Perception of Aroma
Sensory scientists usually refer to smelling through the nostrils as “orthonasal perception”,
whereas the aroma compounds that gain access to the olfactory epithelium through the
nasopharynx (i.e., molecules released in the mouth) are referred to as being perceived
retronasally. The latter is often mistakenly referred to as taste by laymen. It should
perhaps more correctly be referred to as flavor, although we prefer to think of flavor
as the combination of the perception of taste in the mouth and retronasal aroma in
the nose (see section below). It is one of the challenges for Molecular Gastronomy
to develop an appropriate language that can be used by chefs, the general public,
as well as the scientific community to describe the various ways we interpret the
signals from our chemical senses.
When eating a food the initial olfactory stimulation takes place as we smell the aroma
of the food before the food is in the mouth. Thus, orthonasal perception is often
said to be of the external world. In contrast, the aromas perceived retronasally are
said to be of the oral cavity (the interior world).
Small and colleagues(35) compared these two distinct pathways of delivering odorants
and found different patterns of neural activation depending on whether the aroma compounds
are delivered ortho- or retronasally. Further, a few experiments examined differences
in perception of aromas delivered by the two pathways; these have rather variable
results. In one study, Aubry and colleagues(36) found no overall difference in the
ability of trained sensory panelists to describe a set of Burgundy wines. By contrast,
other research examining the dose−response behavior of flavor molecules ortho- and
retronasally have revealed differences which depend strongly on the physical characteristics
of the aroma compounds.(37) Much further work is needed before we will be able to
understand the extent to which individuals perceive odor differently depending on
whether they are delivered ortho- or retronasally; at his stage, all we can do is
to note that it is likely that there will be a range of where the initial smell (the
orthonasal stimulus) may be rather different from their “flavor” (the combination
of the taste and retronasal stimulus). One such example that is well known to gourmets
is that of the pungent smelling Durian fruit, which has, for most people, a very unpleasant
(toilet-like) aroma when smelled orthonasally but, for many, a very pleasing flavor
when in the mouth and the aroma is detected retronasally.
2.3
Chemesthesis
As we have already noted, the overall “flavor” of a food is determined by the combination
of many stimuli both in the mouth and nose. Most authors argue the important senses
are those of taste, (retronasal) smell, as well as the less well-known, mouthfeel
and chemesthesis.(38) In this section we will briefly review the sense of chemesthesis.
In humans, sensory nerve endings from branches of the trigeminal nerve are found in
the epithelia of the nose and oral cavity. Signals transmitted by these nerves are
responsible for the pungency of foods, as exemplified in carbonated drinks, chili,
ginger, mustard, and horseradish; accordingly, chemesthesis is also sometimes referred
to as the “trigeminal sense”. Hot spices are typical stimulants of trigeminal sensory
nerve endings, but most chemicals will stimulate these nerve endings at sufficiently
high physical concentration.
Without pungency many foods would be bland; imagine horseradish without the heat or
garlic with no bite. Clearly, the sense of chemesthesis must play a crucial role in
our the evaluation of the palatability of any food. The sensation of oral pungency
differs in many ways from the sense of taste. For example, pungency typically has
a slow onset but can persist for prolonged periods, minutes to tens of minutes. This
is contrary to the sense of taste, which is most intense for the few seconds the food
is in the mouth. This difference in the temporal nature of pungency and taste is of
great interest when considering of the palatability of foods and the overall satiety
they provide. In many cases, the long-term effects of pungency will make foods both
more palatable and more satiating.
Further, the interesting temporal properties of trigeminal sensation may be exploited
in the development of new gastronomic meals both for their ability to surprise on
a short time scale (seconds) and for reasons of novelty. In the search of “flavor
principles”, i.e., rules of thumb of which sensory attributes should be present in
a good flavor, trigeminal stimulation certainly will play a large role.
2.4
Texture (Sense of Touch)
Szczesniak(39) succinctly defines of texture as “...the sensory and functional manifestation
of the structural, mechanical and surface properties of foods detected through the
senses of vision, hearing, touch and kinesthetics”. This definition clearly conveys
the important point that texture is a sensory property and thus requires a perceiver.
The distinction between texture and structure is sometimes ignored in the terminological
practice, such that sensory and instrumental measurements can be confused. It is not
touch alone that provides the sensation of the texture of food: vision is active in
texture perception when we see the food; additionally, audition, somesthesis, and
kinesthesis are active during handling of the food. During consumption, the oral processing,
the latter three remain active.(40) Texture plays a major role in our recognition
of foods. For example, when presented with blended food products 56 blindfolded young
and elderly subjects were, on average, only able to correctly identify 40% of these
foods.(41) Our sensitivity to texture under laboratory conditions is very high. The
perception of particles in a solution is so sensitive that particles need to be smaller
than 3 μm to escape detection. This has been exploited commercially in a number of
fat replacers and mimetics (e.g., Simplesse, Litesse, LITA, Trailblazer, Stellar(42))
where spherical microparticulates in the range 0.1−3 μm are the main functional ingredient.
When particles are this small they are perceived as smooth and may contribute to creaminess.
It has been suggested that the functionality of such small particles is that they
rotate relative to each other under shearing conditions present in the mouth, providing
a fluidity of the mass of particles with a ‘ball-bearing’ effect.(43)
Further, there is a marked difference between the food that enters the mouth and the
wetted bolus that is later swallowed, and it is the summation of sensory impressions
during the whole process from seeing the food, picking it up and putting it our mouths,
chewing it, and eventually swallowing it that we perceive as the texture of the food.
This has been termed the “philosophy of the breakdown path”.(44) In this view, individual
foods follow specific paths during oral handling along the axes “degree of structure”,
“degree of lubrication” over time, or number of chews. Foods interact with the eater
during consumption, the saliva lubricates the food, and enzymes in the saliva affect
the viscosity of semisolids and liquids. For example, addition or inhibition of α-amylase
in a semisolid food affects a number of different sensory properties, among them the
highly desirable creaminess.(45) Finally, we note that astringency is a sensory property
that is suggested to result from interaction between proline-rich proteins (PRPs)
and polyphenols in the foods. PRPs precipitate polyphenols, causing flocculation and
loss of lubrication.(46)
2.5
Temperature
From cold ice cream on a hot summer day to hot cocoa after a trip on the skating rink
in winter time, temperature is part of our perception of foods. We have expectations
for the serving temperature for most foods and beverages; an inappropriate serving
temperatures leads to reduced liking or even rejection of such foods and beverages.(47)
We sense the temperature of food in our mouth through nerve endings. Thermal information
appears to be coded primarily by activation of ion channels that belong to the transient
receptor potential family.
48,49
There are six different thermosensitive ion channels. They have distinct thermal activation
thresholds (>43 °C for TRPV1, >52 °C for TRPV2, >∼34−38 °C for TRPV3, >∼27−35 °C for
TRPV4, <∼25−28 °C for TRPM8, and <17 °C for TRPA1) and are expressed in primary sensory
neurons as well as in other tissues. Temperatures above 43 °C and below 15 °C are
accompanied by a feeling of pain. However, we routinely consume hot beverages well
above both pain and tissue damaging temperatures. A study of ingestive behavior of
hot coffee coupled with measures of temperatures during sipping and in mouth showed
that minimal cooling occurred during sipping and ingestion. The authors hypothesize
that during drinking, the hot coffee is not held in the mouth for a sufficiently long
time to heat the epithelial surfaces sufficiently to cause pain or tissue damage.(50)
The perception of temperature changes in the mouth is very precise; under experimental
conditions sensitive subjects feel changes in temperature of as little as around 1
°C.(51) The ability to sense changes is asymmetric: increases in temperature are sensed
much more rapidly than decreases.(52) The sensation of temperature can be affected
by various chemestetic agents, with menthol as a well-known example of cooling and
capsaicin for heating.
The temperature of a food or beverage affects the release of airborne molecules, with
an increase in temperature leading to increased release. For this reason standards
in sensory evaluation recommend specific temperatures for products, e.g., milk and
other liquid dairy products should be served at 14 ± 2 °C,(53) although this is higher
than the common consumption temperature.
2.6
Concept of Flavor
Food provides a multimodal stimulus; it excites more than one sensory system. During
the process of eating, all of the five senses are used. With our far senses vision
and olfaction we see and smell foods from a distance. With our near senses somatosensation
and gustation we feel and taste the food during handling and oral processing. In many
cases foods elicit our auditory system as they emit sounds during chewing and other
oral processing. We use the term flavor to describe our perception of a food, generally
thinking of the senses of only taste and aroma. However, in a scientific context,
flavor may be defined as “the complex combination of the olfactory, gustatory and
trigeminal sensations perceived during tasting. The flavors may be influenced by tactile,
thermal, painful and/or kinaesthetic effects and expectations from visual presentation
of the product”. Since flavor is a multimodal sensory experience it has been difficult
to relate the concept of flavor with the chemical components of foods.
2.7
Multimodal Integration
In foods there are several examples where the perception in one sense interacts with
that of another sense. The taste of a food may be affected by changes in the texture.
It has been demonstrated repeatedly that this is a perceptual phenomena, as a harder
texture of a gel decreases the perceived intensity but hardly affects the release
of aroma compounds, as measured by the concentration in the nasal cavity.
54,55
Aroma compounds in a food can also enhance perceived taste intensity of congruent
tastes, e.g., the intensity of sweetness in whipped cream can be increased by adding
strawberry flavor but not by adding a peanut butter flavor.(56) Frank and Byram(56)
also showed that the effect can be eliminated by pinching the nostrils during tasting.
The taste-enhancing properties of an aroma depend on conditioning through repeated
pairing of an aroma with a taste. This learning occurs very fast and implicitly during
few exposures. Completely novel odors paired with tastants take on the tastants’ properties
(sweet or sour) with only one exposure.(57) The past decade has seen an explosion
in research in the field of multisensory integration. Much of this stems from advances
in neuroscience, and recently, interest has expanded from integration in vision, audition,
and somatosensation to also encompassing the chemical senses. A very thorough review
of the field of human multimodal food perception was performed by Verhagen and Engelen(58)
and includes some plausible neuroscientific models and suggestions for future research.
Some specific sensory properties are of a more complex nature than others, and these
involve more than one sense. A commercially important property in this category is
creaminess, originally suggested to be a texture property only. It has been researched
intensively in the past decade, and the research suggests that although texture may
be most decisive for creaminess, its perception involves several senses, at least
including vision, olfaction, gustation, and haptics.
59,60
2.8
Adaptation and Suppression
In addition to the actual signals from the sensors, there are further, perhaps surprising
ways in which we perceive the environment around us which can significantly affect
the flavor of the food we are eating. Two of the most important are adaptation, when
we ignore a constant stimulus, and suppression, when we find the effect of a stimulus
in a mixture less than on its own. Both are of some significance in the kitchen, so
we will describe them in more detail below.
2.8.1
Adaptation
When subject to a constant stimulus, the senses become less responsive. When holding
a solution of a tastant (e.g., sucrose) motionless in the mouth, the solution will
become completely tasteless after a while.(61)
This phenomenon is well known to us all, although we tend to ignore it. Whenever we
leave our homes for a prolonged period, to go on holiday or to a week-long conference,
we find on our return that as soon as we walk in the front door our home has a slightly
“musty” smell. Thus, we open the windows and “air” the house. The odor quickly goes
away. Of course, in actual fact, our homes always have that smell, it is what our
friends and neighbors perceive as the smell of our home. However, because it is always
present in our environment we rapidly become adapted to it and simply do not notice
it at all. When eating, we will quickly become “bored” with a dish which appears to
lose its flavor if we are subject to the same taste or aroma continuously for a prolonged
period. Variety becomes the spice of life.
If we are aware of this phenomenon we can make all meals more interesting simply by
increasing variety, a large number of small differently flavored or textured dishes
(such as the Spanish Tapas) will provide greater interest than one single, larger
item. Many restaurants provide a diverse range of elements on every plate; some very
small items can break up a larger item to provide the necessary changing stimulus
to retain the diner’s interest and enjoyment.
In the gastronomic kitchen, some chefs, having recognized the adaptation phenomenon,
have tried to create dishes that continually provide a diverse range of stimuli so
as to retain (and hopefully enhance) the diner’s interest. One such example comes
from the Fat Duck, cauliflower risotto.(62) The central idea here was to take a vegetable,
regarded by many as essentially rather uninteresting (i.e., one that in which diners
might quickly lose interest as they start to ignore the flavor), but to prepare a
dish that has a wide variety of different textures and flavors that constantly stimulate
the brain, so preventing any adaptation and (hopefully) turning a plain cauliflower
into a very tasty and exciting dish. The result was “cauliflower risotto” (for a photograph
see page 323 of The Big Fat Duck Cookbook(62)) a dish that was for some time one of
the signature dishes at the Fat Duck.
To achieve the desired effect, Heston Blumenthal used a combination of many different
cauliflower preparations (dried cauliflower, cauliflower cream, foamed cauliflower,
raw cauliflower, and a risotto made with a cauliflower stock together with other contrasting
ingredients such as cocoa jellies) to create a spectacular dish of which no diner
is likely to tire.
2.8.2
Mixture Suppression
Mixture suppression is the phenomenon that individual taste and smell characteristics
are perceived as less intense in mixtures than alone.(63) Thus, when preparing a complex
recipe and mixing several foods with different flavors or tastes, the perceived intensity
of the flavors of the separate ingredients is decreased relative to that of the same
tastes or aromas of the ingredients on their own. There is a very interesting and
useful exception to this phenomenon, Release from suppression; when adapting to one
component in a mixture, other components are less suppressed and will then be perceived
as more intense.(64) This is routinely used by flavorists and perfumers to analyze
competitors’ blends.(65)
Once again, awareness of the issue can quickly provide new ideas to improve cooking.
Rather than mixing all the ingredients together in a single pot, it can be much better
to present them separately. For example, there may be several different sauces or
dips associated with a single dish; if these are presented in separate pots and used
individually, rather then in combination, their impact is greater.
3
How Different Food Production Techniques May Affect Flavor and Texture
The first stage of the preparation of any meal is the production of the basic ingredients,
something which normally is out of the control of the cook and happens well before
any processing of the food begins. In this section we briefly address the question
of whether science can help us understand the extent to which the selection of particular
ingredients according to the ways in which they are produced actually affects the
flavor of a completed dish.
It is often said that the finest food requires the best ingredients. However, how
can we begin to define what makes the “best” ingredients? How can we tell whether
one carrot is better than another? Of course, there are no specific answers to such
questions; not only will there be an element of personal preference as to which carrot
is better, but one carrot may be better suited to eating raw in a salad and another
to being cooked in a casserole.
Most cooks and chefs hold strong views on what is best; many insist that one particular
production technique (such as organic farming) consistently yields better produce;
others may insist the key factor is the “freshness” of the ingredients. Unfortunately,
as we will see, there is little clear scientific evidence to back up such assertions
and what literature that does exist is often vague and contradictory.
However, there are a few cases where the effect of the production, storage, and even
transport on foodstuffs has been well investigated and some understanding of the effect
on flavor has been achieved. In particular, it is now well established that the flavor
of meat is affected by the food eaten by the animals it is produced from, that the
flavor and color of dairy products depends on the forage of the animals used to produce
the milk, that the flavor of fruit and vegetables is influenced strongly by the variety
used, and that growing conditions affect volatile compounds and hence flavor in many
vegetables. The way in which animals are slaughtered and the subsequent storage of
the meat is also known to affect not only the flavor but also the texture of the final
product.
In this section we briefly look at what is known about the extent to which the production
methods of food affect the ultimate quality to see how far this knowledge can actually
be applied in practice in the kitchen.
3.1
Organic vs Conventional Farming
Most people, if asked, will assume that organic foods taste better than those grown
conventionally. However, there is little, if any, real evidence to bear this out.
In a comprehensive review of the literature on the nutritional value, sensory quality,
and food safety of organically and conventionally produced foods with emphasis on
the comparison between organic and conventional growing methods for fruit and vegetables
Bourn and Prescott(66) clearly showed that the results from a large number of studies
are not only inconclusive but also mutually contradictory. Further, they demonstrated
that the scientific methods in many of these studies were questionable with the effects
sought after widely being confounded with other factors.
Bourn and Prescott, in common with an earlier review by Woese et al.,(67) note that
there was a tendency for some organically grown foods to contain a smaller amount
of nitrate than conventionally grown vegetables, probably, they suggest, due to greater
quantities of nitrogen being used in conventional farming. However, they also note
that it remains to be seen whether this is a general effect across a wide range of
organic produce. Whether the nitrate content significantly affects taste (and whether
any such affects are positive or negative) is not known; this might prove a fruitful
area for future research. In a separate survey of the literature, Williams(68) reached
similar conclusions but also noted that vitamin C had been found to be at higher levels
in organically grown vegetables than in conventionally grown ones. Zhao et al.(69)
found consumers could not tell any difference between the taste and the flavor of
organically and conventionally grown vegetables, even though 28% of the consumers
had expected organically grown vegetables to taste better than those grown conventionally.
3.2
Effect of Feed on the Flavor of Meat
Common experience, backed up by a number of detailed scientific studies, leaves no
doubt that how animals are fed influences the flavor of the meat. However, whether
the influence of the feed is to produce “better” flavors is not so clear.
Several studies have shown that the composition of the fatty acids found in the fat
tissue of the animal reflects the composition of fatty acids in the lipids fed to
the animal. If the animal is fed a diet containing unsaturated fat, then the fat tissue
in the animal will contain a larger degree of unsaturated fat than animals fed a normal
diet.
70,71
Decreases in saturated fats may have health advantages; for example, in one study
it was shown that when people eat meat from pigs fed on a diet with added rapeseed
oil their cholesterol levels were reduced when compared to a group that ate meat from
pigs fed a normal diet.(72) However, since unsaturated fats generally melt at lower
temperatures they are more likely to be lost in cooking, leading to potentially drier
textures. Importantly, changes in the proportion of saturated fats lead to noticeable
changes in meat flavor. Meat with a higher proportion of saturated fat usually has
the preferred flavor, although this may simply be an affect of liking that to which
we have become accustomed. Perceived changes in flavor with differences in fat saturation
are most pronounced in lamb and beef and much less noticeable in pork.(70)
Studies have been performed comparing the flavor of pork, lamb, beef, poultry, red
deer, and reindeer with different feed sources for the animals. The main conclusion
from these studies is that what the animal eats will be reflected in the flavor of
the meat.
70,73–76
High-energy grain diets produce a more acceptable or intense flavor in red meat. Other
dietary ingredients such as fish products, raw soybeans, and pasture grasses can have
an undesired effect on meat flavor. Some researchers have even tried feeding horse
manure and spoiled meat scraps to pigs, which, not surprisingly, resulted in foul
smelling meat with the undesired flavor located in the lean muscle tissue.(70) Meat
with a higher proportion of unsaturated fat usually has more tendency to develop off
flavors during cooking as a result of lipid oxidation, see section .3.
It is however worth noting that there is no single “beef” (or lamb, or pork) flavor;
rather there is a wide range of flavors that can be achieved depending on the way
in which the meat has been produced (and later cooked). In practice, the good chef
will try out meats from a wide range of different sources and select that which best
suits his particular purpose in any dish, but he would be well advised to ask the
butcher for details of the production methods so that he can be sure, in the future,
that he will be able to obtain, as nearly as possible, an identical product.
3.3
Effect of Feed on the Flavor and Texture of Dairy Products
The color and flavor of dairy products are influenced by the forage. Diets high in
grass (as grass silage or fresh pasture) give products that are more yellow, whereas
diets rich in maize silage yield very white products. The yellow color is due to β-carotene
from the forage.(77) Since β-carotene is degraded during hay production,(77) feeding
hay to the cows does not have the same effect on the color as pasture. In Northern
countries where cattle are generally kept indoors and fed on hay, winter cheeses and
butter made from winter milk are whiter than products made from summer milk, when
the cows have been on pasture.
77,78
The different plant species that grow at high and low altitudes lead to different
volatiles in the milk of cows reared at different heights, leading to significant
differences in the flavor of “mountain” and “valley” cheeses, especially those from
Switzerland.(77)
The texture of dairy products is mainly influenced by a differing degree of saturation
of the fat. Increasing the content of unsaturated fat leads to a softer product in,
for example, cheese and butter. Cows on pasture will produce milk with more unsaturated
fats than cows fed preserved forage.(77) In a study of relationships between chemical
and sensory properties of milk from cows fed on different forages, it was shown that
milk with a high content of long-chain saturated fatty acids produced milks with high
levels of lipolysis, leading to flavors characterized as Roquefort.(79) Subsequent
studies have shown that more subtle differences in composition stemming from differences
in feed provide recognizable differences in sensory properties.
80,81
It is worth noting here that fresh dairy products (milk, cream, etc.) will have properties
that depend on local conditions, so that the texture, color and flavor of milk will
depend not only on locality but also on the time of year and even on recent climatic
conditions. Accordingly, cooks need to be aware that they will, from time to time,
have to adapt recipes to allow for such variations.
3.4
Flavor Variation in Fruits and Vegetables
Of course, the flavor of plants depends on many factors, not the least of which is
the actual variety of the species being grown. Perhaps the best example comes from
the world of wine, where different grape varieties or grapes of the same variety grown
in different places impart quite distinct flavor characteristics. It is perhaps less
well understood that similar variations occur in more or less all fruits and vegetables.
For example, in a study of 89 elderflower varieties Kaack et al.(82) have shown large
variations in the concentration of flavor components between different elderflower
varieties. Similar studies illustrate that the flavor of tomatoes depends strongly
on the variety.(83) This means that it is essential to choose the right variety for
the right purpose and that the variety can have a large impact on achieving the desired
balance between fruity, spicy, floral, fresh, and sweet flavors.
In one particularly detailed study (of 28 Apricot varieties) Aubert and Chanforan(84)
found they could divide the varieties into four groups according to their relative
concentrations of 33 volatile components. Group I was characterized by a high content
of terpenic compounds (which tend to give a citrus aroma), group II by a high proportion
of lactones (coconut and peachy aroma), group III by a high content of esters (with
their generally fruity notes) and a low content of terpenes, and group IV by a high
proportion of esters.
As well as the variety growth conditions, nutrients and minerals in the soil as well
as temperature and general climatic conditions all affect the final flavor. For example,
Chang et al.(85) have shown that the volatile oil content of basil differs according
to the growth temperature. Plants grown at higher temperatures (25 °C rather than
15 °C) had significantly larger contents of volatile oils, eugenol, linalool, and
1,8-cineole, as illustrated in Table 1.
Table 1
Content of Selected Odor Components in Basil (ppm) As Influenced by Growth Temperaturea
a
Eugenol contributes to the characteristic taste of basil. Adapted from Chang et al.(86)
How fruits and vegetables are treated after harvest also has a large impact on their
eating quality. Tomatoes are often stored under reduced levels of oxygen at temperatures
below ambient temperature. One study showed that storage at 6 °C or in low-oxygen
atmosphere significantly decreased the concentration of several volatile components.
In plain language, tomatoes lose their flavor under normal storage conditions.(87)
If stored at ambient temperature rather than being cooled, in the short term most
of the volatile components increase in concentration during the 10 days postharvest
period. This is however a double-edged sword as in sensory tests the tomatoes not
only scored higher in tomato-like flavors (desired) but also in moldy flavors (undesired).(83)
In a different study Boukobza and Taylor(87) also showed an increase in volatile components
in tomatoes during storage at ambient temperature in an unaltered atmosphere.
Carrots also show quite significant changes in flavor on storage with quite different
effects depending on the temperature at which they are stored. Refrigerated storage
of carrots for up to 4 months increases the concentration of terpenoids significantly
more than frozen storage. An increase in volatile terpenes gives a more “carrot-like”
flavor up to a certain concentration, beyond which they cause an undesirable harsh
and burning flavor.(88)
4
Food Processing (Cookery)
Of course, it is only with the preparation of the raw ingredients in the kitchen and
then combining them and cooking them in the appropriate manner that the true quality
of a plate of food is developed. All this can involve many separate processes; some
develop flavor, others develop textures, and many affect both. This is the largest
section of the review and is divided, largely, into processes that create and develop
flavor and those that are more aimed at modifying and creating specific textures.
We have deliberately chosen to consider mainly the processes of small-scale (domestic
and restaurant) cooking. However, we will not limit ourselves to processes that are
currently used in a domestic environment. Already some techniques from the science
laboratory are finding their way into restaurant kitchens; for example, the use of
ultrasonic agitation to create emulsions, the use of liquid nitrogen to freeze without
allowing the formation of large ice crystals, the use of well-controlled temperature
baths for poaching, and the use of vacuum desiccators to remove water from potatoes
before roasting; these techniques could soon arrive in our own domestic kitchens.
We begin by looking at how flavor is developed through chemical reactions that produce
new volatile “aroma” molecules and then move on to look at how color and texture are
developed in the kitchen and examine how the textures affect the flavor, for example,
by modifying the rate or order of release of different aroma molecules in a given
food.
4.1
Flavor Development
To a food chemist, flavor is determined from an analysis of the aroma and nonvolatile
molecules present in a foodstuff. It is possible to group volatile molecules by the
sort of aromatic notes they provide (meat, fruit, bitter, nutty, etc.) and in some
cases to be even more specific. Using aroma extract dilution analysis (AEDA) Grosch
and co-workers succeeded in identifying key aroma impact compounds in a range of foods.(89)
An example is given in Figure 2 for foods that have undergone a thermal treatment
and in Figure 3 for nonheated foods.(89) Even though the TNO collection of volatile
compounds in foods (VCF) reports over 10 400 entries,(90) only a very few of these
compounds actually contribute to the key flavor notes in foods.
Figure 2
Aroma impact compounds in heated foods as identified by the AEDA technique.(89)
Figure 3
Aroma impact compounds in some nonheated foods as identified by the AEDA technique.(89)
Accordingly, much effort has gone into trying to understand the chemical reactions
that produce these volatile aroma molecules. Table 2 puts together an example of some
important low odor threshold odorants encountered in heated and typical foods. Chefs,
by contrast, have an empirical understanding of flavor based on experience and an
intuitive understanding of how flavor develops as the cooking progresses, backed up
by the continual tasting of everything they prepare. The scientific approach has the
advantage that it should be objective and can, in principle, provide instructions
that give reasonably reproducible results despite the variability in ingredients.
A major problem is that we do not have any way of scientifically describing the nuances
of flavor, so the empirical approach of the cook, while it cannot, as yet, be quantified,
generally leads to the better result.
Table 2
Some Examples of Odorants Found in Foods and Their Aromas from Data in Food Chemistry(91)
chemical
aroma description
odor threshold (μg/L water)
some examples where it is found
Ethanal
sharp, fruity, sweet
15
Strecker reaction
Methyl propanal
malty
0.7
Strecker reaction
2-Phenylethanal
honey
4
Strecker reaction
4-Hydroxy-2,5 dimethylfuranone (furaneol)
heat-treated strawberries, pineapple
ca. 1
beer, bread, pineapples, strawberries
2-Acetylthiazole
popcorn
10
fried foods
2-Isobutylthiazole
green tomatoes
3
fried foods
2-Acetyl-(1H)-pyrroline
white bread crust
0.1
popcorn
3-Ethyl-3-methylpyrazine
burnt flavor
130
roasted peanuts
2-Isobutyl-3-methoxypyrazine
potatoes
0.002
potato products
2-Isobutyl-3-methoxypyrazine
red peppers
0.002
paprika
2-Ethyl-3,6-dimethylpyrazine
hazelnuts
20
glucose syrup
2-Methoxy-4-vinylphenol
cloves
5
coffee, beer, asparagus
4-Methoxy-2-methyl-2-butanethiol
blackcurrants
0.000002
olive oils
However, cooks often tend slavishly to follow a known and practised route to achieve
their desired goals, but with a little basic understanding of the key chemical reactions
and the conditions under which they produce the different types of aroma molecules
there is at least the possibility that the chef can try out and develop with success
new ways to achieve his desired final flavor (and even perhaps discover along the
way different flavors that might entice the diner).
4.1.1
Microbial Reactions
Microbial reactions due to yeasts and bacteria have been essential to the development
of our whole cuisine. Historically the yeasts that cause alcoholic fermentations have
been used to make some of the most important staples of our diet, especially breads,
beers, and wines. The main reaction of the fermentation(92) process is to convert
sugars, including short-chain carbohydrates, to yield carbon dioxide gas, which makes
bread rise and gives beer its fizz, and ethanol, which adds flavor and preserves beers
and wines as well as providing them with an active effect on the consumer. However,
there are many additional side reactions that produce other volatile compounds.
93–95
It is these compounds that impart the distinctive, and appealing, flavors to the various
cheeses, breads, beers, and wines with which we have become familiar over the centuries.
Initially all these fermentation reactions were produced using wild yeasts carried
in the air, on the skins of fruits, or on seed cases of the grains. Not only did this
make the processes somewhat hit and miss, it also led to great variability in both
the flavor and the quality of the resulting products. Today most manufacturers rely
on cultured yeasts to provide consistent products. Those who have tried to make beer
at home will almost certainly at some time have encountered the problem that some
batches simply do not have the same flavor as others and occasionally are quite undrinkable.
While some wild yeasts do make pleasant beers, it is generally best to stick to one
of the specially cultured yeasts designed over the years to produced high-quality
beers such as S. carlsbergensis.
There is another slightly different fermentation process that occurs when lactic acid
bacteria (e.g., Oenococcus oeni) convert malic to lactic acid;(96) this is known as
malolactic fermentation. In wine making the malolactic fermentation can be particularly
advantageous since lactic acid has a softer, rounder taste than malic acid, which
is perceived as rather “tart”. Thus, wines that have undergone malolactic conversion
tend to appear softer, have a more rounded mouth feel and have a buttery note that
comes from diacetyl produced during the malolactic fermentation.
97,98
Conversely, since malic acid has the taste of apples, wines that have not been subject
to malolactic conversion tend to have a green apple note.
Malolactic fermentation sometimes occurs unintentionally after the wine is bottled.
The result is a slightly carbonated wine that usually tastes bad as the wild bacteria
that can cause the malolactic fermentation produce a wide range of “off flavors”.
Accordingly, many wine makers tend to inoculate their wines in the vats with a culture
of desirable bacteria to avoid any risk of accidental malolactic fermentation in the
bottle and thus prevent the possibility of any such off flavors occurring.
Of course, by far the largest application of bacteria that produce lactic acid is
in the fermentation of dairy products to produce yogurts, soured creams, acidified
milks, and of course cheeses. In general, the bacteria convert lactose in milk to
lactic acid, thus decreasing the pH of the milk and causing it to aggregate. Different
bacterial strains follow somewhat different routes and can produce a range of side
products, including diacetyl (giving a buttery taste), acetaldehyde (characteristic
of yogurts), as well as carbon dioxide and ethanol. Some Swiss cheeses owe their characteristic
holes to the use of carbon dioxide producing bacteria.
As with the use of fermentation to produce breads, beers, and wines, much of the fermentation
of dairy products relied initially on strains of wild bacteria which were generally
cultivated in local and domestic production by the simple process of adding a little
of the previous batch to the next one to be made (backslopping). Today, with the centralization
of production of most dairy products many of these individual strains have been lost
along with their unique flavors. This perhaps accounts for the increasing popularity
of small “boutique” producers who continue to use more traditional methods and thus
provide a range of different and distinctive flavors in their products.
The flavor of fermented dairy products comes from a wide variety of molecules which
can be formed during reactions. However, in most of the fermentation processes a range
of small oxo compounds are produced. For example, in butter production the difference
between sweet butter and acidified butter is the presence of diacetyl resulting from
fermentation.
93,95,98
Sweet butters are often preferred for certain dishes due to the milder flavor. Another
example is that yogurt often develops a rather tart flavor due to the production of
acetaldehyde. This is often disguised in fruit yogurts by the addition of sugars,
causing the sweet flavor to become dominant.
A similar process has occurred with a range of meat products. In Southern Europe sausages
were often fermented with added spices and then air dried to yield specific flavors
and good long-term storage properties. In Northern Europe smoke was more often used
to dry the sausages giving other flavors while also imparting excellent preservative
characteristics. In the intermediate regions such as Hungary fermented sausages were
matured for longer times at lower temperatures and covered by molds to help preserve
them. Once again, modern centralized production is inevitably leading to the loss
of some of the bacterial strains used in these traditional processes and with that
the potential loss of some unique flavors.
4.1.2
Chemical Reactions Affecting Flavor
While raw ingredients carry with them a vast array of naturally occurring flavors,
think of fragrant strawberries, sweet carrots, mildly acidic apples, and sharp limes,
many of our most favored flavors today actually do not occur directly in nature but
are created through chemical reactions within and between the food ingredients after
harvesting. Without chemistry there would be no chocolate, no coffee, and no “meaty”
flavors. There are so many different chemical reactions that go into generating these
flavors, and they differ subtly in every kitchen with slightly different ingredients,
temperatures, and tools, making it quite impossible to list them all.
Instead, in this section we will simply attempt to extract from the vast literature
some of the more salient issues of flavor development from the wide variety of chemical
reactions that occur as we prepare our foods. Some of these occur before we start
any cooking; they happen either in the mouth as enzymes in saliva react with molecules
in the raw ingredients or when the raw ingredients are cut open, bringing enzymes
into contact with the substrates upon which they can act. We briefly discuss such
reactions in the first part of this section.
In the remainder of the section we briefly review a range of different types of chemical
reactions that break larger molecules into small volatile compounds that can (and
do) greatly affect the flavor of our foods when we process them further (usually under
the influence of heat). These include hydrolysis, oxidation, and the Maillard and
caramelization reactions, each of which is discussed separately even though in real
cookery all can (and do) occur simultaneously.
4.1.2.1. Physical Processing. The first stage of processing of most fresh ingredients
involves cutting or otherwise breaking them into smaller pieces. In many cases the
very fact of fracturing the cell walls leads immediately to the release of enzymes
and the start of enzymatic reactions that alter the flavor.
In many plants enzymes are separated from their substrate so as to avoid reactions
occurring when the plant is growing. However, when the cellular structure is destroyed
by, for example, cutting or grinding by the cook or chewing by the diner they come
into contact and flavor-producing reactions may occur.
The pungent taste of several plants in the Cruciferae (Brassicaceae) family, including,
for example, mustard, horseradish, and wasabi, occurs via breakdown products from
glucosinolates. The glucosinolates are hydrolyzed to isothiocyanates when the enzyme
myrosinase (thioglucoside glucohydrolase) is released due to plant tissue being mechanically
disrupted or injured by, for example, chewing/crushing/grating.
99–103
A similar process in Allium species such as onions, leek, and garlic provides the
characteristic flavor formed when plant tissue is disrupted, releasing alliinase.
The enzyme released in this process breaks down odorless sulfur-containing amino acids
(S-alk(en)-yl-l-cysteines) and their sulfoxides. These are cleaved to yield pyruvate,
ammonia, and sulfur-containing volatiles (Figure 4), giving the characteristic pungency
and lachrymatory effect.
104–109
Figure 4
Formation of potent sulfur-containing compounds in Allium species.(110)
It is important to note that thermal processing of the vegetables denatures the enzymes
responsible for these reactions;
102,111
hence, preparing, for example, onions and garlic whole in the oven gives a much milder
and rather different flavor since the enzyme will never become active. The alliinase
can be inactivated irreversibly at low pH (below 3), preventing these reactions to
occur. Thus, acid marinating directly after cutting and crushing would result in similar
mild flavors.
4.1.2.2. Hydrolysis. Many of the nutritionally important molecules in our food are
large polymers, e.g., starches and proteins; these are neither particularly soluble
or volatile and so have little if any perceivable taste or aroma. We can only detect
their presence in foods though the textures they create and not by any specific taste.
However, when degraded into smaller molecules they generate a wide range of tastants
and aromatics that we can and do enjoy in the taste and smell of our foods. While
there are several different routes through which such degradation can occur, hydrolysis
is probably the most important of these and the subject of this section.
The three macronutrients carbohydrate, proteins, and lipids are all able to react
with water so that the macromolecules are degraded into smaller pieces which are likely
to possess aroma or flavor (which may be pleasant or in some cases unpleasant). Carbohydrates
are polymers in which the monomers (sugars) are linked together by glycosidic bonds,
which may hydrolyze as a result of acid catalysis or via enzymatic reactions. For
example, malting, the initial stage in brewing beer, is an enzymatic hydrolysis of
starch which produces smaller carbohydrates, making them available for the yeast to
convert to alcohol and carbon dioxide. The sweet taste of malted bread is similarly
produced from the small carbohydrates or oligosaccharides produced by hydrolysis.
Proteins may, in addition to the enzymatic and acid-catalyzed hydrolysis seen in carbohydrates,
also hydrolyze via base catalysis since the amino acids are linked together by amide
bonds. For example, proteins can be hydrolyzed during the ripening of cheese or boiling
of meats. In this process shorter peptides are formed which eventually can be cleaved
to individual amino acids. In general, the more hydrophobic amino acids taste bitter,(112)
while the more hydrophilic amino acids have a neutral or sweet taste.
112,113
Thus, peptides with specific combinations of hydrophobic amino acids can be extremely
bitter and may ruin the flavor of cheeses if formed in significant quantities. The
amino acid glutamic acid (and small peptides with glutamic acid) are often formed
during cooking of meat, in soups, and in savory cheeses. These are particularly important
as they excite the umami taste sensation, giving these foods their characteristic
rounded flavors. Most foodstuffs are either acidic or neutral, so base catalysis is
relatively uncommon. However, in fish treated with lime or other alkaline substances
(as is traditional in some Scandinavian and Asian countries) hydrolysis through base
catalysis plays a significant role in flavor development.
Lipolytic enzymes can hydrolytically cleave the lipids to form free fatty acids; these
reactions are particularly important when considering rancidity in oils and fats.
For example, olive oil is produced from fruit pulp which has a high lipid content;
these lipids are easily hydrolyzed by enzymes so that the oil can contain significant
quantities of free fatty acids.(114) A high content of free fatty acids in an oil
lowers its thermal stability during frying which may impair its flavor. Similarly,
butter may go rancid due to lipid hydrolysis of butterfat to produce short-chained
fatty acids such as butyric acid, ruining the flavor.
In some sausages lipid hydrolysis during fermentation and ripening actually provides
a more soapy flavor, which is not necessarily negative for the overall flavor.(115)
Partially hydrolyzed lipids are surface active and find use as emulsifiers, for example,
in bread to reduce staling and starch crystallization.
Mold ripening of cheese yields very complex flavors. The initial ripening occurs through
protein hydrolysis and is followed by lipolysis, while for blue cheeses like Danablue
and Roquefort free fatty acids make significant contributions to the flavor.
4.1.2.3. Oxidation. A second process that can degrade food molecules is oxidation.
Unlike hydrolysis, in nearly all cases oxidation leads to flavors that are not desirable,
so we normally strive to reduce or avoid oxidation during storage and processing of
fresh ingredients.
Oxidation of foods may be initiated enzymatically, for example, in vegetables post
harvest, by catalysis by transition-metal ions, which can occur, for example, when
using copper pans for frying, or by exposure to light, as happens with green plant
oils. Most food components are vulnerable to oxidation, and oxidation may change their
flavor, color, and nutritive value. In the kitchen we are familiar with oxidation
especially in relation to lipids; vegetable oils and lard may go rancid during storage,
while pork and poultry often develop a so-called “warmed-over flavor”(116) upon reheating.
Among the lipids, the degree of unsaturation is crucial. Oils and fats from fish are
extremely sensitive to oxidation, while animal fats from ruminants are less sensitive;
lard and plant oils have intermediate sensitivity. The propensity to oxidation depends
on the degree of unsaturation of the lipids and on the level and nature of any antioxidants
present. A convenient illustration can be seen in the best choice of various plant
oils for cooking. Heavily unsaturated oils such as pumpkin oil and walnut oil oxidize
easily on heating and should never be used for frying, while they are perfect for
dressings with their mild and unique taste. More saturated oils such as palm oil,
rapeseed oil (canola), and olive oil are almost perfect for frying having high monounsaturated
content, while grape-seed oil and soy oil should be used with care due to the increasing
amounts of polyunsaturated fatty acids present in the lipids. Olive oil adds a unique
flavor to fried vegetables or meat, while canola oil is more neutral and accordingly
preferred by some.
The reuse of oil for frying, as is often the case in deep frying, can generate new
flavors that are characteristic of the specific oil used; often these flavors can
be unpleasant and are classed as “off flavors”. Fatty acids belonging to the Ω-9 family
such as oleic acid have the major oxidation product nonanal, while hexanal dominates
the oxidation of the Ω-6 fatty acids and propanal the Ω-3 fatty acids. Hexanal has
been found to be an important compound in warmed-over flavor.
116,117
Another important aspect of oxidation of foods in the kitchen is the oxidative polymerization
of polyphenoles which causes the enzymatic browning in avocados and apples. Control
of enzymatic browning is also important for producing tea. Green teas are obtained
by deactivating polyphenoloxidases by heating freshly picked leaves, black teas get
their color from polyphenoloxidase activity, while oolong teas obtain their light
brown color by strictly controlled heat deactivation of the enzymes. Notably, co-oxidation
of carotenoides in the tea leaves is crucial for the development of the flowery notes
characteristics for the fermented tea.
4.1.2.4. Lipid Oxidation and Antioxidants. Lipid oxidation is a chain reaction with
free radicals as reactive intermediates.(118) Lipid oxidation depends on oxygen activation
or attack by free radicals on the lipids, leading to formation of lipid hydroperoxides
without flavor as the primary oxidative products as illustrated in Figure 5. Oxygen
activation by metal-ion catalysis may produce the hydroxyl radical capable of initiating
the chain reaction (A in Figure 5). Lipoxygenase (D in Figure 5) results in direct
formation of the lipid hydroperoxides, and also photosensitization by chlorophylls
and other pigments (C in Figure 5) is important. Secondary lipid oxidation products
are formed by cleavage of the hydroperoxides to form aldehydes and ketones with low
sensory threshold values.
Figure 5
Lipid oxidation may be initiated by free radicals or enzymatically (A), by transition
metal catalysis (B), photochemically (C), or by lipoxygenases (D).
Metal catalysis cleavage of the hydroperoxides may be due to the redox activity of
heme pigments. The traditional use of thyme to spice blood sausages in traditional
Nordic cooking is an example of the beneficial effect of a natural antioxidant. Thyme
prevents any oxidized flavor as phenolic compounds such as thymol and carvacrol present
in thyme provide antioxidant protection. Phenolic antioxidants, to which class vitamin
E, α-tocopherol, also belongs, are chain breaking as they donate a hydrogen atom to
the lipid radical in the reaction marked by A in Figure 5. Another example of a traditional
source of natural antioxidants is rosemary; many traditional dishes spiced with rosemary
are inherently protected against warmed-over flavor by the phenols from rosemary.(119)
Interestingly, the lipoxygenase enzymes can be active even at the low temperature
of frozen storage.(120) This is the reason why blanching of vegetables prior to frozen
storage to inhibit such lipid oxidation being initiated by lipoxygenase is important
(D in Figure 5).
Carotenoids which are present together with the chlorophylls in the photosynthetic
apparatus in plant cells yield protection against photo-oxidation(121) (C in Figure
5). Photo-oxidation is a comparatively slow process and seldom a problem in the kitchen.
However, changes due to photo-oxidation will occur in some foods when stored in the
light. For example, virgin olive oil and other green vegetable oils should never be
stored in light as they will slowly oxidize. Thus, even though a bottle of olive oil
in the kitchen window can be very decorative it is not a good idea as the oil will
slowly deteriorate.
A third antioxidative mechanism depends on the complexation of pro-oxidative metals
by flavonoides(122) (B in Figure 5). Onions, which are rich in quercetin, may be active
by this mechanism when added to high-fat fresh sausages and pâtés. Green tea rich
in catechin holds the potential for a similar use in other dishes.
123,124
The quality of raw material is important for the oxidative stability. For nonruminants
the feeding regime is crucial. A large supply of unsaturated lipids in the feed increases
the risk of oxidation in the meat, while a rich supply of antioxidants from herbs
or other sources such as acorns yields protection. On the Iberian Peninsula pigs for
the highly praised dry cured hams finish their lives roaming in the cork oak grooves
feeding on acorns rich in antioxidants. Lipid oxidation in such products is characterized
by a lag phase, the duration of which depends on the feeding regime. As long as the
antioxidants are available at a sufficient level, the lipids are protected, while
when they are depleted, oxidation wins and the meat can become rancid very rapidly.
4.1.2.5. Maillard, Strecker, and Caramelization Reactions. We all know from experience
that flavor develops when we cook (heat) our food. Thus, the most important reactions,
from the perspective of flavor development, are those that are thermally activated.
Among these, the Maillard, Strecker, and caramelization reactions are largely responsible
not only for the characteristic flavors of cooked meats; but also for the flavors
of chocolate and coffee as well the caramel flavors of cooked sugars in sweets and
on the crust of freshly baked breads, etc. All of these flavors are produced in chemical
reactions and as such are not natural, although most people would regard these flavors
as completely natural while considering some extracted essential flavors (from, for
example, citrus fruits) which may be added back into food products as artificial.
It will be one of the challenges of Molecular Gastronomy to educate people (domestic
cooks, chefs, and the wider public) to understand better how flavors arise and appreciate
the very subtle differences between so-called “natural” and “synthetic” products.
When foods are heated, reducing sugars and other carbonyl compounds together with
amino acids (or other amines including peptides and proteins) undergo a complex network
of reactions that produce both volatile and higher molecular weight compounds including
colored pigments and heterogeneous polymers. The formation of brown products on heating
aqueous mixtures of amino acids and sugars was first described by Louis Maillard in
1912. Since then, Maillard reactions have been subject to many studies in foods and
model systems. Maillard and related reactions are complex and difficult to understand
fully or predict. However, we do now have a fairly detailed understanding of the reaction
pathways involved as well as the role of the reaction products in flavor, browning,
and nutrition. Further, the antioxidative, toxicological, and antimutagenic properties
of many of the products of these reactions are now reasonably well documented.(125)
Of course, most foods are complex systems with many components that can react when
heated. Accordingly, the Maillard reaction often occurs in parallel with other reactions
such as the Strecker reactions and caramelization of sugars. Maillard reactions have
also been implicated with thermal degradation of lipids, leading to the lack of the
desirable flavor generation by producing heterocyclic compounds with aliphatic side
chains.(126)
4.1.2.5.1. Principal Mechanisms of Maillard and Associated Reactions. The Maillard
reaction has been divided into three stages, consisting of the condensation of an
amine with a reducing sugar, to produce an N-substituted glycosylamine. The subsequent
Amadori (for aldoses) or Heyns (for ketoses) rearrangement then produces isomeric
compounds. These unstable compounds give neither browning nor aroma but serve as important
precursors for subsequent reactions (Figure 6). The second stage involves the enolization
of the Amadori and Heyns products and elimination of the amino compound under formation
of 1-deoxyosones or 3-deoxyosones. These compounds subsequently undergo dehydration
and fragmentation reactions when sufficient heat is provided.
Figure 6
Initial phase of the Maillard reaction showing the formation of Amadori compounds.
The Amadori product can be broken down by either of two pathways. The 1,2-enolization
is promoted by protonization of the N atom in the Amadori compound, which is favored
under acidic conditions and leads to formation of a 3-deoxyosone. In a more alkaline
environment, protonization of the Amadori compound is less effective and, thereby,
indirectly stimulates the 2,3-enolization.
127,128
Alkaline conditions thus favor the formation of hydroxyfuranones via the 2,3-enolization
and formation of a 1-deoxyosone (Figure 7).
Figure 7
Enolization of the Amadori compounds under different pH conditions.
In the dehydration of the deoxyosones, furfurals and/or furanones are formed. Furthermore,
the fission of deoxyosones, e.g., by retro-aldolization, may produce a range of carbonyl
compounds such as acetaldehyde, 2-oxopropanal, hydroxypropanone, and butanedione.(128)
An example of the degradation of the deoxypentosonone is given in Figure 8. It is
at this stage of the Maillard reaction that many different flavor compounds develop
in a network of competing reactions.
Figure 8
Some possible degradation products from deoxypentosones found in cooked foods. These
compounds may also derive from other pathways in the Maillard reaction.
The formation of reductones and fission products in the Maillard reaction is associated
with the Strecker degradation of amino acids. This reaction involves condensation
of α-amino acids with conjugated dicarbonyl compounds to form a Schiff base, which
enolizes into amino acid derivatives that are easily decarboxylated. The new Schiff
base with one atom less is then split hydrolytically into an amine and an aldehyde
which correspond to the original amino acid with one carbon atom less. The net result
of the Strecker degradation is a transamination which could be an important reaction
for the incorporation of nitrogen into melanoidins.(129) Secondary amino acids such
as proline and hydroxyproline resist Strecker degradation because of the blocked transamination.
However, in heating reactions with sugars, proline is an important precursor for compounds
with cereal-like flavor notes. The Strecker reaction of cysteine results in the generation
of ammonia, ethanal 2-mercaptoethanal, and hydrogen sulfide, which are important precursors
for the hetrocyclic flavor compounds. Methionine can also undergo further degradation
by forming ammonia, methional, methanethiol, and dimethyl disulfide.(130) The aminoketones
formed during Strecker degradation can through condensation reactions form different
kinds of alkylpyrazines, which are typically found in toasted/roasted foods such as
nuts, bread, and meat.(131)
The intermediate stage of the Maillard reaction and related reactions provides a complex
pool of reactive compounds, which are subjected to rearrangements and further reactions
producing several classes of heterocyclic (volatile) products, several of which are
important for cooked flavor. This can be characterized as the final stage of the Maillard
reaction, which also comprises the irreversible conversion of carbonyl compounds,
e.g., by retro-aldolization with or without involving amines into high molecular weight
(brown) heterocyclic (co)polymers, the melanoidins.
Caramelization of sugars also gives rise to browning and generation of volatile flavor
compounds and involves enolization, dehydration, and fragmentation pathways. Dehydration
of sugars producing 2-furfural (from pentoses) and 5-hydroxymethyl-2-furfural (from
hexoses) occurs at temperatures greater than 150 °C. At higher temperatures colored
pigments and a number of volatiles are generated, including furans, carbonyl compounds,
etc.(132) In the Maillard reaction, amines allow such reactions to take place under
milder conditions and in addition serve as a source for other heterocyclic compounds
to be generated.(133)
In addition to the classical pathways in the Maillard reaction, alternative routes
have been proposed. Some of these reaction schemes involve the early stage of the
reaction such as the double substitution of the amino-deoxy-ketose with subsequent
regeneration of the amino acid.(134) Other pathways include an early cleavage of the
sugar moiety of the Schiff base prior to Amadori rearrangement. This route seems to
occur at neutral and alkaline pH and involves formation of C2 and C3 carbonyl-alkylamine
fragments that may condense to N-heterocyclic polymers.
135,136
Mechanistic studies using [13C]-labeled sugars have provided more details of possible
fragmentation routes of the sugar moiety in the Maillard reaction.
137–139
4.1.2.5.2. Interactions with Lipids. Several studies have pointed out the participation
of compounds from lipid degradation in the Maillard reaction. A number of volatiles
have been identified, with the majority known as long-chain alkyl-substituted heterocycles
with nitrogen and/or sulfur in the ring. The mechanisms for the formation of these
lipid−Maillard interaction compounds from foods and model systems have been reviewed
elsewhere.(126) The flavor characterisitics of some of these alkyl-substituted heterocycles
have been described by ‘fatty’, ‘tallow-like’, ‘green’, ‘vegetable-like’ terms;(140)
however, their odor threshold values are generally much higher than the Maillard compounds
contributing with desirable flavor notes. Thus, when competing for reaction intermediates,
lipid degradation products may decrease the Maillard flavors during cooking. The levels
of unsaturated fatty acids and conjugated alkadienals may be important for these reactions
to occur.
An example of the reaction of 2,4-decadienal with ammonia or hydrogen disulfide leading
to the formation of 2-pentylpyridine, 2-hexylthiophene, and 2-pentyl-2H-thiapyran
is given in Figure 9. In baked cereals n-ethyl, n-butyl, and n-pentyl 2H-thiapyrans
have been identified when levels of secondary lipid oxidation products were relatively
high.(141) In cooked beef from cattle fed diets supplemented with fish oil or linseed
oil Elmore et al.(142) found many alkylthiazoles and alkyl-3-thiazolines. The substituents
in positions 4 and 5 were methyl or ethyl groups, while the 2-position contained isopropyl,
isobutyl, and C4−C9 n-alkyl groups. The concentrations of 2-n-alkyl-3-thiazolines
were much higher in the steaks from the cattle fed with fish oil supplements than
in the control samples. These 3-thiazolines may also form from the interaction of
intermediates of the Maillard reaction with aldehydes derived from lipid degradation.
Figure 9
Schematic reaction pathways of 2,4-decadienal from polyunsaturated fatty acids with
hydrogen sulfide or ammonia yielding long-chain alkyl-branched heterocyclic compounds
(after Farmer and Mottram(143)).
4.1.2.5.3. Factors Affecting the Rate and Direction of the Maillard Reaction. The
Maillard reaction has been shown to be strongly dependent on the reaction conditions
and availability of reactants throughout the food. The most important parameters affecting
the generation of aroma volatiles are combinations of temperature−time, moisture content,
pH, and type of amine and carbonyl precursors present.
129,141,144,145
Many foods are heterogeneous materials, and the reaction may be favored or inhibited
locally in the product. The reaction often is most severe at the surface of a product,
where the water content locally can be reduced and concentrations of precursors increased
rapidly. Furthermore, the product surface temperature may locally exceed 100 °C. Water
transport from the inner part of the product to the surface during cooking may also
contribute to transport of Maillard precursors, such as monosaccharides and amino
acids, which decrease due to their reactivity at the product surface. Changes in the
physical phases in the food, where Maillard reactions take place, also contribute
to the degree of color and flavor generation in the product. Phase transitions from
liquid to rubbery and glassy states significantly affect the course of the reaction.
At temperatures below the glass transition temperature (T
g) the rates of browning are generally low and increase at temperatures closer to
and above T
g. Different reactivity for monosaccharides has been observed, and the rate of browning
is not clearly related to the rate of loss of amino acid.
146,147
At temperatures below T
g, the limited mobility of small reactant molecules in the glass material may be a
limiting factor for the Maillard reaction to occur efficiently.(148)
The pH is also an important factor in controlling the Maillard reaction, influencing
the reactivity of free amino acids (both thiols and amine groups) and enolization
of the Amadori compounds. In cooked meat with relatively small changes in the initial
pH (4.0−5.6) rather large changes in the aromas and aroma volatiles have been reported.
The total quantity of volatile compounds increased as the pH decreased. A number of
furanthiols and their oxidation products were preferentially formed at acid pH; some
of these compounds had strong meaty aromas. The formation of other heterocyclic compounds
such as thiazoles and pyrazines were favored by higher pH.(149) In extrusion cooking
of cereals, changing the pH by addition of sodium hydroxide in the feed also showed
significant changes in the range and levels of aroma volatiles; however, these changes
were less marked than those achievable by temperature and moisture combinations.(141)
A summary of changes occurring during the different stages of the Maillard reaction
is given in Table 3.
Table 3
Changes Occurring during Different Stages of the Maillard Reaction (adapted from Nursten(125))
changes
initial phase
intermediate stage
final stage
production of color or discoloration
−
+
+++
production of flavor or off-flavor
−
+
++
production of water
+
+
+
lowering of pH
+
+
+
antioxidative activity
+
+
+
loss of vitamin C activity
+
−
−
loss of biological value of proteins
+
+
+
production of fluorescence
−
+
+
4.1.2.5.4. Maillard-Derived Flavor Compounds and Their Presence in Foods. Flavor formation
constitutes quantitatively only a minor pathway in the Maillard reaction, since the
majority of the reactants are converted to carbon dioxide, melanoidins, and numerous
intermediates rather than volatiles. However, a great number of volatile and nonvolatile
compounds may be generated in the Maillard reaction, and many of them have been identified
and characterized sensorial. The aroma volatiles from the Maillard reaction may be
classified into three groups to provide a convenient way of viewing the origin of
the complex mixture of compounds. The classes of compounds in these groups are organized
according to their stage of formation and origin and include (1) simple sugar dehydration
and fragmentation products such as furans, cyclopentenes, and carbonyl compounds,
(2) amino acid degradation products such as Strecker aldehydes, and (3) products from
further interactions.(133)
A number of these compounds have relatively high flavor threshold values and need
to be generated in excess in order to contribute to flavor. However, the compounds
belonging to the third group include a wide variety of small, often branched, heterocyclic
flavor compounds including pyrroles, pyridines, pyrazines, imidazoles, oxaxoles, thiophenes,
thiazoles, thiazines, furanthiols, and sulfides. Several of these compounds have rather
low odor threshold values, and their aromas are often characterized by ‘nutty’, ‘roasted’,
‘toasted’, ‘cooked vegetable’, ‘caramel’, and ‘meaty’ flavor notes typically present
in heat-treated foods as in coffee, chocolate, roasted seeds and nuts, vegetables,
meat, and cooked cereals.
140,150
The mixture of flavor compounds created in the Maillard reaction is usually complex.
The contribution of individual compounds to the flavors of the Maillard reaction is
difficult to predict, since they can give different kinds of perceptual effects in
mixtures, such as suppression, addition, and in few cases even synergy. Furthermore,
the compounds can both contribute to odor impressions as well as taste sensations.
In a few instances Maillard compounds have been attributed as major contributors to
the typical flavor of foods.(151) For example, 2-furanylmethanethiol is an impact
compound in the aroma of freshly roasted coffee,(150) whereas 2-acetyl-1-pyrroline
and to a lesser degree 6-acetyl-2,3,4,5-tetrahydropyridine have been identified as
key aroma compounds in bread crust(152) and popcorn.(153) The chemistry and occurrence
of these and other related potent ‘roasty’ smelling compounds have been reviewed elsewhere.(154)
Alkylpyrazines are often an indication of the degree of Maillard flavor development
during cooking, although very few of them are generated in sufficiently high amounts
to give a contribution to the flavor.
In meat flavor it has been difficult to identify key impact compounds, since meat
can be cooked in different ways and originates from different species. Maillard flavors
in beef, chicken, and pork flavor have been investigated to a greater extent relative
to other species. The Maillard flavors have been shown to depend on the raw meat quality
and cooking method, and many different heterocyclic compounds have been identified.(155)
The flavor precursors in meat are different from those in plant materials. This is
partly due to the fat content and fat distribution throughout the meat, which can
interact with Maillard reactions, but also due to enzymatic reactions post mortem,
which often increase the levels of important Maillard precursors. Enzymatic hydrolysis
of ribonucleotides including adenosyl monophosphate (AMP) post mortem can lead to
relatively high levels of ribose. This aldopentose is an effective Maillard precursor
for typical ‘meaty’ flavor compounds with very low flavor thresholds such as 2-methyl-3-furanthiol
(MFT) and bis(2-methyl-3-furanyl) disulfide (MFT-MFT)(156) (see Figure 10). Although
these compounds have long been known from model reactions of ribose and cysteine,
they have also been identified in cooked beef.(157) Further, phosphorylated monosaccharides
and peptides are hydrolyzed post slaughter and can yield significantly higher levels
during post mortem conditioning of beef.
157,158
In pork it has been shown that ribose is rather unstable and glucose and fructose
are the most likely important Maillard precursors.(159)
Figure 10
Formation of some important meat flavor compounds from the Amadori product of ribose
(modified after Mottram and Whitfield(156)).
Studies on the key aroma compounds in beef have shown the importance of the Maillard
reaction. In boiled beef, MFT, 2-furanylmethanethiol, 2-acetyl-1-pyrroline, MFT-MFT,
and methional were ranked with a relatively high sensory impact together with some
lipid degradation products.(157) In roasted beef other Maillard compounds were indicated
as sensorially more important and included 2-acetyl-2-thiazoline, 4-hydroxy-2,5-dimethyl-3(2H)-furanone,
2-ethyl-3,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, and methional.(160)
Although many volatile compounds have been identified in Maillard reaction systems
and implicated in the aromas of many foods, their impact to bitter tastes should also
be mentioned. Sugar dehydration products such as 2-furfural, 5-hydroxymethyl-2-furfural,
and 2-methyl-3-hydroxy-4H-pyran-4-one are known to possess bitter tastes as well as
a number of alkylpyrazines(161) and some thiazoles and thiazolidines.(162)
Products derived from the reaction of carbonyl compounds with proline provide the
specific bitter compounds cyclo[b]azepin-8(1H)-ones and pyrrolidinyl-2-cyclopenten-1-ones
as identified in model proline reaction systems and beer.(139) Cyclo[b]azepin-8(1H)-ones
and a number of 2,3-dihydro-1H-pyrrolidines have also been found in bread crust from
dough enriched with proline.(163) In addition to the volatile compounds from the Maillard
reaction, which as well as their aroma also possess a bitter taste, a number of nonvolatile
bitter compounds have been identified, e.g., diketopiperazines in cocoa.(164) More
recently, bitter-tasting compounds have been identified by taste-dilution analysis,
and potent bitter compounds from the Maillard reaction with xylose, rhamnose, and
l-alanine have been characterized including 1-oxo-2,3-dihydro-1H-indolizinium-6-olates.(165)
In particular, the thiophene derivatives show extraordinarily low bitter taste thresholds
down to 6.3 × 10−5 mmol/L water.
The Maillard and caramelization reactions provide an excellent means of generating
a variety of desirable flavors during cooking of foods. Although many Maillard precursors
and conditions have been identified to influence the course of the reaction in different
foods, the applications in gastronomy will still depend on experimenting with these
to control the desirable outcomes. Although the positive formation of flavors in the
Maillard reaction has received much attention, control of the generation of toxic
compounds, e.g., imidazoquinolines and acrylamide, also deserve consideration. For
example, tryptophan is effective in preventing the formation of imidazoquinolines,
and cooking methods can be optimized with regard to acrylamide formation from asparagine.
The toxicological aspects of the reaction should deserve attention among gastronomers
both from a consumer standpoint and from a chef’s standpoint during the preparation
of the food. Some toxic compounds may not enter the food, since they are evaporated
during the cooking process, but need to be removed efficiently from the cooking environment
in protection of the chef’s health.
4.1.3
Illustrative Example: Preparing Meat Stocks
It is clear from the foregoing sections that the chemistry of cooking is extremely
complex, making it very difficult to predict with any certainty how flavor will develop
in any real cooking process. Not only will the ingredients themselves vary from batch
to batch but also the actual processing of these ingredients will never be fully consistent
in a real kitchen nor will the temperatures and times of the cooking processes. Thus,
while we may understand some of the general principles it is unlikely we will ever
be able to master cooking simply from the underlying chemistry; this is one reason
why chefs always continually taste samples from their dishes as they prepare them.
However, it is a worthwhile exercise to examine at least one apparently standard cooking
process to see how many of the above reactions occur and attempt to understand how
knowledge of the chemical changes occurring during cooking can be translated into
the kitchen environment. To this end, we have chosen the cooking of a basic meat stock
as an exemple.
One of the most remarkable processes in the kitchen is the preparation of a stock;
initially flavor molecules are simply extracted from the basic ingredients (meat,
vegetables, etc.) in water (or sometimes in wine), but something quite remarkable
happens as the liquid is kept simmering: the flavor changes and apparently new flavors
develop. More astonishing is the fact that as the simmering continues, the stock is
reduced (by evaporation of water), and the kitchen is filled with the aromas of the
cooking stock, even more flavor develops. One would naturally expect that the flavor
molecules, being volatile, would evaporate, leading to a reduction in the overall
flavor of the stock. However, in practice, the process of reducing a stock by slow
evaporation actually concentrates and enhances the flavor.
Such a complex process is clearly too difficult to interpret fully, but it serves
to illustrate several aspects of the development of flavors discussed above and, in
particular, demonstrates the complexity of the nature of the chemistry of cooking.
In the various studies we discuss below different recipes were used; some authors
use the ingredients of a typical stock, meat, bones, and vegetables all cooked together,
while others prefer to try to isolate the importance of particular ingredients and
so tend to prepare a stock using only the meat (or the meat and bones) without the
vegetables.
When studying a recipe for a stock a lot of things can be questioned: does it matter
when the salt is added? Is the initial temperature of the water important? What is
the effect of the time of cooking? How does the ratio of the meat to the bones affect
the final product? And so on. When the stock is further boiled down (reduced) to make
a concentrated liquid, further issues arise including which flavor compounds are lost
through evaporation and which are formed in the pot during cooking.
Recipes will often tell you to start with cold water and bring the pot to simmer,
regularly skimming off fat and scum. According to McGee,(166) starting with cold water
allows soluble proteins to escape the solids and coagulate slowly, forming large aggregates
that either rise to the surface and are easily skimmed off or settle to the bottom
and sides. A hot start produces many separate and tiny protein particles that remain
suspended and cloud the stock, and a boil turns particles and fat droplets into a
cloudy suspension and emulsion. The reason for recommending to leave the pot uncovered
is according to McGee:(166) it allows water to evaporate and cool the surface, which
makes it less likely that the stock will boil. It also dehydrates the surface scum,
which becomes more insoluble and easier to skim. Further, it starts the process of
concentration that will give the stock a more intense flavor.
A lot of research has been concerned with the flavor of beef and attempted to identify
the key aroma components constituting the beef stock aroma.
167–169
There has been considerable similar research on other types of meat such as chicken
and seafood.
170,171
To identify the important aroma and taste compounds in a given food it is necessary
to have appropriate techniques for separating the volatile and nonvolatile compounds
(chromatography) and identifying the components as well as method for determining
the sensory impact of these (GC-Olfactometry, a method where a panel of trained subjects
evaluate the quality and intensity of GC effluents). Less research has been carried
out on the cooking process of the stock, although some studies have been concerned
with the cooking procedures of the stock and the difference it makes for the flavor
of the stock.
On the basis of cookery books, Seuss et al.(172) hypothesized that the temperature
progress during stock cooking is important for the resulting flavor. They investigated
the effect of temperature and cut size (minced meat vs small cubes) on the flavor
of beef stock as well as on several nonvolatile compounds. They found that the best
flavor, as determined by a sensory panel, was obtained when cooking at 85 °C. The
authors recommend cooking the broth at a temperature below the boiling point since
they find that the stock cooked at 95 °C becomes less strong in meat flavor and more
bitter. In general, positive sensory assessments were related to a high concentration
of inosine monophospahate, inosine, lactate, and free amino acids. The difference
between minced meat and small cubes of meat (2 cm) was found not to be important for
the flavor.
Cambero et al.(173) also studied the flavor of beef stocks as affected by cooking
temperature and identified some of the compounds responsible for the brothy flavor.
In correspondence with Seuss et al., they found that the brothy flavor was strongest
when cooking at 85 °C (based on four trained judges). The chemical analyses showed
that a wide mixture of free fatty acids, peptides of low molecular weight (<300 Da),
and IMP all had an important role in the flavor intensity of the beef broth.
The relationship between beef stock components and the flavor developed at different
cooking temperatures has been further studied by Cambero et al.(174) The combined
sensory study (descriptive analysis and rank order test with 9 trained panelists)
with chemical analyses showed a significant increase of small non-amino acid nitrogen
compounds, creatine, GMP, IMP, and AMP with beef stock flavor intensity. The beef
stock obtained at 85 °C was found by sensory analysis to have the most acceptable
flavor, presumably related to the free sugars and amino acids and their reaction products.
Stocks produced at temperatures higher than 95 °C displayed off flavors which were
easily detected by the sensory panel.
Furthermore, Cambero et al.(175) investigated the flavor development of beef stock
in more detail by studying the effect of the cooking temperature, cooking time, ratio
of meat and water, and NaCl concentration. They found that the cooking temperature
is important since less intense heat treatments generated raw meat, bloody, and metallic
flavors whereas stocks prepared at higher temperatures generated sour, astringent,
and warmed-over flavors (WOF). Stock obtained at higher temperatures needed a shorter
cooking time to obtain a good flavor; however, it was concluded that temperature plays
a more important role than cooking time in the generation of the sensory properties
of a good beef stock. The best stock was obtained by cooking at 85 °C for 60 min with
7.5 g/L salt with a ratio of 1:2 (meat:water by weight).
Pereira-Lima et al.(176) performed a similar study on the flavor of beef stock, comparing
the sensory results with chemical data (amino acids, free fatty acids, and the dipeptides
carnosine and anserine) on stocks cooked at various conditions (cooking temperature
and time). A positive relationship is seen between a good beef stock flavor and increased
levels of Glu, Asn, Lys, and Met. An inverse relationship to beef stock flavor was
found with Cys, Pro, Ser, M-His, Tyr, Val, Arg, and Asp, which could be interpreted
as a positive relation of the reaction products (Maillard, Stecker) of these and stock
flavor. The amounts of the dipeptides, carnosine and anserine, increase significantly
with increasing cooking temperature but not with the cooking time. Sensory evaluation
shows that increased carnosine and anserine levels provide an improved flavor.
In a later study by Cambero et al.(177) they investigated the effect of cooking conditions
on the flavor of shrimp stocks. They studied various NaCl concentrations, shrimp−solutions
ratios, cooking temperatures, cooking time, and shrimp preparations in order to determine
the cooking conditions that yield the best stock flavor. Best stock was obtained by
using whole shrimp in 0.5% NaCl solution (1:2) at 85 °C for 30 min. The stocks were
evaluated by a sensory panel (11 trained panelists, rank order test), and the chemical
composition was analyzed. Boiling was found to cause the formation of off flavors.
The free fatty acids were found to be very important for sensory evaluation of the
stocks (the best stocks had the highest level of FFA). Overall temperature was found
to be more important than time, just as with the beef stock study by the same author.
In the kitchen the preparation of stocks is one of the most important operations;
stocks form the basis of nearly all sauces, so that chefs devote a great deal of time
and effort to their preparation. Cookery texts differ greatly on the best methods
to use when preparing meat stocks; for example, some indicate it is essential to put
the meat in cold water and heat up, while others permit meat to be put directly in
hot water; some suggest meat should be browned before boiling whereas others do not.
Most cookery tests suggest “simmering” rather than boiling, which indicates they may
be suggesting using a low temperature to achieve a better result. As yet, it is very
difficult to draw any definite conclusions about the relative merits of such diverse
methods of stock preparation. However, a few general principles do emerge from the
limited scientific studies to date. For example, the fact that the amount of dipeptides
created depends more on the temperature than the time immediately suggests there may
be benefits from cooking stocks in a pressure cooker where temperatures well in excess
of 100 °C can be used; indeed, this is the technique used for most stock production
at several restaurants(178) or for longer times at much lower temperatures (no higher
than 90 °C), which is not a technique used in any kitchen that we are aware of.
4.2
Color of Food
As we shall see later, our perception of the flavor of food is influenced by many
factors other than detection of the actual aroma and taste molecules released by the
food itself. One of these is the color of the food. Color sets up an expectation of
the flavor: red fruits seem to be riper, green vegetables fresher, and (for some)
purple meats more perfectly cooked. Thus, it is worth reviewing some of what we know
about how the color of meats, fruits, and vegetables changes during processing so
that we may, to some extent, control the changes to provide the diner with a dish
that has the most acceptable color. We begin with the better studied area of the color
of meat and then move onto look at fruits and vegetables.
4.2.1
Color of Meats
In modern retail, the quality of meat is often judged by its immediate appearance;
thus, any apparent discoloration of meat is, for many consumers, the primary reason
for rejection of a specific product. During cooking color changes are further used
to follow the progression of heat treatment, and when served, meat is expected to
have a characteristic color appearance depending on the actual meat product, heat
treatment, dish of which the meat is a part, and personal preference of the diner.
For example, the internal color of a steak is used by many as an indicator of its
“doneness”, a purple color indicating a rare steak, red a medium steak, pink well
done, and gray or brown showing the steak is overcooked.
The color of meat is dominated mainly by myoglobin, the heme protein with the physiological
function of oxygen storage in muscles. The attractive cherry red color of meat is
due to a steady-state concentration of oxymyoglobin, which in a specific acid-catalyzed
process is oxidized to the brown metmyoglobin, a process which is often termed autoxidation.(179)
The steady-state concentration of oxymyoglobin is maintained as long as the metmyoglobin
reductase enzyme complex can use the reducing cofactor NADH for reforming the iron(II)
form of myoglobin in a process which notably is less dependent on acidity than the
autoxidation.
Myoglobin, which is violet, is further converted back to oxymyoglobin, when oxygen
is available. The steady-state concentration of oxymyoglobin is thus dependent on
pH, as decreasing meat pH accelerates autoxidation more than it accelerates enzymatic
reduction of metmyoglobin. The steady-state concentration of oxymyoglobin is also
dependent on the presence of reducing cofactors together with enzyme activity, as
depletion of reducing cofactors or inactivation of enzymes will block the reduction
back to the iron(II) state.(180) An example of the absorption spectra of some different
forms of myoglobin is presented in Figure 11.
Figure 11
Absorption spectrum of the various forms of myoglobin.
Further, salting decreases the steady-state concentration of oxymyoglobin, as the
acid-catalyzed autoxidation shows a positive kinetic salt effect in agreement with
protonization of the positively charged iron center as rate determining.
179,180
The balance between the violet and red form of iron(II) myoglobin depends on oxygen
pressure. At the meat surface, the red oxymyoglobin dominates, while in the interior
of the meat, where metabolic activity depletes oxygen, the violet myoglobin determines
the color.
The strong oxidant oxygen is coordinated to the reducing iron(II) center of the porphyrin,
which invariably carries the risk of one-electron transfer, creating the superoxide
radical anion and metmyoglobin, the brown and physiologically inactive iron(III) form
of myoglobin.
180–182
Most people associate a brown color in uncooked meat with spoilage and will reject
it. However, a brown color in cooked meat is associated with doneness; the browner
the meat the “better” it is done and the “safer” it is to keep. Thus, we tend to rely
on smell to detect spoiled cooked meat on storage.
In the kitchen, the spectacular color changes are easily observed by cutting a piece
of beef. Once the violet interior is exposed to the air it soon turns cherry red,
and a similar bloom on the meat surface may be seen when a pack of vacuum packed beef
is opened, allowing oxygen to reach the surface.
A careful inspection of freshly cut beef shows that the three myoglobin forms can
be located within the meat by their colors: in the interior the violet myoglobin is
separated from the cherry red oxymyoglobin on the surface by a narrow brown ribbon
of metmyoglobin. Formation of metmyoglobin has a maximal rate in meat where the partial
oxygen pressure makes oxymyoglobin and myoglobin concentrations equal due to the dominating
bimolecular electron transfer to oxygen to yield hydrogen peroxide:(183)
It is the balance between oxygen diffusion into the meat matrix from the surface (which
generally follows Ficks law) and the rate of oxygen consumption by residual metabolic
activity in the meat that determines the depth at which the two iron(II) forms of
myoglobin have equal concentration and at which the rate for metmyoglobin formation
is highest.(183)
Meats are probably the most perfect and complete protein source for human nutrition
and provide endless possibilities of creation of meals with superior eating experience;
thus, the production of meats of the highest possible quality in an ethical and widely
acceptable fashion is a matter with which we should all be concerned. In this context,
it is notable that an animal which is unstressed at the abattoir prior to slaughter
has a high level of muscle glycogen, which in turn provides high levels of reducing
cofactors in the meat with a high glycogen content, known to have better color quality.
Thus, we can, in principle, use the color of meat before and during cooking and later
during eating the meal as an indicator of the ethical and acceptable production of
the meat. Free-ranging animals such as pigs in the Iberian oak grooves or grazing
cattle in highland regions eat a forage with a high tocopherol content, which further
protects pigments and lipids against oxidation in the meat during storage and cooking,
again providing a direct link between the way animals are raised and the quality of
the meat we experience during cooking and in the meal.
The production of hydrogen peroxide in meat as the result of the dynamics of the meat
color cycle or from lactic acid bacteria present on meat surfaces further affects
the proteins and lipids. Hydrogen peroxide can oxidize metmyoglobin to hypervalent
iron forms of myoglobin, which are highly pro-oxidant. Perferrylmyoglobin, a formal
iron(V) compound, can thus initiate lipid oxidation, while ferrylmyoglobin, an iron(IV)
compound, can cleave preformed lipid hydroperoxides.(183) The peroxidation cycle of
myoglobin, which is linked to color cycle as may be seen in Figure 12, accordingly
provides a coupling between pigment oxidation and lipid and protein oxidation in meat.
184,185
Persisting brown discoloration of meat thus indicates that the reducing cofactors
in the product are becoming depleted and that accumulating metmyoglobin is now available
for conversion to hypervalent and prooxidative myoglobins. NADH and other reductants
are also efficient scavengers of the hypervalent myoglobin in effect protecting the
lipids and proteins against oxidation. Meats with such persistent brown discoloration
are likely to be tough to eat and have unpleasant flavors. The depletion of reducing
cofactors is a direct indication of initiation of lipid oxidation, leading to meat
rancidity, and of oxidative protein dimerization, leading to decreased tenderness.
Figure 12
Color cycle of meat.
In Norway it was legal and a common practice until very recently to pack meat in an
atmosphere with a low level of carbon monoxide, which binds very strongly to iron(II)
myoglobin.(181) Carbonylmyoglobin is intensely red and does not oxidize to metmyoglobin
under normal conditions, so the meat products have a remarkable color stability, which
was appreciated in the country with its many remote and small societies and difficult
transportation of foods.
This practice is now being introduced for master packs of retail meats in the United
States in order to achieve better color stability of fresh meat. The formation of
carbonylmyoglobin, however, blocks the color cycle of meat, and the color is no longer
a direct indicator of the oxidative status of the product. Moreover, the color changes
associated with cooking become different, and color as a doneness indicator becomes
corrupted.(186)
In other countries, some meat products are now being packed for the retail trade in
a controlled atmosphere with a high oxygen content of up to 80% in order to increase
the depth of the oxymyoglobin layer of the meat and accordingly to improve the red
appearance of the meat.(187) Such practice has, however, been shown to increase lipid
oxidation in meat and also oxidative dimerization of myosin, a meat protein of importance
for meat tenderness.(187) For a specific meat product the benefits of using a controlled
packaging atmosphere should accordingly always be compared to any other effects introduced
in relation to product quality.(188)
Over the years the curing of meats has developed from simple salting to preserve meat
into a major industry. Along the way, the introduction of degraded organic material
containing nitrate into the salt serendipitously led to the modern use of nitrite
and nitrate in brine curing. This curing method not only preserves the meat but also
creates an appetizing red color. The importance of the color aspect of the curing
is recognized in the German term “umrotung” for the process. The pink color of cured
meat is due to nitrosylmyglobin, an iron(II) form of myoglobin with nitric oxide coordinated,
formed by reduction of nitrite by added ascorbate or inherent reductants like NADH
present in the meat.(189) Nitrosylmyoglobin is an antioxidant,(190) in contrast to
oxymyoglobin, which is a prooxidant. Discoloration of cured meat as seen upon light
exposure in the presence of oxygen should accordingly be avoided, and a brown or gray
discoloration of cured meats indicates that the oxidative protection by nitrosylmyoglobin
has gone. As long as the meat has a reductive capacity, nitrosylmyoglobin may, however,
be reformed and the cured meat still has resistance against lipid and protein oxidation.
The antioxidative cycle of nitrosylmyoglobin as associated to the product color is
seen in Figure 10.
Figure 13
Nitrosyl myoglobin is rather sensitive to the combination of oxygen and light but
may be reformed as long as there is a residual reducing capacity remaining in the
meat.
Parma ham, by contrast, is produced without nitrite or nitrate, and oxymyoglobin is
converted by an unknown reaction sequence to zinc−protophorphyrin, which is the principal
colorant of this type of dry cured meat.
191,192
For dry cured hams made with the use of nitrite the pigment transformation to the
zinc pigment is completely blocked by an unknown mechanism. In Parma ham from Italy
and in the similar Serrano ham from Spain the iron liberated from myoglobin by zinc
apparently becomes immobilized or otherwise inactivated, since these dry-cured meat
products are surprisingly resistant to lipid oxidation, which is normally catalyzed
by simpler iron compounds including Fe(II) and Fe(III) ions. The high gastronomic
value of these dry cured hams depends on very complex chemical transformations in
the meat matrix during the long maturation period of up to 18 months, which are only
poorly understood but should be explored for use in other processed meats.
4.2.2
Color of Fruit and Vegetables
The pigments of fruit and vegetables play important physiological roles in the living
plants. The chlorophylls match the spectrum of sunlight for optimal conversion of
light energy to chemical energy in the plant. At the same time, we have adapted so
that the green color provides an indication of freshness while a fading green color
acts as a sign of withering. The blue, red, and yellow pigments in plants belong to
several classes of chemical compounds of which the carotenoids and polyphenols are
the most important.(193) Carotenoids are associated with the chlorophylls as auxiliary
light-harvesting pigments in photosynthesis and are quenchers of singlet-oxygen and
scavengers of free radicals for protection during high flux of light.
194,195
Polyphenols have functions as filters for UV light and as protectors against insect
attack, form a part of the enzymatic wound protection system, and are antioxidants.(196)
As we have already noted, man’s appreciation of vegetables is strongly influenced
by the green color. Chlorophyll, in either the so-called a or b form, may degrade
by either of two types of reactions during storage and cooking. In an acid-catalyzed
reaction, chlorophyll will lose the magnesium ion coordinated in the phorphyrin, resulting
in a color change from bright green to dull brown(197) (Figure 14). In the kitchen
this color change may be prevented by using slightly alkaline water for boiling of
green vegetables, which may be obtained, for example, by the addition of baking soda.
Chlorophyll is lipophilic and anchored in the lamellae of the chloroplasts. Enzymatic
hydrolysis of the phytol ester converts chlorophyll to the hydrophilic chlorophyllide
in a process which occurs post harvest and increases leaching of green color from
leafy vegetables during cooking.
Figure 14
Diagram illustrating how the color of chlorophyll is affected through various degradation
mechanisms.
The lipophilic character of chlorophyll is recognized in the much appreciated green
color of virgin olive oil. However, the separation of chlorophyll from the carotenoids
of the chloroplast makes the oil sensitive to light exposure, since the chlorophyll
acts as a photosensitizer generating singlet oxygen, which then oxidizes oleic acid,
resulting in a hay-like off flavor. The (physiological) function of chlorophyll in
vegetable oil becomes uncoupled from the function of carotenoids as protectors against
radicals and singlet oxygen, a protection which is highly important in the fruits
of the olive tree during sun exposure.
Carotenoids are only synthesized in plants and algae and in the kitchen provide the
appetizing red color of ripe tomatoes and carrots. However, carotenoids are transferred
along the food chain, providing color to other organisms.(195) For example, in the
oceans, astaxanthin synthesized by phytoplankton is transferred to krill and eventually
to shrimp and salmonoids, providing them all with their distinctive reddish pink coloration.
In Japanese kitchens the red color of shrimp is highly admired and any white spots
of ikaite, a calcium carbonate hydrate formed during frozen storage in the shell,
is considered as a serious defect although it is without effect on flavor.(198)
In most cultures, a pink salmon color is likewise appreciated in dishes prepared from
smoked or boiled salmon or trout. Trout and salmon are increasingly being farmed in
order to meet demands of the world markets. In such farms the pink pigmentation is
ensured by the addition of carotenoids to the fish feed. The physiological function
of astaxanthin in the flesh is uncertain, but a uniform pigmentation is considered
to indicate high quality.
Carotenoids are, however, distributed unevenly in various tissues in most organisms
that absorb them. Indeed, the pigmentation of many foods is affected greatly by seasonal
(and other) variations in the availability of carotenoids in the diet. Structures
of some of these compounds are illustrated in Figure 15. A good example, which is
likely to be noted by chefs as well as domestic cooks, is the variability of the color
of the yolks of hens’ eggs. Depending on the diet of the hen the yolks may vary in
color from pale yellow to bright red; generally, free-range hens that have access
to a range of feeds tend to eat more colorful foods and produce darker and redder
egg yolks.(195)
Figure 15
Lycopene from tomatoes and carotene from carrots is red, while lutein and zeaxanthin,
classified as xanthophylls (oxygen containing), are yellow. Astaxanthin is the pink
colorant in salmon.
Flavonoids and anthocyanines are examples of polyphenols important for the color of
flowers and fruits. Apples which have been damaged by insects will turn red faster,
indicating the protective role of the polyphenols in the living plants. In the kitchen
the yellow, red, and violet colors of berries are important for deserts of various
types and fruit drinks. Normally, fresh fruits have acceptable color stability, but
upon preservation colors made fade. During canning, tin from metal cans will sometimes
dissolve and form complexes with anthocyanines from the fruits, in effect stabilizing
the color.(193) Anthocyanines have color, which varies with pH, so that red colors
will dominate under acidic conditions as in desserts or fruit smoothies with addition
of lemon juice. Such color varations allow the chef the opportunity to change the
color of such fruits simply by changing the level of acidity. For anthocyanins the
effect of pH on color is due to the complex acid/base equilibria depicted in Figure
16. The acid form of the anthocyanin is in mutual equilibrium with the corresponding
base and with a pseudobase, the latter formed by addition of a hydroxyl ion. The equilibrium
between the pseudobase and the base is independent of pH but depends on water activity.
The heterocycle of the skeleton may open for both the base and the pseudobase. The
equilibria are further complicated by molecular stacking phenomena, which further
affects color. In the kitchen pH is easily controlled, and colors of fruit-based desserts
or drinks may accordingly be adjusted.(199)
Figure 16
Equilibria between different forms of anthocyanins affecting color. AH
+
is a flavylium cation, A is the quinoidal base, A
-
is the ‘anhydro’ base, B is the pseudobase, while C is the chalcone.
4.3
Textures in Food and How To Make Them
The overall appeal of any food is determined not only by its flavor but also, to a
large extent, by its texture. For example, some foods need to be crisp and crunchy
to be properly enjoyed. No matter how good the flavor a “soggy” potato chip (crisp
in the United Kingdom) will not taste right; ice creams that are not properly smooth
due to large ice crystals have an unappealing gritty texture, while a limp salad will
put off even the least discriminating diner. The control and modification of texture
is therefore an important aspect of the kitchen repertoire and well worth discussing
in some depth.
Chefs know how to modify the texture of meats to produce crisp yet moist pastries
and to prepare the lightest soufflés. In many cases they follow long-winded and complex
(but well-tried and -tested) procedures to achieve their desired results. However,
often, with a little understanding of the underlying stability criteria, they can
achieve the same result with far less trouble. A glance at any cookbook on how to
produce a simple mayonnaise shows that many cooks do not have even a basic grasp of
thermodynamics of emulsification. However, there is much more to explore here than
elucidating what makes those things chefs already do work (or fail). The understanding
of physical chemistry to control texture in, for example, emulsions, gels, foams,
and glasses has an enormous range of potential applications to produce foods of novel
and interesting textures in the kitchen.
This is an area where there has already been a good deal of transfer of knowledge
from the science laboratory into the kitchen (“spherification” using calcium to mediate
gelation of alginate systems
200,201
foams that hold and release specific flavors in a controlled way, hot jellies,(62)
etc.).
Our aims in this section are first to explain the physics and chemistry involved in
the development of specific textures in traditional cooking processes (e.g., in roasting
or frying meat, baking a cake, etc.) and second how we can use our understanding of
thermodynamics and materials science to provide different ways to control texture
in the restaurant and domestic kitchen.
We have chosen to divide the section into two parts. In the first part we will look
at how best we can describe the texture of a wide variety of processed foods (breads,
cakes, ice creams, and so on) with a view to providing a physical chemical background
to the production and stability of such processed foods. In the second part we will
look at the texture of naturally occurring foods (in particular meats) and how these
textures can be changed by cooking.
4.3.1
Relationships between Perceived Texture and Measurable Physical Properties
At first sight it would appear that it ought to be a simple matter to relate the perceived
texture of food in the mouth with measurable physical properties of the food (for
example, tensile and shear modulus and fracture stress for solid foods and viscosity
for liquids). However, as with all things matters are far from this simple. First,
very few if any foods have mechanical properties that can be characterized by single-valued
properties. Foods are complex substances and have correspondingly complex physical
properties. They are at best viscoelastic, so that knowledge of properties over a
wide frequency domain would be necessary before any relationship between perceived
texture and measurement is possible. However, most foods display distinct nonlinear
properties, making the problem much greater. To complicate matters further, as we
chew the food in the mouth it interacts with saliva and changes its properties, leading
to changes with time that are more or less impossible to simulate in the laboratory.
Worse still, everyone will chew their food differently, so that the perceptions they
have will differ. Consider, for example, a food that is distinctly nonlinear, one
that has a very low modulus and shear strength at low frequency and small amplitude
but which at higher frequencies and amplitudes becomes a rigid solid with a high shear
strength. Such a food might seem soft and smooth to a person who chews slowly but
hard and brittle to somebody in a rush to eat their food. Accordingly, successful
attempts to relate perceived texture to measurable physical properties are few and
generally limited to specific types of food and textural descriptions.
The best documented cases of direct relationships between measurable physical properties
and perceived texture lie in the area of “semi-solid” foods such as yogurts and custards.
The perceived thickness, T, of such foods can be modeled in terms of the shear stress,
σ, “felt” on the tongue as the food is consumed. Several authors (e.g., Kokini
202,203
) have reported a relationship of the form T = a·σ
b
, where a and b are constants that depend both on the food being consumed and on the
method by which perceived thickness is evaluated by the taste panels. The shear stress
felt by the tongue has itself to be calculated from a knowledge of the viscosity of
the food; again, there are different models to do this (see, for example, the fluid
mechanical calculations of DeMartine and Cussler(204)). Terpstra et al.(205) provided
a detailed discussion of these models together with some comments on their limitations.
Although it is difficult to provide clear predictive relationships between measured
physical properties and perceived texture in the mouth, textural measurements remain
useful for food scientists developing new products, if only to permit elimination
of products that are likely to prove to have unpleasant textures or as quality control
techniques. In both cases, it is possible to create specific measurements that can
be used to select products that may need further testing.
The range of texture-measuring methods that have been devised over the years reflects
on the ingenuity of those working in the field. All manner of devices have been constructed
to simulate what happens as we eat our food, from the simplest of measurements of
stiffness and failure stress using standard testing machines to a specially designed
and built apparatus that simulates the movement of jaws and teeth as we eat.
206–209
4.3.2
Complex Nontissue Foods: Foams and Emulsions
The texture of (solid-like) foods varies from incredibly light and soft foams to very
hard and brittle boiled sugars. The range of possibilities is so vast that it is not
possible to list them all. In this section we first outline the wide range of possible
microstructures that occur in the world of foods and catalog them as foams, emulsions,
and colloids or more complex multiphasic materials. At the same time we try to show
how the different microstructures relate to the physics properties of these foods.
Very few foods and food ingredients can be characterized as being a single phase.
Examples include water and simple solutions. Aqueous solutions include naturally occurring
ingredients such as egg whites (which can be thought of as a 10% aqueous solution
of the proteins ovalbumin, ovotransferring, ovomomucoid, globulins, lysozyme, ovomucin,
and avidin as well as small amounts of salts and carbohydrates) and red and white
wines where the major solute is ethanol, although it is the other minor solutes such
as tartaric acid, flavanoids, tannins, and aldehydes that provide the wine’s flavor.
The various oils used in cooking are nominally mixtures of various liquid triglycerides.
Olive oil, for example, is mainly composed of a mixture of four triglycerides composed
of oleic acid, O, lauric acid, L, and palmitic acid, P; these may be referred to as
OOO (three oleic acids joined together), LOO (one lauric acid together with two oleic
acids), POO, and SOO. These triglycerides further act as a solvating medium for various
minor components: short-chain alcohols, chlorophylls, fat-soluble vitamins, etc. Sugar
and salt are the most common single-phase solid ingredients used in the kitchen. Sugar
can, as we shall see later, be heated to a liquid phase and then spun and cooled quickly
to form a glass: examples of spun glass foods include candy floss. Glassy sugars can
also act as solvents in the case of hard candy. Such products, in their simplest versions,
are single-phase materials where colorants and flavor molecules are dissolved in amorphous
sugar.
While it is possible to find a number of examples of single-phase foods, the vast
majority of the materials in the world of foods are of multiphasic nature. Simply
moving from a still white wine to a sparkling wine such as champagne immediately doubles
the number of phases. Champagne, when in the bottle, may be considered as a pressurized,
supersaturated solution of carbon dioxide. As soon as it is served it is better considered
as an unstable dispersion of rapidly nucleating carbon dioxide bubbles. These bubbles
constitute the second phase of the material. Table 4 shows examples for the development
of the complexity of food materials.
Table 4
Examples To Illustrate the Multiphase Nature of Foodsa
one phase
wine, oils, hard candy, noncarbonized soft drinks
two phases
vinaigrette dressing
oil and aqueous phase
slush ice
crystalline water and aqueous phase
whipped egg white
air and aqueous phase
mayonnaise
oil and aqueous phase
beer foam
air and aqueous phase
three phases
butter
crystalline and liquid fat and aqueous phase
dark chocolate
crystalline fat, crystalline sucrose, cocoa solids
four phases
parfait ice cream
aqueous phase, fat, air, crystalline water
milk chocolate
crystalline fat, liquid fat, crystalline sucrose, cocoa solids
five phases
butter cream
crystalline fat, liquid fat, crystalline sugar, aqueous phase and air
six phases
fudge
crystalline sugar, aqueous sugar solution, air, liquid fat, crystalline fat and cocoa
solids
a
These examples are simplified as some phases can in reality be expanded into more
phases (e.g., crystalline fat really consists of several crystalline phases).
Most foods can be described in terms of a number of dispersed phases, surrounded by
a continuous phase or matrix. Such systems are, in general, termed colloids, and depending
on the properties of the continuous and dispersed phases they can be classified into
the categories solid sols, solid foams, sols, emulsions, foams, solid aerosols, and
liquid aerosols.
Not all of these colloidal states are equally important in foods and cookery. A liquid
dispersed into a gas is called a liquid aerosol. We do not know of any examples of
dishes or foods with this type of structure. Nevertheless, if one sprays a liquid
on some food, an intermediate mist-like material is produced, which indeed is a liquid
phase that is dispersed into a gas phase. The smoke used for smoking foods belongs
to the category of solid aerosols: solid particles dispersed in a gas phase.
Moving to combinations that are found commonly in foods, we begin with the case of
a solid dispersed into a liquid. Such systems are termed sols; examples include melted
chocolate, chocolate sauce, and cold cream (where the fat particles are cool enough
to be solid). Sols are mostly used in cooking as sauces and some soups; the important
properties for the chef are the “thickness” and “creaminess”. In general, a more viscous
liquid matrix and a higher concentration of solid particles will increase the “thickness”
of a sol-based sauce, while higher proportions of small particles will tend to increase
its creaminess.
A solid phase dispersed into another solid phase constitutes a solid sol. Dark chocolate
is an example of a solid sol as both the two solids, sugar and cocoa powder, are dispersed
into the continuous solid fat phase. In the case of solid foods chefs tend to be concerned
about whether they are tough, brittle, hard, or soft. Generally, in systems where
one or more solid phases are dispersed in a single matrix, the properties of the matrix
dominate, so if the matrix has a high tensile modulus the product will be perceived
as hard if it is ductile, with a low yield stress, as soft, and so on. However, the
size and concentration of the solid inclusions also affect the perception of the overall
properties in the mouth, especially if the inclusions are macroscopic so they are
felt by the teeth (or tongue) as the food is eaten.
A gas dispersed into a solid matrix is called a solid foam. The range of properties
that can be achieved with solid foams is vast; the properties of the foam relate mainly
to the properties of the solid matrix. At one extreme are the brittle foams that melt
in the mouth; these can be light and delicate (such as dry meringues) or hard and
brittle (the foamed chocolate “Aero” bars). At the other extreme are the ductile foams
that produce, for example, sponge cakes. Stiff matrices with a high-yield stress provide
hard or tough foams (e.g., stale and toasted breads), while low-modulus matrices with
low-yield stress tend to provide softer weaker foams (e.g., light sponge cakes); brittle
matrices produce foams that burst open when bitten into (e.g., dry meringues), while
ductile matrices produce foams that melt in the mouth as they are chewed. While the
matrix properties dominate the general character of the product (hard, soft, brittle,
etc.), the size and proportion of the dispersed bubbles determine how “light” the
product is perceived, more small bubbles tending to produce a “lighter” result.
When a gas phase is dispersed into a liquid phase it produces foam. Foams are very
common in cookery, both as an intermediate step to a product (bread dough may be considered
a foam prior to cooking), as a part of a dish (the whipped cream on top of the strawberries),
or even a complete dish in its own right (soufflés). The key properties of foams,
from the point of view of the cook, are similar to those of sols. Foams are interesting
low-viscosity fluids, air is dispersed, often in high volume fraction into a fluid
of higher viscosity. The material formed by the two phases; the foam can behave as
a solid (a bowl containing beaten egg whites can often be turned upside down). However,
if we consider its nonlinear rheological behavior then foams are better described
as plastic materials that can be permanently deformed beyond the yield stress limit.
The yield stress as well as the static shear modulus depends on interfacial tension,
the average bubble radius, and the volume fraction of air; a theoretical expression
yield in foams has been derived by Princen and Kiss.
210,211
Emulsions are formed when a liquid phase is dispersed into another liquid phase. Emulsions
constitute perhaps the largest group of food colloids. In gastronomy most sauces and
dressings fall in this category.
The key properties of the sauces for the chef are “thickness”, “creaminess”, and “stability”.
The first two generally follow those of the matrix, but in some cases (e.g., mayonnaises)
the properties of the product can be very different from those of either component.
The third, stability, is far more complex and deserves a section on its own.
Hervé This(8) recently attempted to provide a schema to classify the wide variety
of possible structures in foods. To do this he considers four basic “phases”: gaseous
phases (G), normally simply air but including any gaseous phase, for example, steam
or alcohol vapor; liquid aqueous phases (W), pure water or any aqueous solution irrespective
of the type or amount of solutes; liquid oil-based phases (O), any oil, mixture of
oils, or solutions of molecules of any type in oils (an oil is taken to be any lipophilic
liquid); and any solid phases (S) no matter what their chemical composition or internal
structure, including solid fats, ice, carbohydrates, etc.
Having thus defined four basic continuous phases from which all foods can be made,
This suggests that we may consider the structure of foods in terms of how these are
arranged and provides a formalism to permit a concise description of the internal
structure of a foodstuff. For example, cream (a mixture of solid and oily liquid particles
in a continuous aqueous phase) would be described as (O + S)/W, the “/” symbol indicating
the oil (O) and solid (S) inclusions are contained within a continuous aqueous phase
(W). Whipped cream occurs when gas bubbles are incorporated into the cream, so it
would have structural code G/[(O + S)/W]. This further suggests that such symbolism
can be used to describe the transformations that occur in kitchen operations, so the
production of whipped cream could be represented by the equation
Further, with the use of superscripts and subscripts it is possible to include information
about the sizes (or range of sizes) of the various phases and denote complex cases
where already multiphasic inclusions are contained within one another, building up
complex hierarchical structures. We show in Table 5 a few examples of how particular
foods can be classified in the This schema.
Table 5
Some Illustrative Examples of the Use of the Classification Schema of This
food
This classification
description
cream
(O + S)/W
suspension of liquid and solid fat particles in an aqueous solution
whipped cream
G/[(O + S)/W]
gas bubbles trapped in a suspension of liquid and solid fat particles in an aqueous
solution
egg custard
(G + O + S)/W
liquid fat drops, solid particles (from cooked proteins), and gas bubbles (from air
trapped during beating of the custard) all suspended in an aqueous solution
sponge cake
G/S
air bubbles in a solid matrix
egg yolks
(S/W)@9
nine concentric layers each made up of solids suspended in an aqueous solution; the
example illustrates how the classification can be used to provide information about
inclusions within the various phases
specific emulsion
O100/W10
emulsion of 100 g of oil in 10 g of water; the example illustrates how the classification
can be made quantitative
The utility of such a classification is that it can be used to see the generality
of the various types of products used in the kitchen. For example, This carefully
analyzed all the classical French sauces according to this schema and found that just
23 categories have been used.(8) More interestingly, he found that some simple types
have not been used by traditional French chefs. The simplest sauce not used in traditional
French cuisine is G/W (bubbles in a liquid). Many gastronomic restaurants now use
such foams in a wide range of sweet and savory dishes. While such developments were
not directly inspired by the This schema, it is possible that future generations of
chefs may find novel dishes and variations on existing dishes by examining such classifications.
4.3.2.1. Stability of Food Colloids. From a thermodynamic point of view, more or less
all emulsions and food colloids are unstable. That is, the free energy of food is
higher in the emulsion or colloidal state than it would be if the food were to separate
fully into two (or more) macroscopic regions. For example, a simple vinaigrette dressing
will, given time, separate to form two completely separate regions of oil and vinegar;
the dressing is only temporarily mixed into an emulsion when the oil and water are
shaken together and demixes spontaneously once the agitation stops.
The excess free energy of an emulsion is created by the internal interface area of
the system. The excess Gibbs free energy of creating a surface of area, dA, can be
written as dG = γ(dA), where γ is the surface free energy density or the surface tension
γ = ((∂G)/(∂A))
T,p
. The surface tension is always positive (a hypothetical negative surface tension
would cause an unstable and spontaneously growing surface). Since colloids are systems
of excess surface area, as compared to the macroscopically phase-separated systems,
the dispersed situation is always going to be thermodynamically unstable. However,
it is possible to construct colloids so that they lie at local (rather than global)
minima of the free energy and so are at least metastable.
While preparing an emulsion, foam, or other colloidal dispersion, a surface has to
be formed. The work needed to create the surfaces in emulsions and foams is w = γΔA,
where ΔA is the excess area created. While a chef might not be fully aware of the
thermodynamics of emulsions and foams, he or she will fully acknowledge the work needed
for stirring, whipping, and beating the food materials. Often powerful kitchen machines
are used to obtain the right degree of dispersion.
A simple example serves to illustrate the power needed to achieve an oil−water emulsion.
For a hypothetical oil in water emulsion with a volume fraction of 0.1 and droplet
size of 0.30 mm the surface area is 2 × 103 m2/L. A typical value of the triacylglycerol
water surface tension is 30 mN m−1, so that the resulting excess free energy (or work
needed) is 60 J/L. Thus, to make a liter of such an emulsion at least 60 J of work
needs to be done on the mixture. However, since the emulsion is inherently unstable,
the work needs to be applied in a short time, less than the “relaxation time” of the
droplets, which will typically be significantly less than a second. Thus, any mixing
device used to create emulsions needs to have a power in the region of hundreds of
Watts and to run at high speeds to induce the high shear strain necessary both to
deform the phase to be dispersed and to break it into droplets. Examples of such industrial
devices include rotor stator types with narrow slits and high-pressure homogenizers
using large pressure drops and those that force the liquid through a narrow slit at
high velocity. Neither of these will, on their own, create an emulsion; they simply
disperse an existing emulsion into finer droplets. As yet we are not aware of any
purpose built equipment for the domestic kitchen designed to create emulsions. Rather,
chefs and cooks tend to use inefficient methods such as manual whisking, where most
of the energy is “wasted” going into viscous heating of the liquids rather than into
the creation of (and reduction in size of existing) droplets.
4.3.2.2. Kinetics/Dynamics of Degradation of Structure. The stability of a structure
can be characterized by a lifetime before severe degradation (in this case before
macroscopic phase separation becomes apparent to the diner). This lifetime should
be compared with other relevant time scales for the product, such as those for chemical
degradation, microbiological contamination, and consumption.
Knowledge and empirical understanding of stability have become built into traditional
recipes and procedures; indeed, recipes for many sauces have evolved so that the sauce
remains stable long enough to be taken from the kitchen to the table and consumed
over a prolonged period. However, when chefs develop novel dishes they have to address
the various questions of colloidal stability to ensure the product remains stable
from the time it is completed in the kitchen to the time it is finally consumed in
the dining room. To this end they must select the appropriate ingredients and stabilizing
additives. Further, they need at least an empirical understanding of the conditions
under which the structure will remain stable and how long it will remain sufficiently
stable. In some cases longer time scales become important. Mayonnaise, if well prepared,
has a lifetime of much longer than the time between manufacture and serving, so a
more relevant time scale to consider is that of chemical or microbiological degradation.
4.3.2.3. Mechanisms/Processes That Result in Phase Separation or Severe Degradation
of Structure. The field of liquid−liquid phase separation is well studied and understood
in terms of the underlying thermodynamic drivers. Accordingly, the stability of sols
and emulsions is perhaps best viewed in terms of the various mechanisms of phase separation.
4.3.2.3.1. Sedimentation/Creaming. In the food industry the process of “creaming”
in emulsions is well understood Essentially, provided the droplets are large enough
that Brownian motion is not sufficient on its own to keep them suspended, the droplets
will slowly diffuse either upwards or downwards due to the density difference between
the two phases. This is what happens to whole milk when left to stand—the cream rises
slowly to the top.
The simplest approach is to find the steady-state drift speed of dispersed droplets
assuming spherical droplets and frictional forces obeying Stoke’s equation,
where d is the droplet diameter, ρD and ρC are the densities of the dispersed and
continuous phases, and ηC is the viscosity of the continuous phase.(212) Stoke’s equation
is rarely used quantitatively as several additional effects lead to deviations from
this simple estimate of drift velocity; nevertheless, it can provide some guidance
and insights for the improvement of the stability of colloids and emulsions in the
kitchen.
If the drift velocity can be reduced, the stability will be increased. The simplest
approach is to reduce the particle size; reducing the particle size by a factor of
10 will lead to a 100-fold reduction in drift velocity and correspondingly increased
stability. The simple rule of thumb for chefs is that smaller droplets (generally
made by doing more work) will produce more stable sauces.
One method to reduce droplet size is that used commercially in the homogenization
process of milk.(212) Fat droplets in raw milk have a fairly broad size distribution,
ranging approximately from 1 to 7 µm.(213) After a typical homogenization process
at 20 MPa, the distribution is quite narrow and typically peaks at 0.25 µm. Gastronomic
and domestic kitchens might benefit from similar machinery for efficient homogenization.
Alternatively, the stability of such systems can be manipulated by controlling the
viscosity of the continuous phase perhaps by adding thickening agents (such as the
starches used to thicken some sauces; or the use of sodium carboxymethyl cellulose,
locust bean gum, etc. in some commercial products). Using such additives, the viscosity
and thereby the colloidal stability can, in many cases, be improved a few orders of
magnitude.
4.3.2.3.2. Aggregation and Coalescence. As well as droplets separating under gravity,
they can aggregate into usually undesirable structures of larger size, which then,
due to their increased size, may be subject to enhanced rates of sedimentation/creaming.
On the most basic level, aggregation can be described through Smoluchowski theory,
214,215
which emphasizes the rate of the process as being diffusion limited. However, the
theory does not include the rather important effect of repulsive forces between particles
and thus underestimates the stability of food dispersions.
Coalescence is a small step further and results in the complete fusion of two or more
particles. For example, in the case of a vinegar/oil dressing, coalescence typically
takes place once the droplets become closely packed together, having gained a high
concentration (either at the top or bottom of the container) through the sedimentation/creaming
process. In beer foams, a large quantity of the liquid phase drains back into the
beer under gravity and the air bubbles coalesce in to a coarser structure, which eventually
collapses.
Partial coalescence is an intermediate between complete fusion and aggregation. In
whipped cream the fat globules reach a partial coalesced state and act to form a solid-like
network that gives the whipped cream its physical stability.
216,217
There is little the chef can do to prevent aggregation and coalescence except to be
aware that they will happen and will generally happen more rapidly where the droplets
are closer together and where the surrounding medium is less viscous. But it is worth
noting the importance of electronic repulsion between like charged droplets—this is
often the main stabilizing effect where the droplets become charged—increasing the
charge on the droplets (usually by more and harder beating) can significantly affect
stability—as every chef knows from experience making mayonnaise.
4.3.2.3.3. Ostwald Ripening. Ostwald ripening is due to diffusion of individual molecules
through the continuous phase from smaller, less stable droplets to larger droplets.
The process is driven by the destabilizing surface contribution to the free energy
that makes the chemical potential larger for migrants present in the small particles
as compared to larger particles. This effect creates concentration gradients of the
migrant in the continuous phase and thus a net diffusion. Ostwald ripening is well
understood and has been observed in a wide range of phase-separating systems.
In particular, Ostwald ripening provides for fast growth of the dispersed phase if
it is highly soluble in the continuous phase. The process, therefore, is slow for
dressings such as water-in-oil emulsions. However, it can be very fast in the case
of beer foam since carbon dioxide is highly soluble in the aqueous phase. The mechanism
also becomes relevant for products that are stored for long times, such as commercially
produced frozen desserts. In such cases, the phenomenon can result in large ice crystals
and unpleasant textures.(218)
In systems where one phase is partially soluble in the other, small fluctuations in
temperature (or pressure) will cause changes in the amount of the dispersed phase
as a new equilibrium partition is reached. When the external factors return to the
original state and the net amount of dispersed phase is restored, the size distribution
of particle will change toward larger sizes. The phenomenon can be seen as an enhancement
of the Ostwald ripening process. This sort of process is particularly important for
storage of frozen desserts as temperature fluctuation in freezers can lead to the
irreversible growth of large ice crystals.
To conclude, while chefs have some tools available to combat phase-separation processes
that inevitably destabilize the product they make, it is never possible to eliminate
the tendency of small droplets of dispersed phases from growing larger over time.
All that we can do is to be aware of the time scales involved and make sure that products
remain sufficiently stable over a long enough period that the diner can enjoy them
to the fullest extent possible. It is perhaps for this reason that it can be particularly
challenging to transfer the gastronomic dishes prepared in restaurants to the wider
food industry. In a restaurant environment, it is possible to produce and serve dishes
that have a lifetime of minutes, while the commercial retail industry demands products
with lifetimes in the region of days or even months.
To provide such long lifetimes it is often necessary to introduce additional “stabilizing”
compounds. These stabilizers in turn introduce their own tastes and textures, which
often change the character of the original dishes, often making them less attractive.
However, as we shall see in the gels section, some chefs are now starting to use some
of these “stabilizers” and use them to enhance the range of gastronomic dishes they
prepare. So there is some hope that in the future with superb creative chefs regularly
using the types of food additives that were seen as only fit for “processed” foods
a few years ago, we may soon see gastronomic quality foods appearing on the supermarket
shelves.
4.3.2.4. Ingredients and Compounds That Enhance Colloidal Stability. There is a wide
range of food molecules which can be used to increase the stability of foams, sols,
and emulsions. Generally, these molecules are grouped into two categories: emulsifiers
and surfactants as well as stabilizers. They are widely present in food ingredients.
Typical examples include polar lipids (monoglycerides, diglycerides, phospholipids,
glycollipids, etc.) and globular proteins (such as beta-lacto-globulin); some common
food examples are given in Table 6. These molecules tend to reduce the surface tension
between the phases. The reduced excess surface energy decreases the driving force
for phase separation. This affects both the ease of formation of emulsions and the
stability of the final preparation.
Table 6
Some Commonly Used Ingredients That Contain Significant Amounts of Emulsifiers
ingredient
key emulsifier component
egg yolk
phosholipid (lecithin)
egg white
globular proteins
whey powder
whey proteins
milk powder
caseins and whey proteins
soy bean
phospholipids and proteins
mustard seed (especially yellow mustard)
mustard mucilage, a gum of mainly polysaccharide origin acts as a shear-thinning thickener
and possibly as an emulsifier(219)
flax seed
flax seed mucilage, polysaccharides, which act as shear-thinning thickeners(220)
The presence of an emulsifier decreases the work associated with deforming and breaking
up larger droplets into smaller ones. This usually results in a size distribution
shifted to smaller sizes, and thus, the rate of the creaming/sedimentation process
is lowered. The kinetics of the phase separation process into bulk phases is also
modified by the presence of surface active components. The lowering of the surface
tension also lowers the concentration/pressure differences responsible for the Ostwald
ripening.
The accumulation of surfactant at the internal surfaces of the food colloidal will
also in most cases modify the forces acting between the dispersed particles. Most
prominent is the accumulation of charged surfactant, which gives rise to long-range
electrostatic repulsion between dispersed particles and thus a kinetic stabilization
against aggregation phenomena. In Table 7 we listed some examples of prepared foods
which are stabilized using such molecules together with the stabilizers used.
Table 7
Some Stabilized Systems and the Stabilizers Present
food
stabilizer
mayonnaise, hollandaise, béarnaise
egg yolk lecithin
vinaigrette
can be stabilized by mustard
beer foam
range of proteins from barley that are more or less degraded during boiling and brewing
cappuccino foam
casein and whey proteins of milk
espresso foam (the crema)
foam stabilized by surface active compounds of polymeric nature (proteins, melanoidins,
and polysaccharides) formed during roasting of coffee(221)
bread crumb
complex foam stabilized by among others gluten, before baking, and a solid starch
network of the cold crumb after baking
whipped cream
stabilized by surface active phospholipids and mechanically by partial coalescence
of fat globules of the cream
In restaurant and domestic cooking food needs only to be stable long enough to go
from the kitchen to the table and be consumed. Thus, it is possible to prepare and
use foods that are in a metastable and even an unstable state. An interesting example,
from the Fat Duck, is that of a green tea foam palate cleanser (for a recipe and photograph
see page 133 of The Big Fat Duck Cookbook(62)). The basic concept arose once it was
noted that a mixture of green tea and vodka in an egg white foam acts as an excellent
palate cleanser. Being a foam which disappears quickly it can be served right at the
start of a meal with no risk of spoiling the appetite as might a larger more dense
version of the same ingredients. However, there is a problem of stability: the foam
very quickly collapses, leaving some liquid at the bottom of the glass in which it
is served. Thus, it has to be prepared right at the table and consumed immediately.
However, not everyone is cooperative; some people will leave it to stand (perhaps
to savor it), but the stability is insufficient. In the end a solution was found by
freezing the foam in liquid nitrogen to make a small “poached” hard-shelled meringue-like
sphere that can be put in the mouth where it instantly disappears, cleaning the palate.
4.3.3
Crystalline State in Foods
Many of the ingredients we use in our kitchens are fully or partly crystalline materials
(examples include sugars, fats, many carbohydrates, and salt). During the preparation
of the food these materials will often pass through other liquid or amorphous states
due to action of heat or solvents such as water. Eventually the original ingredients
may, through solvent removal during baking and cooking and by cooling after heat treatment,
be turned into new materials which in many cases also involve (new) crystalline structures.
Often multiple competing crystalline forms can coexist. The recrystallization from
one metastable form into a more thermodynamically stable form is part of the (usually
undesired) aging of food. Overall, an understanding of these phase transitions of
food ingredients can enable the chef to obtain a wide range of textures using the
right combination of ingredients, heating, and drying.
4.3.3.1. Water. Of all the food molecules that occur in crystalline form, water is
the most common. Ice is used in many desserts such as ice creams and sorbets. Ice
is also present in all frozen foods, and so the growth of ice crystals is important
in considering their long-term storage.
In sorbets and ice creams it is important that there is a liquid phase present to
provide enough fluidity to make the dish soft and easy to eat. With no liquid phase
the ice would remain hard and unpleasant. This is achieved through the addition of
sucrose, other sugars, or other solutes such as ethanol to create a sufficient freezing
point depression to leave a substantial portion of unfrozen aqueous solution at the
typical serving temperature from −13 to −6 °C and even at conventional storage temperatures
from −15 to −20 °C. In many ice creams the sugar content is as high as 15%, somewhat
higher than the typical sugar content of other desserts. However, at the low temperatures
we become less sensitive to the sweet taste.
Some frozen desserts such as granites are made with only limited amounts of sugar
or other solutes, and the final snowy texture of such dishes relies on mechanical
treatment of the initially rather solid and hard ice. Recently, novel blenders for
use with frozen foods (such as the Paco Jet, see later, section .5) have been exploited
in restaurants to make finely dispersed ice particles and expanded the range of possibilities
within frozen desserts.
When considering the storage of frozen foods we need to note first that the initial
freezing creates ice crystals that can damage the texture of the food being frozen,
so that it is usually best to freeze as quickly as possible to create the smallest
possible crystals. We discuss how crystallization temperatures and rates affect crystal
sizes later. Second, it is important to realize that these small ice crystals are
not thermodynamically stable. Larger crystals will grow larger at the expense of smaller
crystals that will disappear in a process comparable to that of Ostwald ripening described
previously. Such recrystallization is also promoted by the presence of a liquid phase.
In most foods there are some salts and sugars present in the aqueous phase. On freezing,
as ice forms, so the concentration of the sugars and salts increases, but since the
eutectic temperatures of most salt mixtures are below those found in most domestic
freezers, some aqueous solution usually remains in the frozen foods.
4.3.3.2. Carbohydrates. Simple sugars are generally refined into crystalline powders
and granules of high chemical purity. These are essentially single-component materials
so that, in principle, they possess sharp, well-defined thermodynamic melting transitions.
However, most sugars are not chemically stable; they undergo dehydration reactions
upon heating, which affect the melting behavior. These caramelization reactions have
been discussed previously. There are a number of dishes (such as the créme brulees)
where the flavors and color of the caramelization reactions are desirable. However,
it is also often the case that chefs may wish to produce sugar products that do not
have the caramelized taste or color but which nonetheless need to be heated; examples
include spun sugars and candy flosses.
Two solutions are possible. First, it is possible to select sugars that are more stable;
there are some sugars, in particular, the sugar alcohols, xylitol, sorbitol, and maltitol,
which are stable against dehydration reactions and can be melted without any significant
browning. Second, mixtures of different sugars can be used so as to reduce the melting
temperature range.
For example, boiled sweets are based on a combination of sucrose and glucose (in some
countries glucose syrups). The preparation involves boiling the solution at an increasing
temperature (the temperature rises as water boils off due to pronounced boiling point
elevation of the highly concentrated solutions). The process is stopped at a temperature
of about 146−154 °C, indicating a very low water content. However, due to the presence
of two or more sugars (or glucose oligomers) as well removal of most of the water,
crystallization of sucrose and glucose on subsequent cooling is kinetically suppressed,(222)
so that the process directly provides a supercooled (glassy) sugar melt without entering
the high-temperature regions used to melt sugars that accelerate the dehydration reaction
of sugar. The effect of glucose, glucose oligo- or polymers on the crystallization
of sucrose has been systematically studied by differential scanning calorimetry.(223)
As we shall see later, sugars can also be quenched to form glassy materials. Crystalline
sugars are desirable in many dishes and products (including chocolate, fondant, fudge,
dry meringues), while glassy sugars are required in other dishes (such as hard boiled
candy). The presence of crystalline sugars not only contributes to a sweet taste but
also changes the texture of the product depending on the amount and size distribution
of the crystals. Some sugars, mainly xylitol and to some extent sorbitol, dissolve
quickly in water through a strongly endothermic process and can be used to create
a cold sensation in the mouth.
As we shall see in detail in the gels section starches are present in many basic ingredients
such as flour and potatoes, etc., in the form of small, partly crystalline, micrometer-sized
starch granules. The crystalline parts consist mainly of the macromolecules amylopectin
and amylose. Due to the action of a combination of water and heat the crystalline
parts of the granules will melt and the granule will take up water and swell; eventually
the whole granule can disintegrate. The process is called starch gelatinization. The
melting of the crystalline regions occurs at increasingly high temperatures with decreasing
water content.(224) The melting temperature reaches its lowest level (about 58 °C
in the case of potato starch) independent of water content if excess water is present.(225)
4.3.3.3. Fats and Oils: Triglycerides. Fats and oils are multicomponent materials
with a multitude of crystal phases that can all coexist, making their crystallization
and melting behavior significantly more complex than simple systems such as the sugars.
Mixtures of triglycerides in a liquid state are normally called oils, whereas purely
or mainly crystalline triglycerides mixtures are called fats. Triglycerides show a
richness in polymorphic forms, which can be grouped into three main types, α, β′,
and β. Thermodynamically these polymorphs are distinguished by increasing stability
(decreasing free energy) and increasing melting point in the order α, β′, and β. On
a structural level, they differ in the methylene group packing arrangements as seen
by wide angle X-ray diffraction experiments.(226)
The sizes of individual crystallites are typically a few micrometers. In fats with
a content of liquid triglycerides, the crystals of fat can form a network that penetrates
the materials, making a gel-like system with plastic deformability, giving rise to
the well-known texture and spreadability of ingredients such as butter, margarine,
and pork fat. For most fat types metastable crystals of β′ type give rise to the most
optimal crystal size distribution and crystal network and are therefore desired in
ingredients such as butter, margarine, and some forms of chocolate. A culturally based
exception to this is the middle eastern ingredient, butter ghee (a butter fat without
water and proteins), where a grainy texture based on β crystals is preferred.
Phase diagrams of some triglyceride mixtures have been determined.
227,228
However, in general fats can be considered to be multicomponent mixtures as well as
multiphase materials. For such complex systems the derivation of complete composition
temperature phase diagrams is almost impossible. Instead, for a specific fat or combination
of triglycerides the SFI (solid fraction index), the fraction of crystalline material
present, can be derived as a function of temperature using techniques such as DSC
or low-field nuclear magnetic resonance spectroscopy (NMR).(229) In such studies the
detailed connection between the underlying phase behavior and the SFI-melting profile
remains unclear. However, it is clear that the incompatibility between crystals of
different triglycerides can lead to eutectic-like phenomena. The number of phases
present is comparable to the number of different triglycerides, which for most fats
can be large (>10), making the detailed materials science of fats extremely complex.
In general, all that can be said is that they usually have very broad melting ranges.
However, in some systems with fewer triglycerides and where they have compositions
that lie close to the eutectic-like proportions a comparatively sharp melting temperature
can be seen. Cocoa butter, a chemically simple fat, has three major components: POS,
POP, and SOS (using the notation defined in section ). When cocoa butter is in the
so-called state V, a β′ polymorph (the desired polymorph for chocolate), it has a
melting temperature just below body temperature. This form of cocoa butter lies close
to the eutectic composition for the tempered system of the three triglycerides POS,
POP, and SOS, explaining the sharpness of the melting process of dark chocolate. The
much broader melting interval of butter and pork fat can be understood in this context
in terms of a triglyceride composition further way from the eutectic composition of
the relevant triglycerides.
Finally, we should mention that the presence of metastable polymorphs further complicates
the measurement and observation of phase behavior since the melting behavior becomes
strongly dependent on the thermal history. For example, it is often found that parts
of a material will crystallize and release heat upon heating. Unstable polymorphs
melt at relatively low temperatures on heating (for example, in a differential scanning
calorimeter (DSC)) to form supercooled melts which subsequently crystallize into more
stable polymorphs and finally melt again.(230)
4.3.3.4. Nucleation, Growth, and Other Dynamic Aspects of Crystalline Phases in Gastronomy.
One of the most important issues with crystalline food systems is the size of the
individual crystals. The overall crystallization rate and distribution of crystal
sizes are determined by the rates of both nucleation of new crystals and growth of
existing crystals. Both rates increase with the thermodynamic driving force (which
depends, inter alia, on the supercooling or supersaturation), and both decrease as
the mobility of the molecules decreases. The result is usually a bell-shaped curve
for the rates as a function of, i.e., temperature, with rates at first increasing
and then passing through a maximum before decreasing as the temperature is lowered.
However, the relative rate of nucleation to that of growth tends to increase as the
thermodynamic driving force increases, leading to significantly smaller crystals being
formed at the highest supercoolings (and for crystallization from solution the highest
supersaturations).
Implicit qualitative knowledge is often inherent to classical gastronomic procedures.
In the making of fondants and chocolate, often the hot sugar melt/solution or parts
of the melted chocolate are poured on to and kneaded on top of a marble block, which
ensures fast cooling and with it formation of very small crystals. When making ice
cream the ice cream mix is usually stirred vigorously while being cooled in order
to maximize the supercooling and break up any large crystals that may grow to produce
a very fine dispersion of ice crystals, which is very important to the perceived texture
of this frozen dessert. Dry meringues are baked for a long time at sufficiently low
temperature (≤105 °C, well below the melting point, ca. 185 °C, of sucrose) in order
to ensure that sucrose will crystallize as the water is removed.
In cases where prevention of crystallization is desired, as in the making of hard-boiled
sweets, we exploit the fact that crystallization can be quenched by sufficiently fast
cooling through the temperature interval of overall fast crystallization and into
a more kinetically stable temperature region. However, it should be noted that crystallization
of such impure material must involve accumulation of impurities and diffusion over
increasingly longer distances as crystallization progresses (rather than forming a
supersaturated/supercooled melt that eventually will be glassy).
4.3.4
Glassy State in Foods
Some of the macrocomponents of food, such as sugars, larger carbohydrates, and to
some extent proteins, form glasses when melts or solutions of these food components
are cooled or dried sufficiently fast to avoid the crystallization processes. Glass
formation is important for the description and understanding of hard-boiled candy,
candy floss, cookies, crusts, and crackers, etc. In fact, more or less all crunchy
or crisp foods are in a glassy state.
Briefly, a glass is, from a structural point of view, a molecularly disordered material
that behaves from a simple mechanical point of view as a solid, characterized by a
high shear modulus of the order of 1012 Pa. However, food glasses are often more like
very viscoelastic liquids which show very slow relaxation processes (with relaxation
times, τ, of many hours) and can thus also formally be assigned a viscosity >1012
Pa s. Glasses are also brittle, which is very important for the perceived crunchy
textures they produce.
The rates of the relaxation processes as well the viscosity of food glasses are heavily
dependent on temperature with a strongly non-Arrhenius temperature dependency. When
cooling amorphous food materials, the very steep dependency on temperature gives rise
to a moderately well-defined transition from liquid behavior to solid behavior at
the so-called glass transition temperature, T
g. Differential scanning calorimetry using standardized scanning rates of 5−10 deg/min
is the preferred technique for the determination of glass transition temperatures
in food systems, although other methods can be employed.(231)
The strong temperature dependence of the viscosity near the glass transition is most
commonly described by the Williams−Landell−Ferry expression (the WLF equation):
where η is the viscosity, T
0 is some reference temperature, and C
1 and C
2 are constants pertinent to the combination of the glass-forming material and reference
temperature. If no specific knowledge is available of the material under investigation
a combination of a reference temperature of T
g and the so-called universal values of C
1 (17.44K) and C
2 (51.6 K) can be employed.
Although, accurate numerical prediction within gastronomy is not relevant in most
cases, the WLF equation gives an impression of the changes of the material properties
as food materials are cooled during gastronomic processing. A set of constants, close
to the so-called universal values, predict that the viscosity will change by 10 orders
of magnitude over a temperature interval of about 20 °C.
Table 8 shows that the (calorimetrically measured) glass transition temperature depends
strongly on the nature of the food components. For sugars the calorimetric glass transition
ranges between −29 °C for xylitol and 110 °C for trehalose. The glass transition temperature
of dry starch and dry starch components is high from the viewpoint of cookery (>200
°C). The glass transition temperature of water has been heavily debated(232) and reported
in a temperature range which can be considered irrelevant from the viewpoint of gastronomy.
The diversity of glass transition temperatures gives the creative chef possibilities
of manipulating textures of foods by interchanging components and changing the composition
of ingredients.
Table 8
Calorimetrically Determined Onset Glass Transition Temperature of Various Dry Food
Components
component
T
g/°C
xylitol
−29
sorbitol
−9
fructose
5
glucose
31
sucrose
62
trehalose
110
lactose
101
nonsugars
water
−133
starch
243
gelatin
100
Unlike the pure food components of Table 8, food is usually made out of mixtures.
The glass transition temperature of such mixtures can be calculated as a weighted
average of the glass transition temperatures of the individual components. Various
expressions have been reported for the calculation of the glass transition temperature
of mixtures.(231) Most importantly, components with low glass transition temperatures
strongly depress the glass transition temperature of mixtures; the effect is known
as plasticization. The ever present solvent water is the most prominent and efficient
plasticizer in food systems. The uptake of moisture is responsible for the loss of
crispiness in many foods, for example, crackers, which can be treated as starch-based
composites with glassy regions.
In the kitchen many sweet dishes involve boiling sugar syrup to various “stages”.
Essentially, as water is driven off, so the solution concentration increases with
a corresponding increase in boiling point (and decrease in the plasticization). The
progress of the process is usually monitored by measurement of the boiling temperature
and terminated at a specific temperature to obtain the required consistency (or “stage”).
Of course, the glass transition temperature and viscosity of both the hot melt/solution
and the cooled product all depend on the water content. The various stages of syrups
are usually called thread, soft ball, firm ball, hard ball, soft crack, and hard crack.
The glass transition temperatures and viscosities of the various stages is given in
Table 9.
Table 9
Boiling Temperatures, T
b, Water Content W, Glass Transition Temperature, T
g, and viscosity, η, of Various Stages of Syrupsa
stage
T
b/°C
W (% w/w)
T
g/°C
η(25 °C)/Pa s
thread
110−111
20
−50
101
soft ball
112−115
15
−30
102
firm ball
118−120
13
−25
103
hard ball
121−130
8
0
106
soft crack
132−143
5
20
1010
hard crack
146−154
1
50
1019
a
Although not quantitatively accurately, these data give an impression of the ranges
of material properties embraced by mixtures of the two components, sugar and water.
Water contents were estimated from McGee,(233) Chapter 12. Glass transition temperatures
were estimated using the Gordon−Taylor equation from glass transition temperatures
of water and dry sucrose. The viscosity was estimated using the WLF equation and the
set of universal constants.
4.3.5
Gels and Gelation
4.3.5.1. Introduction to Gels. The original jellies are the aspics, meat-based jellies
that arise naturally from the juices of boiled meats and bones. They get their name
from the Latin gelare (to freeze), presumably as the hot clarified meat juices eventually
set in a transparent solid on cooling. Thus, when the underlying structure was found
to be that a solution of long molecules formed a three-dimensional network on cooling
it is not surprising that the molecules identified as being responsible for the formation
of the jelly were named gelatin.
Today we understand a gel to be a system where a large volume of liquid is stabilized
in a solid-like form by a network of partially dissolved long-chain polymer molecules.
Provided these long molecules form a complete three-dimensional network throughout
the system it will have a solid-like behavior and becomes a gel. The properties of
the gel then depend largely on the properties of this polymeric network. Since this
network is normally very dilute and made up from molecules that are, more or less,
random (although swollen by the presence of the surrounding liquid) it is reasonable
in many cases to treat the network as a rubber-like system.
Most gels start off as a solution of polymeric molecules in a fluid (in food gels
nearly always water, although alcohol- and oil-based gels are also possible). Some
process whereby the molecules become cross-linked occurs; this may be chemical or
physical. As more and more cross-links are formed, so the effective molecular weight
of the dissolved polymer molecules increases and the viscosity increases correspondingly.
As molecules form links to one another it soon becomes possible to build a full three-dimensional
network throughout the solution, at which point it forms a gel. The cross-link density
at which this full 3-D network occurs is sometimes referred to as the percolation
limit.(234)
In a rubber the stiffness and extensibility are determined by the molecular weight
between the cross-links and the equivalent segment length of the polymers. The simplest
theories assume the molecules between the cross-links adopt random configurations
with an equivalent segment length (l) being the length of a segment of a hypothetical
molecule of the same total length (L) as the real molecule, which would ensure its
end to end distance (r) is that predicted by that of a simple random walk. The stiffness,
G, of the rubber is then given by G ≈ (3k
T)/(2L
l) and the maximum extensibility λmax by λmax = n
1/2, where n is the number of segments in the molecule. Thus, a gel made from a more
rigid molecule will have a longer equivalent segment length and, thus, for the same
cross-link density will be correspondingly stiffer and less extensible. Similarly,
for the same system if the cross-link density is lower the gel will be softer and
more extensible. A further complication is that not all cross-links are stable. In
some gels they are labile, so that the gel may flow to some extent and if broken has
the chance to reform.
4.3.5.2. Methods of Gelation. There is a very wide range of possible gelation mechanisms
depending on the type of junctions or cross-links and their relative stability. We
can divide the junctions into two broad categories: chemical and physical. Chemical
junctions are irreversible, while physical junctions can usually be undone as easily
as they are made.
Examples of gels with chemical junctions in food are well illustrated in the cooking
of eggs. Both the egg white and the yolks can form gels as they are heated. In both
cases the covalent cross-links are created between the proteins once they have denatured.
Both the denaturation and the cross-linking are thermally activated processes with
different activation temperatures for different proteins. Egg whites will form a gel
due to the denaturation and cross-linking of the albumin proteins at temperatures
above ca. 52 °C, while the proteins in the egg yolks require a higher temperature
(>ca. 58 °C). Conveniently, this provides a method of preparing perfect soft boiled
eggs: simply place the eggs in a temperature-controlled water bath for a long time,
enough for them to reach thermal equilibrium (∼30 min) at a temperature above that
at which the albumin will cross-link but below that at which the yolk proteins do
so.
However, most of the gels we encounter in the kitchen are physical rather than chemical
in nature. The most common is probably the gelatin gel. Despite the common nature
of gelatin gels the details of the mechanisms by which the individual gelatin molecules
become associated is still at best poorly understood.
235,236
It seems that most if not all junctions involve several molecules and may be semiordered.
It is probable that there are actually a range of different types of junctions, simple
molecular entanglements, regions where pairs of molecules interact via perhaps mutual
intertwining, and zones where several molecules come together to form semiordered
structures. Further, not all gelatins are equal; they come from different sources
and have widely differing molecular weights, so the range of possible junction types
may differ significantly between products. Overall, this leaves a complex situation
for the chef. The minimum concentration of gelatin required to make a system gel depends
on the ability to form a three-dimensional cross-linked network; so, shorter (lower
molecular weight) molecules need higher concentrations. Thus, the gelatin gels can
form and melt over quite large temperature ranges. For example, the concentration
of gelatin appears to affect the melting temperature, more concentrated gels tending
to have higher melting ranges. However, other properties of the solution (e.g., pH)
can also affect the propensity for junctions to form as can the temperature. Some
junctions only form at comparatively low temperatures, and others form very slowly,
so a gel can change its properties on storage, usually by increasing the cross-link
density, hence both making the gel stiffer and less extensible, in food terms providing
a harder tougher gel. The only hard and fast rules are that the more gelatin used
the stiffer the resulting gel will be.
Other gel-forming food molecules use a range of mechanisms to create the junctions.
These include simple entanglements, electrostatic forces, e.g., the use of counterions
to bind specific sites (alginates), local precipitation caused by pH changes, and
crystallization.
237–248
In recent years a wide variety of gelling agents have found new uses in gastronomic
restaurants. Here we give just a couple of examples. First, work at El Bulli by Ferran
Adria using the fact that alginates can be made to gel simple by changing the counterion
environment led to the process that has become known in gastronomic circles as spherification.(200)
The process can be used to make, for example, small spheres with a tough outer skin
and a liquid center that look like and have a texture similar to caviar photographs
of some examples, and the process can be found in A day at el Bulli,(249) but which
have any chosen flavor. An alginate solution with the desired flavor is prepared and
then dropped into a water bath with a suitable solution of a calcium salt, as the
solution falls so it forms into small spheres, as these fall into the calcium solution
the outer layer gels quickly to create the “caviar”. One example of the use of this
technique in the restaurant is the spherical green “olives” served on a spoon at El
Bulli.(201)
Another application of the properties of gelling agents is to exploit the fact that
they can be quite temperature resistant; unlike gelatin gels which typically melt
around 30−40 °C, some gels such as agar can have a melting temperature up to almost
100 °C. Such gels have been used as flavored layers in hot dishes to keep different
foods apart. However, perhaps the most spectacular use is the flaming sorbet invented
at the Fat Duck.(62) Here a sorbet is doped with a suitable gelling agent (pectin)
so that it will keep its shape even as the ice melts. Such a sorbet can then be flambéed
at the table, providing a sorbet that is truly hot on the outside and completely frozen
in the middle.
4.3.5.3. Gel Properties. Gels may be characterized by their main properties, e.g.,
hard, elastic, brittle, fluid, etc. Most of these properties are inherent to a particular
system; within a particular system the degree of cross-linking can sometimes be controlled
(up to a limit), and the concentration of the gelling agent can be varied. These two
variables usually provide control over the stiffness of the gel but not over whether
it is “brittle” or “elastic”; such properties are determined by the rigidity of the
gelling agents themselves.
There is a wealth of gelling agents now available to the cook. Thus, it is possible
to make gels that retain strength even when very hot (using, for example, agar as
a gelling agent) as well as gels that form “crusts” on small liquid drops (using,
for example, alginates that gel under changes in counterion content). The possibilities
are almost endless. A list of readily available gelling agents that are approved for
food use is provided in Table 10 together with some notes on the type of gels they
produce and the conditions under which they form gels.
Table 10
Some gelling Agents Used in the Preparation of Foods
250–252
gelling agent
conditions for gelation
alginates
sets in the presence of divalent counterions
agar
sets on cooling; thermoreversible mechanism involves formation of double helices
carageenan
gels when mixed with proteins
locust bean gum
gels on addition of various counterions including borates
gum arabic
gels at high concentrations and in acidic environments by partial precipitation of
entangled molecules
xanthan
provides shear thinning gels; thermoreversible gelation
gellan
thermoreversible double-helix formation
pectin
gels at low pH and with divalent ions
cellulose derivatives
various derivatives form gels by swelling, or even wicking
gelatin
thermoreversible gels form on cooling
It is unfortunate that the precise conditions under which any gelling agent will form
a gel always depends on the molecular weight of the specific material, but manufacturers
rarely supply this vital information, so that the chef is usually reduced to carrying
out some trial and error experiments to establish the best conditions for each specific
application.
A particularly interesting class of gels are those formed by starches; these not only
can be made in the kitchen but more often occur inside foods while they are being
cooked. In the following section we will look at the structure of starch granules
and how they form small gel particles when they are swollen and heated.
4.3.5.4. Swelling Starch, Including Potato, Rice, and Flour Cookery. Starch is formed
by many plants in small granules; a typical granule may be a few micrometers across.
Within a granule the plant lays down successive rings, each with a higher or lower
proportion of amylopectin.(224) In rings with a lower amylopectin content the molecules
are packed close together in a well-ordered form, making these parts of the granules
more resistant to attack from enzymes; such layers are often referred to as “crystalline”.
The granules are not purely amylopectin and amylose; the plants also incorporate some
proteins as they make the granules. Importantly, different plants (and different varieties
of the same plant) incorporate widely differing amounts of protein in their starch
granules.
The amount of protein and where it is located in the starch granules is crucial when
cooking with starchy foods. Cold water added to starch granules will be absorbed by
the proteins but hardly penetrate the amylose and amylopectin. Accordingly, high-protein
granules absorb significant amounts of moisture at room temperature compared to low-protein
starches.
Water absorption can be important as it affects greatly how the starch granules are
used. If there are sufficient proteins around the outside of a starch granule and
they absorb enough water they can bind granules together. Once a large group of granules
become bound, those near the center are unlikely to be further swollen by any additional
water; this can be the origin of “lump” in sauces, etc.
While cold water will not greatly affect the amylose or amylopectin in a starch granule,
hot water certainly will. When starch granules are heated the ordered “crystalline”
layers start to melt as the temperature exceeds 60 °C. The actual melting temperature
depends on the relative amounts of amylopectin and amylose and on how well the amylose
molecules pack together to form small crystals inside the granules. This disordering
and opening up of the structure of the granules allows water to penetrate. The linear
amylose molecules are quite soluble in the water and the branched amylopectin less
easily dissolved. As the molecules overlap with one another to a significant degree
they do not fully dissolve in the water but rather form a soft gel. Starch granules
can absorb an enormous amount of water without losing their integrity; this is one
of the reasons why they are such good thickening agents. For example, potato starch
granules can swell up to 100 times their original volume. This swelling provides the
thickening effect of starches used in the kitchen.
The thickening of a sauce by ingredients such as flour or corn starch is an example
of the gelatinization of starch granules into an optimal swollen state. Overcooking
the sauce can result in a complete disintegration of granules, thereby releasing amylopectin
and amylose into solution with the consequence of an undesired thinning of the sauce.
Cooling a suspension of gelatinized starch results in gels where crystalline regions
serve to link the amylopectin and amylose into an overall nonliquid structure. Further
storage of such a gel can lead to the, usually undesired, so-called retrogradation,
which is caused by recrystallization of amylopectin into a thermodynamically more
stable form. The gel expels water and becomes denser and more heat stable during this
process.
The baking of breads involves gelatinization of starch granules; during the cooling
and initial storage of the fresh bread the amylose partly crystallizes and transforms
the doughy texture of the crumb of very fresh or still warm bread into the more desired
texture of a fresh bread. Further storage will lead to crystallization of amylopectin
and formation of stale bread, which has a dry and hard sensation despite the fact
that staling does not necessarily imply the loss of water.
Potatoes provide an excellent system to see what can be done with a starch gelling
system. One can simply cook the potatoes and then mash them (mechanically break the
major structures) so as to allow the starch molecules to absorb whatever liquid is
present; typically potatoes can easily hold more than three times their own weight
of additional liquid while still remaining firm enough to be eaten with a fork. Thus,
it is possible to create a wide range of flavored potato dishes; traditionally fats
such as butter or milk are often added to provide a creamy texture and flavorants
such a garlic to provide a suitable savory note. However, in practice it is possible
to use more or less any liquid to make mashed potatoes, so the water from red cabbages
provides a distinctly flavored pink mash or the use of a dark beer provides a slightly
bitter and malty tasting brown mash.
However, if the water content is low enough then, as we have seen previously, the
starchy component can become glassy; then we have a crisp texture, as in chips (crisps
in the United Kingdom). A further refinement is to create a glassy texture on the
outside, leaving a smooth creamy gel in the interior. This can be done in heavily
processed dishes such a croquette potatoes, where mashed potato is rolled into suitable
shapes and then fried to remove water from the outer layer, or in pieces of potato
where no additional liquid has been added, such as French fries (chips in the United
Kingdom). The key to crisp fries is to prevent water from the interior gelled starch
migrating to the outer glassy layer and so reducing its glass transition temperature,
rendering it soggy. A remarkable solution, pioneered at the Fat Duck, is to begin
by cooking the fries in water until they are just about to break up; they will have
absorbed some water during this stage and will form firm gels on cooling. Next, the
cooled fries are dried by placing them in a vacuum desiccator; during this stage the
outer layers become quite dry to the touch but of course still retain significant
amounts of water. These fries are then put in oil at a temperature of around 110 °C
to boil off the water in the outer layers and turn them glassy. Then, in a third stage
the fries are put into hotter oil at ca. 190 °C; in this stage the glassy outer layer
is temporarily softened and as steam is generated from the gel within it pushes much
of the outer now softened glassy layer away from the interior, leaving a distinct
puffed up nature to the fries. On subsequent cooling and serving the small air gap
between the interior and exterior slows down diffusion of water, ensuring the crisp
texture remains for at least as long as it takes the diner to eat the fries.
A similar interesting phenomenon is the popping of cereal grains; we are all familiar
with popcorn. On heating, the outer layer of the corn, which is initially glassy in
nature, softens to become rubbery. At the same time, pressure builds up inside the
grains as some water in the slightly swollen starch turns to steam. If the balance
is right, then there is sufficient pressure inside the grains to make them explode
just as the glassy outer layer is soft enough that it can no longer withstand the
pressure. The key to the process is to ensure there is enough water in the interior
of the grains to generate the pressure and little enough water in the outer layers
that they remain glassy until the temperature is high enough to convert enough water
to steam to allow the grains to “pop”. More or less any grains can be popped if they
are initially cooked in water to increase the water content at the center and then
dried in a cool oven (at around 50 °C) to reduce the water content of the outer layers
while leaving the interior well hydrated.
4.3.6
Cooking of Meat
4.3.6.1. Denaturation of Protein and the Associated Textural Changes. At the start
of cooking meat has a flaccid feel. During the cooking process the most obvious changes
are the shrinkage in muscle volume with a consequent loss of fluid and development
of a rigidity absent in raw meat. The texture changes in the meat are related to all
stages in the denaturation of the fiber and connective tissue proteins. The myosin
begins to coagulate at about 50 °C, which gives the meat some firmness. The myosin
squeezes out some of the water molecules which are further squeezed out of the cell
by the sheath of connective tissue. In steaks and chops the water also escapes out
of the cut ends of the fibers. At this stage meat is firm and juicy. At around 60
°C more of the proteins inside the cell coagulate and the cells become more segregated
into a solid core of coagulated protein and a surrounding liquid. In this temperature
range the meat progressively gets firmer and more juicy. At 60−65 °C the denaturation
of the collagen in the connective tissue happens. The collagen shrinks and forces
the liquid out of the cells. At this stage the meat releases lots of juice, shrinks,
and becomes more chewy and dry. A continued cooking leads to dryer and continuously
more compact meat. However, at around 70 °C the connective tissue collagen begins
to dissolve into gelatin and the muscle fibers are more easily pushed apart. At this
stage the meat seems more tender, although the fibers are still stiff and dry because
the fibers do not any longer form a substantial mass and because the gelatin provides
a juiciness on its own.(253)
4.3.6.2. Meat Tenderness and Appropriate Cooking Methods. The ideal cooking of meat
would minimize moisture loss and toughening of the fibers while also maximizing the
conversion of collagen to gelatin. In ordinary kitchen practice this is difficult,
and hence, the cooking method is usually adjusted to the meat’s tenderness.(253) The
main difference between tender and less tender cuts of beef is the relative quantity
of connective tissue. Tender meat has a small and the less tender a large proportion
of the connective tissue. When cooking meat with a high content of connective tissue
the point is to bring about hydrolysis of collagen and leave the fibers free to fall
apart, in which case the meat appears to be tender. This can be accomplished by cooking
the meat in the presence of water. If a little acid is added, the hydrolytic process
is accelerated. Steam is even more efficient than water, and if it is under pressure
so that the temperature is above that of boiling, hydrolysis is brought about rapidly.
The methods using water or steam include braising and stewing. The dry-heat methods
of broiling and roasting are used for the tender cuts since these presumably have
so little connective tissue that none of it needs to be removed to make the meat tender.(254)
Cooking of tender meat can be a challenge since the desired temperature range is very
narrow and it is difficult to obtain a uniform temperature in a piece of meat. When
frying or grilling meat at a high temperature, a temperature gradient from the outside
to the center will be present, which means the meat will dry out on the outside before
reaching the desired temperature at the center. By using a long cooking time at a
lower temperature this problem can be eliminated; however, a high temperature is needed
for the desired browning reactions on the outside. This issue leads to the common
kitchen procedure of heating the meat at a high temperature for a short time to obtain
the browning (Maillard) reactions and finish the cooking at a lower temperature. The
cook can also remove the meat from the heat before it is fully cooked and rely on
the afterheat to finish the cooking more gradually. The ideal cooking time is affected
by a number of factors such as the meat’s starting temperature, the cooking temperature,
flipping of the meat, fat content of the meat, and the extent of bones in the meat.
Hence, there is no way of making a precise prediction of the ideal cooking time.(253)
4.3.6.3. Effect of Cooking Conditions on the Texture of Meat. In order to determine
the optimum conditions for cooking of meat scientifically it is necessary to understand
the heat-induced processes like the denaturation of the various proteins and the textural
consequences, as described above. Several studies have investigated the effect of
time and temperature on meat tenderness and the textural changes.
255–257
Differential scanning calorimetry (DSC) is being used to investigate the denaturation
of proteins by measuring the energy input when heating up the meat sample. The Warner
Bratzler method, which measures the shear force when cutting meat, has often been
used to determine the tenderness objectively. However, the results do not often correlate
to a sensory evaluation.(258) Nevertheless, the shear force is a common method for
meat quality assessment.
4.3.6.3.1. Detailed Description of Processes during Heating. An increase in shear
force with increasing temperature occurs in two distinct phases. The first occurs
at 45−50 °C and has been associated with denaturation of myofibrillar proteins (actin
and myosin). Denaturation of actinomyosin leads to a release of tension and fluid
is forced out of the space between the endomysium and the denatured myofibrils, accounting
for the observed loss of fluid at these temperatures. The higher temperature (65−70
°C) increase in shear force is attributed to shrinkage of the perimysial collagen.
The fibers are denatured and may be seen to change from an opaque inelastic fiber
with characteristic banding pattern to a swollen fiber with elastic properties. The
extent of shrinkage and loss of fluid depends on the nature of the intermolecular
cross-links, which stabilize the perimysial collagen fibers. For this reason the extent
of shrinkage is greater in older animals. Heating for prolonged periods at temperatures
above 70 °C eventually causes a reduction in shear value, probably due to cleavage
of peptide bonds in the molecule. The residual strength of fibers binding the muscle
together contributes to toughness of the meat, in addition to the tension generated
by the thermal shrinkage of perimysial collagen. Overall, the mass of the meat depends
on the amount of denatured myofibrillar protein, while the texture is mainly determined
by the collagen fibers of the perimysium. Two effects are involved: compression of
the muscle bundles during collagen shrinkage and binding of the muscle bundles due
to the residual strength of the denatured collagen fibers. In each case the effects
are determined by the nature and extent of collagen cross-linking.(259)
4.3.6.3.2. Texture Changes Due to Denaturation (Time and Temperature). Martens et
al.(257) investigated the texture changes for beef during cooking. Table 11 shows
the results interpreted in terms of thermal denaturation of the three major proteins.
Table 11
Texture Changes in Bovine Muscle during Cooking, Relative to Thermal Denaturation
of the Three Major Muscle Proteinsa
molecular process
myosin denaturation
collagen denaturation
actin denaturation
firmness
++
(−)
+++
fiber cohesivity
−−−
bite-off force
+
−−−
+
residual bolus
−−
+
juiciness
(−)
−
−−−
total chewing work
++
−−−
++
total texture impression
++
−−
denaturation temperature
40−54 LMM 53−60 HMMb
56−62
66−73
a
Sensory score increases and decreases with increasing temperature are represented
by + and −, respectively. Number of signs represent relative size of respective texture
changes.(257)
b
LMM, HMM, represent low and high molar mass myosin respectively.
From Table 11 it can be seen that both myosin and actin denaturation results in increased
firmness, bite-off force, and total chewing work (usually referred to as toughness).
Actin denaturation also results in increased amounts of residual bolus and has a negative
effect on juiciness and total texture impression (the meat becomes drier). Denaturation
of collagen leads to reduced fiber cohesivity, bite-off force, residual bolus, juiciness,
and total chewing work and increased total texture impression (in everyday terms,
tenderness). This means that to achieve the optimal eating experience the meat should
be heated to a temperature where collagen is denatured but actin is still native,
i.e., between 62 and 65 °C.(257) The rate of actin denaturation is dependent on time
as well as temperature; at 66 °C denaturation of 10% of the actin is accomplished
in approximately 10 min, 50% denaturation in approximately 40 min, and 90% denaturation
in approximately 100 min.(257)
Tornberg(260) suggests that the denaturation of sarcoplasmic proteins has a big impact
on the texture of the meat, as the denaturation below 50 °C causes increased toughness
of the meat but in the temperature range 50−65 °C makes the meat appear more tender
as these proteins form a gel in the space surrounding the fibers and fiber bundles.
At temperatures above 65 °C this gel is much firmer and leads thus to a less tender
texture.(260) This is in accordance with the results found by Martens et al.(257)
4.3.6.3.3. Pressure. Ma and Ledward(261) studied the effect of pressure (0−800 MPa)
on the texture of beef muscle. They found that a pressure of 200 MPa caused a large
significant decrease in hardness, chewiness, and gumminess (evaluated by texture profile
analysis: Stable Micro System Type) in the temperature range of 60−70 °C. By using
DSC the texture changes were further analyzed. Collagen was, as expected, inert to
pressure, whereas myosin was unfolded by both pressure and temperature. From the results
obtained it was concluded that the structurally induced changes are unlikely to be
a major cause of the significant loss of hardness observed when beef is treated at
60−70 °C and 200 MPa. Instead, accelerated proteolysis under these conditions is suggested.
4.3.6.3.4. Heat Transfer. The heat-induced processes, as described above, all depend
on the heat transfer in a piece of meat when cooking. Some studies have worked on
the modeling of heat transfer in meat. Most studies are mainly concerned with the
safety issue (i.e., when does the meat reach 75 °C in the center
262–266
), but a few studies have been conducted on the impact of heat transfer on the textural
properties.
267–270
Modeling heat transfer in meat is an immensely difficult task. Meat is seldom uniform
in size or shape; it consists of several fractions of protein each with their own
thermal properties, and a large number of processes occur during heating. These processes
will cause changes in the thermal properties, e.g., changes in heat capacity, thermal
conductivity, etc., and changes in dimensions, water-holding capacity, etc. Furthermore,
water is transported in the meat during heating, which leads to transport of heat
and, if the temperature is high enough, evaporation of water at the surface. To complicate
things further the fiber direction also plays a role in the heat transfer. However,
understanding the basics of heat transfer in meat (and using a thermometer when cooking)
can lead to improved gastronomic quality of cooked meat, as this will reduce the risk
of overcooking.
271–273
Califano et al. simulated the heat transfer during the cooking process of beef and
the related textural changes. They propose a model for the cooking process as a tool
for analyzing the effect of cooking operations on the texture of meat. The effect
of heat transfer coefficient, temperature of the cooking medium, and size of the meat
piece on the hardness (by the Warner Bratzler method) of the cooked meat was studied.
If the cooking medium was above 85 °C the average hardness was high regardless of
processing time. Conversely, a low temperature gives a more tender product.(267)
4.3.6.3.5. Marinating Meat: Softening. Marinating, immersing meat in a fluid medium,
has been used for many years to flavor meat as well as tenderize it in the domestic
kitchen. Marinating of meat can be considered as a chemical method to help tenderize
meat, which also changes the flavor of the meat. The literature on this topic is however
driven mostly by the demand in the food industry to tenderize meat. Marinade solutions
can improve the perceived juiciness and tenderness of the meat as well as increase
the weight of the product. However, there is a particular problem with marinating
meat in that the marinade penetrates the meat very slowly and hence only works on
the outer layers. This problem is sometimes overcome by injecting the marinade into
the meat.
Several studies have investigated the properties of tenderizing agents, focusing on
obtaining the optimal juiciness and tenderness without causing any undesirable effect
on color or flavor. The mechanisms for increased tenderness and juiciness are generally
connected with higher water holding capacity (WHC) and swelling of myofibrils.
Acids, like vinegar, lemon juice, or wine, are very common ingredients in a marinade.
Sour marinating of meat has been found to improve tenderness and juiciness and increase
the weight of the product due to retention of water. However, sour marinating is also
found to affect the flavor, giving an unpleasant sour taste.
274,275
The mechanism behind the tenderizing action of acidic marinades is shown to involve
increased proteolysis and increased conversion of collagen to gelatin.(276) The pH
value is important to the swelling capacity and hence the WHC of meat. Both low and
high muscle pH after marinating have positive effects on texture and give an increase
in water binding capacity and swelling of myofibrillar protein.
277,278
As the pH moves further from the isoelectric point, the water-holding capacity increases
due to an increase in the amount of negative charges on the meat proteins that can
bind water.
279,280
Since an increase in WHC on either side of the isoelectric point is seen on the myofibrillar
protein, alkaline solutions also have tenderizing properties. Alkaline marination
is a common method of tenderization in Chinese cookery,(281) and it is likewise commonly
used as a marinade in household cooking in India.(282) A study by Hsieh et al. in
1980 showed that that bicarbonate treatment caused swelling and fusion of the myofibrils
and obscured the structures within the sacomer. Although alkaline marinating generally
has been overlooked in the West, recent work has focused on using bicarbonate to minimize
the problem of pale, soft, and exudative (PSE) pork meat.
Wynveen(283) found that postmortem injection of sodium bicarbonate and sodium phosphate
improved WHC and color in pork. Likewise, Yang(284) found that postmortem injection
of sodium bicarbonate in meat gave an increase in WHC and solubility of myofibrillar
protein as well as decrease in drip loss, weight loss during cooking, and shear force.
Sheard(285) compared the effect of sodium bicarbonate with that of salt and phosphate,
which are also common tenderization agents in the food industry. All solutions gave
a significantly higher yield and a decrease in shear force. The effect of sodium bicarbonate
on shear force and cooking loss was as great as salt and phosphate. However, the bicarbonate-treated
samples contained air-filled pockets between fibers, giving an unusual appearance
that might not be appreciated by consumers. This problem might be caused by generation
of carbon dioxide produced during cooking, which is likely also to be a contributing
factor for the texture change. A recent study by Anna et al.(286) investigated the
synergistic effect of sodium bicarbonate and blade tenderization on tenderness of
buffalo rumen meat. They found that sodium bicarbonate gave improved tenderness based
on analytical and sensory analysis.
The alkaline marinating techniques used in some places in Asia can be compared to
the traditional Swedish/Norwegian way of preparing dried fish (lutefisk).(166) Drying
was a common way of preserving fish (it can keep for years in Arctic climate), and
preparation of lutefisk was a primitive way of reconstituting the moistureless tough
fish. Lutefisk is made by soaking the dried cod in fresh water for a week, then submerging
it in a alkaline solution for several more days, and resoaking it in fresh water for
another 2 days.(287) Originally the ashes from a wood fire (rich in carbonate and
minerals) or lime stone (calcium carbonate) and later lye (sodium hydroxide) was used.(166)
The fish becomes very soft after cooking, which can be explained by the fish proteins
accumulating a positive charge in the alkaline solution. This causes the proteins
to repel each other, and only weak bonds are formed between the muscle fibers, giving
a soft texture.(166)
Another chemical way of tenderizing meat is by using enzymes. For many years the Mexican
Indians have known that the latex from the papaya leaves has a tenderizing effect
on meat during cooking. The major enzyme, papain (a cysteinyl-proteinase), has been
investigated. It has little effect at room temperature, so the main action occurs
while cooking. Above 50 °C the collagen structure is loosened and vulnerable to attack
from the proteinase;(288) unless the enzyme is deactivated it will continue to attack
the collagen until none is left and the meat has fallen apart. Ashie et al. in 2002(289)
compared the use of papain and an aspartic protease (expressed by Aspergillus oryzae)
for their tenderizing effect on meat.(290) They found that aspartic protease has an
advantage over papain by having a limited specificity on meat proteins and being readily
inactivated by cooking. Additionally, the use of proteases from kiwi fruit,(291) bromelain
from pineapple, and ficin from figs(292) has been reported.
4.4
Cooking Methods and How They Work
Most cooking techniques have been around in one form or another for many centuries,
but often they are not well understood by those who use them. In this section we try
to describe the basic physics and chemistry in the traditional cooking techniques
used in the kitchen so that chefs will be able to avoid such “schoolboy” errors. Then
we shall move on to describe how “new” methods can be introduced by adapting the equipment
found in the science laboratory. Indeed, this process has already started to have
a real impact in some kitchens where, for example, the introduction of laboratory-style
temperature-controlled water baths permits much finer control of the cooking process
and thus at the same time both improves consistency and reduces waste.
4.4.1
Traditional Cooking Methods
4.4.1.1. Use of Heat. Many cooking methods or gastronomic unit operations involve
heating the food material to induce chemical and/or physical changes that favor the
development of pleasant flavors and textures and create microbiologically safe food.
However, we also often heat food simply so as to serve a hot meal. Gastronomic foods
are often served above room temperature (for example, warm salads) even when there
are no specific physical, chemical, or microbiological reasons for the heating. However,
heating the food enhances the release of aroma compounds, changes the perception of
the food, and can create the direct sensation of heat.
During heating the elevated temperatures will shift the thermodynamic stability of
food components and make chemical changes and especially phase transitions possible
(e.g., melting, evaporation, gelatinization, denaturation of proteins). Through such
transitions the food itself can be transformed: plant cell walls can be broken, making
vegetables soft, meat protein can be denatured, rendering it tougher, etc. For the
gastronomic cook it is important to be aware of the range of possible transformations
and their impact on the flavor and texture of the food being heated.
Elevated temperatures change the rates of chemical reactions; chemical compounds that
are otherwise metastable may become unstable, and severe changes may take place within
the time scale of the cooking process (minutes to days). For example, proteins may
be hydrolyzed into peptides and amino acids during the cooking of stocks, Maillard
reactions will take place during the frying process at elevated temperatures, etc.
The phase transitions that occur in foodstuffs are mostly strongly endothermic. The
demand for significant amounts of heat can in turn influence the cooking process by
perturbing the dynamics of transport of heat and moisture. Since most foods have rather
high water content, the evaporation of water is a particularly important phenomenon
for understanding most cooking methods.
Water has a particularly high specific heat (heat capacity), 4.19 kJ/K. Accordingly,
the energy needed to heat 100 mL of water from room temperature to (normal) boiling
point is about 31 kJ. However, a much more important issue is the high latent heat
of evaporation of water (2.26 MJ/kg). Thus, the energy needed to evaporate 100 mL
of water is about 222 kJ; about 7 times more heat is needed in order to evaporate
water as compared to heating it from room temperature to boiling temperature. This
large heat requirement is one of the more important limiting factors in the kitchen,
especially when scaling up recipes.
The temperature distribution in food during preparation can be strongly influenced
by evaporation phenomena. Since many food ingredients are very moist their thermal
behavior can be modeled, to a first approximation, by the behavior of pure water.
In cooking operations where the surface temperature can exceed 100 °C (e.g., frying)
a dry crust has to be developed in order for the local temperature to be above that
of boiling water; the loss of water enables high temperature and accompanying acceleration
of chemical reactions such as the Maillard reactions specific to the frying process.
The presence and evaporation of water affect the transport of heat through foods as
they are being cooked. In particular, they tend to reduce the rate at which the temperature
at the center of foods being cooked increases and thus also increase temperature gradients
within the food.
4.4.1.2. Heating Methods.
4.4.1.2.1. Boiling. Boiling (heating in boiling water) is perhaps the simplest of
all kitchen techniques. For most vegetables we can assume that the temperature is
close to 100 °C (of course, addition of salt or changes in atmospheric pressure will
affect the boiling temperature, but such changes are generally small). We can then
treat the cooking process as a relatively straightforward heat transfer problem with
the boundary conditions of a uniform constant temperature (100 °C) at the food surface.
Thus, cooking times will be reproducible.
The time required to cook different foods will vary greatly; the actual temperature
required will differ between foods. In many cases, food is cooked for a short time
in boiling water, creating a significant temperature gradient across the food. Consider,
for example, green beans cooked for 3 or 4 min. The surface temperature will be 100
°C, while the center will have a temperature of only around 30 °C. As the food is
left on a plate heat continues to flow to reduce (and eventually remove) this temperature
gradient.
In some foods we wish to ensure that some physical or chemical process proceeds. In
such cases the time scale of the cooking process may be much longer than that of heat
transport, leading to fewer temperature gradients within the food during the cooking
process. Examples where longer cooking times are used include the need to gelatinize
starch in cooking of rice (and to a lesser extent potatoes) and hydrolysis of proteins,
which is required to provide the flavor of stocks.
Some complete dishes are prepared in boiling water. We can consider stews and some
sauces simply as a dilute solution with solutes of low molecular weight (salts, amino
acids, etc). The boiling takes place close to the normal boiling point of water (any
boiling point elevation due to the dissolved salts is of no practical importance).
The temperature will remain uniform as long as there is no accumulation of solid material
at the bottom of the pan near the heat source, in which case it can start to overheat
and “burn”; stirring is required to prevent this from happening.
In the kitchen there are many different ways to describe boiling water. The main differences
are simply in the rate of heat input. The minimum heat input is that which will just
maintain the temperature at 100 °C with the minimal amount of evaporation of steam.
As the heat input is increased, so the rate of creation of steam and the rapidity
of bubbles rising to the surface increases; at some heat input the flow caused by
the bubbles becomes rather chaotic, leading to violent movement and mixing of food
in the pan.
Gentle boiling with minimum heat input, just enough for ensuring the boiling of water,
is usually termed “simmering”. Some chefs claim that simmering causes less toughening
of meat than more “rapid” boiling, although to our knowledge there is no actual evidence
for such an effect. However, it may be that the heat input in these cases is not sufficient
to maintain the water at boiling point and the actual temperature was significantly
lower, in which case a more tender product could be quite possible.
At the opposite extreme with a high heat input chefs often say water is at a “rolling
boil”, indicating the large-scale movements in the water. If food is added to a pan
at a “rolling boil” the heat input may be sufficient to avoid a significant temperature
drop as the cold food has to be heated, thus reducing the cooking time, and the violent
movement of the water may ensure the food is kept moving and prevented from sticking
to itself. However, there is no literature where any clear distinctions are drawn,
and personal experience suggests these are not important effects.
4.4.1.2.2. Steaming. In the steaming process the food (such as vegetables, fish, or
bread dough) is in contact with steam above boiling water. Heat is transferred to
the food as the gaseous water condenses at the food surface and releases its latent
heat. The steam is thus a very effective medium for transferring heat as opposed to
hot air at the same temperature (as seen in the baking process). One could consider
steaming as the opposite of baking, condensing rather than evaporating water at the
food surface. No crust is formed, and the food is not dehydrated, which creates very
different results. In steaming the surface temperature is 100 °C due to the equilibrium
between the two states of water.
Steaming thus resembles boiling and simmering in many respects, although some differences
may be important. The food is not immersed in water, so that probably fewer soluble
compounds are lost from the food. Steaming is generally believed to create textures
with a better bite and is a more gentle form of heating compared to boiling.
4.4.1.2.3. Frying and Deep Frying. In frying a much higher temperature is used than
when cooking in water. In deep fat frying the oil temperature is usually around 160−180
°C. The surface temperature of food is thus well above the boiling point of water,
and rapid boiling occurs. This boiling requires a large amount of heat, so the temperature
of the cooking oil normally falls rapidly, leading to a reduction in this surface
boiling. In practice, the amount of food put in a deep fat fryer will greatly affect
the way in which it cooks. If too much food is put in at one time the temperature
is reduced below 100 °C and the food will not become “crisp”.
Shallow (or dry) frying uses a high (surface) temperature (typically 200 to 240 °C)
and, usually, some frying oil to ensure a good heat conduction. The surface temperature
is (unlike boiling) controlled by a balance between the heat needed to evaporate water
and the supplied heat from the stove. Unlike the boiling of water, there is no phase
transition to control the temperature of frying oil or the pan surface. The temperature
remains nonuniform throughout the frying process due to evaporation of water.
Two main transport processes take place: heat conduction from the hot surface toward
the center and transport of water toward the surface where water is evaporated (of
course, it can evaporate from any surface, but most evaporation will come from the
hot surface in contact with the pan). A dry, hot, and relatively well-defined frying
crust is built up. Just below the crust the temperature is expected to be close to
100 °C due to evaporation of water. Thus, a large temperature gradient is created
over the frying crust. The temperature of the crust surface is comparable to the frying
oil. If the temperature in the crust is high enough, browning reactions can take place
at considerable rates.
The temperature at the center of the food item will however only slowly increase with
time as heat is conducted inward. For example, in the case of steak frying, the process
is terminated when the temperature has reached 40−60 °C, giving the center an appropriate
red color. This can take (for a typical 3 cm thick steak) between 5 and 12 min.
During heating the food (especially meat) releases considerable amounts of water (due
to changes of meat at temperatures above the denaturation temperature of muscle proteins).
The energy input must match this release in order to quickly evaporate all the water
so as to keep the surface temperature of the food well above 100 °C. Too little power
will result in a cooking/broiling process where the surface temperature will be close
to 100 °C, giving rise to less browning and a different flavor formation. It is often
forgotten by cooks that they should not attempt to cook too many steaks (or brown
too much meat) at a time. The amount that can be cooked is limited by the power of
the burner used on their stove. The power of the burner must be significantly greater
than that required to boil off the water that is being released from the cooking meat.
This problem is particularly important when scaling up recipes. A recipe that works
well when cooking for four may completely fail when doubled for a dinner party of
eight all because the pan temperature cannot get above 100 °C with the increased amount
of meat.
The heating power of the burners on modern domestic stoves is typically up to 2.5
kW, while restaurant stoves can often provide up to 8 kW. The power limits the amount
of meat that can be browned at a time; for a domestic stove the limit is around 800
g, and for a commercial stove it is nearer 2.5 kg.
In a wok or when using any continuously stirred process the surfaces in contact with
the pan will heat up briefly. The surfaces facing away from the pan will quickly be
cooled down due to evaporation of water. The average surface temperature during stir
frying is thus quite low (below 100 °C). Heat conduction into the vegetable is thus
quite slow and gentle despite the high surface temperature of the wok. The center
temperature of the end product will be low compared to a boiled vegetable; this will
ensure less transformation of the cell wall carbohydrates, etc. However, the vegetables
are subjected to severe dehydration. The combination of gentle heating and dehydration
gives the product its texture or bite. In meats, the Maillard reactions and pyrolysis
at the surface ensure development of taste and aroma compounds.
4.4.1.2.4. Baking. In a baking oven the air temperature is kept fixed in the range
150−250 °C by a thermostat system. Air circulates the oven due to convection or forced
circulation. Confusingly, some manufacturers (particularly in the United States) refer
to ovens where the air is circulated using a fan as “convection” ovens.
Oven surfaces and heating elements may have much higher temperatures so that some
radiant heating can also occur.
(a) Leavening of Cakes, Breads, and Soufflés. The expansion of various baked goods
during the heating can be created by several kinds of endothermic processes where
the thermodynamic equilibrium is shifted by the increasing temperature. For example,
in the endothermic decomposition of baking powder into gaseous compounds carbon dioxide
is formed from sodium bicarbonate (baking soda) or carbon dioxide and ammonia are
created from ammonium bicarbonate. Dissolved gases can be released and water can evaporate.
At elevated temperatures the equilibrium of these processes will be shifted toward
the formation of more gaseous compounds, so that the baked goods rise due to an increase
in the gas content.
Industrially, but not so much in home cooking, leavening acids are used. These leavening
acids (such as sodium aluminum sulfate, dicalcium phosphate dihydrate, potassium acid
tartrate, or δ-gluconolactone) enable the decomposition of bicarbonate at lower temperature
and prevent the pH elevation and soapy flavors due to formation of carbonate ions.
The action of leavening acids is kinetically determined and can be classified into
slow, medium, and fast reacting.
(b) Role of Water Evaporation for the Expansion of Bread Cakes and Soufflés. The dough
or soufflé base is very moist with water activity (relative humidity) close to 1.
Therefore, in many respects they can be treated as if they behave like water. The
doughs belong to the colloidal category of foams containing many small air bubbles.
The air is trapped within a soft material and is thus approximately subjected to pressure
very close to the external pressure, which usually is close to 1 atm. The entrapped
dry air can be a result of mechanical treatment of the dough, leavening using yeast,
or leavening using baking powders. In the case of soufflés since no yeast or baking
powder is used, evaporation of water is particularly important and the main cause
of the rising of the soufflé.
During the baking process the macroscopic transport of heat and moisture over distances
comparable to the cake/bread/soufflé size are rate limiting and responsible for the
overall duration of the baking process. On a much shorter time scale we can expect
water to evaporate into the air bubbles of the dough in order to attain a local equilibrium
between gaseous and liquid water. Some water will be lost from the surface of the
soufflé, but the inside will remain a very moist structure. It is reasonable to assume
the pressure of water will attain its equilibrium-saturated value inside the bubbles.
At elevated temperature the saturated water pressure will increase and ultimately
approach the external pressure (1 atm at 100 °C). Likewise, the molar and volume fraction
of water in the gas phase increase (ultimately to 1). Due to the dilution effect of
the gaseous components already present (N2, O2, Ar, etc.) a large amount of water
has to evaporate, making the overall gas phase expand. Under these somewhat simplified
assumptions the gas volume will in principle diverge when the equilibrium partial
pressure of water approaches the external pressure (at the boiling point temperature
of water)
where V
g(T
0) is the volume of dry gases at low temperature (T
0), p
w
o(T) is the temperature-dependent saturated vapour pressure of water, p
ex is the external pressure, and T is the absolute temperature.
The total gas volume is proportional to the initial volume of dry gases. The expansion
mechanism due to water will only work if other gaseous components are present. In
other words, evaporation of water will boost other leavening mechanisms. In a soufflé,
the chef beats in air in the egg white before cooking; during baking this increased
air volume is further boosted by evaporation of water. The normalized gas volume as
a function of temperature for an idealized model air/water soufflé is shown in Figure
17. The trivial temperature expansion of the dry air contributes in itself with an
almost negligible expansion, whereas evaporation of water contributes dramatically
when approaching the boiling temperature.
Figure 17
Expansion due to evaporation of water shown as normalized gas volume as a function
temperature. The volume is normalized with respect to the volume of dry air at T =
25 °C. The external pressure is taken to be 1 atm, and the volumes are calculated
using data for saturated vapor pressures of water from the Handbook of Chemistry and
Physics, 79th edition. The initial volume of dry air shows an expansion which is proportional
to the absolute temperature, whereas the volume taken up by the gaseous water will
diverge when the temperature approaches the boiling point of water. The expansion
of a soufflé is of the order of about 3 times corresponding to an (average) temperature
of close to 90 °C, which is somewhat higher the typical center temperature for white
breads (close to 70 °C).
All the various mechanisms of heat-assisted rising of bakery products, do not work
independently but mutually boost one another other. For example, the expansion due
to evaporation of water will influence the equilibrium between dissolved and chemically
bound carbon dioxide and gaseous carbon dioxide. The expansion due to evaporation
lowers the partial pressure of carbon dioxide and thus favors release of more gaseous
carbon dioxide in order to restore equilibrium. At the same time, the released carbon
dioxide boosts the expansion due to evaporation of water.
4.4.2
“New” Cooking Techniques
4.4.2.1. Microwaves. It is the dipolar nature of water molecules that permits them
to be heated by microwaves. The alternating field (typically 2.45 GHz in a domestic
microwave oven) causes the dipoles to rotate; the inability of the dipoles to keep
up with the field leads to the heating effect. Any dipolar material will be heated
in a microwave oven, but water having the strongest dipoles in common food stuffs
displays the greatest heating effect. In general, any material with hydroxyl groups
will display dipolar heating in a microwave field; thus, oils, sugars, proteins, and
carbohydrates can all be heated to a greater or lesser extent with a microwave oven.
However, materials where the dipoles are not able to rotate (such as ice) are much
less susceptible to microwave heating.
It is worth realizing that the microwave energy will always be absorbed from the outside
inward (in general, the higher the dielectric constant the greater the absorption).
In wet foods the typical penetration depth of microwaves is a few centimeters. The
common myth that microwaves heat from the inside out probably arises from reheating
foods such as jam doughnuts where the outer regions have significantly lower water
content than the center so that the center can become hotter than the outside.
An interesting example of the use of a microwave oven to produce a novel food was
described by Nicholas Kurti in his highly influential 1969 Royal Institution lecture(9)
which perhaps can be traced as the origin of the modern science of Molecular Gastronomy.
In the lecture he demonstrated the concept of an “inside out” Baked Alaska, by freezing
ice cream at low temperatures, leaving very little liquid water, and placing some
very high sugar content jam which retains a significant liquid content at the center;
he was able to use a microwave oven so that the microwaves were hardly absorbed by
the ice and passed through heating the jam center, thus producing a novel dessert
with a cold outside and a hot center.
However, despite this early potential novel use of a microwave the device has not
found many real gastronomic applications, perhaps due to the nonuniformity of the
heating (there are always nodes and antinodes separated by a few centimeters in a
microwave oven) or maybe to the fact that it is very difficult to achieve temperatures
above 100 °C so that browning and Maillard reactions do not usually occur. Of course,
a variety of methods have been introduced to permit browning in microwave ovens: the
use of containers that themselves heat up or the use of combination ovens which have
conventional as well as microwave heating.
It is instructive to note that boiling water in a microwave oven is (in certain cases)
very different than boiling water in a kettle. In a conventional kettle the element
(or for a kettle on a stove the base) is heated to a temperature well above 100 °C
and the water starts to boil readily at this hot surface. However, in a microwave
oven the water is heated directly, so that it can superheat if no bubbles are nucleated.
In practice, tap water has a good deal of dissolved air which starts to come out of
solution as the water is heated (the solubility of the gas being lower at elevated
temperatures), and this leads to the nucleation of bubbles and allows the water to
start to boil as soon as the temperature reaches 100 °C. However, in previously boiled
water in which there is no dissolved air, it is quite possible for the water to superheat
significantly. If this happens and you remove say a cup of reheated coffee from the
microwave oven that has superheated and not yet started to boil it can at the least
disturbance (for example, adding a little sugar) boil over in an explosive fashion.
4.4.2.2. High Pressure. Some useful processes in cooking, such as extruding, pressure
cooking, and homogenization, use moderately elevated pressures. In these cases it
is the increase in the boiling temperature of water with pressure that provides the
useful effects. In this section we are concerned with the effects of cooking at much
higher pressures where more interesting effects can be found.
The use of high-pressure techniques in the pressure range from 2000 to 10 000 atm,
where pressure effects become significant, has been limited for technical reasons;
the exploration of the technique is not new. Hite(216) used pressure as a means of
microbial control in milk as early as 1899. Pressure treatment normally has no effect
on covalent bonds but denatures proteins, as denatured proteins have smaller partial
molar volume than native proteins owing to differences in solvation. High-pressure
processed foods commercially available today included juices with noncooked flavor
but with long shelf life, dried cured hams, where residual microbial contamination
following slicing has been removed without heat treatment, and products such as guacamole
where browning enzymes are deactivated by pressure. Equipment for high-pressure processing
is becoming less costly and will eventually find its way to the restaurant kitchen,
opening up possibilities for new types of dishes. Meats can be decontaminated by pressure
treatment prior to shorter heat treatment such as in a wok or heat treatment at lower
temperatures. Eggs can be hardened by pressure treatment instead of boiling but with
preservation of the fresh egg flavor. Milk can be solidified by pressure without requiring
acidification and addition of sugars, creating new pH-neutral types of deserts. The
effects of pressure on fat crystallization deserve further attention for optimization
of spreadability and smoothness. The drawbacks seem few except for the cost of equipment.
One potential drawback is that lipids in poultry show increasing oxidation following
pressure treatment and subsequent cooking.(293) It should, however, be possible to
define pressure−temperature−time windows, where microbial decontamination is acceptable
without pressure-induced lipid oxidation.
Pressure effects on ice are unique as pressure lowers the melting point of normal
ice down to almost −20 °C around 2000 atm. Pressure-assisted freezing involves pressurizing
up to 2000 atm followed by cooling to below zero. Upon pressure release, water in
the subzero liquid food item solidifies all at once, not just from the outside, to
form smaller ice crystals than normal methods. Pressure-assisted thawing, which has
already been used to prepare of raw fish dishes from frozen fish without lengthy thawing
at room temperature or thawing at high temperature, depends on an initial pressurization
of the frozen fish during which the ice melts at the low temperature followed by a
controlled pressure release with simultaneous temperature compensation to reach ambient
pressure and temperature. The technique produces a fish which is thawed uniformly
and without high-temperature domains.(294)
High-pressure techniques should find their way to the kitchen for creation of new
dishes, where freshness and uncooked flavor can be combined with changed texture.
4.4.2.3. Improved Temperature Control. One area where commercial kitchens have already
learned from the science laboratory is in the use of accurately controlled temperature
baths. Until quite recently a kitchen “bain marie” was simply a warm water bath; the
temperature might have been anything from 40 to 80 °C. Temperature control (if any)
was via a crude bimetallic strip type of thermostat, and temperatures were set using
a simple variable resistor, usually with no calibration at all. However, in the past
decade or so many restaurant kitchens have started using recirculating water baths
with modern, accurate temperature controllers (the same as those found in any good
laboratory). The results are immediately noticeable. Precise temperature control permits
all sorts of cooking that is not otherwise possible. For example, an egg can be cooked
in water at 52 °C, a temperature where the white will set but the yolk will remain
fluid; thus, perfect poached or soft-boiled eggs can be served to order with complete
confidence that the product on the plate will be quite perfect.
Another use for such baths is in sous vide cooking of meats. Meat is placed in a plastic
bag and sealed under vacuum to exclude any air and provide a barrier between it and
the surrounding water (or other heat transfer medium). Of course, care is taken to
ensure any bacteria are killed prior to the low-temperature cooking, either by quick
pasteurization in a hot (85 °C) bath for a couple of minutes or by passing the flame
of a blow torch over the surface of the meat before putting it in the bag. The meat
is then cooked at a low temperature, where the myosin and actin proteins are scarcely
denatured, but for a sufficiently long time that the collagen is slowly softened.
The required temperatures and times vary according to both type and cut of meat. Thus,
for example, a lamb fillet might require 90 min at 56 °C, while belly pork might take
6 h at 60 °C. The best results are found by trial and error, but chefs who use the
technique find significant differences in the texture and flavor of meats cooked at
temperatures that differ by as little as 1°C.
4.4.2.4. Low Temperatures. Another area where modern laboratory equipment can be of
real use in the kitchen is in cooling. Freezers used in the kitchen typically can
only reach temperatures down to −20 °C. However, many processes demand lower temperatures.
For example, to kill some parasites, etc., in some fish it is necessary to cool them
below −30 °C. Similar low temperatures are needed to prepare ices with high alcohol
content and foods for freeze drying. In this context, freeze dryers are also potentially
useful kitchen appliances (and are already in use at some restaurants).
Liquid nitrogen can be particularly useful in the kitchen; it not only provides quick
and easy access to low temperatures but also permits rapid cooling of all sorts of
foods and so prevents the growth of large ice crystals that so often damage frozen
foods. Two particular uses for liquid nitrogen are to permit the easy grinding of
herbs; simply mixing the herbs with some liquid nitrogen in a mortar quickly freezes
them into brittle solids, and grinding with the pestle turns these into a useful powder
that can be used to provide an instant hit of fresh flavor in any dish. However, perhaps
the best use of liquid nitrogen was pioneered in 1901 soon after nitrogen was first
liquefied by Wroblewski and Olszewscki in Warsaw in 1887(295) by Mrs. Agnes Marshall.
296,297
Mrs. Marshall was the celebrity chef of her day, giving large-scale demonstrations
and selling her recipes, books, and equipment along the way. Unfortunately for us
on her death all the rights to her work were bought up by Mrs. Beeton’s publishers
and never again saw the light of day. However, she describes in some detail how to
use liquid gases at the table to prepare ice cream. The principle is extremely simple:
you just add liquid nitrogen to the ice cream mixture and stir while it freezes; it
is possible to freeze a liter of ice cream in under 20 s in this way.
298,299
The advantage is that the speed is so great and the temperature so low that only very
small ice crystals can form, making a wonderfully smooth ice cream.(300)
4.4.2.5. High-Power Mixing and Cutting Machines. A further group of items of modern
equipment that is finding increased use in the kitchen are powerful mixing, grinding,
and cutting tools. One of the most interesting of these is one that was indeed designed
for the kitchen, the Paco Jet. The Paco Jet consists of a very sharp knife that rotates
at around 2000 rpm and is driven slowly into a solid frozen block of food, shaving
layers of thickness (ca. 1 μm) in each revolution.(301) The machine is most commonly
used to produce ice creams and sorbets. It can produce particularly smooth ices by
ensuring the crystals are kept very small. To ensure all the ice crystals are cut
into pieces that are small enough that we can hardly detect them it is important that
the block is completely solid (i.e., frozen to a temperature below the eutectic point
for the solutes, sugars and proteins, present), which for most ices would mean a temperature
below about −18 °C. The machine has a suitable headspace above the solid block to
permit aeration of the resulting mixture, while it is still fully frozen so as to
produce light ices.
Another useful high-power technique can be to use ultrasonic agitation to induce emulsification.
A simple ultrasonic probe placed in a small container of oils and aqueous liquids
will very quickly emulsify the mixture; provided suitable emulsifiers are present,
the resulting emulsions can be very stable. This is a particularly easy way to prepare
small emulsions; typical probes have diameters of 1 cm or less, so volumes of 1 or
2 mL can be prepared to order.
5
Enjoyment and Pleasure of Eating: Sensory Perception of Flavor, Texture, Deliciousness,
Etc
In this section we start to deviate from chemistry and move into psychology and sensory
science. In particular, we want to begin to understand what factors influence our
perception of “pleasure” and our enjoyment of the meal.
5.1
Flavor Release
The chemistry of the formation and formulation of flavor molecules in foods is important
in almost all culinary practices. However, the presence of these flavor components
alone is not sufficient to describe the perception of food flavor. Flavor molecules
need to be delivered to the chemical senses during eating in order to create sensations
and perceptions. Accordingly, the binding, release, and transport of these molecules
are all important factors contributing to flavor perception. The flavor perceived
during eating arises from a complex time-dependent pattern of releasing volatile and
nonvolatile components from the food matrix, a process which has different characteristics
for particular foods. Nonvolatile compounds may be dissolved in the saliva surrounding
the taste receptors and stimulate the receptors cells by reversible binding to the
gustatory receptor proteins. Volatile compounds are transported back up through the
nasopharynx into the nasal cavity. The transport of volatiles is facilitated by mouth
movements, swallowing and breathing, which allow air to be moved retronasally from
the mouth to the nose. Besides the release in the mouth, the release of volatiles
from residuals of the bolus left in the pharynx after swallowing is also an important
pathway.
5.2
Matrix Interactions and Thermodynamic Aspects
Flavor release from a food during eating will only occur if the partition equilibrium
between the gas−product phases is disturbed. An important factor is the horizontal
movement of the tongue pressing and releasing the food from the palate during chewing.
This process creates pressure differences and temporal generation of a “fresh” or
new air−water surface area, thus stimulating the kinetics release of volatiles from
the product phase to the gas phase.
The interaction of the components with the food matrix has a significant impact on
the variation of flavors in foods. An important factor in the relation governing flavor
perception is the relation between concentration in some solvent (oil or water) and
equilibrium partial pressure of some aroma component. Due to the nonpolar character
of most aroma volatiles even a small concentration in water will generate a relative
large partial pressure, and thus, odor molecules can be considered as very volatile
in foods with an aqueous character. On the contrary, when the solvent is oil, a larger
concentration is needed to generate the same partial pressure. To our knowledge a
detailed thermodynamic analysis of odor in water and oil in terms of the size Henry’s
law constant, partitioning coefficients, and nonideality is not available in the literature.
The differences between interactions with oil and water as solvents can be noted in
the fact that the odor recognition threshold concentration in general is much higher
in the case of oil as a solvent as compared to water. Many foods are dispersed systems
containing both aqueous and lipid phases, and the effect of lipids content on flavor
release will reduce the concentration of headspace volatiles, but removal of lipids
in low-fat foods may also result in increased release.(302)
On top of a solvent-like interaction flavors may be bound to food components and,
since only the free dissolved flavor molecules exert a vapor pressure, fixation of
flavors can have a significant effect on flavor perception. An example of fixation
is the embedding of flavor molecules in glassy materials, like candies and cereals.
Certain flavor compounds, e.g., aldehydes, may also become covalently bound to proteins
during storage. Other types of binding include physicochemical interactions such as
van der Waals, hydrogen bonding, ionic bonding, and hydrophobic interactions in proteins
and hydrophobic coils of carbohydrates. Relatively high levels of monosaccharides
and salts increase the release of the less polar volatiles, a phenomenon attributed
to as a ‘salting out’ effect. A higher serving temperature will generally shift equilibrium
toward more gaseous odor as binding and solubilization reactions in general are exothermic
reactions.
5.3
Transport of Volatiles and Kinetic Phenomena
Besides the interactions with the food components, kinetic aspects of the transport
of volatiles from the food matrix to the aqueous or gas phases are important. Once
the food has entered the mouth it becomes wetted by a thin layer of saliva. Even the
fragments during chewing will be rapidly coated by saliva. Accordingly, the flavors
in solid foods must be transported though the aqueous layer to the air. It is unlikely
that simple diffusion alone accounts for the release through these respective phases,
since the mastication disturbs diffusion gradients and generates new interfaces. Depending
on whether the food is a liquid, semisolid, or solid, different transport mechanisms
apply and some of them have been modeled from a kinetic viewpoint.
303–305
A higher serving temperature will again favor a greater rate of release of odor and
taste.
5.4
In Vivo Flavor Generation
Besides the flavor compounds present in the food matrix, enzyme activities in the
mouth from both digestive enzymes in the saliva, those from micro-organisms and those
present in the food as prepared, all play a significant role in the modification and
generation of particular flavor components. Thiols like 2-furanmethanethiol found
in coffee undergo rapid oxidation, leading to a change in the profile of volatiles
entering the nose via the retronasal pathway. Similarly, esters are rapidly hydrolyzed
by enzyme activities in saliva.
306,307
Further, aldehydes may be formed during the mastication process by lipoxygenase activities.
Therefore, characterization of aroma volatiles in foods does not necessarily reflect
the volatiles stimulating the chemical senses during eating. Instrumental methods
for measuring flavor release in the nose, such as ACPI-MS and PTR-MS, made it possible
to evaluate such changes in the concentration of flavor components at low sensitivity
and high selectivity in real time.(308)
There are many factors influencing the retronasal flavor release and delivery, some
of which have been briefly presented here. It is a challenge within molecular gastronomy
to bring the physicochemical principles into practice, designing particular flavor-delivery
systems contributing to the quality and timing of the sensory experience of the dish.
5.5
Sensory Perception of Flavor: Complexity and Deliciousness
Even though we talk of “the taste of food”, senses other than that of taste contribute
significantly to the sensory experience we have when we eat food. The “taste” of an
orange, for example, is a result of the integration, in higher areas of the brain,
of taste, smell, and tactile signals. Besides the sense of taste, which begins when
taste receptors on the tongue are stimulated with substances that give rise to the
experience of sweetness, saltiness, sourness, bitterness, or umami; olfactory (smell),
tactile (touch), and chemestesis (e.g., hotness) sensations all make major contributions
to the “taste” of food.
309,310
Indeed, the sense of smell is of crucial importance for the perception and enjoyment
of food, as may easily be demonstrated by blocking its function while eating. If you
simply pinch your nose (or better that of your partner) when eating, the overall flavor
quickly fades away, leaving only a sensation of sweetness, sourness, etc. This is
the reason why food loses its ‘taste’ when we are suffering a cold; the reduced flow
of aroma molecules into the nose means we sample far less of the aroma compounds and
so are not able to integrate the full flavor.
The sense of taste has five independent components (salty, sweet, sour, bitter, umami),
but the sense of smell has many more dimensions. Humans have around 500 different
receptor types in the nose, and these open up a very rich space of smell/aroma experiences.(311)
Further, the texture of food contributes greatly to our perception of its ‘taste’.
Perception of hardness, elasticity, viscosity, brittleness, etc., is made possible
by the action of the sense of touch; in the context of food, texture perception is
often referred to as “mouthfeel”.
311,312
Some dishes completely lose their appeal without the perception of hotness (e.g.,
much of Thai and Indian cuisine). These sensations arise by stimulation of the trigeminal
nerve (fifth cranial nerve).(313) The senses of vision and hearing are of lesser importance
for the actual experience of a meal but can, via the expectations they bring about
in the eater, influence how the meal is perceived.
The brain has a somewhat modular functional architecture, especially at the first
processing steps toward extraction of information about the environment. Each of the
senses has its own neural system in the brain, and only later along the chain of processing
is there massive integration of signals from the different sensory systems.
309,314
From a functional point of view the brain’s tasks are often subdivided into categories
of perception, cognition, emotion, and action. Each of the senses performs tasks from
each category. Perception allows the perceiver to obtain knowledge about what is where
and when. Cognition is responsible for higher mental states (e.g., rational thinking,
planning), which are often of a kind that allow for introspection and consciousness.
Even though some cognitive processes are embedded in each of the senses, they are
much more central to the so-called “higher senses”, vision and audition. Emotional
processes, on the other hand, seem to be more central to the “lower senses” of taste,
smell, touch, and chemestesis. Any judgment of the pleasantness or hedonic quality
of a stimulus is of an emotional nature, and such processes are central to the senses
which are strongly involved in the perception and evaluation of foods and drinks.
Emotional activity in these senses is of crucial importance for all animals and constitutes
the drive to satisfy the most basic needs of living beings: food and sex.(315) It
is therefore not surprising that man shares many of the neural structures responsible
for emotional processes with evolutionarily older animals. Man’s larger brain houses
structures in the neocortex, which he does not share with other animals and allows
for other feats, such as language. However, the important feature here is the fact
that we share emotional neural structures with other animals. These structures are
found in the limbic system in the midbrain and in a number of structures in the forebrain
and basal ganglia (orbitofrontal cortex, striatum, and nucleus accumbens).
Reward mechanisms are emotional in nature, and these mechanisms might have evolved
to guarantee engagement in behaviors important for survival. A varied energy supply
is necessary for survival, and eating food in most cases leads to rewarding feelings
and pleasure.
316,317
Dopaminergic pathways in the brain, i.e., neural structures depending on dopamine
as neurotransmitter, have long been known to be crucial for reward mechanisms.(318)
Recently, however, a new neurology of reward has emerged in which reward is suggested
to consist of distinguishable processes in separable neural substrates. In this account
liking (emotion or affect) is separated from wanting (or motivation), each having
explicit as well as implicit components. Explicit processes are subjectively aware
to us, whereas implicit processes exert their influence without being conscious to
us.
319,320
Contrary to previous belief, the pleasure of eating palatable food is not mediated
by dopamine but rather by opioid transmission in a neural network including the nucleus
accumbens, ventral pallidum, parabrachial nucleus, and nucleus of the solitary tract.
Wanting (appetite, incentive motivation), on the other hand, is suggested to rely
on a dopaminergic system with projections from the ventral tegmental area to the nucleus
accumbens and circuits involving areas in the amygdala and prefrontal cortex.(319)
The distinction between liking and wanting was originally based on work on rodents,(319)
but psychophysical and neuroimaging studies on humans support the distinction.
320,321
Since eating and drinking are crucial to survival the motivational mechanisms and
rewards related to feeding are strong. It might therefore not be very surprising that
the biological mechanisms of feeding and addiction overlap throughout evolutionary
history. Work in rodents has demonstrated increases in dopamine in nucleus accumbens
induced by food and by amphetamine. The dopamine response to the two types of stimulation
are qualitatively identical, although the size of the response is an order of magnitude
larger for amphetamine.(322) Similar results have been obtained from neuroimaging
studies on humans.
323,324
Besides dopaminergic systems, several cholinergic systems in the brain have been implicated
in both food and drug intake.(325)
Eating leads to satiety and termination of a meal. Humans have a number of satiety
mechanisms, all eventually controlled by the brain, but the classical homeostatic
satiety mechanism begins with events in the gastrointestinal system. Homeostatic satiety
is a biological negative feedback system that works much like a thermostat. Hunger
is signaled by a number of hormonal substances such as ghrelin in the stomach and
NPY, orexin, and AgRP in the hypothalamus. Different nuclei in the hypothalamus are
thought to control hunger and satiety and the associated relevant behaviors. Intake
of food depresses the hunger signals and leads to an increase in satiety signals such
as CCK, GLP-1, PYY, insulin, and leptin. Food intake cannot, however, be fully controlled
by homeostatic regulatory circuits in the hypothalamus. If this were the case, we
would not expect the average weight to have steadily increased in the western world
for many years to an extent that in many countries one-half of the adult population
is now overweight or obese (BMI > 25). Berthoud(317) has argued that human food intake
control is guided by cognitive and emotional rather than metabolic aspects in the
obesogenic environment of affluent societies. As he succinctly has stated “The mind
wins over the metabolism in the present affluent food world”.(317) This state of affairs
is often referred to as “the obesity epidemic” and constitutes a serious problem to
the individual suffering from obesity as well as severe challenges to society, both
in terms of lost work years and escalating expenses to treat the effects of obesity.(317)
The hypothalamic system, classically believed to control food intake, has an abundance
of connections to other parts of the brain involved in sensory and reward processing,
and evidence seems to suggest that these cortico-limbic processes can dominate the
homeostatic regulatory circuits in the hypothalamus. A more precise understanding
of the interactions between these different systems participating in the control of
food intake is important and necessary for the development of behavioral strategies
and pharmacological interventions to curb inappropriate food intake.(317)
Besides the homeostatic satiety system humans possess so-called sensory-specific satiety
mechanisms. Sensory-specific satiety denotes the decline in liking of a food eaten
to satiety without effects on the appreciation on other foods.(34) An animal endowed
with such mechanisms will tend to eat a varied diet, which in turn will counteract
the risk of malnutrition. These mechanisms also highlight the importance of reward
for food intake. Work to understand the neurobiological and psychological underpinnings
of inappropriate eating will accordingly have importance for the endeavors to produce
superb experiences and pleasure from meals and vice versa. In short, a better understanding
of what it is in a meal that can cause people to experience great pleasure and how
to physically and chemically transform the raw materials to arrive at such an end
product may have much to contribute to the task of developing strategies to overcome
the obesity epidemic.
Recent work has demonstrated that there are different types of sensory-specific satiety
mechanisms. Some are product specific, some are product-category specific, and some
are genuinely sensory specific, determined by basic sensory attributes such as sweetness,
sourness, and fattiness. These insights can be used to guide the composition of meals
put together to produce maximal satisfaction as well as meals which produce maximal
satiety with the lowest amount of energy content. Neuroimaging work has also begun
to delineate the underlying neural structures and mechanisms responsible for sensory-specific
satiety and hedonic pleasure related to eating.(34) Closely connected to these efforts
is work that has demonstrated the effects of hot spices (irritants like capsaicin,
piperine) on metabolism as well as the feeling of satiety. Use of hot spices not only
can increase the pleasure gained from meals but also can lead to higher metabolism
and increased feelings of satiety.
326,327
The foods we eat are to a large extent determined by our preferences. Other factors
such as price, social context, etc., are also important, but within boundaries set
by these factors, we eat what we prefer or like.
Research has demonstrated that we are born with very few specific preferences.(328)
Newborn babies have a strong preference for sweet and fatty taste and a dislike for
bitter taste. From a developmental point of view the preference for sweetness and
fat facilitate breastfeeding. The dislike for bitter has been interpreted as an inborn
defense against bitter-tasting toxic alkaloids in nature. Most people have to reach
adulthood before they have learned to appreciate the bitter taste of beer, coffee,
and many vegetables. Thus, besides these few examples, all other preferences are incidentally
learned by being exposed to them in the food culture one is born into. Such a system
allows man to be omnivorous and able to adapt to whatever eatable materials are found
in his environment. There are no differences between the nervous systems of different
human races and cultures, but the different cultures have nevertheless developed radically
different cuisines or food cultures based on what nature provides. This demonstrates
quite clearly that food preferences are learned and not in born.
Learning starts in the fetal state(329) and during breastfeeding(330) and continues
through childhood and later life. Conditioned learning, where an unconditioned stimulus
(which is unconditionally appreciated) is paired with a conditioned stimulus, is an
important mechanism in forming human preferences for foods.(331) Learning (and memory)
plays a major role not only in forming preferences but also for choice behavior when
preferences have been formed. Recent work on food memory has demonstrated that elderly
people have as vivid memories of foodstuffs as young people in line with the incidental
learning and implicit nature of the memories. These memories also seem to be based
on a kind of “novelty detection”, where people are especially adept at detecting slight
changes from a food stimulus they have previously been exposed to rather than determining
that a presentation of the previous stimulus is indeed identical to the learned stimulus.
332–335
Memories of stimuli and events are important to raise expectations and can have a
strong influence on what is perceived and how it is hedonically evaluated, as exemplified
in a neuroimaging study by De’ Araujo and Rolls, in which they demonstrate that one
and the same chemical used as a smell stimulus activates different parts of the olfactory
brain, depending on whether subjects are led to expect the smell of “cheddar cheese”
or “body odor”.(336)
Learning and memory in the chemical senses, important for food behavior, might work
very differently from learning and memory in the higher senses, vision and audition,
and this might have important implications for how to produce foods that are either
manufactured to give maximal pleasure or produced more with health concerns in mind.
What makes a particular meal pleasurable? As preferences are learned and we have our
own idiosyncratically coded memories, we should not expect to come up with an “ultimate
meal” which will appeal to members of all food cultures, but there might nevertheless
be more fundamental underlying principles which determine what brings pleasure to
humans. Unfolding of such principles will most likely require different physicochemical
materials and processes in different food cultures, but there might be more fundamental
determinants of what activates the reward systems in our brains the most.
Various lines of recent psychological and neuroscientific work suggest that this is
indeed possible. Perceived complexity as defined by Berlyne
337–339
is a general concept that has recently been applied to the study of (changes of) food
appreciation.
340–342
There is, in a very general sense, an inverted U relationship between “perceived complexity”
and liking (of most stimuli) as shown in Figure 18. This is the case for rats, monkeys,
and human beings. Perceived complexity is thus an important determinant of pleasure,
and ongoing attempts to develop ways of quantifying it precisely are being performed.
Figure 18
Inverted U relationship between liking and the arousal potential of a stimulus suggested
by Berlyne’s arousal theory (solid curve), and the shift (broken curve) of the original
inverted U curve and of the optimal individual level of psychological complexity upon
exposure to a ‘Pacer’ (B).
Novelty seems to be another general concept which might be strongly related to reward.
343,344
Recent neuroimaging findings indicate that midbrain regions preferentially respond
to novelty and suggest that novelty can serve as its own reward.(345) The mere anticipation
of novelty seems to recruit reward systems.(346) Novelty per se, of course, will not
on its own guarantee a reward. It is easy to imagine novel foods which will generate
a feeling of disgust rather than pleasure. Novel dishes with a clear relationship
or reference to familiar dishes have in one recent experiment been shown to be appreciated
more than those that do not.(347)
As an integral part of molecular gastronomy studies of, presumably universal, principles
of how to join different sensations in space and time to obtain optimal pleasure are
central. The application of physics and chemistry to the study of the soft but very
complex materials that make up foods, in conjunction with modern psychophysical and
neurophysiological studies of pleasure and satiety, could both contribute to a deeper
understanding of human reward systems as well as to development of new foodstuffs
that could bring more pleasure and healthier lifestyles to the eater.
6
Summary and the Future
For anyone who has read through all the preceding sections, it should be readily apparent
that the overall effect of any individual foodstuff, let alone a complete dish or
meal, is influenced by a diverse and complex set of factors that start with the production
of the ingredients and via their processing, both physical and chemical, to produce
aroma and tastant molecules as well as change the texture and color end as the food
is eaten and digested with the sensations sent from all our senses to our brains,
where we decide whether or not we enjoyed the experience and degree of pleasure imparted.
Accordingly, if we are ever to be able to predict, a priori, how delicious a food
might be, it will require serious collaborative efforts from scientists of all the
chemical (and other) disciplines.
As we have seen, some areas are much better understood than others. Some of the chemical
reactions are generally well understood. Others, such as the Maillard reactions, are
much well less understood; although general schemes exist, the full details of these
reactions still remain far too complex to permit any complete description. We will
later in this section outline some of the major challenges which we believe are possible
to tackle in the short term as well as lay out a potential long-term strategy for
the future development of Molecular Gastronomy as a scientific discipline in its own
right.
However, before we get carried away with grandiose schemes, we should return to a
more basic and most important issue. We should address the question of why should
the pursuit of all this research be worthwhile. Is Molecular Gastronomy necessary?
To answer this question, we describe in the following sections a few aspects where
MG may be able to make significant contributions in the near future. These range from
questions that concern chefs and cooks, such as why do some food pairings enhance
flavor while others can be quite unpleasant, to socially important issues such as
how we might persuade people to adopt healthier diets and how we can encourage more
youngsters to take up careers in the sciences.
6.1
Complexity and Satiety: Relationships between Liking, Quality, and Intake
It is obvious that the extent to which individuals enjoy the food they eat depends
on a number of factors related to the food itself as well as their own individual
set of experiences and memories. It is also clear that given choice and opportunity
individuals will tend to eat that which they enjoy. Thus, it should be possible, if
we can gain a better understanding of how enjoyment relates to the food preparation
itself, to influence the diet of individuals in a positive, more healthy, fashion.
There is no inherent reason why high palatability should necessarily lead to a larger
intake. One can easily argue that it is the other way around: if eating is as much
an activity engaged in to obtain pleasure as it is a means to secure the necessary
energy intake, high palatabilty in a meal will lead to a smaller energy intake because
sufficient satisfaction is obtained with smaller portion sizes. If this could be demonstrated
in a number of cases, so that “quantity” could be replaced by “quality”, it may become
possible to encourage more appropriate eating behavior in an environment with high
food availability. More work to investigate the existing anecdotal evidence that we
eat and drink less of high-quality products than we do of more mediocre products could
indeed provide evidence for such a possible replacement of “quantity” with “quality”.
We think that Molecular Gastronomy is particularly well suited to make a contribution
here because of the precise definition and high sensory quality of the foods we work
with in Molecular Gastronomy. For example, particular sensory dimensions can be defined
much more precisely by means of the methods used in Molecular Gastronomy. Experimenting
with new physical and chemical methods which are used in Molecular Gastronomy kitchens
and laboratories allows us to acquire many new insights about the individual senses’
contribution to satisfaction and satiety. Along the way we will also need to develop
new methods to quantify ‘satisfaction’, both psychophysically and neurophysiologically
and by means of biochemical and neuropharmacological measurements. In this way one
might hope that MG could also become a driver of development in the more psychological,
neurological, and biochemical aspects of eating behavior.
One area of some immediate potential is the investigation of the relationship between
food complexity and satiety. In particular, by using highly palatable, real, and complete
meals, rather than the more usual simple single food (such as fruit juices or yogurts)
approaches, we believe it should be possible to make a direct impact on food choice
and intake.
Over the last 50 years the Western diet has changed enormously. For example, the British
population eats less red meat, more poultry, and more processed food than 30 years
ago. Although British Government statistics suggest that the consumption of fats,
carbohydrates, and protein is falling and that of vegetable and fruit intake is increasing,
there has still been a 10% rise in the incidence of obesity in the last 10 years.(348)
Data from the United States suggest that a key determinant in the increase in obesity
is consumption of processed and “fast” foods; higher weight is associated with more
food eaten away from home.(349) In the United Kingdom about 30% of food expenditure
is on food eaten outside the home. When eating out, people tend to consume larger
portion sizes and more calories.(350) Accordingly, we should ask are there opportunities
for Molecular Gastronomy to provide routes to improved diet and consequently through
that the health of indivduals.
Foods vary tremendously in their energy density. During eating we need to predict
how much food to consume in order to satisfy current and future short-term needs.
One possibility is that meal size is governed by a simple feedback mechanism such
as a gut hormone that is released or reaches a critical level when enough energy has
been absorbed. However, the system needs to be more adaptable because meal termination
occurs well before a significant proportion of food is emptied from the stomach for
absorption.(351) In part, we overcome this problem by learning to predict the likely
consequences of consuming individual foods (for a review, see ref (352)). However,
in addition to these food-specific associations, meal size also appears to be influenced
by a range of other factors including food palatability,(353) serving size,(354) and
the number of people present at a meal(355) irrespective of the specific food that
is being consumed.
Food complexity is not a straightforward concept and can be defined in many ways.
If complexity is defined as the sensory experience, it might be argued that increasing
flavor complexity could provide for increasing enjoyment and satiation. Conversely,
it could be argued that highly processed foods, despite being potentially nutritionally
less valuable, can also be extremely complex but in terms of their constituents rather
than their flavors. For example, a simple pasta sauce prepared at home might contain
olive oil, onion, garlic, tomato, tomato puree, and seasonings (salt, pepper, maybe
some herbs), whereas a preprepared sauce might also contain sugar, modified maize
starch, an acidity regulator such as citric acid, white wine vinegar, onion extract,
and other nonspecified flavorings.
In addition to the potential effect of flavor or constituent complexity on food intake
and satiety, there is an additional dimension to the sensory experience of eating
that is textural complexity and palatability. Controlling for taste quality, a lessened
desire to eat “hard” foods has been reported following consumption of a “hard” lunch,(356)
and other work suggests that satiety can be affected by somatosensory features (texture,
feel, quality) in addition to taste quality. It is not at all clear how different
complexities in food may contribute to food intake and satiety.
On the basis of the above lines of evidence, it is at least possible that different
aspects of food complexity play an important role in determining the satiating quality
of foods and that the specific effect of complexity might be mediated via particular
kinds of taste profiles. In recent years our affection for processed and manufactured
foods has increased markedly. Yet, little is known about the complexity of these foods
relative to ‘home prepared’ equivalents. By elucidating a role for complexity we may
be better placed to explain the commonly held view that processed foods are overconsumed
and regarded as unhealthy on this basis. Of course, this is a very simplistic approach
to satiety and enjoyment of food; complexity is at best just one of many factors affecting
our appreciation of the food we eat. Nevertheless, the role of sensory complexity
has not been previously measured or explored with respect to pleasure, palatability,
food intake, and satiety.
It is therefore clear that the links between sensory complexity, on the one hand,
and pleasure, palatability, food intake, and satiety, on the other, are areas worthy
of a proper scientific investigation. Such a study could be an excellent opportunity
for Molecular Gastronomy to demonstrate a societal useful role.
6.2
Models for Cooks and Chefs
While there are some models already in the literature that attempt to describe the
heat transfer in food as it cooks, these are not, in general, of much utility for
the domestic cook or the chef in a restaurant. However, these people are those who
are most often in need of simply and clear guidance on the optimum temperature and
time to cook particular foods. This situation provides a wealth of opportunities for
research with the objective of producing straightforward to use models that can be
used in the kitchen. For example, we can envisage a computer package that provided
with the dimensions, type, and cut of a piece of meat can suggest a range of different
cooking methods (times and temperatures and even temperature profiles) that will give
a range of different textures, colors, and flavors in the finished product. Such a
tool could prove invaluable in any busy kitchen.
Such a system might be developed either from a purely theoretical standpoint or by
a purely experimental approach. However, neither alone is likely to prove successful.
The pure theory will find it hard to deal with the variability of meats, the different
mass transport that will occur with different cuts, and the odd shapes and sizes of
real pieces of meat. At the same time a purely empirical approach would involve the
need to test such a large range of different examples that it is unlikely ever to
provide for all the possible examples met in a real kitchen. It will only be through
a combination of theory and experiment and collaborations between chefs and scientists
working together that truly useful models and systems can be produced.
However, it is not only in the area of meat cookery that predictive cooking models
can prove useful; the whole area of gels and gelation (under what conditions will
the diverse range of food-approved gelling agents actually form gels, what properties
will these gels have, and how stable will they remain) is another area that is ripe
for development of a detailed set of models. While much of this information is in
the scientific literature, much is not and still needs to be discovered. For example,
most phase diagrams of gelling systems are produced for just a single example of the
gelling agent; other batches (or batches from a different source) are likely to have
quite different molecular weights and distributions of molecular weights, making their
gelling characteristics quite different. Formulating a schema which permits simple
measurements (such as intrinsic viscosity) to be made and using these to predict gelling
behavior in the kitchen would be another step forward.
Another area where MG may be able to contribute significantly and directly to the
kitchen is that of food pairings. Chefs continually search for novel and interesting
food and flavor combinations. Currently much of this work is hit and miss, although
a number of empirical models have started ot emerge and led to Web sites that offer
suggested food pairings based on a concept of synergy between foods that contain similar
components. It is interesting to note that here chefs are beginning to propose models
that scientists should be able to test as MG develops.
Molecular gastronomy attempts to bridge the gaps between work in physics and chemistry
over technology and food preparation to sensory perception and pleasure. Within this
framework there is ample space to experiment with well-known (and less well-known)
perceptual effects in space and time. Under this heading, studies of “flavor principles”,
in the most general sense of the phrase, are of great interest. Why do some foodstuffs
go well with some others but not all? What are the not well-understood chemical and
perceptual principles underlying these phenomena? Do “flavor principles” transcend
“food cultures”? Not such that the very same dishes should be highly appreciated all
over the world but such that certain combinations of basic sensory perceptions would
be enjoyed universally. Elucidation of these problems would not only be highly interesting
from a scientific point of view but could potentially also make a major contribution
to more appropriate eating behaviors worldwide.
6.3
Language of Sensory Properties: Engaging the Public
It is, however, not enough just to prepare food; we want to prepare the best and most
delicious, nutritious, and healthy food we possibly can. However, how can we explain
that the food really does taste good? How can we describe flavor? Is it possible to
create a language of taste and flavor so that we understand how different people appreciate
the same food differently? We have seen that the perception of flavor depends not
only the taste and aroma molecules present in food but also on the way in which they
are released in the mouth, which in turn depends on the individual eating the food.
These are all hard questions and involve not only the scientific community but importantly
chefs, cooks, and of course the public at large. Here are opportunities to engage
the public directly in research. In sensory science much progress has been made in
naming food characteristics and finding descriptions and reference materials for sensory
perceptions. Furthermore, many studies have been made to describe relationships between
physical/chemical stimuli in foods and perceptions, although mainly under strict laboratory-controlled
conditions in a sensory laboratory.
As we increase the communication between those who cook and those who are interested
in the scientific basis of what happens as we cook (and consume) our food, so mutual
benefits can arise. Chefs will persistently ask difficult questions that can lead
to new research opportunities: why do some pairs of foods work so well together while
others do not? Is there a way we can predict whether a particular pair of foods will
make a “good” flavor combination or food pairing? The fact that chefs have asked this
question has led some to hypothesize that if a pair of foods share similar aroma molecules
they make good combinations in a dish. Although nothing more than a simple hypothesis,
the concept has gained considerable approval within the gastronomy community; there
are now several Web sites devoted to trying to suggest food pairings based on detailed
analyses of the major aroma compounds found in the ingredients. However, there is
no hard evidence for or against such a simple model; here then is another area ripe
for proper research, research that of necessity would involve tasting some very fine
foods indeed!
Then there is the question of how ingredients are perceived as being “natural” or
“artificial”, such terms can have very different meanings to scientists and the general
public. It seems obvious to most in the scientific community that if a food molecule
is healthy to use and imparts desirable characteristics to a dish, then whether it
is extracted from a fruit or synthesized in a laboratory should not matter at all.
Here those practicing MG should perhaps engage with the public and help them understand
that, for example, chocolate is a highly processed food that is far from the general
public perception of a natural foodstuff “natural” while the much maligned and often
perceived as “artificial” monosodium glutamate (E621) occurs naturally in a wide range
of foods from mother’s milk and tomatoes to cheese.
6.4
Science Education Using Food as Exemplars
One area where Molecular Gastronomy is already having an impact is in schools. In
the United Kingdom, chemistry classes now often take experimental examples from the
world of food science. This has been much encouraged and enhanced as some of the finest
chefs (especially Heston Blumenthal) have not only acknowledged the usefulness of
chemistry in their own cooking but produced materials that can be used in schools.
For example, the Royal Society of Chemistry has produced a text book for use in schools
which draws heavily on a TV series “Kitchen Chemistry”, which was presented by Heston
Blumenthal.
Current health and safety legislation often makes the use of “traditional” experiments
in schools difficult, and many teachers are wary of using potential harmful chemicals
in a school environment. However, foodstuffs are not seen as harmful chemicals, so
that many reactions and processes can be demonstrated and even reproduced by pupils
in a school laboratory. Returning chemistry in schools to a hands-on experience can
only be a good thing.
The endorsement by celebrity chefs is a clear way forward for the engagement of youngsters
with chemistry which sometimes is thought to be “boring”, “difficult”, and most importantly
“uncool”. In the United Kingdom there is increasing, anecdotal evidence from schools
that students are finding chemistry more approachable and distinctly “cool”.
Examples of the use of Molecular Gastronomy in school chemistry lessons include the
following. The use of salt in cookery (illustrating boiling point elevation, titration,
color reactions, monovalent and divalent ions). Why do pans stick (providing an introduction
to polymer chemistry and the structure of fats and oils)? The science of ice cream
(illustrating the structure of ice and water and introducing concepts of enthalpy
of fusion, nucleation, crystallization, and phase changes). It should remain an objective
of Molecular Gastronomy research to retain the link to education and wherever possible
develop more educational resources to encourage youngsters into the chemical sciences.
6.5
What Is Molecular Gastronomy? Where Will It End Up?
Finally, we should ask where Molecular Gastronomy might lead. Perhaps the most important
objective of MG should be to delineate the essential principles that underpin our
individual enjoyment of food. We hypothesize that there are a number of conditions
that must be met before food becomes truly enjoyable, some trivial (e.g., the food
should have some flavor), some very subtle (e.g., we may need to be in the “right
frame of mind” to enjoy a meal), and many highly speculative (e.g., we may need a
minimum number of different simultaneous or temporally related stimuli before a particular
dish becomes interesting). The long-term aims of the science of MG should be to elucidate
these minimal conditions, to find ways in which they can be met (through the production
of raw materials, in the cooking process, and in the way in which the food is presented),
and hence to be able to reasonably well predict whether a particular dish or meal
would be delicious.
We can see many areas where MG can and should develop. For example, there are many
traditional processes used by chefs in their kitchens. We can legitimately ask why
do we do use these processes? Are they really the best possible methods or have they
just been handed down from chef to apprentice over many decades without any real development
or optimization? Such systematic and scientific studies of gastronomic procedures
could form the basis for the rationalization and improvement of basic kitchen processes.
Similarly, there are many classical dishes or components of dishes that have earned
a reputation for excellence and remained on menus with little experimental development.
A scientific study comparing classical dishes that have stood the test of time with
historic dishes that have not may throw some useful light to help us understand why
it is that some dishes do indeed achieve greatness and stand the test of time.
Another important aspect of Molecular Gastronomy and one that is much practiced by
Dr. Herve This at INRA is the systematization of recipes or procedures. In a similar
way to the systematization of sauces by This it should be possible to rationalize
other processes. For example, the boiling of sugar solution is traditionally described
in terms of “stages”, which could perhaps be rationalized using glass theory.
It may even become possible to give some quantitative measure of just how delicious
a particular dish will be to a particular individual. Thus, in the future, we may
be able to serve different variants of the same dish to our dinner party guests so
that each has their own uniquely pleasing experience. If MG can achieve such a goal,
it will go a long way to changing forever the public perception of chemistry.