Mechanisms of change in gel water-holding capacity of myofibrillar proteins affected by lipid oxidation: The role of protein unfolding and cross-linking

Meat and meat products are a fundamental part of the human diet. The protein and vitamin content, as well as essential fatty acids, gives them an appropriate composition to complete the nutritional requirements. However, meat constituents are susceptible to degradation processes. Among them, the most important, after microbial deterioration, are oxidative processes, which affect lipids, pigments, proteins and vitamins. During these reactions a sensory degradation of the product occurs, causing consumer rejection. In addition, there is a nutritional loss that leads to the formation of toxic substances, so the control of oxidative processes is of vital importance for the meat industry. Nonetheless, despite lipid oxidation being widely investigated for decades, the complex reactions involved in the process, as well as the different pathways and factors that influenced them, make that lipid oxidation mechanisms have not yet been completely understood. Thus, this article reviews the fundamental mechanisms of lipid oxidation, the most important oxidative reactions, the main factors that influence lipid oxidation, and the routine methods to measure compounds derived from lipid oxidation in meat.

Protein oxidation (P-OX) is an innovative topic of increasing interest among meat researchers. Carbonylation is generally recognized as one of the most remarkable chemical modifications in oxidized proteins. In fact, the quantification of protein carbonyls by the dinitrophenylhydrazine (DNPH) method is the most common procedure for assessing P-OX in meat systems. Numerous studies have investigated the occurrence of protein carbonylation right after slaughter and during subsequent processing and cold storage of meat. However, the significance of protein carbonylation in meat systems is still poorly understood. Beyond their role as markers of protein oxidation, specific protein carbonyls such as α-aminoadipic and γ-glutamic semialdehydes (AAS and GGS, respectively) are active compounds that may be implicated in several chemical reactions with relevant consequences on meat quality. The formation of protein carbonyls from particular amino acid side chains contribute to impair the conformation of myofibrillar proteins leading to denaturation and loss of functionality. Recent studies also highlight the potential impact of specific protein carbonyls in particular meat quality traits such as water-holding capacity (WHC), texture, flavor and its nutritional value. As a truly emerging topic, the results from current studies provide grounds from the development of further investigations. The present paper reviews the current knowledge on the mechanisms and consequences of protein carbonylation in meat systems and aims to encourage meat researchers to accomplish further investigations on this fascinating research topic.

The involvement of oxidized proteins to the development of biological diseases has been studied for a few decades, but the effects and the mechanisms of protein oxidation in food systems are largely unknown. Protein oxidation is defined as the covalent modification of a protein induced either by the direct reactions with reactive oxygen species (ROS) or indirect reactions with secondary by-products of oxidative stress. ROS can cause oxidation in both amino acid side chains and protein backbones, resulting in protein fragmentation or protein-protein cross-linkages. Although all amino acids can be modified by ROS, cysteine, and methionine that are the most susceptible to oxidative changes due to high reaction susceptibility of the sulfur group in those amino acids. Oxidative modifications of proteins can change their physical and chemical properties, including conformation, structure, solubility, susceptibility to proteolysis, and enzyme activities. These modifications can be involved in the regulation of fresh meat quality and influence the processing properties of meat products. Oxidative stress occurs when the formation of oxidants exceeds the ability of antioxidant systems to remove the ROS in organisms. Increased levels of protein oxidation have been associated with various biological consequences, including diseases and aging, in humans and other animal species. The basic principles and products of protein oxidation and the implications of protein oxidation in food systems, especially in meat, are discussed in this review.