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Abstract
Cohesive gels have been obtained by de-esterification of 1.0wt% high-methoxy citrus
pectin (degree of esterification approximately 68%) in the presence of Ca(2+) cations,
using a commercial preparation (NovoShape) of fungal methyl esterase cloned from Aspergillus
aculeatus. A convenient rate of network formation (gelation within approximately 30min)
was achieved at an enzyme concentration of 0.2 PEU/g pectin. At a Ca(2+)-concentration
of 40mM and incubation temperature of 20 degrees C, severe syneresis (>7% of sample
mass) was observed, but release of fluid decreased with decreasing concentration of
Ca(2+) and increasing temperature of incubation, becoming undetectable for 10mM Ca(2+)
at 30 degrees C. Under these conditions, progressive development of solid-like character
(storage modulus, G') was observed during 160min of enzymic de-esterification, and
the mechanical spectrum recorded at the end of the incubation period had the form
typical of a biopolymer gel. On subsequent heating to 70 degrees C, dissociation of
the gel network (sigmoidal reduction in G' and G'') was observed. At or above the
midpoint temperature of this melting process ( approximately 50 degrees C), there
was no indication of gel formation on enzymic de-esterification (at 50 or 60 degrees
C). At lower temperatures (20, 30 and 40 degrees C), the rate of gelation (assessed
visually) showed no systematic increase as the incubation temperature was increased
towards the temperature-optimum of the enzyme ( approximately 50 degrees C). This
unexpected behaviour is attributed to competition between faster de-esterification
and slower formation of Ca(2+)-induced 'egg-box' junctions.