Gelation of high-methoxy pectin by enzymic de-esterification in the presence of calcium ions: a preliminary evaluation

Cohesive gels have been obtained by de-esterification of 1.0 wt% high-methoxy citrus pectin (degree of esterification approximate to 68%) in the presence of Ca2+ cations, using a commercial preparation (NovoShape) of fungal methyl esterase cloned from Aspergillus aculeatus. A convenient rate of network formation (gelation within similar to 30 min) was achieved at an enzyme concentration of 0.2 PEU/g pectin. At a Ca2+-concentration of 40 mM and incubation temperature of 20 degrees C, severe syneresis (>7% of sample mass) was observed, but release of fluid decreased with decreasing concentration of Ca2+ and increasing temperature of incubation, becoming undetectable for 10 mM Ca2+ at 30 C. Under these conditions, progressive development of solid-like character (storage modulus, G') was observed during 160 min 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 (similar to 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 (similar to 50 degrees C). This unexpected behaviour is attributed to competition between faster de-esterification and slower formation of Ca2+-induced 'egg-box' junctions. (C) 2008 Elsevier Ltd. All rights reserved.