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 ≈ 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 ∼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 °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 °C, dissociation of the gel network (sigmoidal reduction in G′ and G″) was observed. At or above the midpoint temperature of this melting process (∼50 °C), there was no indication of gel formation on enzymic de-esterification (at 50 or 60 °C). At lower temperatures (20, 30 and 40 °C), the rate of gelation (assessed visually) showed no systematic increase as the incubation temperature was increased towards the temperature-optimum of the enzyme (∼50 °C). This unexpected behaviour is attributed to competition between faster de-esterification and slower formation of Ca2+-induced ‘egg-box’ junctions.

Cohesive gels are formed when high-methoxy pectin is de-esterified by fungal pectin methyl esterase in the presence of calcium ions.Figure optionsDownload as PowerPoint slide