Effect of de-(methyl)esterification on network development and nature of Ca2+-pectin gels : towards understanding structure-function relations of pectin

Pectins of varying degree and pattern of methylesterification were produced through controlled de-esterification of highly esterified citrus pectin, using carrot pectin methylesterase (PME) (P-pectins), Aspergillus aculeatus PME (F-pectins) or sodium hydroxide saponification (C-pectins). Estimation of the degree of methylesterification (DM) and quantification of the pattern of methylester distribution in terms of absolute degree of blockiness (DBabs) enabled the characterisation of pectins. Characterised pectins were used for the preparation of Ca2+-pectin gels with varying calcium ion (Ca2+) concentration. The rheological characteristics of produced gels were evaluated by means of small-amplitude oscillatory tests. During gel formation, gel strength was monitored so as to allow assessment of network development. Based on the evaluation of mechanical spectra, the nature of the cured gels was established. Depending on Ca2+ concentration as well as DM and DBabs, gels prepared from specific C-pectins (48 = DM = 26%; 9 = DBabs = 37%) and F-pectins (64 = DM = 29%; 9 = DBabs = 50%) showed a striking decrease of the gel strength with time, while gel networks produced from other pectins either displayed a continuous increase of the gel strength or verged towards pseudo-equilibrium within the observation time of 5 h. Furthermore, the DM, DBabs and Ca2+ concentration influenced the evolution of Ca2+-pectin networks from "structured liquids" to "strong gels". Based on the experimental results, specific mechanisms of Ca2+ interactions with pectins were (also) considered.