Characterization of pectin gelation under conditions of low water activity, by circular dichroism, competitive inhibition and mechanical properties

Abstract

Pectin is a significant polysaccharide component of plant tissues, where it occurs mainly in the intercellular regions and as a component of certain specialized, thickened primary walls. It usually, and perhaps invariably, exists as a branched polymer in which the backbone is based on linear, partially methyl-esterified sequences of ~(1--.4) linked D-galacturonate residues with periodic interruptions by L-rhamnose residues 1. The side-chains are composed principally of neutral sugars. Technological utilization of pectin centres on its gelation behaviour. For preparations of low ester content ('low methoxy pectin'), gels are usually formed by controlled introduction of calcium ions. Recently, we have shown 2 that the primary mechanism of this gelation involves extended chain sequences which adopt a' regular two-fold conformation and dimerize with specific interchain chelation of Ca 2 + (~egg-box' binding) 3. Pectins of both high and low ester content form gels without the necessary involvement of divalent cations, at low pH and decreased water activity, presumably because these conditions minimize intermolecular electrostatic repulsion and promote chain~zhain rather than chain solvent interactions. In this work we have explored the nature of these intermolecular associations using techniques which proved effective in our earlier studies 2 --5 of cation-mediated gelation.