Effect of polymeric cosolutes on calcium pectinate gelation. Part 1. Galactomannans in comparison with partially depolymerised starches

The effect of galactomannans on gelation of low methoxy pectin (DE 31; 2.0 wt%; pH≈2.9–3.0) on cooling from 90 to 5 °C in the presence of stoichiometric Ca2+ has been characterised by low amplitude oscillatory measurements of G′ and G″ for five samples of guar gum of different molecular weights and for a single sample of locust bean gum (LBG). All samples caused an increase in G′ and G″ in the solution state at 90 °C and a reduction in final moduli at 5 °C, with cooling curves crossing those for calcium pectinate alone at ∼55 °C. The increase in moduli at high temperature is attributed to segregative interactions promoting formation of calcium-mediated ‘egg-box’ junctions between pectin chains. The loss of gel strength at low temperature is attributed to excessive association of pectin into large aggregated bundles, again driven by segregative interactions with the galactomannan. Both effects increased in magnitude with increasing concentration of galactomannan. At fixed concentration, the effectiveness of the galactomannan samples in promoting self-association of pectin was found to be inversely proportional to their hydrodynamic volume, as characterised by intrinsic viscosity (i.e. with the smallest molecules having the greatest effect). The changes in moduli caused by LBG were closely similar to those observed for guar gum of comparable molecular weight. The effect of galactomannans of high molecular weight was similar in magnitude to that of oxidised starch and substantially greater than that of potato maltodextrin, whose interactions with calcium pectinate have been studied in detail in previous published investigations.