Co-gelation of high methoxy pectin with oxidised starch or potato maltodextrin

In a continuation of the work reported in the preceding paper, the effect of oxidised starch on gelation of high methoxy pectin was studied under different experimental conditions. The main changes in procedure were: (i) gelation was induced by slow cooling (1 degrees C min(-1)) instead of rapid quenching; (ii) the pectin concentration was reduced from 1.0 to 0.5 wt%; and (iii) mixtures were prepared at pH 4 and subsequently acidified to pH 3, rather than being prepared directly at pH 3. The temperature-course of structure formation (increase in G' and G ") during cooling (from 95 to 5 degrees C), and the final moduli at 5 degrees C, are consistent with the conclusions reached for the mixtures gelled by quenching: (i) the effective concentration of both polymers is raised by mutual exclusion; and (ii) the initial reduction in moduli observed on partial replacement of sucrose by oxidised starch is due to the reduction in sucrose content, rather than to the presence of starch. An analogous series of experiments, under identical conditions, was carried out using potato maltodextrin in place of oxidised starch. In contrast to the pectin-oxidised starch systems, there was no evidence of any significant exclusion effects in the pectin-maltodextrin mixtures, but the changes in moduli observed during cooling indicate that maltodextrin, unlike oxidised starch, is almost as effective as sucrose in promoting self-association of high methoxy pectin at acid pH. These differences in behaviour are attributed to a high population of short, linear chains and oligomeric fragments in maltodextrin, facilitating segmental contacts with pectin, in contrast to the densely branched (partially depolymerised amylopectin) structure of oxidised starch, which will limit segmental contacts, but resist penetration by pectin chains, with consequent increase in effective concentrations of pectin and starch. (C) 2000 Elsevier Science Ltd. All rights reserved.