Application of Cascade theory to the description of microphase-separated biopolymer gels

Abstract

Studies of the kinetics of gelation of an acid-modified waxy maize amylopectin have been performed using gel-time measurements and small- and wide-angle X-ray diffraction. The gel time–concentration relationship for gels set at 5 °C showed a high negative C^–5.2 power law dependence over most of the concentration range considered (ca. 14–55% w/w), with a decrease in this exponent to –7.4 as the gelation temperature increased to 20 °C. The effective critical concentrations, at which the gel time–concentration relationships diverged, shifted from ca. 14 to 25% w/w for this temperature increases. For the 5 °C gels, changes in X-ray scattering continued over periods in excess of one month, with the kinetics of this process showing a C^–4.1 concentration dependence. Comparison of the X-ray results at low and wide scattering angles showed a clear correlation between the growth of B-form starch crystallinity in the gels, and the aggregation of individual amylopectin molecules. From these, and some microscopical observations, a picture emerges of the gelation in such systems, involving the rate-determining formation of large semi-crystalline aggregates (of diameter 0.1–1.0 µm) which subsequently percolate. The simple cascade theory approach to biopolymer gelation will require substantial modification if it is to provide a realistic description of this situation.