文摘
The effect of sucrose on gelation of oxidised starch (partially depolymerised amylopectin with 1 carboxyl group per 30 residues) has been studied by small-deformation oscillatory measurements (1 rad s−1; 0.5 strain) of storage and loss moduli (G′ and G″). Solutions were prepared at 95°C, cooled to 5°C at 1°C/min, and held at 5°C for 500 min. In the first series of experiments, the combined concentration of sucrose and starch was held fixed at 65 wt % . Sharp increases in moduli, attributable to double-helix formation, were observed during cooling, and moved to progressively higher temperature as the concentration of starch was increased (from 10 to 40 wt % ). At all concentrations, however, G″ on reaching 5°C was higher than G′, indicating that intermolecular interactions at this stage are largely topological. Formation of “true” gel networks (G′G″) was observed during holding at 5°C, and is attributed to intermolecular helix–helix aggregation. The holding time at the onset of gelation decreased as the concentration (c) of starch was increased, and at concentrations in the range 20 to 30 wt % showed the c−2-dependence expected for a simple dimerisation process. At higher concentrations the slope became steeper, consistent with some limited aggregation during cooling. In a second series of experiments, where c was held fixed at 40 wt % and sucrose concentration was varied (between 0 and 25 wt % ), the time-course of gelation at 5°C showed little change with increasing sucrose content, but there were large increases in the moduli attained on completion of cooling. Measurement of the same samples by differential scanning calorimetry showed that the extent of ordering during cooling varied from 2 to 15 % of the helix content in the ungelatinised granules as the sucrose concentration was increase from 0 to 25 wt % . Replacement of sucrose by glucose or fructose decreased the rate of conformational ordering during cooling, in the sequence: fructose<glucose<sucrose, but on holding at 5°C the order was reversed, with fructose causing rapid gelation and sucrose having least effect. A similar inverse correlation between ordering and aggregation has been observed for other biopolymer systems, and is tentatively ascribed to stability of individual helices inhibiting slight changes in conformation required for efficient packing.
