Effect of sucrose, glucose and fructose on gelation of oxidised starch

The effect of sucrose on gelation of oxidised starch (partially depolymeris ed amylopectin with similar to 1 carboxyl group per 30 residues) has been s tudied by small-deformation oscillatory measurements (1 rad s(-1); 0.5 stra in) of storage and loss moduli (G' and G "). Solutions were prepared at 95 degrees C, cooled to 5 degrees C at 1 degrees C/min, and held at 5 degrees C for 500 min. In the first series of experiments, the combined concentrati on of sucrose and starch was held fixed at 65 wt%. Sharp increases in modul i, attributable to double-helix formation, were observed during cooling, an d moved to progressively higher temperature as the concentration of starch was increased (from 10 to 40 wt%). At all concentrations, however, G " on r eaching 5 degrees C was higher than G', indicating that intermolecular inte ractions at this stage are largely topological. Formation of "true" gel net works (G' >> G ") was observed during holding at 5 degrees C, and is attrib uted to intermolecular helix-helix aggregation. The holding time at the ons et of gelation decreased as the concentration (c) of starch was increased, and at concentrations in the range similar to 20 to similar to 30 wt% showe d the c(-2)-dependence expected for a simple dimerisation process. At highe r concentrations the slope became steeper, consistent with some limited agg regation during cooling. In a second series of experiments, where c was hel d fixed at 40 wt% and sucrose concentration was varied (between 0 and 25 wt %), the time-course of gelation at 5 degrees C showed little change with in creasing sucrose content, but there were large increases in the moduli atta ined on completion of cooling. Measurement of the same samples by different ial scanning calorimetry showed that the extent of ordering during cooling varied from similar to 2 to similar to 15% of the helix content in the unge latinised granules as the sucrose concentration was increase from 0 to 25 w t%. Replacement of sucrose by glucose or fructose decreased the rate of con formational ordering during cooling, in the sequence: fructose < glucose < sucrose, but on holding at 5 degrees C the order was reversed, with fructos e causing rapid gelation and sucrose having least effect. A similar inverse correlation between ordering and aggregation has been observed for other b iopolymer systems, and is tentatively ascribed to stability of individual h elices inhibiting slight changes in conformation required for efficient pac king. (C) 2000 Elsevier Science Ltd. All rights reserved.