Thermal equilibrium state of starch-water mixture studied by differential scanning calorimetry.

Abstract

The process to a thermal equilibrium state of nonglutinous and glutinous rice starch-water mixtures around the gelatinization temperature (40-85°C) was studied by differential scanning calorimetry (DSC). The starch-water mixture was held at a constant temperature for an appropriate duration between 5 min and 140 hr, and then the usual DSC was carried out. Since the low temperature side of the peak of the DSC curve after the holding shifted to a higher temperature as the holding time proceeded, the peak gradually narrowed and grew higher. Even after a 100-hr holding, the DSC spectrum continued to change, indicating that an equilibrium state had not yet become established, so far as the constant holding temperature was concerned. By contrast, the gelatinization enthalpy reached a constant value corresponding to a holding temperature of less than 5min. In the course of holding, the crystalline structure of the X-ray diffraction pattern decreased. Only an amorphous component was observable in the pattern after holding for 22 hr at 75°C, corresponding to a state which showed a sharp endothermic peak in the DSC curve after holding for that time.