Water absorption and starch gelatinization in whole rice grain during soaking

Hydration of rough rice grain in hot water as a function of time was studied at temperature range 25–90 °C. A simple model which considers simultaneous unsteady-state water diffusion and first-order irreversible water–starch reaction phenomenon, was used to evaluate the kinetics parameters from experimental curves. The values of the diffusion coefficients and reaction rate constants were between 1.40×10−11 and 9.36×10−11 m2 s−1 and 2.29×10−10 and 3.72×10−5 s−1, respectively. Both parameters followed a Arrhenius-type equation with distinct activation energies below and above a break temperature of 60 °C. It was 25.4 and 289.3 kJ mol−1 for the activation energies of diffusion and reaction, respectively, below 60 °C. Above this temperature the respective values of the activation energies of diffusion and reaction were 30.0 and 16.6 kJ mol−1. This break temperature was in agreement with the gelatinization temperature determined experimentally.