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 degrees 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 x 10(-11) and 9.36 x 10(-11) m(2) s(-1) and 2.29 x 10(-10) and 3.72 x 10(-5) s(-1), respectively. Both parameters followed a Arrhenius-type equation with distinct activation energies below and above a break temperature of 60 degrees C. It was 25.4 and 289.3 kJ mol(-1) for the activation energies of diffusion and reaction, respectively, below 60 degrees 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. (c) 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.