Kinetic Studies on Cooking of Rice of Various Polishing Degrees

The mechanism of cooking rice was investigated using a japonica type rice variety, Akibare, of 50%, 70% and 90% polishing degrees. The hardness of rice cooked at various cooking temperatures () was measured with a Texturometer. The cooking rate followed the equation of a first-order reaction. The reaction rate constants were in the increasing order of 50%, 70% and 90% polished rice. The temperature coefficient of the reaction rate constant at cooking temperatures of () was about 2 in all rice samples. The activation energies of cooking at temperatures below and above were about 17,000 and 9,000 cal/mole, respectively. The polishing degrees and water soaking time of rice did not affect the activation energy of cooking; however, the lower polishing degrees and shorter soaking increased the cooking time The experimental results suggested that the cooking process of rice comprises two mechanisms: At temperatures below the cooking rate is controlled by the reaction rate of rice constituents with water, and at temperatures above , it is controlled by the rate of diffusion of water through the cooked portion (or layer) toward the interface of uncooked core in which the reaction is occurring.