Kinetics of Starch Retrogradation in Rice (Oryza sativa) Subjected to State/Phase Transitions

Rice is an important component of many ready-to-eat frozen and refrigerated products. Changes in temperature during storage or handling of frozen cooked rice may lead to thermal transitions, ice recrystallization, and subsequent adverse changes in physicochemical properties. In this study, we explored the kinetics of starch retrogradation over a 12-week storage in frozen cooked rice of two varieties: 8% amylose and 0% amylose. Cooked rice was frozen at - 32 +/- 1 degrees C for 5 days to achieve a glassy state and then subjected to temperature fluctuations of varying amplitudes to achieve different physical states (e.g., glassy, rubbery, and thawed states). The temperature fluctuation refers to regular/cyclic change from storage temperature to preset temperature. Starch retrogradation enthalpy was measured using differential scanning calorimetry. There was significant (p < 0.05) increase in retrogradation enthalpy with the 12-week storage time, in both varieties of rice. Among the treated samples, with increased degree of temperature fluctuation, there was significant (p < 0.05) increase in retrogradation enthalpy: rice with temperature that fluctuated to the thawed state showed the highest increase in retrogradation enthalpy. A lower degree of starch retrogradation was observed in 8% amylose rice, indicating a lower susceptibility to changes in physicochemical properties as a result of retrogradation during frozen storage.