浸泡、烹煮及儲存條件對糙米飯物化特性與消化性質的影響

Abstract

Rice bran impedes water penetration into the kernels, thus soaking is considered as an essential process for cooking to improve the palatability of brown rice. During soaking, water diffuses into the rice kernels while some materials leach out, both phenomena are functions of time and temperature. The rate of water absorption and the amounts of carbohydrates and proteins leached from rice kernels increased with an increase in soaking time and temperature. There was more than 3.5 fold increase in the diffusion coefficients of polished rice (2.84x10-10 - 1.32x10-9 m2/s) compared to those of brown rice (5.14x10-11 – 3.66x10-10 m2/s), indicating rice bran acted as a barrier for water diffusion into the center of rice kernels. Starch granules in rice endosperm are polygonal in shape, during the course of soaking the edges of the granules became less distinct and the granules appeared to be more rounded due to hydration and swelling, as observed by scanning electron microscopy. The texture and eating quality of cooked rice is strongly influenced by the degree of gelatinization, which is induced by hydration and heating. Soaking for extended time resulted in more water uptake by the kernel, and the resulted cooked rice had higher degree of gelatinization, higher digestibility, higher stickiness and lower hardness with poorer eating quality. Cooked rice stored at refrigerated temperature exhibited lower values in the degree of gelatinization and harder texture. For brown rice cooking, soaking at moderately high temperature (30-50°C) for less than 2 hours is recommended.