Physicochemical factors affecting cooking and eating quality of nonwaxy rice

摘要

The physicochemical properties of isolated rice starch and milled rice from the twelve varieties of nonwaxy rice with diverse and extreme characteristics were investigated;The stickiness of cooked milled rice was only moderately correlated with amylose content (r = -0.58[superscript]*), but relatively more with protein content (r = -0.72[superscript]**) among nonwaxy rices with amylose content ranging from 10.4 to 27.8%. Amylograph breakdown (r = 0.87[superscript]**), setback (r = -0.93[superscript]**), and consistency (r = -0.91[superscript]**) of milled rice flour (10%, as-is basis) showed strong correlations with cooked rice stickiness, and can be used as good indices of cooked rice stickiness, especially among rices with a narrow range of amylose content;The changes of physical appearance of isolated rice starch and milled rice were observed during heating by scanning electron microscopy (SEM). Rice starch granules begin to swell radially, then undergo radial contraction and random tangential expansion to form complex structures in the gelatinization temperature range. At the end of the gelatinization temperature range, starch granules begin to lose their granular structures. At higher temperatures, rice starch granules soften and melt into thin flat disks and then stretch into thin filaments to form a three-dimensional network. During this transition, rapid increases in swelling power and solubility occur. These progressive configurational changes are reflected in the amylograph curves. Susceptibility of the starch granules dispersed in excess water to changes during heating is dependent on gelatinization temperature range during initial swelling and on the ratio of amylose to amylopectin in the latter stage of swelling. Birefringence end-point temperature (BEPT) was affected mainly by the degree of crystallinity (r = 0.67[superscript]**);The progressive structural changes of milled rice grains proceed from the periphery toward the center of the endosperm as water diffuses. The rate of water diffusion seems to depend on cell arrangement of the endosperm and protein content of milled rice, and gelatinization temperature range and amylose content of starch. These factors affect the cooking rate of milled rice. Low protein content of milled rice tends to increase cooked rice stickiness by increasing the solids lost in cooking-water.