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The distribution of chemical constituents is known not to be even within a rice kernel. To improve the eating quality of rice or to give it some special traits by adjusting the milling intensity, we investigated the distribution of amylose, nitrogen (N), and specific minerals (P, K, Mg, Ca, and Mn) in rice kernels of 11 cultivars with various characteristics cultivated under similar conditions. The distributions of these constituents were determined using flour samples prepared consecutively by abrasive milling from the outer to the inner portions of hulled rice. In all the cultivars tested, N and the minerals were found to be more abundant in the outer than in the inner portion, but amylose was rich in the inner portion. P, Mg, K, and Mn were extremely rich in the outer portion, while N and Ca were only relatively rich there. Koshihikari, which is the most popular cultivar in Japan because of its excellent eating quality, showed the highest Mg/K ratio in the outermost portion of polished rice. The color of flour samples became pure white going from outside portions toward the center of the kernel, even if the sample was from red rice or purple-black rice because only the surface of hulled rice contains pigments. These findings suggest that the outer portion contains various compounds other than starch and the inner portion contains relatively pure starch. Rice palatability and other characteristics can be improved through controlling the degree of milling using the biased distribution of chemical constituents within a rice kernel.
Itani et al. (Fri,) studied this question.