Studies on Diagnosis of Rice Grains by Photometric Method : V. Color of hulled rice and method for color estimation

Abstract

A convenient method for estimating kernel color was investigated using the "rice inspector". And examinations were also conducted on the coloring substances in rice bran layers. In these studies, special attentions were directed to the specific photo-absorbing characteristics of colored rice. 1. Relative transmittance values (RTV) increased in accordance with the wave length of illumination light. And the inclination of this spectral transmittance curve was steeper for deeper-colored variety of brownish rice. 2. Spectral curve of RTV for greenish-colored kernel was featured by a peak at 540 nm, a bottom at around 660 nm and a remarkable ascent toward infra-red range. 3. Hulled rice with dark violet color containing anthocyanic pigment showed a quite different pattern of RTV curve. 4. Ratio of RTV at 710 nm and 660 nm was shown to be a suitable color index (green color index) showing the degree of greenish coloring of rice sample. And RTV ratio of 660 nm and 420 nm (brownnish-color index) made good estimations of coloring degree of brownish rice. 5. These two color indexes could estimate the respective degree of coloring independently without being obstructed by kernels of the other characters. 6. Brownish color of rice deepened during storage, and the deepening of color was more remarkable in darker-colored varieties. 7. Considering from light absorbing features of solvent extractive and coloring reaction in chemical reagents, brownish coloring substances were supposed to be polyphenolic tannin-like materials. 8. Classifying rice varieties into 4 groups, that is, yellowish-white, brown-colored, reddish-brown-colored and anthocyanic varietal groups was proposed according to the degree of coloring of kernel, anthocyanic pigmentation and ferric coloring reaction in rice bran.