Studies on Diagnosis of Rice Grains by Photometric Method : VI. Changes in translucency and color of rice kernel in the progress of maturation and dehydration

Abstract

Changes in translucency and coloring of rice kernel in the course of maturation were examined photometrically by the "rice inspector". And the effects of dehydration, variety and vigor of kernel were also investigated. The results are summarized as follows: 1. Regardless of glutinosity, translucency of fresh kernel increased rapidly with the progress of ripening, until 15 to 20 days after anthesis, and thereafter the increment gradually declined toward maturity. The increase of translucency was considerably rapid in vigorous kernels as compared with weaker kernels. Some of the vigorous kernels of glutinous rice showed a little decline in their translucency in the last stage of maturity in accordance with the decrease of kernel moisture. 2. Fresh kernels of non-glutinous rice slightly increased their translucency by artifical drying in the later stages of ripening. On the other hand, those of glutinous rice markedly lost their translucency and turned opaque by drying. And this phenomenon (Ryokka) characteristic of glutinous rice was seen throughout the entire ripening period. 3. Another experiment on Ryokka cleared that in the course of artificial drying, fresh kernels of glutinous rice started to diminish their translucency at around 20% of moisture content and rapidly increased their opacity in the moisture range of 15 to 17%. 4. Greenish coloring of hulled rice attained its peak about 15 to 20 days after anthesis, and thereafter declined toward maturity in accordance with chlorophylous decomposition. 5. Red-brownish hue of the colored rice was remarkably deepened during maturation, especially in vigorous kernels. And this coloring tendency was more conspicuous for deeper-colored varieties. A test on artiticial drying of fresh kernel obviously showed that the dehydration of hulled rice encouraged its brownish coloring. 6. Results of some spectrophotometric investigation on rice extractives suggested that brownish coloring of rice kernel during ripening might be ascribed to bathochromic shift or bathochromic effect due to structural changes of polyphenolic tannin-like substances in rice bran.