Studies on Alkali Decomposition of Rice Kernel : 4) Some alkali tests using "powder method"

Abstract

Powder method of alkali test on which the author has reported in the previous paper, was applied for an investigation of varietal difference in alkali digestibility of rice flour. Effect of air temperatures on the alkali digestibility was also tested during the ripening periods of rice. Special emphasis was laid upon the comparison of digestibility of milled white rice, rice flour and rice starch. The results were summarized as follows. 1. Alkali decomposition values for rice flours of 108 varieties showed analogus tendency to the values for milled white rice which had been tested previously. All the materials being obtained from the sama source. A highly significant correlation was found between the decomposition values of the two materials, but some difference in regression line was shown between japonica and indica varieties. 2. The values of the powder test were negatively correlated to the mean air temperatures during the ripening periods of 20 day after heading. High temperature treatment (30°C) in the ripening period decreased the decomposition values, whereas low tempertures (23 and 13°C) increased. The effect of temperature was more conspicuous for japonica varieties than for indica ones, indicating higher sensitivity to temperature of the former. 3. Difference in cultural season resulted in different values of alkali decomposition of rice. Late-season-cultured rice showed higher digestibility as compared with early- or mid-season-cultured rice. 4. Alkali digestibility of rice starch which had been separated and refined by alkylbenzen-sulphonate solution was quite analogous to that of rice powder of the same source. 5. The results mentioned above may lead to the conclusion that alkali digestibility of rice is exclusively depending on the digestibility of starch granule. 6. Air temperature which seemed to be the most influential of all the environmental factors in ripening season may cause some changes in micellar structure with in the developing granules of starch, and as a result, a change in swelling character which is connected with alkali digestibility.