Studies on the Phisical and Chemical Properties of Soft Rice : I. On hardness, 1000 grain weight, specific gravity and water-content

Abstract

Two groups of samples were used as materials. One of the groups consisted of the crops of variety Norin No.1 from eight prefectures, each representing the soft-rice-producing region in Japan, and the other group consisted of the crops of the same variety from eight prefectures representing the hard-rice-producing regions. The water-content was measured by means of the KETT infra-red ray water-content meter of the Sanwa Electric Institute (calibrated with Mr.NAGAHARA's desiccation-method at 135°C), the specific gravity by means of the REISHAUER's pycnometer, and the hardness by means of the hardness meter of the Kiya type. The results obtained are as follows : (1) The water content of soft rice was about 1% higher than that of hard rice. (2) The 1000 grain weight of soft rice was about 1.7 g. higher. (3) The specific gravity of soft rice was a little (0.01) higher. (4) The fracture hardness was 1.0 kg. lower and the breaking hardness, 0.8 kg. lower. (5) The comparison of its correlation coefficients with those of hard rice showed no significance. Then, the water-content was changed between 10 % and 20 % at an interval of 2 %, and the other properties above mentioned corresponding to each level of the water-content were measured. As the results : (6) The 1000 grain weight of soft rice was invariably about 2g. heavier than that of hard rice. (7) The specific gravity of the former was invariably about 0.02 higher. (8) In the hardness of fracture and breaking, there was no difference between the two kinds of rice, either of the properties depending on water-content. (9) The correlation coefficients were found to exsist significantly only (a) between the water-content and the 1000 grain weight ; (b) between the water-content and the fracture hardness ; and (c) between the 1000 grain weight and the fracture hardness. Soft rice was more or less lower than hard rice, in (a) and there was much difference in (b) and (c) between the two sorts of rice. Nearly all the researches on rice quality in the past have left the physical properties of soft and hard rice and their differences dark. But the present research has clarified that at least the 1000 grain weight and the specific gravity of soft rice, whether in a natural state or at a level of water artificially controlled, are higher than those of hard rice. The present author believes that it can be ascribed to the quantitative difference in solid materials, that is, largely in starch, and also in protein, between soft rice and hard rice.