On the Grain Texture of Rice : 2. Varietal differences of cooking characteristics of milled white rice.

Abstract

To examine the varietal difference of swelling behaviour and total solids content in residual cooking liquid, milled white rice sampled of 58 varieties were treated with excess water fqr 30 minutes at temperatures ranging from 60° to 96°C. Swelling behaviour was experssed as the water-uptake ratio (weight of cooked rice)/(weight of raw rice), because the water-uptake ratio was colosely related to the swelling in the volume as shown in fig. 1. The total solid content was experssed as percent of raw rice, (dry weight of total solids) × 100/(dry weight of raw rice). The results are summarized. as follows: 1) In general, Japonica varieties began to swell at the lower temperatures than did Indica varieties, and among Indicas many of the semi-hard varieties (medium-grain), swelled at the lower temperatures than did the hard varieties (long-grain). Therefore, at the lower temperatures than 75°C, Japonicas and many of semi-hard varieties had higher water-uptake ratios than had Indicas and the hard varieties respectively, wereas at the temperature of 96°C, the hard varieties had the higher water-uptake ratios than any of the other varieties tested. (fig. 2) The varietal differences of water-uptake ratio were most distinct in the range from 65° to 75°C. 2) Among Japonicas, the varieties matured under lower temperatures began to swell at the lower cooking temperature than did those matured under higher temperatures. This relation was observed among the different samples of the same varieties matured under different temperatures.(table 1. fig. 3) Therefore, a highly significant negative correlation existed between the water-uptake ratios at cooking temperatures from 65° to 75°C and the mean temperatures during ripening.(r-0.956 at 70°C) 3) Microscopic observations of cooked rice revealed that swelling of rice grains was closely connected with swelling of starch granules and disruption of cells. In Towada and Fujisaka-5, swelling of starch granules and disruption of cells were observed on the outer layers of rice cooked at 70°C and on the fairly inner layers of rice cooked at 80°C. On the other hand, in Blue-bonnet, these changes were not observed when cooked at 70°C, but existed at 80°C. (photo. 1, 2). 4) The swelling of rice grain at the low cooking temperatures (60-75°C) will be attributed to the low geratinization temperature of starch and the fragility of cell membrane, and the swelling at the high temperatures (85-96°C) may be affected by the structure of starch and density of starch in endosperm (hardness distribution in endosperm¹⁸⁾). The rice which has high water-uptake ratio at low cooking temperature is considered to have a tendency to be cooked more easily and to become tender boiled rice. 5) Figure 4 shows that the maxima of total solids contents were at 65°C in glutinous varieties, 70°C or 75°C in Japonicas and 90° or 96°C in Indicas. Then the total solids contents at temperatures from 65° to 75°C were markedly higher in Japonicas than in Indicas and among Indicas some of the semi-hard varieties had fairly high total solids contents. (table 1) 6) Among Japonicas, highly significant negative correlation existed between total solids contents at cooking temperature from 65° to 75°C and mean temperature during ripening (r-0.789 at 65°C). This correlation was not recognized among Indicas in fig. 5 and 6, but according to the previous report¹⁹⁾, when ripening temperatures were artificially coctrolled to 23° and 30°C, the rice matured at 23°C had higher total solids contents than had those matured at 30°C in all Indicas examined. Accordingly, swelling behaviour and amounts of total solids in residual liquids depend to a great extent on varieties and to some extent on climatic conditions. [the rest omitted]