Variation of the Hardening Speed of Pounded Rice-Cake, Gelatinization of Rice Flour and Physical Property of Cooked Rice of Glutinous Rice Among Years and Among Areas, and Its Occurrence Factors in Coldest Region for Rice Cultivation in Hokkaido, Japan

Hardening speed of pounded rice-cake and gelatinization property of glutinous rice were investigated in six areas of Hokkaido during four years. The range of the yearly variations (differences between the two extreme values and coefficients of variation) was larger than that of regional variations. Using the ratio of yearly to regional variations and values of correlation coefficient among those properties, in ascending order of ratio, the properties were classified as: Group 1: maximum viscosity and break down; Group 2: minimum viscosity, final viscosity and consistency; Group 3: gelatinization initiation temperature, temperature at which maximum viscosity was shown, time in which maximum viscosity was shown and hardening speed of pounded rice-cake. Among years, for gelatinization property, a positive relationship was seen for properties in Group 2 and Group 3 with hardening speed of pounded rice-cake, and the clearest positive relationship was seen for properties in Group 3. Moreover, the higher the cumulative daily mean temperature during 40 days after heading (grain-filling temperature), the higher the values in Groups 2 and 3 and hardening speed of pounded rice-cake; and, the clearest positive relationship was seen for properties in Group 3 and hardening speed of pounded rice-cake. However, the relationship with the area was not clear. Moreover, the higher the protein content; the lower the values in Group 1 and Group 2 among years and among areas, and the lower the values in Group 3 among years only. In addition, physical properties of cooked rice produced in the normal grain-filling temperature year (NGFTY) were compared with those in the high grain-filling temperature year (HGFTY). The cooked rice produced in NGFTY was softer and less sticky, namely the eating quality was lower. However, it was less stiffened and was kept soft by storage at 5 ℃ for 24 hours after cooking. On the other hand, cooked rice in HGFTY was stickier, and became hard and less sticky with storage at 5 ℃ for 24 hours. Moreover, the higher the protein content, the lower the stickiness, namely the eating quality was lower in both NGFTY and HGFTY. Therefore, the cultivation technique to obtain low protein content rice was considered important to obtain glutinous rice with good eating quality.