Average Temperature during the Ripening Period to Obtain First Grade Rice and High Enzymatic Digestibility of Steamed Rice of Brewer’s Rice Cultivar ‘Ginnoyume’ in Kochi Prefecture

Abstract

We conducted a transplanting time experiment to improve the inspection grade and enzymatic digestibility of steamed rice of brewer’s rice cultivar ‘Ginnoyume’ in Kochi prefecture. We examined the relationship between inspection grade, enzymatic digestibility of steamed rice, and temperature during the ripening period, as well as changes in yield and protein content of brown rice in ‘Ginnoyume’ transplanted in mid-May, early June, mid-June, and late June in 2016, using five transplanting times in 2017 and 2018, including mid-July. In ‘Ginnoyume’, a lower average temperature during the ripening period due to later transplanting time improved the inspection grade by increasing 1000-grain weight, the percentage of whole grains, white-core grains, and by decreasing the percentage of white immature grains such as white-back grains, and also improves the enzymatic digestibility of steamed rice by changing the starch characteristics. Furthermore, the regression relationships between inspection grade, enzymatic digestibility of steamed rice, and average temperature during the ripening period, indicated that the average temperature during the ripening period to obtain first grade rice and high enzymatic digestibility of steamed rice was less than 23.6ºC. However, since later transplanting time also resulted in yield reduction and increased protein content of brown rice, it is necessary to consider appropriate cultivation methods to mitigate these negative effects.