Leaching of amylose and amylopectin during cooking of rice grains and their effect on adhesiveness of cooked rice

Composition and structures of materials leached from rice grains during cooking were investigated in relation to their contribution to the adhesiveness of cooked grains, an Important determinant of eating-quality preference. Two cultivars of rice, Koshihikari and Hoshiyutaka, are compared because of their well known sticky and less sticky cooked grains, respectively, and a waxy cultivar Hiyokumochi was also included. The amount of materials leached from rice grains increased with the increase of temperature at which the materials were collected. Hiyokumochi, Koshihikari and Hoshiyutaka yielded leached materials of 0.9, 0.8 and 1.3%, respectively, of rice grams (200 g) cooked at 85 deg C. More than 93% of the materials were carbohydrate. Fluorescent labeiing/GPC showed that the materials comprised two major populations with degree of polymerization (DP) of -6000 and -1500. The populations were degraded by alpha-amylase or isoamylase, indicating they were starch-derived alpha-glucans. Amylose (AM) and amylopectin (AP) fractions were prepared by 1-BuOH precipitation of the leached materials. AM content was 1, 9 and 21% for Hiyokumochi, Koshihikari and Hoshiyutaka, respectively. In comparison with amylose and amylopectin fractionated from starch, both AM and AP had about 1/2 of number-average DP, and chain-length distribution of AP was essentially the same. Replacement of leached liquid of Koshihikari at 85 deg C to that from Hoshiyutaka resulted in the decrease of adhesiveness of cooked grains of Koshihikari from 334 to 173 erg (average of 40 grains) and the increase of ratio of grains with lower adhesiveness. These results suggested that the higher the ratio of the small amylose leached, the lower the adhesiveness of cooked grains.