Slow starch digestion in the rice cooked with adzuki bean: Contribution of procyanidins and the oxidation products

Adzuki bean is often cooked with non-glutinous rice in Japan, and the dish is called adzuki-meshi. By the cooking, flavonoids in adzuki bean are transferred to rice, and the color of the rice becomes pale red. However, it has not been reported on starch digestion of the rice of adzuki-meshi. The purpose of this study is to elucidate that the transferred flavonoids, especially procyanidins could slow down the digestion of rice starch. The principal results obtained are (1) that pancreatin-induced starch digestion, which was observed as the liberation of reducing sugars and starch fragments from the rice, was slower in the pale red rice of adzuki-meshi than the rice cooked without adzuki bean, (2) that the starch fragments liberated from the rice of adzuki-meshi were digested slowly, and (3) that procyanidins and the oxidation products, which were not extracted by methanol, were present in the pale red rice. From the results, it was concluded that adzuki bean procyanidins and the oxidation products, which bound to rice starch during cooking, could contribute to slow down the starch digestion.