Black soybean (BSB), which contains cyanidin-3-O-glucoside (C3G) and procyanidins, is cooked with rice in Japan. The color of the cooked rice is purplish red due to the binding of C3G and reddish oxidation products of procyanidins. These components can slowdown pancreatin-induced hydrolysis of amylose more significantly than the hydrolysis of amylopectin, and can react with nitrous acid in the stomach. This manuscript deals with the effects of nitrous acid on pancreatin-induced hydrolysis of amylose heated with BSB extract. The hydrolysis of amylose heated with BSB extract was slow, and the slowdown was due to the binding of C3G/its degradation products and degradation products of procyanidins. The amylose hydrolysis was slowed down further by treating with nitrite under gastric conditions. The further slowdown was discussed to be due to the binding of the products, which were formed by the reaction of procyanidins with nitrous acid, to amylose. In the products, dinitroprocyanidins were included. In this way, the digestibility of amylose heated with BSB extract can be slowed down further by reacting with nitrous acid in the stomach.
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