Slowing down of starch hydrolysis of glutinous rice and non-glutinous rice flours by black soybean extracts: Cooperation between cyanidin 3-O-glucoside and procyanidins

Abstract

• Black soybean components inhibited starch hydrolysis by binding to rice flours. • The inhibition was greater in non-glutinous rice flour than glutinous rice flour. • In the components, cyanidin 3- O -glucoside and procyanidins were included. • The inhibition by cyanidin 3- O -glucoside and procyanidins were cooperative. • The cooperation took place in the digestion of amylopectin as well as amylose. Black soybean (BSB), which contains cyanidin 3- O -glucoside (C3G) and procyanidins, is an ordinary ingredient in Japan. The BSB extracts slowed down pancreatin-induced reducing sugar formation and starch hydrolysis more significantly in non-glutinous rice flour (joshin-ko) than glutinous rice flour (mochi-ko), suggesting that BSB components inhibited amylose digestion more effectively than amylopectin digestion. In the components, C3G, a procyanidin dimer (proB2), and procyanidin oligomers were included. C3G could cooperate with proB2 for the inhibition. Procyanidin oligomers inhibited the amylose hydrolysis significantly but slightly the reducing sugar formation in joshin-ko, and inhibited the both reactions slightly in mochi-ko, suggesting that the oligomers could effectively inhibit amylose digestion. C3G could also cooperate with the procyanidin oligomers to inhibit amylose digestion in joshin-ko. The above results suggest that the cooperation of C3G and procyanidins might also be included in the BSB extract-dependent inhibition of amylose digestion in joshin-ko.