Slower liberation and digestion of amylose in high-amylose rice cooked with adzuki bean: contribution of procyanidins

Adzuki bean is cooked with rice in Japan, and the color of rice becomes pale red by the cooking. In previous papers, we showed that pancreatin-induced starch liberation was slower in red rice prepared from non-glutinous rice. In this study, it was shown that pancreatin-induced amylose liberation from high-amylose rice was divided into two phases independent of cooking with or without adzuki bean; the first phase was amylose liberation from starch leached from rice during the cooking and the second phase was the liberation of amylose from starch in rice grains. The slower amylose liberation from rice cooked with adzuki bean was due to the binding of cyanidin-producible components derived from procyanidins to rice. The binding was confirmed by treating rice with a 1-butanol/HCl/Fe(III) system. The digestion of amylose liberated from high-amylose rice cooked with adzuki bean was also slower, and the liberated amylose could produce cyanidin by the 1-butanol/HCl/Fe(III)-treatment. Furthermore, it was shown (i) that the heating of rice flour with reagent procyanidin B2 resulted in the binding of the oxidation products to the flour, and (ii) that the binding resulted in the slower digestion of amylose in the rice flour. It was concluded that the slower liberation and digestion in high-amylose rice cooked with adzuki bean were due to the binding of cyanidin-producible components, which were derived from adzuki bean procyanidins, to amylose during the cooking.