Purple pigments, catechinopyranocyanidins A and B, in the seed coats of red adzuki beans elute during processing to yield purple-colored an-paste

Abstract

Red adzuki beans (Vigna angularis) are primarily used in producing sweet an-paste for use in Japanese confectionery, where the quality of the an-paste is evaluated based on its purple color. Catechinopyranocyanidins A and B (cpcA and cpcB) are seed coat pigments and might be responsible for the purple color of an-paste, thus, analyzing them in an-paste is crucial. However, no quantitative analyses of these compounds in an-paste have been reported. We examined extraction conditions for cpcA and cpcB from an-paste and established a quantitative analytical method. Additionally, we developed a pre-vacuum processing procedure for preparing purple an-paste. The more purple the an-paste was, the higher the content of cpcA and cpcB extracted from it. The purple color of an-paste originated from the catechinopyranocyanidins eluted from the seed coats upon boiling. Using the proposed procedure, we prepared more intensely purple wet sarashi-an compared with that generated via a conventional procedure.

The purple color of an-paste is due to catechinopyranocyanidins in seed coat