In collaboration with Hirosaki University and Meijo University, a research group led by Professor Kumi Yoshida of the Graduate School of Informatics, Nagoya University extracted pure pigment from the seed coat of red red beans and succeeded in determining its structure for the first time in the world.This is a novel substance different from anthocyanins, which are well known as pigments for flowers and fruits, and it was also found that this pigment is responsible for the purple color of red bean paste.
Research on seed coat pigments of red beans has a long history, and in 1934 Chika Kuroda et al. Of RIKEN reported that they consisted of water-insoluble pigments and brown tannins.After that, many studies were conducted, but the chemical structure and properties of the seed coat pigment were completely unknown.Moreover, it is still misunderstood that the base of the pigment is anthocyanin, which is also contained in kidney beans.
This time, two kinds of purple pigments were taken out from the red beans, and the complete structure including the three-dimensional arrangement was determined.It was found to be a novel substance in which catechin and cyanidin were bonded (condensed) with a pyran ring (a 2-membered ring containing oxygen), and was named "catechinopyranocyanidin A, B".This pigment is almost insoluble in water and turns from strongly acidic (pH 6) to neutral (pH 1) to a beautiful purple color.In addition, the dye solution is decomposed even with weak light such as room light, but it is stable and does not decompose in a dark place.Furthermore, it was found that the bean paste prepared from red bean also contained this purple pigment, and it was proved that the purple color of the bean paste was due to this pigment.
When boiling azuki beans, it is usually cut astringently.This removes the water-soluble, brownish tannins, during which the dye is adsorbed on the bean paste particles, and the colorless bean paste particles are colored purple.The rationality of this bean paste process can also be understood from the chemical properties of the dye.In the future, it is expected to be applied to the method of obtaining high-grade purple bean paste and the breeding of new varieties of azuki beans.
Paper information:[Scientific Reports] Structure of two purple pigments, catechiopyranocyanidins A and B from the seed-coat of the small red bean, Vigna angularis
