Characterisation of anthocyanins and proanthocyanidins of adzuki bean extracts and their antioxidant activity

A novel extract powder purified from the boiled water of adzuki bean paste production was developed to better utilize this resource. The compounds contributing to pigmentation of purified adzuki bean extract powders were investigated in order to compare their antioxidant activity in vitro with (+)-catechin. When a normal extract was exposed to the air under heat treatment, the colour of adzuki bean extract became more strongly reddish, which was associated with polyphenol polymerization. Anthocyanins also contributed to the pigmentation of the purified adzuki bean extracts. Especially, two anthocyanin compounds, peonidin-3-rutinoside and malvidin-3-O-glucoside were newly identified in the adzuki bean extract. The reducing powder, iron chelating activity and free-radical scavenging capacity of the adzuki bean extract were greater than that of (+)-catechin while its total antioxidant value was lower. Thus, adzuki bean extract powders are promising alternatives to replace synthetic antioxidants and potential dyes.