Decrease in proanthocyanidin levels upon astringency removal treatment of yacon root

Abstract

Yacon (Smallanthus sonchifolius) root is known as a functional food material because it contains antioxidants high in polyphenols and fructooligosaccharides. However, yacon root is astringent. We reported that while the astringency was removed by exposure to ethanol (EtOH) vapor, as previously observed in persimmons, the antioxidative activity and fructooligosaccharides were retained. In this study, we investigated the mechanism of astringency removal in yacon root. Levels of proanthocyanidin (PA), which contains the gallate ester structure responsible for astringency, in yacon were decreased during EtOH treatment. The decrease in PA was proposed to originate from changes in its water solubility. About 57.7% of PA was insoluble in water after the treatment. This change in solubility might be caused by the reaction with acetaldehyde, which is generated by the action of alcohol dehydrogenase (ADH). ADH activity increased during the treatment period, as previously observed in the case of persimmons. These results suggest that the removal of astringency in yacon root is a result of the increase in insoluble PA.