Chemical evidence for the de-astringency (insolubilization of tannins) of persimmon fruit

After artificial removal of the astringency from persimmon fruit by treatment with ethanol, thiol-promoted degradation of the insolubilized proanthocyanidin polymers with 2-sulfanylethanol yielded 4β-(2-hydroxyethylsulfanyl)-6- and -8-[1-(2-hydroxyethylsulfanyl)ethyl]-flavan-3-ols 9–14. Furthermore, when deuteriated ethanol was used for de-astringency, the deuterium atoms were incorporated into the C2 unit attached to the A-ring of these compounds. These findings evidently show that acetaldehyde formed in situ from ethanol plays an important role in polymerization (insolubilization) of water-soluble proanthocyanidins, causing the loss of astringency.