On the nature of coagulated tannins in astringent-type persimmon fruit after an artificial treatment of astringency removal

There are many reports concerning the astringency removal of persimmon fruit by ethanol or carbon dioxide treatment. However, none are concerned with the chemical characteristics of tannins after astringency removal, so the chemical properties of coagulated tannins after the treatment were investigated. For this purpose, the astringency of 'Hiratanenashi' persimmon (Diospyros kaki Thunb.) fruit was removed by enclosing them for several days on the tree in polyethylene bags containing 5 ml of 5% ethanol in water. On analyzing these fruit, we found that the coagulated tannins caused by this treatment can be resolubilized in 1% HCl-MeOH (methanol containing 1% hydrochloric acid) with about 80% efficiency. However, as the browning reaction of coagulated tannins in the fruit flesh proceeded gradually after astringency removal, the degree of resolubilization decreased to 20%. Enzymatically isolated tannin cells placed in 1% HCl-MeOH solution revealed that coagulated brown tannin cells did not show any changes, while coagulated colorless tannin cells swelled spherically by resolubilization of tannins. Furthermore, gel chromatography of tannins resolubilized with 1% HCl-MeOH from ethanol-treated fruit showed much smaller molecular size when compared with tannins extracted with 80% MeOH in water from untreated astringent fruit. Tannins extracted with 1% HCl-MeOH from untreated astringent fruit also showed smaller molecular size than tannins extracted with 80% MeOH in water from the same fruit. The involvement of noncovalent bonds such as hydrogen bonding in tannin coagulation is discussed.