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 conce rned with the chemical characteristics of tannins after astringency re moval, so the chemical properties of coagulated tannins after the trea tment were investigated. For this purpose, the astringency of 'Hiratan enashi' persimmon (Diospyros kaki Thunb.) fruit was removed by enclosi ng 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 th e 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 de gree of resolubilization decreased to 20%. Enzymatically isolated tann in cells placed in 1% HCl-MeOH solution revealed that coagulated brown tannin cells did not show any changes, while coagulated colorless tan nin cells swelled spherically by resolubilization of tannins. Furtherm ore, gel chromatography of tannins resolubilized with 1% HCl-MeOH from ethanol-treated fruit showed much smaller molecular size when compare d with tannins extracted with 80% MeOH in water from untreated astring ent fruit. Tannins extracted with 1% HCl-MeOH from untreated astringen t 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.