A Model Experiment for De-astringency of Persimmon Fruit with High Carbon Dioxide Treatment:<i>in vitro</i>Gelation of Kaki-tannin by Reacting with Acetaldehyde

Abstract

In order to explain the mechanism for removing the astringency from persimmon fruit by high carbon dioxide treatment, we studied in vitro whether purified kaki-tannin, a kind of polymeric proanthocyanidin, reacts with acetaldehyde to become a gel under mild conditions or not. Kaki-tannin reacted with acetaldehyde in a relatively short time to become a gel in phosphate buffer at pH 6 to 8. Phosphate, malate and citrate accelerated the gel formation, whereas ethanol, ascorbate and Tricine buffer prevented it. Formaldehyde was more effective, and propylaldehyde was less effective than acetaldehyde in causing gelation. We suggest that the de-astringency is due to the insolubilization of kaki-tannin, which occurs by means of reacting with acetaldehyde produced in the fruit during high carbon dioxide treatment.