1) An increase of ethyl alcohol, methyl alcohol and acetaldehyde was recognized gas chromatographically by immersing immature astringent fruits in warm water to remove astringency.
2) When non-enzymatic fruit was treated with ethyl alcohol, methyl alcohol and acetaldehyde, only acetaldehyde was effective in removal of the astringency.
3) During the warm water treatment, as the acetaldehyde was formed in the fruit, the astringency disappeared from the fruit.
4) Observing the treated fruit tissue histochemically, it was found that the protoplast of tannin cells coagulated and the acetaldehyde stain was confined to the tannin cells.
5) When mature astringent fruit was treated with acetaldehyde, the astringency disappeared very rapidly. For example, when the fruit was treated with 20% acetaldehyde solution the astringency disappeared in 12 hours.
6) Therefore, it seems that acetaldehyde which is formed as the result of anaerobic respiration during the warm water treatment coagulates protoplast in the tannin cells and removes the astringency.