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Ellagic acid

The structural feature of a tannin responsible for the phenomenon of astringency has been stated as "the accumulation within a molecule of moderate size, of a substantial number of phenolic groups, many of which are associated with 1,2-dihydroxy or 1,2,3-trihydroxy orientation within a phenyl ring" (16).

The human diet basically includes two groups of tannins: the ellagic acid esters of glucose (ellagitannins) found in the persimmon, pomegranate, and chestnut; and flavanol-derived substances such as condensed catechin oligomers occurring in many fruits, and compounds produced by the enzymic oxidation of the flavanols accompanied by condensation reactions. These are found in wine, beer, and black tea.

The more water-soluble tannins, such as the ellagitannins, are less efficient in precipitating protein than the polygalloyl glucose esters. Molecular size is also important because of its effect on the orientation of the di/trihydrox-ylphenyl groups with respect to the proline-rich proteins. Astringency decreases because of steric hindrance as the number of epicatechin units in an oligomer exceeds six or seven. The loss of astringency that occurs during fruit ripening may be caused by further polymerization or by com-plexation with increased amounts of sugar.

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