Polyesters

Most of the hydrolyzable tannins are essentially polygal-lates of glucose. Hydroxyl groups of the gallate segments can be further esterified with gallic acid to build up large complex molecules. Gallotannins occur in chestnut, oak, sumac, and in several plant galls. The Peruvian plant, tara, is an important commercial source. The gallotannins of commerce are known as tannic acid. They contain 5 to 10 esterified gallic acid residues. Tannic acid is used as a mordant in the dyeing of cotton, as a clarifying agent for wine and beer, and as a food flavorant at low concentrations. It is approved for use in the United States as a GRAS additive as shown in Table 2.

When galloyl groups of glucose esters are appropriately oriented, oxidative coupling may take place during plant metabolism to form ellagitannins that contain hexahy-droxydiphenic acid groups. Hydrolysis yields ellagic acid as well as gallic acid (10). The ellagitannins are rigid molecules in contrast to the gallotannins, which exhibit a great degree of flexibility. The structural relationships among the ellagitannis, casuaricitin, and pentagalloyl glucose that are depicted in Figure 4. provide an indication of the biosynthetic pathway for the formation of ellagitannins.

Many ellagitannins yield additional phenolic compounds on hydrolysis such as chebulic, dehydrodigallic,

Table 1. Proanthocyanidin Content of Foods

Food

Proanthocyanidins (mg/100 g)

Strawberries

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