Tannins
Tannins Tannins, also tannic acid, common name applied to a group of vegetable products, both amorphous and crystalline, obtained from various plants, and important commercially in the tanning of leather . Tannins have variable composition. Some, called condensed tannins, are phenols of moderately complex structure, and others are esters of glucose or some other sugar with one or more trihydroxybenzoic acids. The empirical formula, C14H14O11, often given for common tannin, is only an average. Tannins occur in many trees, and the best sources include oak galls and the bark of sumac. Extraction with water, or water and alcohol, is the first step in the preparation of tannin. Settling, followed by evaporation at a low temperature, yields the commercial product. Tannins have a yellow-white to brown color and a faint, characteristic odor. Exposure to light deepens the color. They all taste bitter and are astringent. Water, acetone, and alcohol dissolve tannins readily, but benzene, ether, ...