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gallotannic acid,astringent vegetable product found in a wide variety of plants. Sources include the bark of oak, hemlock, chestnut, and mangrove; the leaves of certain sumacs; and plant galls. Tannin is also present in tea, coffee, and walnuts. A solution of tannic acid is obtained from one of these natural sources by extraction with hot water; in particular, gallotannic acid is obtained from plant gall. Tannin varies somewhat in composition, having the approximate empirical formula C76H52O46. Tannic acid is a colorless to pale yellow solid; it is believed to be a glucoside in which each of the five hydroxyl groups of the glucose molecule is esterified with a molecule of digallic acid. Tannin is used in tanning animal skins to make leather; it transforms certain proteins of animal tissue into compounds that resist decomposition. It is also used in manufacturing inks, as a mordant in dyeing, and in medicine as an astringent and for treatment of burns.
any of a group of phenol compounds that are widespread in plants and are capable of forming strong bonds with proteins and certain other natural polymers, such as cellulose and pectins.
Originally (late 18th century), the term “tannin” was applied to a mixture of substances extracted by water from a number of plants that was capable of transforming animal hides into leather. In modern usage, the term includes all naturally occurring compounds having molecular weights from 500 to 3,000 and containing a large number of phenolic hydroxyl groups. (In the Soviet Union, tannins of plant origin are also called tannides.)
Tannins are divided into two classes. The first comprises the tannins formed by a polyhydric alcohol, such as glucose, in which the hydroxyl groups are partially or completely esterified by gallic acid or related compounds. These tannins are referred to as hydrolyzable; an example is seen in structural formula I. The second includes the tannins formed through the condensation of such phenol compounds as catechins. These tannins are described as condensed; an example is seen in structural formula II.
Tannins are contained in galls and in bark, wood, and leaves and/or fruits of tannin plants. They are used in tanning leather, and they also serve as mordants in dyeing cotton fabrics. In medicine they are used as astringents. When used on burns, aqueous solutions of tannins bind the toxic protein products of tissue decomposition and facilitate healing. In the body, tannins can bind bacterial toxins, as well as toxic salts of silver, mercury, and lead.
REFERENCESBiokhimiia rastenii. Moscow, 1968. Chapter 22. (Translated from English.)
Zaprometov, M. N. Osnovy biokhimii fenol’nykh soedinenii. Moscow, 1974.