Tannin
Tannin, any of various chemical compounds that are obtained from plants and have the ability to convert animal hides into leather. (It is this ability that gave tannins their name.) Virtually all plants contain some tannin. Tannins obtained from different plants vary slightly in physical properties and chemical behavior. For example, gallotannin, which is obtained primarily from galls (abnormal growths) on oak trees, can be hydrolyzed (chemically decomposed by water), while other tannins cannot. In general, tannins are noncrystalline solids with a bitter taste.
In the tanning process, tannins transform animal hides into leather by producing chemical changes in the collagens (fibrous proteins) in the hides. Tannins have various other uses as well. Tannic acid, derived from gallotannin, is used in medicine as an astringent to shrink skin tissues, to retard the flow of blood, and to control diarrhea. It is also used as a mordant (a substance that causes fabrics to accept dyes readily), and as a clarifying agent (a substance that renders liquids clear) in wine and beer manufacture. Because tannic acid reacts chemically with iron salts (for example, ferrous sulfate) to form dark blue or greenish-black compounds, it is used to make permanent inks. Gallic acid is also derived from gallotannin.
Most of the world's supply of commercial tannin is obtained from the wood of quebracho trees in Argentina and Paraguay. Other important tannin sources include oak galls; the bark of various species of oak, chestnut, mangrove, wattle, and hemlock; the leaves of sumac; the roots of the canaigre plant, found in Mexico and the southern part of the United States; the seed pods of the divi-divi tree of the West Indies; and myrobalan nuts of India.