Tannins are astringent, bitter-tasting plant polyphenols that bind and precipitate proteins. The term tannin (from the Celtic word for Oak) refers to the source of tannins used to convert animal skin into leather, however the term is applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong complexes with proteins and other macromolecules. Tannins have a molecular weights ranging from 500 to over 20,000.

Tannins are usually divided into hydrolyzable tannins and condensed tannins (proanthocyanidins). At the center of a hydrolyzable tannin molecules there is a polyol (usually D-glucose) and the hydroxyl groups of the carbohydrate are partially or totally esterified with phenolic groups like gallic acid (in gallotannins) or ellagic acid (in ellagitannins). Hydrolyzable tannins are hydrolyzed by weak acids or weak bases to produce carbohydrate and phenolic acids. Condensed tannins, also known as Proanthocyanidins, are polymers of 2 to 50 (or more) flavonoid units that are joined by carbon-carbon bonds, which are not susceptible to being cleaved by hydrolysis. While hydrolyzable tannins and most condensed tannins are water soluble, some very large condensed tannins are insoluble.

Examples of a gallotannins are the esters of tannic acid (C76H52O46) with glucose, found in the leaves and bark of many plant species.

The tea plant (Camellia sinensis) is an example of a plant with a naturally high tannin content, though in the case of green tea the leaf does not release its tannin into the infusion. Tannins have a calming effect on the human central nervous system, countering the stimulating effect of the caffeine in tea. New varieties of tea have been specifically bred for a lower tannin content. If ingested in excessive quantities, tannins inhibits the absorption of minerals such as iron into the body. This is because tannins are metal ion chelators, and tannin-chelated metal ions are not bioavailable.

Tannins (mainly condensed tannins) are also found in wine, particularly red wine. Tannins in wine can come from many sources and the tactile properties differ depending on the source. Tannins in grape skins and seeds (the latter being especially harsh) tend to be more noticeable in red wines, which are fermented while in contact with the skins and seeds.

Modern winemakers take great care to minimize undesirable tannins from seeds by crushing grapes gently to extract their juice. Pressing the grapes results in press wine which is more tannic and might be kept separately. Wines can also take on tannins if matured in oak or wood casks with a high tannin content. Tannins play an important role in preventing oxidation in aging wine and appear to polymerize and make up a major portion of the sediment in wine.

Tannins are an important ingredient in the process of tanning leather. Oak bark has traditionally been the primary source of tannery tannin, though synthetic tanning agents are also in use today. Tannins are used to bind and precipitate proteins.

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