Tannin

Tannins are astringent[?], bitter-tasting esters of tannic acid[?] with glucose, found in the leaves and bark of many plant species. Tannins vary in their exact composition, C₇₆H₅₂O₄₆ being one example.

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.

Tannin has 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, tannin inhibits the absorption of minerals such as iron into the body.

Tannin is also found in wine, particularly red wine. Tannin 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 tannin 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.

Tannin is 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.