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Tannins are usually divided into hydrolyzable tannins and condensed tannins (proanthocyanidins). At the center of a hydrolyzable tannin molecule, there is a polyol carbohydrate (usually D-glucose). The hydroxyl groups of the carbohydrate are partially or totally esterified with phenolic groups such as 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.
Tannins may be employed medicinally in antidiarrheal, hemostatic, and antihemorrhoidal compounds. Also, they produce different colors with ferric chloride (either blue, blue black, or green to greenish black) according to the type of tannin.
Examples of gallotannins are the esters of tannic acid (C76H52O46) with glucose, found in the leaves and bark of many plant species.
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Tea
The tea plant (Camellia sinensis) is an example of a plant with a naturally high tannin content. Green tea leaves are unquestionably a major plant source of tannins, as they not only contain the tannic and gallic acid groups, but also a proanthocyanidin (a type of flavanol) named prodelphinidin. When any type of tea leaf is steeped in hot water for an excessively long time period it brews a "tart" (astringent) flavor that is characteristic of tannins (and other components). New varieties of Camellia sinensis have been specifically bred for a lower tannin content. If ingested in excessive quantities, tannins inhibit 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.
Wine
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. Tannins extracted from grapes are condensed tannins, which are polymers of procyanidin monomers. Hydrolysable tannins are extracted from the oak wood the wine is aged in. Hydrolysable tannins are more easily oxidised than condensed tannins.
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.
Pomegranates
Pomegranates contain a diverse array of tannins, particularly hydrolysable tannins. The most abundant of pomegranate tannins are called punicalagins. Punicalagins have a molecular weight of 1038 and are the largest molecule found intact in rat plasma after oral ingestion (Biomed. Pharmacother. 2002, 56, 276-82) and were found to show no toxic effects in rats who were given a 6% diet of punicalagins for 37 days. (J. Agric. Food Chem. 2003, 51, 3493-3501). Punicalagins are also found to be the major component responsible for pomegranate juice's antioxidant and health benefits (J Agric Food Chem 2000 48 (10) 4581-89).
Several dietary supplements and nutritional ingredients are available that contain extracts of whole pomegranate and/or are standardized to punicalagins, the marker compound of pomegranate. Extracts of pomegranate are also 'Generally Recognized As Safe' (GRAS) by the United States. It has been recommended to look for pomegranate ingredients that mimic the polyphenol ratio of the fruit, as potent synergistic effects have been observed in 'natural spectrum' extracts, especially pomegranate concentrate normalized to punicalagins (J Nutr Biochemistry 2005 (16) 360-367).
Leather
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.
