Tannin
class of astringent, bitter plant polyphenolic chemical compounds
Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term tannin is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong compounds with various macromolecules.
The term tannin (from scientific French tannin, from French tan "crushed oak bark", tanner "to tan", cognate with English tanning, Medieval Latin tannare, from Proto-Celtic *tannos "oak") refers to the abundance of these compounds in oak bark, which was used in tanning animal hides into leather.
The tannin compounds are widely distributed in many species of plants, where they play a role in protection from predation (acting as pesticides) and might help in regulating plant growth. The astringency from the tannins is what causes the dry and puckery feeling in the mouth following the consumption of unripened fruit, red wine or tea. Likewise, the destruction or modification of tannins with time plays an important role when determining harvesting times.
Tannins have molecular weights ranging from 500 to over 3,000 (gallic acid esters) and up to 20,000 daltons (proanthocyanidins).
Structure and classes of tannins
There are three major classes of tannins: Shown below are the base unit or monomer of the tannin. Particularly in the flavone-derived tannins, the base shown must be (additionally) heavily hydroxylated and polymerized in order to give the high molecular weight polyphenol motif that characterizes tannins. Typically, tannin molecules require at least 12 hydroxyl groups and at least five phenyl groups to function as protein binders.
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