Tannins are substances with high molecular weight and are water soluble polyphenols able to precipitate proteins. Tannins were classified for the first time in 1922 by Freudemberg, depending on their chemical structures
Hydrolysable tannins are further divided into gallic and ellagic tannins. The first, the gallotannins release glucose and gallic acid by hydrolysis at low ambient pH. They are mainly extracted from Tara (Caesalpinia spinosa), sumac (Rhus coriaria) and gallnuts (Quercus infectoria and Rhus semialata). The ellagitannins, made from ellagic acid glycosides, are one of the components of oak wood (Quercus robur, Quercus petraea and Quercus alba), chestnut wood (Castanea sativa) and myrobalan (Terminalia chebula).
Condensed tannins have a reduced astringent power when compared to hydrolysable tannins. They are further divided into proanthocyanidin and profisetinidin. Proanthocyanidins are naturally present in grapes (Vitis vinifera) consisting of various flavonoids which release anthocyanins and other insoluble molecules when they are treated under acid hydrolysis. They are mainly diffused in the skins and seeds of grapes, and therefore you can find them in red wines. Profisetinidins are extracted from the quebracho wood (Schinopsis lorentzii) and mimosa (Acacia mollissima).
Recently a third class of tannins has been identified, the phlorotannins, present in many species of dark algae.
Along with many other substances of a mainly phenolic nature, the tannins enter into the class of semiochemicals, which is a generic term used for a chemical substance or mixture that carries a message. These chemicals act as messengers within or between species. The semiochemicals are divided into two classes: pheromones and allelochemicals. Pheromones are involved in the communication between the same species while allelochemicals interact with different species.