The word “tannin” is very old and is originally used in the context of the traditional work “tanning”. Tanning is a process used in the manufacture of leather. Tannins are widely known and widely used in plant life and are of particular importance in legumes
Tannins are mainly localized in the superficial wax layer of the plants, inside the cell they are in vacuoles and are delivered as extracellular excretion to the cell wall material; they are not necessarily for plant life required (HANS LAMBERS, 1993) They do not affect the metabolism of plants; only when the cells die off can the tannins exert metabolic effects.
The definition of tannin was first given by HASLAM in 1989: “Tannins are water soluble phenolic metabolic products of plants of molecular weight 500 and larger and with the ability to precipitate gelatin and other proteins”.
As there are other phenols that bind and precipitate the proteins (such as pyrogallol, resorcinol) and also phenols that have higher molecular weights and similar chemical structures like Tannins, but proteins can not precipitate (BATE-SMITH, 1973; STAFFORD and CHENG, 1980), HORVATH 1981 developed a broader definition: “that compound has sufficiently higher molecular weights and contains sufficient phenolic Hydroxyls and other appropriate groups (such as carboxy-carboxylates) make effective strong complexes with proteins and other macromolecular substances among the particular ecological Conditions to form “.
Tannins are usually divided into two groups:
1- Hydrolyzed Tannins (HT) (Tannic Acid):
It contains D-glucose as a central nucleus whose hydroxyl group, such as gallic acid or ellagic acid. Hydrolysed tannin, unlike condensed tannin, can be affected by the action of weak acid or base, enzymes (tannase) or hot water and produce carbohydrates and phenolic acid.
2- Condensed tannins (KT) (proanthocyanidins)
Condensed tannins are polymers of flavan-3-ol that are linked together by c-c Connect bonds.
KTs differ in their degree of polymerization: from dimer to polymers with hundreds of flavanol units. (mostly from 4 to 40) (WILLIAMS et al., 1983) The large condensed tannins have little nutritional significance, possibly due to their limited solubility. (MEHANSHO et al., 1987)
In strong mineral acid, the KTs depolymerize to anthocyanidin as a result of oxidation (not hydrolysis). KT is thus called due to its condensed structure; but also HT can go through a condensation process. The term KT is therefore not completely unique (JESS, 1995).