Tannins (Tannic Acid): High molecular weight between 500-3000) compound containing sufficiently large number of phenolic hydroxyls or other suitable groups to enable it to form effective cross linkage between proteins and other macro molecules is called tannins.
Occurrence of Tannin:
1) In plant parts-bark, wood, leaves and fruits, maximum in galls.
2) Indian berry oke, tea leaves, bark of acacia
Chemical Nature of Tannin:
1. Complex derivative of Gallic acid and are poly hydroxyl phenols
2. Upon hydrolysis yield Gallic acid, di-Gallic acid + sugar (gl)
Properties of Tannin:
1. Lusterous, faintly yellowish, amorphous, powder, spongy mass.
2. Darkens when exposed to air, odourless.
3. It decomposes at 21°C
4. Strong Astringent taste
5. PPT glue, protein, alkaloid
6. Soluble in boiling water, alcohol and acetone
7. Tannins form blue colour with ferrous sulphate later on converted to blue black colour.
Tannic + FeSO4 → Ferrous tannate (Blue) → Ferric tannate (Blue, black)
Classification of Tannins:
According to their Behaviour on Dry Distillation:
1. Condensed Tannins: Condensed Tannins are formed by process of polymerization of flavonols
A. Catechin polymer e.g. does not contain glucose and ester bonds, catechin, epigall ocatechin.
B. Diol Condensed Tannins- e.g. Teracacidin, Melaccidin
2. Hydrolysable Tannins: A hydroxyl acid is esterified with glucose. These tannins are hydrolyzed to form Gallo tannic acid (pyrogallol).
a. Gallo tannic acid hydrolysis Gallic acid e.g. Acertannin, sumac tannin.
b. Ellagi tannin hydrolysis Ellagic acid e.g. Jugalin tannin, corigolagin tannin.
Physiological Role of Tannins:
1. It inhibits the enzymatic activity in plant in dead cells.
2. As reserve food material and means of removal of toxic compounds.
3. It takes part in the formation of pigments.
4. Tannins may serve as a protective agent for preventing the germination and growth of pathogenic fungi or organism in plants.
Uses of Tannins:
1. Tannins are used in leather industries (Preparation of leather from raw hids)
2. Preparation of paints and dyes, paper ink.