Tannin:
Tannins are astringent, bitter plant polyphenols that either bind and precipitate or shrink proteins and various other organic compounds including amino acids and alkaloids. tannins comprise a large group of complex substances that are widely distributed in the plant kingdom; almost every plant family embodies species that contain tannins. When tannins occur in appreciable quantities, they are usually localized in specific plant parts such as leaves, fruits, barks or stems.
Characteristics of Tannins:
- Tannins are complex chemical substance and consist of mixtures of polyphenols.
- Most of the time they occur in glycosidic combinations with sugars.
- They are non-crystallisable compounds and form colloidal solutions with water.
- Tannins readily combine with proteins rendering them resistant to proteolytic enzymes.
- They also precipitate protein, gelatin and alkaloids from solutions.
- They form dark blue or greenish black compounds with ferric chloride.
- Tannins produce a deep red colour with potassium ferricyanide and ammonia.
Classification of Tannins:
Chemically, Tannins are classified into two classes-
- Hydrolyzable tannins
- Non-hydrolyzable tannins or condensed tannins
Hydrolyzable tannins:
Hydrolyzable tannins are tannins on heating with hydrochloric or sulphuric acids yield gallic or ellagic acids. At the center of a hydrolyzable tannin molecule, there is a 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.
Examples of gallotannins are the gallic acid esters of glucose in tannic acid (C76H52O46) found in the leaves and bark of many plant species.
Non-hydrolyzable tannins or condensed tannins:
Non-hydrolyzable tannins or condensed tannins are tannins on heating with hydrochloric acid yield phlobaphenes like phloroglucinol. 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.
Example- Condensed tannins from Lithocarpus glaber leaves have been analyzed through acid-catalyzed degradation in the presence of cysteamine and have a potent free radical scavenging activity.
Uses of Tannins:
- Tannins may be employed medicinally in antidiarrheal, hemostatic and antihemorrhoidal compounds.
- The anti-inflammatory effect of tannins help control all indications of gastritis, esophagitis, enteritis and irritating bowel disorders.
- tannins not only heal burns and stop bleeding, but also they stop infection while they continue to heal wounds internally. The ability of tannins to form a protective layer over the exposed tissue keeps the wound from being infected even more.
- Tannins are also beneficial when applied to the mucosal lining of the mouth.
- Tannins are an important ingredient in the process of tanning leather. Oak bark has traditionally been the primary source of tannery tannin, though inorganic tanning agents are also in use today.
