Tannins 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 refers to the use of oak and other bark in tanning animal hides into leather.
By extension, the term tannin is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups to form strong complexes with various macromolecules.
The tannin compounds are widely distributed in many species of plants, where they play a role in protection from predation (including 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 (proanthocyanidins).
Classification of tannins
1. Hydrolysable tannins
2. Condensed tannins
1. Hydrolysable tannins:
These tannins are hydrolyzed by acids, or enzyme and produce gallic acid and ellagic acid.
Chemically, these are esters of phenolic acid like gallic acid and ellagic acid.
The tannins derived from gallic acid are known as gallitannins and from that of ellagic acid are known as gallitannins.
The gallic acid is found in rhubarb, clove and ellagic acid is found in eucalyptus leave and myrobalans and pomegranate bark.
These tannins treated with ferric chloride to produced blue or black colour.
2. Condensed tannins:
These tannins are resistant to hydrolysis and they derived from the flavonols, catechins and flavan-3, 4-diols.
On treatment with acids or enzymes they are decomposed into phlobaphenes.
On dry distillation condensed tannin produce catechol.
These tannins are called as catechol tannins.
These tannins are found in cinchona bark, male fern, areca seeds, tea leaves and wild cherry bark, bahera fruits, Amla, etc.
they produce green colour with ferric chlorides.
They are phenolic compounds of lower molecular weight and do not show the goldbeater‘s test.
They are found in catechu and nux- vomica, etc.
(i) Catechins: Catechu, cocoa, etc.
(ii) Ipecacuanhic acid: Ipecacuanha
(iii) Chlorogenic acid: Coffee and nux-vomica
Properties of tannins:
Tannins are freely soluble in water, alcohol, glycerol, and acetone and dilute alkalies.
They are sparingly soluble in chloroform, ethyl acetate and other organic solvents.
They have an astringent taste.
They yield purple, violet or black precipitate with iron compounds.
They are precipitated by number of metallic salts notably potassium dichromate, and lead acetate and sub acetate.
They combine with skin and hide to form leather and with gelatin and isinglass to form an insoluble compound.
They combine with alkaloids to form tannates, most of which are insoluble in water.
Chemical tests of tannins:
1. Gelatin test:
To a solution of tannin, aqueous solution of gelatin and sodium chloride are added.
A white buff coloured precipitate is formed.
2. Goldbeater’s skin test:
A small piece of goldbeater skin is soaked in 20% hydrochloric acid, ringed with distilled water and placed in a solution of tannin for 5 minutes.
The skin piece is washed with distilled water and kept in a solution of ferrous sulphate.
A brown or black colour is produced on the skin due presence of tannins.
3. Phenazone test:
A mixture of aqueous extract of a drug and sodium acid phosphate is heated and cooled and filtered.
A solution of phenazone is added to the filtrate. A bulky coloured precipitate is formed.
4. Match stick test (Catechin test):
A match stick is dipped in aqueous plant extract, dried near burner and moistened with concentrated hydrochloric acid.
On warming near flame, the matchstick wood turns pink or red due to formation of phloroglucinol.
5. Chlorogenic acid test:
An extract of chlorogenic acid containing drug is treated with aqueous ammonia.
A green colour is formed on exposure to air.
6. Vanillin-hydrochloric acid test:
Sample solution and added vanillin- hydrochloric acid reagent (Vanillin 1 gm, alcohol 10 ml, concentrated hydrochloric acid 10 ml).
A pink or red colour is formed due to formation of phloroglucinol.
Isolation of Tannins
Both hydrolysable and condensed tannins are highly soluble in water and alcohol but insoluble in organic solvents such as solvent ether, chloroform, and benzene.
Tannin compounds can be easily extracted by water or alcohol.
The general method for the extraction of tannic acid from
various galls is either with water-saturated ether, or with mixture of water, alcohol, and ether.
In such cases, free acids such as Gallic and ellagic acid go along with ether while true tannin gets exracted in water.
If the drug consists of chlorophyll or pigment, it may be removed by ether.
After extraction, the aqueous and ethereal layers are separately concentrated, dried, and subjected to further isolation and purification using various separation techniques of chromatography
Medicinal properties and uses
Tannins occur in crude drugs either as major active constituent as in oak bark, or as a subsidiary component as in clove.
In many cases, they synergistically increase the effectiveness of active principles.
Tannins are medicinally significant due to their astringent properties.
They promote rapid healing and the formation of new tissues on wounds and inflammed mucosa.
They are used in the treatment of varicose ulcers, hemorrhoids, minor burns, frostbite as well as inflammation of gums.
Internally tannins are administered in cases of diarrhoea, intestinal catarrh and in cases of heavy metal poisoning as an antidote.
Recently, these compounds have demonstrated their antiviral activities for treatment of viral diseases including AIDS.
They are used as mordant in dyeing, manufacture of ink, sizing paper and silk, and for printing fabrics.
They are used along with gelatin and albumin for manufacture of imitation horn and tortoise shell.
They are widely used in the leather industry for conversion of hide into leather, the process being known as tanning.
They are also used for clarifying beer or wine, in photography or as a coagulant in rubber manufacture.
They are used for the manufacture of gallic acid and pyrogallol, and sometimes as a reagent in analytical chemistry.