Tannins are complex natural organic compounds that are polyphenolic, polyhydroxy benzoic acid derivatives, or flavonol derivatives, having astringent action. They are widely distributed in nature (leaves, barks, immature fruits but, they disappear during the ripening process). They protect the plants especially during growth from herbivorous animals, insects, bacteria, and others. They have large molecular weight but some have low molecular weight. The general uses of tannins are:

  1. Anti-diarrheal
  2. Anti-bacterial
  3. Antidote in alkaloidal and heavy metal poisoning
  4. Hemostatic
  5. Mild diuretic
  6. Leather industry
  7. Astringent for inflamed mucous membrane
  8. Stomachic etc.

Classification of Tannins

Tannins are classified into:

1. True tannins:

(a) Hydrolysable group:

  • Hydrolyzed by acids or enzymes (Tannase).
  • They are esters of a sugar (β-D-glucose), with one or more trihydroxy-benzene carboxylic acid.
  • The tannins which are derived from Gallic acid called Gallo-tannin or gluco-gallin like in Rhubarb rhizomes, Clove buds, etc.
  • Tannis is derived from ellagic acid called ellagotannin or glucoellagin like in Pomegranate roots bark and Eucalyptus leaves (which also contain Tannic acid which is Gallotannin).
  • They form a dark blue color with FeClз.
Chemical structures of Gallic acid and Ellagic acid
Fig.1: Chemical structures of Gallic acid and Ellagic acid

(b) Non-hydrolyzable group of tannin:

They are known as condensed tannins or proanthocyanidins.

They have resistance to acid or enzymatic hydrolysis.

They are Flavanol derivatives:

  • Flavan-3-ol like catechin.
  • Flavan-4-ol like leucocyanidin.

They are founded:

  • In barks like Cinnamon (Phlobatannins), Cinchona (Cinchotannin), Wild cherry.
  • In seeds: Cacao, Cola (Colacatechin).
  • In leaves: Tea.

They produce dark green color with FeClз.

They produce Catechol with conc. HCl and vanillin.

2. Pseudo tannins:

They have low molecular weight.

They occur:

  • As Gallic acid like in Rhubarb rhizomes.
  • As catechin like in Cacao, Acacia.
  • As chlorogenic acid like in coffee.
Chemical structures of Gallic acid, Hexahydroxydiphenic acid, Ellagic acid, Chebulic acid
Fig.2: Chemical structures of Gallic acid, Hexahydroxydiphenic acid, Ellagic acid, Chebulic acid

Physico-Chemical Properties

  1. They form colloidal solutions with water.
  2. They are non-crystalline substances.
  3. Their aqueous solution is acidic.
  4. They have a sharp puckering taste.
  5. They have astringent property.
  6. They are precipitated by: alkaloids, gelatin, salts of heavy metals, proteins (enzymes).

Chemical Tests of Tannins

1. With FeClз: Upon addition of ferric chloride solution in tannins aqueous solution:

  • Hydrolyzable Tannins give dark blue colour.
  • Non Hydrolysable tannins develop dark green colour.

2. It forms a precipitate with alkaloids, gelatin, proteins, and salts of heavy metals like lead acetate, copper acetate.

3. Tannins extract yields yellow colour with sulphuric acid.

4. When vanillin hydrochloride reagent is added in tannin solution, red to pink colour is seen.

Extraction and Isolation of Tannins

Tannins (hydrolyzable and condensed both) are highly soluble in water and alcohol but insoluble in non-polar solvents like ether, chloroform, and benzene. So tannins can be easily extracted by water or alcohol.

Extraction and isolation procedure of tannins
Fig.3: Extraction and isolation procedure of tannins
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