Tannins - Pharmacognosy and phytochemistry 2 (BP503TP)

 Tannins are organic compounds which are polyphenolic, polyhydroxy benzoic acid derivatives or flavanol derivatives, having astringent action. They are widely distributed in the nature. They provide protection to 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. Haemostatic
5. Mild diuretic
6. Leather industry
7. Astringent for inflamed mucous membrane
8. Stomachic etc.

CLASSIFICATION

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 trihydroxybenzene carboxylic acid.
• The tannins which is derived from Gallic acid called Gallo-tannin or glucogallin like in Rhubarb rhizomes, Clove buds etc.

(b) Non hydrolysable group of tannin

• They are known as condensed tannins or proanthocyanidins.
• They have resistance to acid or enzymatic hydrolysis.
• They are Flavanol derivatives:

1. Flavan-3-ol like catechin.
2. Flavan-4-ol like leucocyanidin.

• They are founded:

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

• They produce dark green color with FeClз.

2. Pseudo tannins

• They have low molecular weight.
• They occur:

1. As Gallic acid like in Rhubarb rhizomes.
2. As catechin like in Cacao, Acacia.
3. As chlorogenic acid like in coffee.

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 sharp puckering taste.

5. They have astringent property.

6. They are precipitated by: alkaloids, gelatin, salts of heavy metals, proteins

CHEMICAL TESTS

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

           (a) Hydrolysable Tannins gives dark blue colour.

           (b) Non Hydrolysable tannins develops dark green colour.

2. It forms 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 can be easily extracted by water or alcohol.

Powdered crude drug  >  Extracted with water-saturated ether Or mixture of water, alcohol and ether (Gallic, ellagic acid, chlorophyll or  Pigments extracted in ether) >Tannins gets extracted in water

 >Separate aqueous and ethereal layers (Concentrate and drying) > Isolation and purification

(Various separation techniques of chromatography)

CATECHU (BLACK CATECHU) (click here)

PALE CATECHU (click here)

PTEROCARPUS (click here)