TANNINS
- Chemically tannins are derivatives of polyhydroxy benzoic acid, capable of combining with proteins. They are complex, high molecular Wt. Organic nitrogenous substances.
- They are non- crystallizable, soluble in alcohol & water
- 1. They have tanning property.
- 2. They precipitate proteins and alkaloids.
- 3. They are acidic in nature.
- 4. They have an astringent test.
- 5. They cause the contraction of smooth muscle.
- 6. They give dark blue, green black color with ferric salt or iron.
- 7. They give the Goldbeater skin Test positive.
- 8. They prevent putrification of leather.
Classification: -
- Hydrolysable. 2. Condensed.
Hydrolysable tannins
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Condensed tannins
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They are hydrolysable by acids or enzymes
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They are more resistant to hydrolysis
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Chemically they are esters of Phenolic acids
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They are related to flavonoid pigments.
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On treatment with acids or enzymes they produce Gallic
acid or Ellagic acids.
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On treatment with acids or enzymes, they decompose into insoluble red
compound known as phlobaphenes
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On dry distillation they are converted into pyrogallols
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On dry distillation they produce catechol
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With iron salts they produce blue colors.
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With iron salts they produce green colors.
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Ex. Clove, Myrobalon
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Black catechu, Acacia bark.
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Chemical Test of Tannins: -
1) Gold beater’s skin Test: - Goldbeater skin is a membrane prepared from intestine of ox.
Gold beaters skin is treated with 2 % HCl. Wash the skin with water & keep the skin in the test solution for 5 minute. Again wash the skin with water. Treat the skin with 1 % FesO4 soln
If true tannins are present brown or black spot is produced on the skin.
2) Gelatin Test: - If solution of tannin is added to 1 % solution of gelatin containing 10 % NaCl, ppt. is produced.
3) Aqueous solution of tannins + Aq. solution of ferric chloride.
Hydrolysable tannin gives blue black colors & condensed tannin gives brownish green colors
4) Tannins give red color with vanillin hydrochloride reagent.
Isolation of Tannins (extraction): -
- Raw material------ >Extract with ether + alcohol------> Filter ---->Shake Filtrate with water----------->Aqueous solution Takes most of tannin------->Evaporate aqueous extract to dryness under vacuum.
- Crude Tannin dissolves in alcohol----- > Alcoholic solution id shaken with ether-------> Ppt of pure product is obtained from ether solution.