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Tannins
Tannins are "phenolic natural products that precipitate proteins from their aqueous solutions".
Classification:
1- Hydrolysable tannin (pyrogallol tannin):
Classified into gallitannin and ellagitannin
2- Condensed tannin (catechol tannin).
04/09/23 1Dr. Harish Kakrani & Purvi Kakrani
04/09/23 2Dr. Harish Kakrani & Purvi Kakrani
1- Hydrolysable Tannin
Members of this class consist of several molecules of phenolic acids united by ester linkage to a central glucose residue. Such esters are readily hydrolyzed by acids or enzymes so called hydrolysable tannins.
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Types of hydrolysable tannin:
A- GallitanninOn hydrolysis , it gives gallic acid and glucose.Present in rhubarb, galls, clove, hamamelis.
B- Ellagitannin On hydrolysis it gives ellagic acid and glucose Present in pomegranate and eucalyptus.
Gallic acid
COOH
OH
OH
HO
OH
OH
HO
Pyrogallol
OH
OH
OH
HO
OH
COOHHO
HOOC
Hexahydroxydiphenic acid
O
OO
O
OH
OH
OH
HO
Ellagic Acid
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2- Condensed tannin
On dry distillation they give catechol.They differ from pyrogallol tannin where
their structures are related to flavonoids and they lack sugar in their structures
Present in cinnamon, cinchona, tea.Catechin:Flavan-3-ol and leucocyanidin:
1- tannin + FeCl3 → bluish black colour with hydrolysable tannin and brownish black colour with catechol.
2- Gelatin test: solution of 1% tannin precipitate gelatin from its solution at conc. 1%.
3- Gold beater's skin test: soak a small piece of Gold beater's skin membrane in 2% HCl, rinse with H2O, place the piece for 5 minutes in the tannin solution to be tested, wash with H2O, transfer to 1% FeSO4 solution, a brown colour is formed.
4- Bromine water: only condensed tannin gives ppt. with bromine water.
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Quantitative determination of tannin
1- Hide powder method:The difference in the dry weight of the
extract before and after treatment with hide powder is taken as a measure for tannin content.
2- Copper acetate method:Gravimetric method using copper acetate
as precipitant.04/09/23 10Dr. Harish Kakrani & Purvi
Kakrani
Uses of Tannins
Industrially; in leather tanning, ink manufacture.
Medicinally; astringent, haemostatic, antidiarrheal, antidote in alkaloid and heavy metals toxicity.
In labs; reagent for detection of proteins, alkaloids and heavy metals because of their precipitating properties.
(pathological outgrowth) resulting from the deposition of the eggs of Cynips gallae tinctoria F. Cynipidae on the young twigs of Quercus infectoria F. Fagaceae. Collected before the escape of the insect and known as blue galls.
Morphology: subspherical, 12-20 mm in diameter, external surface is bluish green to olive green in colour, showing blunt short projections. Internally it is buff in colour.