2. TANNINS
Definition:
Tannins are naturally occurring complex organic compounds
possessing nitrogen free polyphenols of high molecular
weight.
Tannin, also called tannic acid, any of a group of pale-
yellow to light-brown amorphous substances in the form of
powder, flakes, or a spongy mass, widely distributed in plants
and used chiefly in tanning leather, dyeing fabric, making ink,
and in various medical applications.
They form colloidal solution with water giving acid reactions.
They also precipitate proteins and alkaloids.
The astringent in nature of tannins is due to the fact that they
can precipitate proteins and render them resistant to
enzymatic attack.
When applied on a wound or injury, tannins form a protective
3.
4. The tannin compounds can be divided into two major groups on
the basis of Goldbeater’s skin test.
A group of tannins showing the positive tanning test may be
regarded as true tannins
whereas those, which are partly retained by the hide powder and fail
to give the test, are called as pseudotannins.
Most of the true tannins are high molecular weight compounds.
These compounds are complex polyphenolics which are produced
by polymerization of simple polyphenols
They may form complex glycosides or remains as such which may
be observed by their typical hydrolytic reaction with the mineral
acids and enzymes.
Two major chemical classes of tannins are usually recognized
based on this hydrolytic reaction and nature of phenolic nuclei
involved in the tannins structure.
The first class is referred to as hydrolysable tannins, whereas the
other class is termed as condensed tannins.
5. Hydrolysable tannins:
These tannins are hydrolyzed by acids, or enzyme and produce
gallic acid or ellagic acid.
On dry distillation condensed tannin produce pyrogallol
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 ellagitannins.
The gallic acid is found in rhubarb, clove and ellagic acid is
found in eucalyptus leaves, myrobalans and pomegranate
bark.
These tannins treated with ferric chloride to produced blue or
black colour.
Condensed tannins or Non-Hydrolysable or Phlobatannins:
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 polymerised or
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.
6. Pseudo tannins:
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.
PROPERTIES OF TANNINS:
Tannins are freely soluble in water, alcohol, glycerol, and
acetone and dilute alkalis.
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
to form an insoluble compound. They combine with alkaloids
to form tannates, most of which are insoluble in water.
7. “The complex organic non nitrogenous, poly
phenolic substances of high molecular weight.”
Medicinal & Biological Properties
Tannins are the drugs which can stop hemorrhage
They precipitate proteins.
They are used as an astringent.
They are used in the treatment of burns.
They act as anti diarrheal.
They are employed as an antidote for heavy
metal poisoning.
They are most potent antioxidant.
8. CHEMICAL TESTS
Gelatin test:
To a solution of tannin, aqueous solution of gelatin and
sodium chloride are added. A white buff coloured precipitate
is formed.
Goldbeater’s skin test (Important test of tannins)
A small piece of goldbeater skin (membrane prepared from
the intestine of an ox) is soaked in 20% hydrochloric acid,
rinsed 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.
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.
9. 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
Chlorogenic acid test:
An extract of chlorogenic acid containing drug is
treated with aqueous ammonia.
A green colour is formed on exposure to air.
10.
11. CatechuSynonyms – Pale catechu, gambier, catechu
Biological source – It consists of the dried aqueous extract prepared from
the leaves of Uncaria gambier
Family - Rubiaceae
Chemical constituents –
Pale catechu contain from about 7 to 30% of pseudotannin catechin
and 22 to 55% of a phlobatannin catechutannic acid
It contains flavonoids like quercetin and fluorescent substances
Gambier fluorescein.
It also contains catechu- red, pyrogallol, fixed oil and waxes
Catechin forms white, needle like crystals, which dissolves in alcohol
and hot water. Catechutannic acid gives green colour with ferric chloride.
Uses –
Used as an astringent
Used in the treatment of diarrhoea
Large proportion of gambier is used in dyeing and tanning industries
12. BLACK CATECHU
Synonym: Cutch, black catechu, kattha
Biological source –It consists of the dried aqueous extract prepared
from heart wood of Acacia catechu
Family - Leguminosae
Chemical constituents –It contains about 2–12% of catechin and
about 25 to 33% of phlobatannin catechutannic acid.
The principle fraction of cutch has been identified as a mixture of
catechin isomers which includes (-)epicatechin, acacatechin, DL-
acacatechin, L-acacatechin and D-isoacacatechin
It contains flavonoids like quercetin.
Others like Catechu- red and gum
Uses –Used as an astringent
It cures troubles of mouth, diseases of the throat and diarrhoea.
In India and eastern countries, it is used in betel leaves for chewing
Marketed Products
It is one of the ingredients of the preparation known as Koflet lozenge
(Himalaya Drug Company) as cough expectorant, and Gum tone
(Charak Pharma Pvt. Ltd.).
13. CHEMICAL TESTS FOR CATECHU
1. Gambir Fluorescin test-Alcoholic extract of drug, few drops
of sodium hydroxide
Mix and add petroleum ether, shake and kept aside for few
minutes – petroleum ether layer shows green fluorescence (
+ ve for pale catechu due Gambir Fluorescin )
2. Match stick test ( Catechin test) –A match stick is dipped in
aqueous plant extract , dried near burner and moistened with
Hcl
On warming near flame, the match stick wood turns pink or
red due to the formation of phloroglucinol
3. Vanillin–Hydrochloric acid test –Test solution and few
drops of Vanillin–Hydrochloric acid reagent – A red or pink is
formed due the formation of phloroglucinol
4. Chlorophyll test -Powdered drug is heated with chloroform
on a water bath for 1-2 minutes .The organic layer is filtered
in a china dish and evaporated on the water bath– green
14. Pterocarpus
Synonyms-bijasal, Indian kino tree, Malbar kino
Biological source- it can consist dried juice obtained by
making vertical incisions to the stem bark of the
plant Pterocarpus marsupium belonging to family
leguminosace.
Geographical source-Gujrat, Madhya pradesh, uttar
pradesh, bihar
Chemical constituents-
70% to 80% kinotannic acid , kino red , k-pyrocatechin
Kinotannic acid is glucosidal tannin, whereas kino-red is
anhydride of kinoin. Kinoin is an insoluble phlobaphene
Chemical test-drug treated with ferrous sulphate shows
green colour. Drug + alkali shows violet colour. Mineral
acid shows ppt.
Use-– it is used as astringent of diarrhoea and local
