Tannins

Phenolic compounds and tannins: Chemical nature and test for tannins some taruin
containing drugs such as nutgall and catechu.

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Nuzhat Tasnim Amin, Lecturer, Dept. of Pharmacy, Varendra University

Tannins

Tannin
Tannin is a type of biomolecule, is an astringent, bitter plant polyphenolic compound that binds
to and precipitate proteins and various other organic compounds including amino acids and
alkaloids.
Biomolecule is a molecule that is produced by a living organism. There are four major classes of
Biomolecules – Carbohydrates, Proteins, Nucleic acids and Lipids.
An astringent substance is a chemical compound that tends to shrink or constrict body tissues.
Polyphenolic compounds are usually referred to as a diverse group of naturally
occurring compounds containing multiple phenolic functionalities.

Properties of Tannins
1. Many of the tannins are glucosidal in nature. The gallotannins on hydrolysis yield a sugar
and gallic acid.
(A glucoside is a glycoside that is derived from glucose)
(Gallotannins are polymers formed when gallic acid, a polyphenol monomer, esterifies and binds
with the hydroxyl group of a polyol carbohydrate such as glucose)
2. Tannins are non-crystalline substances, occurs as mixture of polyphenols and form
colloidal solution with water.
(In a colloidal solution, the units are bigger and they don't liquefy.)
3. Their aqueous solution is acidic in nature and possess sharp puckering taste.
(Pucker taste that is, a bitter or astringent taste)
4. They are precipitated with solution of gelatin and alkaloids.
5. The protein tannin complex is resistant to proteolytic enzymes. This property is known as
astringent action.
6. Tannins are freely soluble in water, alcohol, glycerol, acetone and dilute alkalis.
7. They are sparingly soluble in chloroform, ethyl acetate and some other organic solvents.


Use of Tannins
1. Tannin containing drugs are used in medicine as astringent.

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Nuzhat Tasnim Amin, Lecturer, Dept. of Pharmacy, Varendra University

(Astringent medicine used to reduce swollen mucous membranes that result from inflammations of
the nasal, gastrointestinal, and urinary passages, astringents are also frequently employed to dry
up excessive secretions and to stop bleeding.)
2. Tannins are used in the tanning process of animal skin to convert them into leather.
3. Aqueous solution of tannins is used to precipitate gelatin, proteins, alkaloids in the
laboratory.
4. They are used as healing agents in inflammation, gonorrhea.
(Gonococcal infection, also known as gonorrhea is a common sexually transmitted infection caused
by the bacterium Neisseria gonorrhoeae and burns.)
5. It is also used as antidote in the treatment of alkaloid poisoning.
6. The deep red color complexes of tannins with iron salts are used to manufacture inks.

Classification of Tannins


Tannins are classified as
1. Pseudo tannins: Pseudo tannins are phenolic compounds of lower molecular weight and
they do not show the goldbeater’s skin test. They occur as gallic acid, catechin, chlorogenic
acid.
2. True Tannins: These are complex phenolic compounds of high molecular weights ranging
from about 1000 to 5000. They display the general properties of tannins and precipitated
by gelatin in 1% solution. They can be sub-classified into hydrolysable tannins,
condensed tannins and complex tannins.
i. Hydrolysable tannins: These tannins are esters of a sugar, usually glucose with one
or more trihydroxy benzene carboxylic acids. They are hydrolyzed by acids or enzymes

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Nuzhat Tasnim Amin, Lecturer, Dept. of Pharmacy, Varendra University

to yield several molecules of phenolic acids such as gallic or ellagic acids. The phenolic
acids are combined to a central glucose residue by ester linkage.
ii. Condensed tannins: Condensed tannins are resistant to hydrolysis and they are derived
from the flavanols, catechins and flavon-3, 4- diols. These tannins are called catechol
tannins.
iii. Complex tannins: Complex tannins are defined as tannins in which a catechin unit
(1.39) is bound glycosidically to either a gallotannins or an ellagitannin unit. As the
name implies, the structure of these compounds can be very complex.
a) Gallotannins: Gallotannins are polymers formed when gallic acid, a polyphenol
monomer, esterifies and binds with the hydroxyl group of a polyol carbohydrate such as
glucose.
b) Ellagitannins: The ellagitannins are a diverse class of hydrolysable tannins, a type of
polyphenol formed primarily from the oxidative linkage of galloyl groups in 1,2,3,4,6-
pentagalloyl glucose.

Chemical test of Tannins
1. Goldbeaters skin test: A small piece of goldbeater’s skin is soaked in 2% 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 color is produced on the skin due to the presence of tannins.
(Goldbeaters skin is the outer membrane of a calf's intestine, is a parchment traditionally used in
the process of making gold leaf by beating.
Gold leaf is gold that has been hammered into thin sheets by goldbeating)
2. Gelatin test: To a solution of tannin (0.5 -1%) aqueous solutions of gelatin 1% and sodium
chloride 10% are added. A white colored precipitate is formed.
(Gelatin is a translucent, colorless, brittle, flavorless foodstuff, derived from collagen obtained
from various animal by-products. It is commonly used as a gelling agent in food, pharmaceuticals,
photography, and cosmetic manufacturing)
3. Phenazone test: A mixture of aqueous extract 5 ml of a drug and sodium phosphate 0.5
gm is heated, cooled and filtered. A solution of phenazone 2% is added to the filtrate. A
bulky colored precipitate is formed.
4. Catechin test: A matchstick is dipped in aqueous plant extract, dried near burner and
moistened with concentrated hydrochloric acid. On warming near a flame, a matchstick
would turn pink or red due to formation of phloroglucinol.
5. Chlorogenic acid test: An extract of chlorogenic acid containing drug is treated with
aqueous ammonia. A green color is formed on exposure to air.
6. Vanillin- Hydrochloric acid test: Vanillin 1 g, alcohol 10 ml, concentrated HCl 10 ml.
When a drug is treated with vanillin hydrochloric acid reagent, pink or red color is formed
due to formation of phloroglucinol.

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Nuzhat Tasnim Amin, Lecturer, Dept. of Pharmacy, Varendra University

Catechu

Biological source: Acacia catechu.
Family: Leguminosae
Geographical source: The tree is a native of India and found in Burma.
Chemical constituents: Black catechu contains acacatechin, catechu tannic acid, gum and water.
It contains flavonoid like quercetin.


acacatechin

Uses:
1. Black catechu possesses cooling and digestive properties.
2. It is used in relaxed condition of throat and in cough and diarrhea.
3. Externally as an astringent medicament it is applied to ulcers, boils, and skin eruption and
in a number of medicinal preparations.


Nutgall

Botanical source: Quercus infectoria.
Family: Fagaceae
Geographical source: The plant is found in turkey, Syria, Cyprus and Greece.
Chemical Constituents: Galls contain tannin known as gallotannic acid (50-70 %), gallic acid (2-
4) %, ellagic acid, β-sitosterol, starch, calcium oxalate.

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Nuzhat Tasnim Amin, Lecturer, Dept. of Pharmacy, Varendra University



Uses:
1. Galls are used in the tanning and dyeing industry.
2. Used externally in the treatment of burn.
3. Used as an antidote for alkaloid poisoning.
4. In the manufacture of inks.