Tannins are a group of complex polyphenolic compounds found in many plants. They have the ability to form strong complexes with proteins, polysaccharides, alkaloids, and other macromolecules. Tannins are known for their astringent taste and their ability to precipitate proteins, which is why they are often used in tanning leather and in the clarification of wine and beer.
Classification of Tannins
Tannins are classified into two main groups based on their chemical structure and the nature of their hydrolytic breakdown products:
1. Hydrolyzable Tannins: These tannins are esters of gallic acid (gallotannins) or ellagic acid (ellagitannins) with a sugar core, usually glucose. Hydrolysis yields gallic acid or ellagic acid along with glucose.
Examples: Tannic acid, Punicalagin.
2. Condensed Tannins (Proanthocyanidins): These tannins are polymers of flavan-3-ols, such as catechin and epicatechin. They do not hydrolyze easily and form insoluble precipitates. Hydrolysis yields anthocyanidins.
Examples: Procyanidin, Prodelphinidin.
Properties of Tannins
Tannins exhibit a range of physical and chemical properties influenced by their complex structure:
1. Solubility: Tannins are generally soluble in water, alcohol, and acetone. They are insoluble in non-polar solvents like benzene and chloroform.
2. Astringency: Tannins have a characteristic astringent taste due to their ability to precipitate proteins.
3. Complex Formation: Tannins can form strong complexes with proteins, polysaccharides, alkaloids, and heavy metals. This property is utilized in leather tanning and in medicinal applications to reduce the absorption of toxins.
4. Antioxidant Activity: Tannins exhibit strong antioxidant properties due to their polyphenolic structure.
5. Color: Tannins can impart color to solutions, ranging from yellow to dark brown, depending on their concentration and structure.
6. Precipitation: Tannins can precipitate proteins and alkaloids, which is a basis for several of their biological activities and identification tests.
Tests for Identification
Several tests can be used to identify the presence of tannins based on their chemical reactivity and ability to form complexes:
1. Ferric Chloride Test:
Procedure: Add a few drops of ferric chloride solution to the tannin solution.
Positive Result: Formation of a blue-black or green-black color indicates the presence of tannins.
2. Gelatin Test:
Procedure: Add a 1% solution of gelatin containing 10% sodium chloride to the tannin solution.
Positive Result: Formation of a white precipitate indicates the presence of tannins.
3. Goldbeater’s Skin Test:
Procedure: Soak a piece of goldbeater’s skin (ox skin) in 2% hydrochloric acid, rinse with distilled water, and place it in the tannin solution for a few minutes. Then rinse it again with distilled water and immerse it in a 1% ferrous sulfate solution.
Positive Result: Formation of a blue-black or green-black color on the skin indicates the presence of tannins.
4. Lead Acetate Test:
Procedure: Add a 10% lead acetate solution to the tannin solution.
Positive Result: Formation of a white precipitate indicates the presence of tannins.
5. Stiasny’s Test:
Procedure: Heat the tannin solution with formaldehyde and hydrochloric acid.
Positive Result: Formation of a precipitate indicates the presence of condensed tannins.
6. Phenazone Test:
Procedure: Add a 2% phenazone solution to the tannin solution, followed by sodium chloride solution.
Positive Result: Formation of a bulky precipitate indicates the presence of tannins.
7. Matchstick Test:
Procedure: Dip a matchstick in the tannin solution and dry it. Then moisten it with concentrated hydrochloric acid and warm it over a flame.
Positive Result: Formation of a pink or magenta color indicates the presence of condensed tannins.
8. Vanillin-HCl Test:
Procedure: Mix the tannin solution with vanillin and hydrochloric acid.
Positive Result: Formation of a red or pink color indicates the presence of condensed tannins.
These tests help in the identification and confirmation of tannins in various plant extracts and products.
