Tannins

What are tannins?

Polyhydroxy phenolic compounds found in some plants.

Why was the term tannin originally used?

For plant substances that react with animal hide proteins, prevent putrefaction, and convert hide to leather.

What are the main physical properties of tannins?

-Dark brown or reddish-brown

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- non-crystalline

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- a puckering taste

Solubility of tannins

-Soluble: Water, alcohol, dilute alkalis, glycerol, and acetone (polar solvents)

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-sparingly soluble: chloroform, ethyl acetate and other organic solvents

(Tannins have polar hydroxyl groups)

List 6 chemical properties of Tannins

(i)Precipitation

(ii)Anti-oxidizing properties

(iii)Astringent

(iv)Carcinogenic

(v)Reaction with salts

(vi)Reaction with potassium ferricyanide and ammonia

What substances can tannins precipitate?

Gelatin, alkaloids, glycosides, heavy metals, and proteins.

Precipitation of Tannins explain

-React with animal hide to form leather (tans leather)

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-React with alkaloids to form Tannates, most of which are insoluble in water (antidote for alkaloid poisoning)

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-React with gelatin and isinglass to form an insoluble compound

Why do tannins have antioxidant properties?

Their small nuclei contain many accumulated OH groups which donates electrons or H atoms to neutralize free radicals thereby reducing their harmful effect

How do tannins produce an astringent effect?

They react with proteins in human mucous membranes resulting in precipitation

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-Therefore, tightening effect shrinks and protects underlying tissue

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-used medically to dry up secretion/soothe inflamed surface

Carcinogenicity

prolonged use of plant materials containing Tannis results in cancer as polyphenols cause cellular irritation

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-Areca catechu can cause oral and esophageal cancer

Reaction of Tannins and salts

Hydrolysable tannins:

-Due to gallic or ellagic acids; hydrolysable tannins form dark complex binds with iron

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-Condensed tannins

Potassium ferricyanide and ammonia with tannins

The phenolic hydroxyl groups of the tannins reduce the ferricyanide compound, and the resulting complex changes the color of the liquid dramatically, signaling that polyphenolic tannins are present.

What are the medicinal uses of tannins?

-Antidote: They act as an antidote for alkaloidal poisoning; prevent the stomach and intestines from absorbing toxic plant alkaloids

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-Antiseptic: They help protect tissues from infection by precipitating microbial proteins

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-Algicides: They can destroy or inhibit algae

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-Astringents: By precipitating the proteins in mucous membranes and damaged tissue, they shrink and tighten tissues, helping to stop bleeding, reduce secretions, and soothe inflammation

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-Anti-carcinogenic: Certain tannins exhibit properties that can inhibit cancer development.

Biological Activities in the Body

• -Inhibition of Lipid Peroxidation: They act as strong antioxidants that stop fats from oxidizing, protecting cells from oxidative stress.

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• Decrease in Blood Urea Nitrogen (BUN): They can help lower BUN levels, as lower levels indicate great marker kidney function

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• Inhibition of Plasmin: They can inhibit plasmin, an enzyme responsible for dissolving blood clots

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• -Lipolysis in Fat Cells: They stimulate the breakdown of fats (lipolysis) in fat cells(used for energy.

What are the industrial uses of tannins?

• Ink Manufacture: Historically and commercially, tannins (like those from oak galls) are combined with iron salts to create dark iron-gall inks

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• Vegetable Tanning: Converting raw animal hides and skins into durable, non-putrefying leather

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• Preservatives: Their ability to bind proteins and act as antioxidants allows them to be used to preserve various materials.

Function of tannins in plants

• Energy Source: They serve as an internal energy source for the plant due to their high oxygen content

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• Plant Protection: They act as natural plant antiseptics and defense mechanisms. Their bitter taste and puckering texture discourage herbivores and insects from eating the plant

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• Respiration: They may play a mechanical role in respiratory activity by assisting with hydrogen transfer inside plant cells

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• Flavor and Consumer Acceptance: They heavily influence the taste profile, color, and mouthfeel of many foods and beverages we consume, such as tea, cocoa, wine, and fruit.

How are tannins chemically classified?

Based on the identity of their phenolic nuclei and how they are joined.

Chemical Classification

What are hydrolysable tannins?

Tannins that are hydrolyzed by enzymes or acids.

What are the precursors of hydrolysable tannins?

Gallic acid, ellagic acid, and a glucose residue.

What type of linkage occurs between phenolic acids and glucose in hydrolysable tannins?

An ester linkage.

What are the two types of hydrolysable tannins?

Gallitannins and ellagitannins.

Where do gallitannins occur?

Rhubarb, clove, and Hamamelis.

What are the main properties of gallitannins?

They are rapidly soluble in water, and free gallic acid can form gluco gallitannins.

What are the main properties of ellagitannins?

They are slowly soluble in water and occur as open and ring forms of hexahydroxy diphenic acid.

What are the precursors of condensed tannins?

Flavonoids, catechin, and flavonol-3,4-diol.

What happens to condensed tannins when heated with acids?

They self-condense and polymerize into insoluble red complexes called phlobaphenes.

What are examples of plants containing condensed tannins?

Hamamelis (witch Hazel), Cinchona, and cinnamon.

What are complex tannins and egs

-Mixtures of hydrolysable and condensed tannins

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-Tea, Quercus, and Castanea.

How are tannins determined quantitatively?

By adsorption on standard hide powder.

How are tannins detected qualitatively?

By the Gold beater's skin test.

Gold beater’s skin test

membrane prepared from intestine of Ox that behaves similarly to untanned skin

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-Soak a small piece of Gold beater’s skin in 2% hydrochloric acid

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- Rinse it with distilled water

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- Place it in solution to be tested for 5 minutes

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-Wash in water and transfer to 1% solution of ferrous sulphate

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Black or brown color of skin indicates presence of tannins.

What indicates a positive Gold beater's skin test?

Black or brown coloration of the skin.

two classification of complex tannins and explain

What is Hamamelis?

Witch hazel, botanically known as Hamamelis virginiana.

What part of Hamamelis is used?

Dried leaves.

What are the main constituents of Hamamelis?

Gallitannins, ellagitannins, free gallic acid, pro-anthocyanidins, and hamamelase.

What are the uses of Hamamelis?

Astringent, anti-hemorrhagic, anti-inflammatory, and anti-carcinogenic.

What is Catechu and what is its botanical name?

Catechu; Uncaria gambir.

What part of Catechu is used?

Dried aqueous extract from leaves and young twigs.

How is Catechu prepared?

Leaves and twigs are boiled in water, evaporated to a paste, cubed, and sun-dried.

What are the uses of Catechu?

Astringent; gargle for sore throats and inflamed gums; and for diarrhoea and dysentery.

What is Nutt gall and what is its botanical name?

Nutt gall; Quercus infectoria.

What family does Nutt gall belong to?

Fagaceae.

What part of Nutt gall is used?

Galls obtained from twigs.

How are Nutt galls formed?

They are pathological outgrowths caused by eggs deposited by Adleria galactinctoria.

What are the main constituents of Nutt gall?

Tannic acid, gallic acid, ellagic acid, roburic acid, syringic acid, and calcium oxalate crystals.

What are the uses of Nutt gall?

Haemostatic, antidote for alkaloidal poisoning, and astringent.

Which part of cranberries is used?

The fruit.

What are the botanical names of the cranberries listed?

Vaccinium erythrocarpum, V. macrocarpon, V. microcarpum, and V. oxycoccos.

How may cranberry tannins help prevent urinary tract infections?

They help prevent E. coli from adhering to urinary tract walls.

Which other berries contain tannins?

Strawberries and blackberries.

Where are grape tannins found?

In the skins, seeds, and stems of wine grapes.

Why do red wines generally contain more tannins than white wines?

Their grape skins remain in contact with the juice longer, allowing more tannins to dissolve.

What are the listed botanical names of wine grapes?

Vitis amurensis, V. labrusca, V. mustangensis, V. riparia, and V. rotundifolia.

What are the listed medicinal benefits of wine grapes?

They are high in vitamins, antioxidants, and fiber.