Tannins: Chemical Properties and Uses

Tannins

• Tannins are complex organic compounds with high molecular weight.
• They are derived from the French word "tanin" (tanning substances).
• They combine with proteins in animal hides, preventing decomposition and turning them into leather.
• Tannins are astringent due to their ability to precipitate proteins, making them resistant to enzymatic breakdown.

Sources of Tannins

• Beverages: Red wine, green tea
• Grains: Barley, chickpeas, beans, legumes
• Fruits: Strawberry, blueberry, pomegranate, grapes, apple, jamun, guava, orange
• Herbs and spices: Clove, cinnamon, cumin, catechu, vanilla, mint, basil
• Nuts: Walnut, almond, cashew, apricot
• Tannins are generated through the shikimic acid pathway, phenylpropanoid pathway, and flavonoid pathway.

Classification of Tannins

• True Tannins: High molecular weight compounds, positive Goldbeater's skin test
  • Hydrolysable Tannins: (Pyrogallol) hydrolyzed by acid, alkali, enzymes, and hot water. Produce gallic or ellagic acid.
  • Condensed Tannins: (Catechol) Polymerized flavan-3-ols, resistant to hydrolysis.
  • Complex Tannins: A combination of both.
• Pseudo Tannins: Low molecular weight compounds, do not give a strong positive reaction to the Goldbeater's skin test.
  • Includes: Gallic acid in rhubarb, catechins in catechu.

Uses of Tannins

• Antioxidant: Free-radical scavengers, prevent oxidation, but at high levels can be prooxidants/carcinogens.
• Antidiarrheal: Astringent, helps with heavy metal poisoning.
• Treatment of burns: Contract body tissue and precipitate proteins.
• Stop bleeding: Astringent use in hemorrhoids.
• Anti-inflammatory
• Anti-viral
• Mouthwashes, eyewashes, and bowel conditions

Tannins in Berries

• Proanthocyanidins in blueberries affect the striatum (memory), and Ellagitannins in strawberries reduce hippocampus aging effects on spatial orientations.
• Cranberry tannins prevent urinary tract infections by inhibiting E. coli adhesion and preventing kidney stones.
• Cranberries are a source of proanthocyanidins which have a protective effect against urinary tract infections.

Tannins in Herbal Teas

• Tannins can inhibit the absorption of iron in plants, particularly non-heme iron.
• Some herbal teas, notably higher-tannin ones, are believed to contain tannins which may inhibit iron absorption.

Green Tea vs Black Tea

• Both are from Camellia sinensis plant.
• Black tea leaves are rolled and exposed to air to cause oxidation (darker colour), enhancing flavours.
• Green tea isn't exposed to air to prevent oxidation (lighter colour).
• Black tea generally has a higher tannin level than green tea. More oxidisation means more tannins and polyphenols.

Witch Hazel (Hamamelis Virginiana)

• A source of tannin used in skin care, treating bee stings and abrasions.
• Used in mouthwashes, eyewashes, and some bowel conditions.

Tannin Facts

• Unripe fruits contain high levels of tannins, discouraging animal consumption until the seeds mature.
• Tannins are present in coffee, tea, red wine, and beer.
• Tannins contribute to the colours of flowers and autumn leaves.

This quiz explores the origins, characteristics, and applications of tannins. Delve into the differences between true tannins and pseudotannins, as well as their chemical reactions and medicinal uses. Test your knowledge on these fascinating compounds and their significance in chemistry.