Tannins

Tannins are a diverse group of plant-derived polyphenolic compounds known for their astringent taste. They occur in many species and tissues, including bark, wood, leaves, fruits, and seeds. Chemically, tannins are defined by their ability to precipitate proteins and alkaloids from solution, a property that underpins both their ecological roles in defense and their industrial uses. Tannins are broadly divided into two main classes: hydrolyzable tannins and condensed tannins.

Hydrolyzable tannins consist of a sugar core (usually glucose) esterified with gallic or ellagic acid. They include gallotannins and ellagitannins and can be broken down by acids, bases, or enzymes to release sugars and phenolic acids. They tend to be more rapidly hydrolyzed and can contribute strong astringency and bitterness.

Condensed tannins, or proanthocyanidins, are polymers of flavan-3-ol units such as catechin and epicatechin. They resist hydrolysis and persist in plants and foods. Condensed tannins influence astringency, bitterness, and color stabilization in beverages like wine and tea, and they play roles in plant defense against herbivores and pathogens.

Occurrences and uses: Plants rich in tannins include oak, chestnut, sumac, cacao, tea, coffee, grapes, and many hardwoods. In industry, tannins are used in leather tanning, where they cross-link collagen in hides, and as preservatives and antioxidants in some foods and wines. Ecologically, tannins deter herbivory and microbial attack and can influence soil nutrient dynamics.

Health and safety: as dietary polyphenols, tannins can have antioxidant activity and may contribute to health benefits in moderation. They can also bind minerals and proteins, reducing digestibility, and very high intakes can cause adverse effects in sensitive individuals or livestock. Bioavailability and effects depend on tannin type, dose, and food matrix.

Summary: Tannins are a structurally diverse class of plant polyphenols with astringent properties that affect taste, texture, and chemistry in foods and industries. Their hydrolyzable and condensed forms differ in structure, reactivity, and tolerance to processing, but both contribute to plant defense and human use.