tannines

Tannins, or tannines, are a diverse group of plant-derived polyphenolic compounds known for their ability to bind proteins and other macromolecules. They contribute astringency to many foods and beverages and occur in seeds, bark, wood, leaves, and fruit skins. Chemically, tannins are commonly classified into hydrolyzable tannins, which include gallotannins and ellagitannins, and condensed tannins, also called proanthocyanidins. Hydrolyzable tannins can be hydrolyzed to sugars and phenolic acids such as gallic or ellagic acid, whereas condensed tannins resist hydrolysis and consist of polymers of flavan-3-ol units.

Tannins occur across many plant lineages and are especially abundant in chestnut and oak woods, gall nuts, tea (Camellia sinensis), grape skins and seeds, and various bark extracts. Their biosynthesis follows the phenylpropanoid pathway, with gallotannins formed from gallic acid units and ellagitannins built from hexahydroxydiphenic acid; condensed tannins are polymers of catechin and related flavan-3-ols.

Biological roles and uses: In plants, tannins function as defenses against herbivores and pathogens and may protect against ultraviolet radiation. In humans, they influence taste and texture, notably in tea and wine, contributing to astringency and mouthfeel. Industrially, hydrolyzable tannins have a long history in leather tanning, and tannins also appear in dyes and inks.

Health and safety: Dietary tannins can act as antioxidants but may interfere with mineral absorption or protein digestibility at high levels. Some tannins can be toxic in very large quantities or with certain medications, though typical dietary exposure is considered safe for most individuals. Ongoing research examines potential health effects, including antimicrobial and anti-inflammatory properties.