Tannins are water-soluble polyphenols found across a wide range of vascular plants, concentrated in bark, heartwood, leaves, fruit skins, and seed coats. They are broadly divided into two chemical classes: hydrolysable tannins (esters of gallic or ellagic acid with a polyol core, typically glucose) and condensed tannins, also called proanthocyanidins, which are oligomeric or polymeric flavan-3-ol chains. A third, smaller group , phlorotannins , occurs exclusively in brown algae.
The defining chemical property of tannins is their affinity for proteins. They form hydrogen bonds and hydrophobic interactions with proline-rich peptide sequences, causing precipitation or conformational change. This is the same mechanism responsible for the dry, puckering sensation of unripe fruit or strong black tea on the palate. Molecular weights across the class range widely, from roughly 500 Da for simple gallotannins to over 20,000 Da for some condensed forms.
In botanical ecology, tannins function primarily as defensive compounds , deterring herbivory and resisting microbial degradation of plant tissue. High tannin concentrations in oak galls (Quercus spp.) and in the bark of Acacia mearnsii (black wattle), a species commercially significant in Australia, reflect this role. The astringency tannins impart to foods such as persimmons, pomegranates, and wine grapes is a direct consequence of their protein-binding activity acting on salivary proteins.
Research into tannin chemistry is ongoing across food science, phytochemistry, and materials science. Studies examine their behaviour in the digestive tract, their interactions with dietary minerals such as iron, and their roles in plant-based food processing. As with all phytochemical research, findings from in vitro and animal studies require careful interpretation and do not directly establish effects in humans; no health claims are made here.
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