proanthocyanidins

Proanthocyanidins, or condensed tannins, are a class of polyphenolic compounds formed by the polymerization of flavan-3-ol units such as catechin and epicatechin. They occur as oligomers and polymers with a wide range of degrees of polymerization, and their properties depend on polymer size and linkage patterns rather than on a single structure.

The chemistry centers on carbon–carbon and, in a subset known as A-type, carbon–oxygen–carbon linkages between flavan-3-ol units. The most common bonds are C4–C8 or C4–C6 (B-type); A-type includes an additional ether bond such as C2–O–C7. Terminal and extender units determine composition and mDP, the mean degree of polymerization.

Proanthocyanidins occur widely in the plant kingdom, particularly in grapes, apples, berries, cocoa, tea, and legumes. In grape and wine chemistry, they influence astringency, mouthfeel, and color stability in conjunction with anthocyanins and pigments. They also contribute to bitterness and antioxidant capacity in foods and supplements.

Biosynthesis draws on the flavonoid pathway in plants; polymerization is largely a storage phenotype in seeds and tannin-rich tissues. Orally ingested proanthocyanidins are not uniformly absorbed; high-DP polymers are largely degraded by gut microbiota to smaller phenolic metabolites that may be absorbed and exert systemic effects.

From a nutritional perspective, proanthocyanidins display antioxidant activity in vitro and are investigated for anti-inflammatory and cardioprotective effects, but human evidence remains inconclusive. They can interact with proteins and minerals, potentially reducing iron and mineral absorption when consumed with meals.

Analytical methods such as thiolysis, phloroglucinolysis, and various spectroscopic approaches are used to determine unit composition, linkage type, and mean degree of polymerization. Practical applications include beverage quality assessment, dietary studies, and quality control in products rich in tannins.