Condensed tannin

Condensed tannins (proanthocyanidins, polyflavonoid tannins, catechol-type tannins, pyrocatecollic type tannins, non-hydrolyzable tannins or flavolans) are polymers formed by the condensation of flavans. They do not contain sugar residues.

They are called proanthocyanidins as they yield anthocyanidins when depolymerized under oxidative conditions. Different types of condensed tannins exist, such as the procyanidins, propelargonidins, prodelphinidins, profisetinidins, proteracacinidins (from mesquitol), proguibourtinidins or prorobinetidins. All of the above are formed from flavan-3-ols, but flavan-3,4-diols, called (leucoanthocyanidin) also form condensed tannin oligomers, e.g. leuco-fisetinidin form profisetinidin, and flavan-4-ols form condensed tannins, e.g. 3',4',5,7-flavan-4-ol form proluteolinidin (luteoforolor). One particular type of condensed tannin, found in grape, are procyanidins, which are polymers of 2 to 50 (or more) Catechin units joined by carbon-carbon bonds. These are not susceptible to being cleaved by hydrolysis.

While many hydrolyzable tannins and most condensed tannins are water-soluble, several tannins are also highly octanol-soluble. Some large condensed tannins are insoluble. Differences in solubilities are likely to affect their biological functions.

Tannins of tropical woods tend to be of a catechin nature rather than of the gallic type present in temperate woods.

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