Nonhydrolyzable tannins

Nonfoaming surfactants Nonhydrolyzable tannin polymer functional 123 ... 

Nonhydrolyzable or condensed tannins are also named proanthocyanidins. These are polymers of flavan-3-ols, with the flavan bonds most commonly between C4 and C8 or C6 (Figure 6-23) (Macheix et al. 1990). Many plants contain tannins that are polymers of (+)-catechin or (-)-epicatechin. These are hydrogenated forms of flavonoids or anthocyanidins. Other monomers occupying places in condensed fruit tannins have trihydroxylation in the B-ring (+)-gallocat-echin and (-)-epigallocatechin. Oligomeric and polymeric procyanidins are formed by addition of more flavan-3-ol units and result in the formation of helical structures. These structures can form bonds with proteins. 

The B- and A-type procyanidins and proanthocyanidins (condensed or nonhydrolyzable tannins, Fig. 6.4) are polymers of flavan-3-ols present in the skin and seeds of the grape berry. In winemaking, they are transferred to the wine, and the sensorial characteristics of astringency and bitterness of wine are linked to the galloylation degree (DG) and degree of polymerization (PD) of flavan-3-ols (Cheynier and Rigaud, 1986 Vidal et al., 2003). 

The most important members, however, are proanthocyanidin polymers, also called nonhydrolyzable or condensed tannins. Proanthocyanidin polymers exist as chains of C-4-C-8 (or C-6) linked flavan-3-ol units (Fig. 318). The monomer unit may be based on either of two stereochemistries designated as cis and trans and on either of two B-ring hydroxylation patterns, the procyanidin (PC) and the prodelphinidin (PD) units. Thus the polymer chains are characterized by the average stereochemistry and the PC PD ratios (Table 56). 

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