Procyanidins heterogeneous polymerization

Fig. 6.27. Diagram of heterogeneous polymerization of procyanidins (P) in the presence of ethanal (Glories, 1987)...

In wine, this type of reaction occurs at the same time as the heterogeneous polymerization of the procyanidins (Section 6.3.7), as a result of the controlled oxidation during barrel aging, when traces of ethanal are produced by the oxidation of ethanol. The color of the wine becomes more intense and changes tone, becoming darker after a few months in the barrel. 

These results may be compared with viscosities obtained in a similar way from conifer bark extracts which, while heterogeneous, contain polymeric pro-cyanidins or mixed polymeric procyanidins and prodelphinidins as their predominant components (2). For example, Weissman (25) reported a viscosity of 65 mPa-s for a 30% solution of the water extract from Pinus oocarpa bark, and Dix and Marutsky (26) obtained a value of 31 mPa-s for a similar solution from Picea abies bark. These viscosities are similar to those observed for the 30% procyanidin polymer solutions. They indicate that the viscosities of these bark extract solutions are dominated by the proanthocyanidins and that there is little influence from any accompanying polysaccharides-as already suggested by Weissmann (25)-in contrast to wattle extracts where gums play an important role in determining solution viscosities (7). 

The tannins in red wine are made up of chains of more-or-less polymerized fiavanols (procyanidins). These procyanidins are either homogeneous, with regular linking, or heterogeneous, linked by different bonds (Section 6.3.7). In both cases, certain bonds are broken when these molecules are heated in an acid medium, and the resulting carbocations are partially converted into cyanidin if the medium is sufficiently conducive to oxidation. This property has been used in tannin assays (FA method) for many years (Ribereau-Gayon and Stonestreet, 1966). 

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