Scientific research

Dietary polyphenols show a great diversity of structures, ranging from rather simple molecules (monomers and oligomers) to polymers. Higher-molecular-weight structures (with molecular weights of > 500) are usually designated as tannins, which refers to their ability to interact with proteins. Among them, condensed tannins (proanthocyanidins) are particularly important because of their wide distribution in plants and their contributions to major food qualities. All phenolic compounds are highly unstable and rapidly transformed into various reaction products when the plant cells are damaged (for instance, during food processing), thus adding to the complexity of dietary polyphenol composition. The polyphenol composition of plant-derived foods and beverages depends on that of the raw material used but also on the extraction process and subsequent biochemical and chemical reactions of plant polyphenols. The occurrence of specific tannin-like compounds (ie, thearubigins and theaflavins) arising from enzymatic oxidation is well documented in black tea. Various chemical reactions involving anthocyanins and/or flavanols have been demonstrated to occur during red wine aging. Current knowledge regarding the reaction mechanisms involved in some of these processes and the structures of the resulting products is reviewed. Their effects on organoleptic and nutritional quality are also discussed.

On astringency: [the] conversion of grape tannins to new products, particularly tannin-anthocyanin adducts, is generally reported to reduce wine astringency (Somers, 1971*). This was classically ascribed to an increase in molecular weight, because larger tannins were thought to be insoluble and thus non-astringent. However, recent studies showed that higher molecular-weight proanthocyanidins are both soluble and more astringent than the oligomeric proanthocyanidins (Vidal et al, 2003). Consequently, the decrease in astringency observed during wine aging is likely to involve acid-catalyzed processes leading to lowermolecular- weight species, rather than polymerization reactions. However, the taste of polyphenol reactions products and the effect on astringency of incorporating anthocyanin units into a tannin structure remain to be investigated.

* Somers, T. (1971). The polymeric nature of wine pigments. Phytochemistry, 10, 2175-2186 (abstract not publicly available)