Condensed Tannin Reacts with SO2 during Wine Aging, Yielding Flavan-3-ol Sulfonates

Numerous monomeric and oligomeric flavanol sulfonation products were observed in 10 wines. Levels of 0.85-20.06 and 0-14.72 mg/L were quantified for two monomeric sulfonated flavan-3-ols and, surprisingly, were generally higher than the well-known native flavan-3-ol monomers. Increasing SO2 levels during wine aging increased the sulfonate-modified flavan-3-ol monomers and dimers along with higher concentrations of native monomers. The results indicate that >10% of SO2 is reacting with the C-4 carbocation, formed from acid cleavage of the interflavan bond, perhaps by a bimolecular SN2-type reaction, and as a reducing agent. In addition, the high SO2 wine had the lowest protein-binding tannin levels, tannin activity, and mean degree of polymerization (mDP), and acidic SO2 treatment of condensed tannin abolishes protein binding. Thus, SO2 changes tannin composition during wine aging, and the substantial formation of sulfonate-modified flavan-3-ols may provide an additional explanation for the reduction in astringency of aged red wines.