Fractionation of Red Wine Polymeric Pigments by Protein Precipitation and Bisulfite Bleaching

Bovine serum albumin (BSA) precipitates tannins from red wine and also removes some of the red pigments. The pigments that bind to BSA are not released from the precipitate by washing and they are stable in the presence of bisulfite. Together these observations suggest that the pigments removed from wine by BSA precipitation are polymeric pigments. The pigments removed from wines by BSA do not account for all of the polymeric pigments in the wine. After removal of the precipitated pigments by centrifugation the supernatant fraction still contains pigments that are stable to bisulfite bleaching. Thus, protein precipitation fractionates the polymeric pigments into two distinguishable classes; large polymeric pigments (LPP) that precipitate along with the tannins, and small polymeric pigments (SPP) that do not. The number that best expresses the relative amounts of the two classes of polymeric pigment distinguished by protein precipitation is the LPP/SPP ratio. This ratio was found to be highly variable in 454 commercial red wines and could vary by more than a factor of 20 even in wines from a single variety (Cabernet Sauvignon). Composition of must and conditions during fermentation favor formation of LPP compared to SPP, and during barrel aging LPP is also preferentially formed compared to SPP.