Measurement of Polymeric Pigments in Grape Berry Extract sand Wines Using a Protein Precipitation Assay Combined with Bisulfite Bleaching

Abstract

By combining protein precipitation and traditional bisulfite bleaching to distinguish monomeric anthocyanins from polymeric pigments, two classes of polymeric pigments in grapes and wines were observed: small polymeric pigments (SPP) that do not precipitate with protein and large polymeric pigments (LPP) that do. The combined assay was used to study the polymeric pigment composition of Cabernet Sauvignon and Syrah berries during ripening and to compare the composition of the fruit at harvest with the wine made from that fruit. Although fruit of Cabernet Sauvignon and Syrah contained very little polymeric pigment compared to the corresponding wines, nearly all of the polymeric pigment found in fruit was SPP with very little LPP. The percentage of the color at pH 4.9 due to LPP was 7-fold higher in Syrah wines as compared to the corresponding fruit, whereas in Cabernet Sauvignon and Pinot noir the difference was greater than 2-fold. This suggests that most of the LPP found in wines is formed during winemaking. By contrast, the percentage of color due to SPP in wines compared to fruit was variable, decreasing in Syrah, increasing in Cabernet Sauvignon, with little change in Pinot noir. The combined protein precipitation/bisulfite bleaching analysis used in this study allows the parallel determination of tannins and polymeric pigments in grape extracts and wines.