Chemical Characterization of Small Polymeric Pigments in Wines and Red Grape
Polymeric pigments are important because they are the stable form of color in wines. They are thought to be formed by reaction of monomeric anthocyanin pigments with tannins or flavan-3-ols, such as catechin or epicatechin. During the 1999 season we observed a class of low molecular weight polymeric anthocyanin pigments in wines. In new wines these pigments were a large percentage of the anthocyanin color not bleached by SO2, thus classifying them as polymeric pigments. On the other hand they were not precipitated by protein, which suggested that they had a low molecular weight compared to typical tannins. This observation raised many practical questions, but also brought up several important questions regarding the chemical nature of the small polymeric pigments (SPP), which we addressed in this project during the 2000 season. One of the objectives of the work was to devise a purification scheme that would permit separation of small polymeric pigment (SPP) from monomeric anthocyanins and large polymeric pigment (LPP). We were successful in developing a procedure to purify SPP based on column chromatography on a Toyopearl HW-40(F) column and eluting the monomeric anthocyanins and the tannin fraction separately with different solvents. Using standard SO2 bleaching to assay polymeric pigments, we were able to show that the SPP actually resides in the monomeric fraction from the column. In the course of this work we discovered that the SPP could be partially bleached by SO2. This was an unexpected result, but indicates that further work needs to be done with polymeric pigments to determine the extent to which they are affected by SO2 bleaching. This is important because most assays for polymeric pigment rely on SO2 bleaching to distinguish them from monomeric anthocyanins. If polymeric pigments are indeed bleached by SO2, it means that such assays give an underestimate of the amount present, and that monomeric anthocyanins are overestimated. A major accomplishment in this work during the past year has been the synthesis of an SPP dimer containing catechin as the extension unit and malvadin-3-glucoside as the terminal unit. The availability of this dimer by an unambiguous chemical synthetic route will enable us to confirm the structure of the naturally occurring dimer found in grape skin extracts and wine. The availability of the synthetic dimer will also enable us to determine if anthocyanin pigments having that configuration (catechin-malvadin-3-glucoside) are affected by SO2 bleaching. The chemical reaction by which we created the dimer may also point the way to understanding how polymeric pigments are formed in wines during aging.