Identification of Factors that Influence the Level of Large and Small Polymeric Pigments in Grapes and Wines

During the 2001 season we set out to identify the most important factors that influence the level of large and small polymeric pigments in wines. Polymeric pigments are important because they are the stable color compounds that form during fermentation and wine aging. We found that Pinot noir wines made after cold soak had more polymeric pigment at the time of pressing but that they had less after a period of barrel aging. All wines in our experiments exhibited dramatic compositional changes from pressing until the first racking, showing a decline in tannin and a large increase in both large and small polymeric pigments. Experiments with maximum fermentation temperatures showed that temperature probably plays the biggest role in tannin extraction and polymeric pigment formation. Three temperatures were used, 25C, 30C and 33C. Tannin, large polymeric pigment and small polymeric pigment were all greater at 33C than 25C with 30C intermediate. In all cases there were declines in tannin levels during barrel aging but a concomitant increase in the amount of polymeric pigments that formed. It is clear that important compositional changes occur during barrel aging with regard to tannins and polymeric pigments. It will be important to follow all of the wines made this season through to the level of finished wine in order to adequately evaluate all of the treatment effects imposed on the experimental wines.

Preliminary work indicated that polymeric pigments bleach to some extent with SO2. It was important to determine the extent to which they bleach so that correction factors could be derived for calculating the actual amount of polymeric pigments in wines. This is particularly important in aged wines (3 years or greater) where nearly all of the pigments can be present in the polymeric form. Using aged Pinot noir and Cabernet Sauvignon wines (8 to 28 years old) that contained no monomeric pigments we performed experiments using column purified polymeric pigments to show that they were indeed bleached extensively by SO2. Using combined protein precipitation and SO2 bleaching we were able to derive correction factors that we can now use to more accurately calculate the amount of polymeric pigments present in grape extracts and wines.

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