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

One of the objectives of the work during the 2002 season was to establish the relationship between values for tannin obtained in a protein precipitation assay using bovine serum albumin (BSA) and the amount of tannin that actually binds to salivary proteins. We found that the quantity of tannin in a wine that binds to BSA proteins and the amount that binds to human salivary proteins are virtually the same. Our work also showed that salivary proteins are much more efficient at binding tannin from wine than is the BSA protein. We found that salivary protein is about three times as efficient as BSA in binding tannins. We also found that the protein concentration in saliva from different donors can vary as much as 4 fold. Since people also have different salivary flow rates, the tannin binding capacity of different people vary greatly.Another objective was also to study how polymeric pigments, formed by the reaction of anthocyanins with tannins, bind to salivary proteins as compared to BSA. In this case we found that BSA and salivary proteins bind slightly different amounts of polymeric pigment, but have essentially the same efficiency of binding. Furthermore, salivary proteins bind tannins 12 times more efficiently than they bind polymeric pigments. This result suggests several possibilities. It may be that polymeric pigments are not simply typical tannins with anthocyanins attached. An alternative interpretation could be that anthocyanin reaction with tannin to form polymeric pigments drastically reduces the tannin molecule?s ability to bind to protein. Since reaction of anthocyanins with tannins to form polymeric pigments is one of the major changes that occurs during red wine aging, this may provide an explanation for why tannins become less astringent with aging. This would be because reaction of anthocyanin with tannin to form polymeric pigments reduces the ability of the tannin to bind to proteins and elicit the astringent sensation in the mouth.In studying the binding of tannins to salivary protein we found two different patterns of binding among the saliva donors in our study. Some people?s saliva exhibited typical linear binding while others showed cooperative tannin binding. This could reflect cooperative binding among the different proteins present, or might be restricted to the predominant species found in the saliva. The important point for our work is that regardless of the characteristics of tannin binding by different individuals, BSA binds the same amount of tannin as salivary proteins when protein is in excess. Since in our tannin assay the protein concentration is always in excess, we can be confident that regardless of the mode of binding we are able to measure the amount of tannin in grape extracts and wines that is responsible for astringency.