Adaptation of a Simple Assay to Measure Tannins in Grapes and Wines

Premium red wine production is dependent upon proper extraction of pigments and tannins from grape berry skins and seeds during winemaking. While many methods are available to estimate the level of polymeric phenols in grapes and wines, none of the simple ones give an independent estimate of the amount of tannin present. Our aim was to provide a simple tannin assay that would be suitable for analysis of grape berries during maturation as well as for monitoring tannin extraction during fermentation and extended maceration. We have taken a tannin assay that was originally developed for measuring tannins in persimmon and adapted it so that it can be used for measuring tannins in grapes and wines. We studied many of the compounds in wine that could potentially interfere with this analysis and have found that while some can indeed inhibit the assay at very high concentrations, they do not interfere at the levels found in wines. Thus, the plate binding assay has been successfully adapted. In the course of working with the plate binding assay we realized that with some simple modifications we might be able to develop a much simpler solution assay that would not require a specialized piece of equipment like a microtiter plate reader. Although our initial research objectives were only related to adapting the plate binding assay, we were able to quickly demonstrate the feasibility of a solution assay, and we have now successfully developed a new assay method for tannins. The solution assay is simpler than the plate binding method we originally set out to adapt for grapes and wine. It is also much less variable than the plate-binding assay and is useful over a much wider range. Because it requires only a spectrophotometer and a small table-top centrifuge rather than a specialized piece of equipment like a microtiter-plate reader, this analysis will be suitable for use in many winery laboratories. One of our primary objectives in this project was to provide a simple tannin assay. The new analysis that we have developed meets that objective better than the plate binding assay ever could have.