Influence of Grape and Wine Production Practices on Tannin Extractability and Activity.
The objectives of this proposal are to do the following: I. Develop optimized analytical methods for the measurement of: A. Grape skin tannin extractability and concentration potential in red wine. B. Predicted concentration-independent interaction of red wine tannin with protein. II. Explore the influence of grape and wine production practices on developed analytical methods These objectives are consistent with the highest priority research objective as outlined by the American Vineyard Foundation as well as the National Grape and Wine Initiative Part A of the first objective addresses observations that higher quality wines are associated with an increase in skin tannin concentration. Presumably this is a vineyard-derived aspect of wine quality in that the data collected to date were designed to understand vineyard management practices. Historically it has been difficult to predict tannin extraction into wine from grape-based data. Part of this difficulty can be attributed to the lack of accounting for the dependence of extraction on cellular integrity. In order to take into consideration the cellular integrity of the grape berry, an analytical approach which exposes grape berry samples to mild mechanical force is being employed. It is expected that incorporating an analytical component that accounts for cell integrity will provide an improved ability to predict red wine tannin extraction and quality, from fruit samples. Part B of the first objective is designed to improve our understanding of how tannins are perceived and address the generally held perception that tannin quality has a concentration-independent component. Recently published research has found that the enthalpy change of a protein-tannin interaction experiment is dependent in part on the “age” of the tannin in that apparent interaction between tannin and protein declines as tannin structure modification increases. This research was performed using Isothermal Titration Calorimetry on previously purified tannins. This portion of the project will focus on the development of an analytical method that can predict protein-tannin interaction from wines directly. In the second objective, optimized methods will be used to understand experiments in progress by other research programs, vineyards, and wineries. IV. Objective(s) and Experiments Conducted to Meet Stated Objective(s): Part A, Objective #1 The objective is to develop an extraction method that is mild yet predictive of potential skin tannin extraction. A successful method will compare favorably with wines made consistently from the same fruit. A further criterion for success will be that the analytical approach is as rapid as practically possible. Fruit samples have been collected from the Fresno State vineyard and have been subjected to readily-available equipment designed to introduce various degrees of sheer to the sample. The specific target was the introduction of sufficient sheer to differentiate between berries having variable cellular integrity. We initially investigated a Kitchenaid kitchen mixer equipped with various implements. After exploring this mixer with and without added ethanol, a decision was made to test a new instrument due to insufficient extraction. We next transitioned to a Cuisinart food processor. The food processor with a dough kneader worked well for our purposes and all extracts prepared were found to be sufficiently extracted. E&J Gallo was approached and they expressed interest in cooperating on this project. To provide a meaningful data set, they offered to provide us with berry samples and wines from Cabernet Sauvignon vineyards from warm interior valley sites, to cool north coast sites. Wines were also prepared from this fruit to provide comparison. To date, extracts have been prepared and have been analyzed. Also to be analyzed by May 2012 will be exhaustive skin extracts and the red wines produced from samples. Extracts prepared with the food processor and preliminary comments from E&J Gallo indicate that the results are positive. Extracts were analyzed by acid-catalyzed degradation in the presence of excess phloroglucinol. This analytical approach was used because it could provide information on the composition of tannins in the extracts and hence the tissue of origin. Of specific interest was determining the prodelphinidin content which is skin tissue-derived.