The effects of row orientation and cluster exposure to sunlight on the microclimate and fruit composition of Cabernet Sauvignon was studied in a commercial vineyard in the Napa Valley. Eight clusters per vine (one cluster/shoot) were selected for use immediately after berry set. Clusters were divided into two groups of four, with each group located on either the morning exposed (north or east side of the vine row, respectively, for east-west and north-south oriented rows) or afternoon exposed (south or west side of the vine row, respectively, for east-west and north-south oriented rows) portion of the vine row. Four cluster sunlight exposure categories were established: (1) full exposure, (2) moderate to high exposure, (3) moderate to low exposure, and (4) shaded. A positive, near linear relationship was found between berry temperature and PAR incident to the cluster surface following veraison. Fully exposed clusters on the south (E-W rows) and west (N-S rows) sides of the canopy had the greatest mid-day berry temperatures in the experiment. Berry weight was generally greatest for fully exposed berries and least for shaded berries, regardless of row orientation. Soluble solids followed a similar trend, with the lowest °Brix found in clusters on the east side of N-S rows and the highest clusters grown on the south of E-W rows. Titratable acidity was generally similar among exposure treatments, while malate levels in shaded fruits were significantly greater compared to exposed fruits. Observed differences in both skin anthocyanins and total phenolics among the treatments reflected differences in berry temperature resulting from sunlight exposure; fully exposed clusters on the south (E-W rows) and west (N-S rows) exposures had lower anthocyanin concentrations compared to shaded clusters. The results indicate that berry exposure to direct sunlight during the afternoon (west or south facing exposures) leads to elevated berry temperatures and undesirable reductions in berry color, total phenolics and malate.

This research project has been aimed at developing an assay for tannins that can be used in wineries. The first such assay we developed was based on tannin binding to a protein coated onto 96-well micortiter plates. This assay is easy to perform, it is sensitive to very low levels of tannins and can provide very high throughput for situations where large numbers of samples must be analyzed. One disadvantage of the plate-binding assay is the well-to-well variability which was minimized by introducing high salt washes into the procedure. A further disadvantage is that the procedure requires a specialized piece of equipment in the form of a microtiter plate reader which is uncommon in most winery laboratories. We used the principle of the plate-binding assay to develop a solution assay which requires only a spectrophotometer to perform. The variability seen in the plate binding assay is eliminated in the solution assay. The solution assay is less sensitive than the plate binding assay, but it has a much wider useful range, and is still sensitive enough to measure tannins even in light red wines. The solution assay is based on measuring the activity of an alkaline phosphatase enzyme (AP) which co-precipitates with bovine serum albumin (BSA) when tannin is added to a solution containing both proteins at a pH near the pi of BSA. The solution assay requires measuring the rate at which AP hydrolyzes a substrate and is thus a kinetic-based assay. In the current year we developed a third assay for tannins in grapes and wines. It takes advantage of protein precipitation by tannins, but is an end point assay rather than kinetic. Furthermore it measures the amount of tannin directly, by formation of a colored complex with ferric ion, rather than indirectly by the amount of protein precipitated by the tannin. This assay is even easier to perform than the solution assay with AP and is less expensive because the AP enzyme is replaced with a suitable ferric chloride solution. Throughput is also higher because it is an end point assay rather than a kinetic-based one. During the 1998 season we used the end point assay to determine the amount of tannin in skins and seeds of Cabernet Sauvignon and Pinot noir berries during development, from three weeks prior to veraison until harvest. In both varieties the amount of tannin in skins changed very little during this time, but the amount in skins of Cabernet Sauvignon was greater than in Pinot noir. Seed tannin was found to be highest shortly before veraison and showed a rapid decline just afterward. In Cabernet Sauvignon seeds the tannin level declined soon after veraison and remained constant during the four weeks prior to harvest, at which time the amount of tannin in skins and seeds was nearly the same. Comparison of tannins in berries with tannin levels in wines made from the fruit showed that less than 25%of the tannins in the berries at harvest was present in the finished wine.

Grape samples were collected at two Oakville area vineyards during an 8 week period for analysis of phenolics. Wines were also made from these two vineyards at three different maturity stages. All samples are being analyzed for phenolic content using a relatively new procedure which separates phenols based on size. This year of the project was designed to replicate last year’s effort in order to test for year to year differences, and to expand the use of a new analytical method which is capable of measuring polyphenols from both skins and seeds. At this point, all the samples have been collected and extracted. The chromatographic analysis of the seeds is complete, but the skin and wine analyses are still underway. Preliminary analysis of sensory data on the wines prepared by Mondavi winery for both bitterness and astringency shows increasing astringency with ripeness, the same as last year’s wines, but also shows a trend for increasing bitterness. While these trends are interesting, further statistical analysis will establish the significance of the sensory data. The goals of this year of the project were to: 1) Repeat the measurement of the levels and polymer distribution of phenolic compounds during ripening in vineyards selected in consultation with NCVRG representatives. 2) Compare maturity with specific chemical levels. 3) Prepare, in consultation with NCVRG representatives, wines from the same vineyards at different stages of maturity. 4) Analyze the wines using the same procedure to see if the changes observed in the grapes follow through to the wine.

Sauvignon blanc and Chardonnay grapevines grown at the North Oakville Experimental Vineyard (OEV) generally responded favorably to increased rate of nitrogen fenilization up to 90 lbs/acre as demonstrated by higher yield, shoot growth, pruning weight and total leaf area per vine. Differences in fruit composition between low and high nitrogen fertilization were relatively small. However, high N generally increased the pH, TA, malate, K, and arginine levels of must of both cultivars. The field fertilization did certainly improve the ability of the juices to ferment. Without the nitrogen additions, the juices took twice as long to ferment. Otherwise, the routine must and wine analyses show little differences.