Influence of Grape and Wine Production Practices on Tannin Extractability and Activity

The primary objective of this proposal is to develop an analytical method that predicts tannin interaction with salivary protein. This method is unique in that it moves away from tannin concentration as a predictor of astringency so that the impact of tannin structure variation (e.g.: color incorporation, oxidation of tannin structure) on interaction, can be measured. This analytical approach follows previous work which found that tannin structure variation related to grape maturity and wine age, could be related to thermodynamics of interaction.

Coupled to the development of an analytical method, this project is also focused on thedevelopment of a rapid reproducible method for preparing extracts from grape berries. This method deviates from many extraction methods developed to date in that it does not rely on the addition of solvents to mimic a wine-like system. Instead, this extraction method imposes a mechanical stress on berries for a short period of time (5 min), thereby testing the robustness of plant cells and hypothesized durability of diffusional barriers. The two methods above are expected to provide new and novel information on tannin development, from grapes to wine.

The objectives of this proposal are consistent with the highest priority research objective as outlined by the American Vineyard Foundation. The results to date have been very positive. First, a new analytical method has been developed and is now being applied to grape extracts and wines. The analytical method has the ability to measure tannin interaction variation that would be consistent with “softening” and therefore has significant potential in managing grape and wine production operations. Importantly, the analytical method is able to measure the impact of tannin modification on the “stickiness” of tannins. With regard to the new extraction method, the results have also been successful in that extracts prepared from the developed extraction procedure have been associated with predicted differences based upon growing region and historical block differences in tannin quality.

The results from the first two years have led to the development of analytical methods for tannin assessment in grapes and wine. In year three of this project, research efforts are being directed to the development a more complete understanding of the utility of these new methods.

Effect of Cluster Temperature on the Composition of Berries Grown Under Field Conditions

Because climate influences grape composition, a considerable amount of research has
been directed towards understanding how grape composition responds to light and
temperature. Studies that have separated light from temperature affects however are few.
This project is investigating the effect of cluster temperature on berry composition.
Using equipment designed to precisely manipulate in-situ cluster temperature,
temperatures were manipulated to achieve daytime cooling, nighttime heating, and a
variation in diurnal temperature range (DTR) while maintaining constant light exposure.
Grapes were monitored for various phenolic compounds (flavonols, flavan-3-ol
monomers, tannins, and anthocyanins), organic acids, and sugars. The results of the
project indicate that the period between fruit-set and véraison is sensitive to temperature.
For tannins, an increase in temperature was associated with an increase in tannin
production and a reduction in average degree of polymerization. During fruit ripening,
berries were much less sensitive to changes in temperature. Finally, a reduction in DTR
led to an apparent increase in the rate of fruit development. The results of this
investigation are designed to provide grapegrowers with information that can assist then
in managing grape components that are important to wine quality.

Relationship between Nitrogen Demand by Fruit, Nitrogen Uptake and

During the period of 2003 to 2007 my laboratory examined the concept of nitrogen use efficiency for Merlot on two wine grape rootstocks, Vitis berlandieri x V. rupestris cv 1103P and V. riparia x V. rupestris cv 101-14 Mgt. Both are extremely important rootstocks of relatively wide distribution in the North Coast and other grape growing regions of California. There are numerous ways one might define nitrogen use efficiency (NUE). The classical definition in physiology generally concerns the instantaneous rate of photosynthetic carbon dioxide assimilation per unit of leaf nitrogen. We defined NUE in more agronomic terms concerning the quantity of nitrogen (N) absorbed and its influence on total above ground productivity and yield, plus N translocated into the fruit, and therefore must, and its influence on fermentation dynamics and wine quality. Further, the quality of must N was evaluated in terms of its amino acid composition and yeast assimilable nitrogen content (YAN). Two N fertilization regimes were sustained that generally consisted of application of 17 or 35 kg N per hectare in either spring or fall following harvest, or no nitrogen applied. These treatments are important since storage in trunks and roots, atmospheric N deposition and soil mineralization pools of N could supply sufficient N for sustainable viticulture in this region, but information is lacking concerning longterm N fertility experiments.

Overall, our results indicated that the two rootstocks differed vastly in their ability to acquire N and translocate it to fruit and must. Rootstock 1103P generally had more than two times the concentration of amino acids and thus two times YAN content in must as 101-14 Mgt. This held true for every vintage where amino acids and YAN were evaluated, and was independent of N fertilization rate or whether the N fertilizer was applied in Spring or Fall. Changes in AA contents generally resulted in more substantial alterations to the glutamine family of amino acids (glutamate, glutamine, arginine, proline) and this has provided background for a major research initiative and collaboration being led by Dr. Douglas Adams with respect to its influence on phenolics metabolism. Fermentation dynamics were much faster for fruit harvested from 1103P in comparison to 101-14 Mgt. The combined effect was that N had a major influence on wine quality irrespective of irrigation. Our results ultimately indicate that a re-evaluation of the UC critical values concept of vine N nutrition in favor of an approach based on the nutrient budget concept (Bowen 1990).

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

In previous years we developed a method to assess tannin binding by grape berry cell walls. Using this method during the 2006 season, we studied tannin binding by the cell walls of skin and pulp from five different varieties of wine grapes grown in the Davis experimental vineyard: Cabernet Franc, Sauvignon blanc, Cabernet Sauvignon, Merlot, and Refosco. Among the varieties studied, tannin binding by the skins was found to be dependent on the amount of skin cell wall material produced by the fruit rather than differences in the binding capacity of the material. This is an important result because it indicates that skin cell wall mass should be considered along with berry size in thinking about the relative tannin extractability among different varieties. The way in which the skin cell wall mass varies with cultural conditions or vintage might be an important consideration in understanding the level of tannins extracted during winemaking. We also measured the amount of tannin in the skins and seeds of each of the varieties listed above. This was done so that we could compare the tannin binding capacity of the cell wall material with the amount of tannin in the skins and seeds of the fruit. By comparing the tannin binding capacity of the skin and mesocarp with the amount of tannin in the fruit we found that the cell wall material has the potential to bind from 55 to 140{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} of the tannin in the fruit at harvest. We also studied Dijon clones of Pinot noir from five vineyards in California and found that the cell wall material had the potential to bind from 43 to 70{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} of the tannin in the fruit. The work is useful because it can give us an idea of how important the binding of tannins by cell wall material might be in the extraction process during winemaking, but at this point we must be very cautious with our interpretation. For example, the assays to determine binding potential were conducted using a fixed concentration of skin tannin from Cabernet Sauvignon. The actual amount bound will depend on several variables including the nature of the tannin in the fruit and the alcohol level in the final wine. Furthermore, elevated temperatures lead to more tannin release from cell walls and these experiments were all conducted at room temperature. Despite these and other caveats this analysis serves to underline the importance of berry cell walls in the extraction phenomenon.

Evaluation of the Influence of Common Viticultural Practices on the Chemical and Sensory Characteristics of Wines

The goal of this project is to find critical viticultural parameters that affect the chemical and sensory characteristics of the final wine. Once these parameters are identified, winemakers and vineyard managers can use computational methods developed in our lab in order to suggest the best vineyard practices to use to achieve specific target characteristics in their final wine. We are taking two complementary approaches to this problem. First, we are developing tools for searching through large existing databases of viticultural information to find the most critical factors. A Decision Tree Analysis algorithm has been developed in our lab for this purpose. Second, we are producing Cabernet Sauvignon wines from existing viticultural trials at the Oakville Experimental Station. The 37 wines from the 2000 harvest are made from grapes harvested from vines differing in parameters such as rootstock, irrigation, trellis, pruning, row orientation, and vine density. Juice analysis has begun on these wines, and wine analysis will commence as soon as the processing is completed.

Effects of Berry Size and Crop Yield on Wine Composition and Sensory Quality

We draw these conclusions about the dependence of composition on size and irrigation: 1. Imposing significant water deficits can decrease berry size and increase skin: juice ratio by approximately 25{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} compared to conventional irrigation regimes. From two separate yield experiments, it appears that smaller berries are not produced when yields are increased from 3.5 to over 10 tons/acre by leaving longer spurs at pruning. 2. Berry size is highly correlated with seed mass/berry. However, berry growth did not keep pace with seed growth such that the amount of seed per unit berry mass increased with berry size. Low irrigation had less effect on seed growth than on mesocarp growth for any size of berry. Hence, Low irrigated fruit had greater relative seed mass, implying a greater concentration of seed tannin in the resultant wines. 3. The mass of skin tissue bore a constant relation to berry size, i.e. the mass of skin per unit berry mass was relatively constant over the 3-fold range of berry sizes encountered. Low irrigated fruit had greater relative skin mass per berry. Low irrigation inhibited mesocarp growth more than skin growth such that the relative amount of skin per unit berry mass was always greater than for Control and High irrigated fruit. 4. The amount of skin tannin per berry was a fairly constant fraction of berry size. Thus, the concentration of skin tannin in must was only slightly lower for large fruit than for small fruit (approximately 10{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}). However, the concentration of skin tannin was significantly greater in Low irrigation fruit of any size (approximately 30{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} greater in Low fruit). 5. The amount of anthocyanin per berry increased with berry size, but was a slightly decreasing fraction of berry size. The resulting anthocyanin concentration was approximately 20{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} less from the largest fruit compared to small fruit. Again, the concentration was significantly greater (approximately 30{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}) in Low irrigated fruit for all fruit sizes. These results show that the amount of each skin solute is not a constant that is diluted to varying degrees dependent upon fruit growth. Accumulation of skin solute is coordinated with growth. Accordingly, oft observed changes in fruit composition caused by water deficits are not attributable simply to differences in berry growth. There is an independent effect of deficit irrigation on composition. Because deficit irrigation produces lower yield, it is important to evaluate whether there is an independent effect of yield on these quality parameters. The fruit size and irrigation differences in fruit composition were carried through to the resulting wines. The differences in wine composition were slightly less dependent upon fruit size than the juice composition, suggesting a possible difference in “extractability”. And, both fruit and wine composition were much less sensitive to fruit size than the theoretical dependence that is predicted from differences in surface: volume of the berries. A preliminary investigation found differences among the low, control and high irrigation wines in the intensities of green beans aroma, red fruit by mouth, bitterness and astringency as shown below. When wines were made from different sized berries that were irrigated similarly, small berry wines were more astringent than large berry wines.

Effects of Selected Vineyard Management Practices on Nitrogen Status of Grape Juice

The research team has established relationships with seven vineyards, Virginia, as well as the central coast, central and southern San Joaquin Valley and the Lodi area of California, with respect to vineyard trials, sampling and small lot wine production. A total of 528 berry samples (excluding duplicates) corresponding to crop load and canopy orientation and multiple sample periods from veraison to harvest have been collected and analyzed by Formol, OPA and ArOPA procedures for FAN and NH3. Comparison of the analytical methods by crop load level and canopy orientation indicated no statistically significant differences (T-test, p<0.05) between high and low crop load levels for that season. There were significant differences in OPA amino acids and OPA arginine values between east and west sides of the canopy. Higher levels were observed on the west side. A preliminary comparison of Formol values with summed OPA and ammonia from 1999 Virginia Cabernet Sauvignon samples indicated that the Formol method produced significantly higher values for assimilable nitrogen than did OPA + ammonia for the same sample (p <0.05). Initially, two methods were evaluated for expression of juice from collected fruit. A comparison of blender and stomacher processing methods was performed to evaluate possible effects of seed breakage or analyte stratification on analytical results. Early season and post-harvest samples of 1999 Virginia Cabernet Sauvignon fruit were tested. No significant differences (p <0.05) in Formol values were observed. For the OPA measurements, no significant differences were observed in the early season sample set, but the stomacher method was found to produce higher OPA nitrogen values for the late season samples. In conjunction with Formol titrations, spectrometnc (OPA) analyses for general total amounts of the alpha amino acids, arginine specific spectrometric (ARGOPA) analyses, and ammonia, we are attempting to also conduct HPLC analyses for all the amino acids present. The purpose for the latter analyses is to allow us to look at proline/arginine ratios, and specific amounts of cysteine, methionine, and other specific amino acids, as a function of vineyard or winemaking study. Currently we are investigating the use of phenylisothiocyanate, dimethylamino-azobenzene sulfonyl, and dimethylaminoazobenzene thiohydantoin amino acid derivatizations to determine which of these procedures will work best with our juice and wine samples. A Hewlett-Packard model 1050 HPLC with diode array detection is being used in this work. To date the instrument operational conditions have been established for the derivatives being run and chromatograms of some of the derivatized amino acids run. Analysis of fermentation volatiles (fusel alcohols and esters) has begun.

Evaluation of the Influence of Common Viticultural Practices on the Chemical and Sensory Characteristics of Wines

The goal of this project is to find critical viticultural parameters that affect the chemical and sensory characteristics of the final wine. Once these parameters are identified, winemakers and vineyard managers can use computational methods developed in our lab in order to suggest the best vineyard practices to use to achieve specific target characteristics in their final wine. We are taking two complementary approaches to this problem. First, we are developing tools for searching through large existing databases of viticultural information to find the most critical factors. A Decision Tree Analysis algorithm has been developed in our lab for this purpose. Second, we are producing Cabernet Sauvignon wines from existing viticultural trials at the Oakville Experimental Station. The 37 wines from the 2000 harvest are made from grapes harvested from vines differing in parameters such as rootstock, irrigation, trellis, pruning, row orientation, and vine density. Juice analysis has begun on these wines, and wine analysis will commence as soon as the processing is completed.

Influence of Row Orientation and Cluster Exposure to Sunlight on the Microclimate and Composition of Cabernet Sauvignon Fruits

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.

Effect of Viticultural Practices on Flavor Formation in Grapes

Norisoprenoids are volatile secondary metabolites which are important aroma constituents of a number of grape and wine varieties. The norisoprenoids are thought to arise from carotenoid breakdown and occur in grapes as glycosidically bound precursors. However, the factors which influence carotenoid breakdown and norisoprenoid formation in grapes are not well understood. In this study we evaluated the effect of seven levels of sunlight exposure (5{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, 20{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, 27{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, 37{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, 50{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, 70{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, and 100{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, expressed as percent of full sun exposure) on norisoprenoid concentrations in three grape varieties (Chardonnay, Sauvignon blanc, and White Riesling). Our results suggest that viticultural practices which affect fruit exposure to sunlight may influence norisoprenoid composition of grapes and wines. In White Riesling, TDN (l,l,6-trimethyl-l,2-dihydronaphthalene) levels increased at sunlight exposures above 20{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}.