November - December, 1997

It is important to carefully review each management and processing step to help determine how each influences both style and quality. How is each vineyard and wine production decision influencing your wines? Are processing steps undertaken simply because that=s the way you have always done it? It is imperative that regional wine quality move forward not laterally from vintage to vintage if we expect to compete. This requires a continued, careful review of all wine production practices, viticultural and enological. Grape-derived secondary metabolites are the principal sources of wine aroma, flavor, color and taste (6). These sensory responses arise from various types of aroma, flavor and phenolic compounds. An understanding of the nature of these compounds, factors which influence their production, extraction from the fruit and management during processing and aging is critical to successful premium wine production. The following reviews some viticultural and enological factors influencing the production of grape-derived secondary metabolites, specifically aroma, flavor and phenolic compounds.

The production of wine grapes with desirable concentrations of secondary metabolites at harvest is the result of the convergence of two broad influences (6).The first is the reproductive process that, through seed development, determines potential berry size. Because the skin contains a high concentration of most aroma/flavor and phenols, the berry surface to volume ratio is important in determining the concentration of these metabolites. This was an important feature this year as berry weights (and therefore, size) were relatively low. The second influence comes from environmental factors and viticultural management that conditions the genetic potential and influences berry growth. Environmental and viticultural conditions that influence secondary metabolites are listed below. Table 1 lists some viticultural and environmental features to maximize the production of aroma/flavor and phenols.

<Meso climate <Canopy climate

<Soil moisture <Temperature

<Berry size <Asynchronous development

<Fruit maturity <Leaf area/fruit weight or fruit

Climate

Among the viticultural options affecting the production of aroma/flavor and phenoliccomponents either directly or indirectly, mesoclimate (site climate) is considered one of the most important. The production of secondary metabolites is influenced by the temperature during Stage 3, or the final period of fruit maturation (9). Maximum concentrations are produced when fruit ripens within the temperature ranges given in Table 1. Therefore, it has been suggested that the best variety for a siteis one that matches the length of the growing season so that fruit maturation occurs during the portion of the season that is cool but warm enough to allow the fruit to continue to accumulate aroma/flavor. Mesoclimate has been divided into two general temperature zones, Alpha and Beta (7). In Alpha zones, maturity occurs just before the mean monthly temperature drops to 10 EC (8). Specifically, Alpha zones are those where the mean temperature at the time of ripening, for a particular variety, is between 9-15 EC. In warm climates the length of the growing season is more than adequate to ripen most grape varieties which, therefore, mature in the warm part of the season. In Alpha zones, day temperatures are moderate and night temperatures usually cool, creating desirable conditions for the development of secondary metabolites.

On the other hand, in Beta zones the majority of grapes ripen well before temperatures begin to drop. Specifically, Beta zones are those with a mean temperature above 16 EC at the time of ripening for a particular variety. Thus, days and nights are still warm, and attainment of adequate minimum EBrix (either by sugar production or dehydration) is not a problem. In many Beta zones grape varieties with short, moderate and long seasons are grown, but perhaps should not be.An additional value of this mesoclimate classification is that it can allow a positive correlation between soluble solids and wine quality. This relationship is much more direct in Alpha, compared to Beta zones.Also, a more negative correlation exists between yield and quality in Alpha vs Beta zones.This is an important reason why I have suggested that ^oBrix in our climate is neither a good predictor of maturity or ultimate wine quality.

The belief that the best site is one which just ripens the crop has given rise to the belief that cool wine-growing sites produce superior wines. Wines from reportedly cool regions have been presented as being comparable to their prestigious European counterparts (2). Is it true that regions with long, cool ripening periods are required to produce superior wines? If so, is this the result of the quantitative or qualitative nature of secondary metabolites? Most Pauillic vineyards do not produce Chateau Margaux quality wines,yetfrequently haveclimates vary much like Margaux. The other obvious confounding factor is that many wines of quality are produced from warm to hot climates.

There are several reasons why comparisons between climates, secondary metabolite production and grape and wine quality have been confounded. First, is the effect of crop load. Crop load, and most significantly, the ratio of exposed leaf area-to-crop load, can have a profound effect on the rate of maturity. Fruit maturity and the rate of fruit maturity can influence grape and wine quality. Another factor often overlooked is asynchronous growth (either berry, cluster or vine) (2). This will also delay maturity, and few comparisons of climate and wine quality have taken this into account. An additional confounding factor that obscures the relationships between quality and climate is the degree of fruit maturation. Ripeness is a relative term which depends, in part, on the region. For example, 'Ripe' Cabernet Sauvignon in many regions of California has a higher sugar concentration than Cabernet Sauvignon in Virginia. The perceived importance of sugar content is illustrated by its use as a synonym for quality in many countries. Ripeness is also style and site dependent. The chemical composition of the fruit at optimum ripeness will vary.

There is no direct evidence that a cool climate is a prerequisite for superior wine quality. However, climate does have a significant influence on wine style, due not only to the effects of primary metabolites such as sugar, but also to secondary metabolites such as aroma/flavor and phenolic compounds. Reported cool climate features adopted from Jackson and Lombard (9) are listed below:

<Wines fresher, more delicate and more aromatic <pH lower

<Phenolics lower <Methoxypyrazines may be higher

<Terpenes increase more slowly but may be higher <Increased color

<Cool sites in warm districts and warm sites in cool districts produce higher scoring wines

Becker (1) and others have shown that white wines produced from cool climates are frequently fresher in aroma and have lower alcohol than their warm-climate counterparts. Warm regions tend to produce wines which have less varietal aroma/flavor, and potentially harsher on the palate, due primarily to the increased concentration of alcohol and phenols. The belief that warm climates produce wines with less and/or different aroma/flavor is well accepted,but has been difficult to quantify. Ewart (3) compared cool and warm climate vineyard sites in south Australia and found that in the former, the free monoterpene content (the compounds which provide for the floral aroma of White Riesling, Gewurztraminer, and Muscats) increased more slowly, but was in greater concentration at the end of the season. On the other hand, the concentration of methoxypyrazines, responsible in part, for the herbaceousness of some wines, may be undesirably high in fruit from cool climates. This isparticularly true with certain cool climate varieties grown in dense canopies with excessive leaf and cluster shading (11). Climate can influence the phenal content of the fruit, a reason why many warm climate producers whole cluster press their white grapes.

Throughprocessing, aroma/flavor and phenolic compounds can be attenuated to help produce a balanced, integrated product. Processing considerations which may influence bound aroma/flavor are listed.

<Yeasts <Post-fermentation use of enzymes

<Sur lie storage <Post-fermentation thermal storage

The liberation of free volatiles from bound components can occur as a result of acid or enzyme catalyzed hydrolysis.S. cerevisiae is known to produce - glucosidase enzymes that may hydrolyze bound compounds.Most studies, however, have demonstrated that wine yeasts have either limited enzyme activity or excrete little into the medium. Despite this, several yeast manufacturers market strains promoted as >enhancers of varietal expression=. The implication is that these yeasts have increased ability to hydrolyze grape-derived bound aroma precursors, improving wine aroma and aroma intensity. Zoecklein et al. (12, 13) conducted several studies evaluating four strains of Saccharomyces cerevisiae on bound Riesling conjugates. Fermentation resulted in a significant decrease in bound glycosides (reduction signifies hydrolysis and the liberation of free aroma volatiles into the wine). The greatest reduction (and presumably greatest increase in the free aroma volatiles) with cultured yeasts occurred with the Fermiblanc (FB) strain, although the maximum difference among yeasts was only 7% (Table 2). This would suggest that the influence of a particular yeast strain (at least those used in this study) on grape glycosides is limited. The other yeasts shown in Table 2 are Prise de Mousse (PDM), D47 and VL1. It is interesting to note that native yeasts (NTL) were shown to have a greater influence on lowering the glycoside concentration (again presumably increasing the free aroma volatiles) than culture strains of Saccharomyces cerevisiae (12).

Significance of LSD test of treatment means at P # 0.05, N=20.Means with the same letter are not significantly different.

In a study we are conducting with California State University - Fresno, native yeast were isolated from a sonoma - cutrer vineyard. Twelve of these native organisms are being evaluated for _-glucosidase activity (ability to hydrolize bound grape aroma compounds) by my graduate student, Heather McMahon. It may be possible to select native yeast which could grow in co-culture with cultured saccharomyces to optimize grape-derined aroma and flavor.

Pectic enzymes have been used in the wine industry for some time as an aid to color extraction and enhanced juice yield. Most commercial pectic enzymes have some level of -glycosidases, the enzymes that can break the glycosidic bond, releasing bound aroma/flavor components (5). -glycosidases are generally inhibited by small concentrations of glucose and therefore must be added post-fermentation into dry wines. These products must remain in the wine for at least several weeks - longer if the wine is cool or has a low pH. Aroma enhancing enzymes are generally not inhibited by the alcohol content of table wines but can be removed from solution by precipitation with tannin phenols.They must not be used in the presence of bentonite, which will also remove them from solution. These enzymes are not very discriminatory, they release bound phenol glycosides which may increase the bitterness of some white wines. Since all cultivars contain conjugated or bound components, aroma/flavor enhancing enzymes may prove to be a major technological advancement.

Post-fermentation Thermal Processing

There is great interest in understanding the hydrolytic mechanism by which odorless bound glycosides generate aroma volatiles. Heat has been used as a means of increasing or accelerating wine aging. The compounds associated with aging are enhanced by mild anaerobic heating as a result of glycoside hydrolysis. Figure 1 (14) shows the affect of post-fermentation thermal storage on total glycosides of Riesling wines (reduction signifies hyrolyses and the possible liberation of free aroma volatile into the wine). Thermal storage reduced the concentration of grape glycosides by an average of 29% for all yeasts. While not all volatiles released have a desirable affect, post-fermentation thermal processing has been shown to positively influence wine sensory scores of Chardonnay and Semillon (10, 4).These wines were reported to have a decrease in the floral character and enhanced perception of oak, honey and smoke. Our research confirms these results. I will have wines treated with post-fermenation thermal processing available at our next winemaker round table meeting.

The annual meeting of the American Society for Enology and Viticulture, Eastern Section is scheduled for July 22-24, 1998 at the Crowne Plaze Hotel in Grand Rapids, Michigan. An international symposium on sparkling wines B Issues in Sparkling Wine Production will be held in conjunction with the meeting. Mark your calendars!