Vol. 11, No. 4 July - August, 1996
Department of Food Science and Technology
VPI & SU
Blacksburg, VA 24061-0418
I. Wine Quality Components, A Review 1
Grape Phenols 2
Grape Aroma Components 2
II. Grape Maturity 3
Maturity Sampling 4
III. Department of Food Science and Technology Internship 5
IV. VVA Annual Membership and Technical Meeting 6
I. Wine Quality Components
As this harvest season approaches I invite each producer to carefully review their wines in terms of specific quality parameters. Red wine complexity remains our greatest challenge. It is equally important to carefully review each management and processing step to help determine how each influences both style and quality. How are 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 our wine quality move forward not laterally for vintage to vintage if we expect to compete. This requires a continued, careful review of all wine production practices, viticultural and enological.
Wine quality depends largely on terroir and is influenced by fruit maturity and processing technology. The universal problem, of course, is that there are few objective criteria for assessing quality of grapes prior to/or at harvest. Traditional methods such as sugar, pH and acid offer only broad indices of fruit ripening and even less information about potential quality. The organoleptic evaluation of wines, although quite helpful in understanding seasonal and vineyard varietal expression, is an after-the-fact approach which must cope with the inherent variables of winemaking and fermentation products.
The two most important components of red and white grapes influencing wine quality are secondary plant metabolites - phenols and aroma/flavor components. It makes sense, therefore, that evaluation of phenols and aroma/flavor play a primary role in fruit quality assessment.
Grape Phenols. The importance of optimal phenol maturity and the methods of evaluation have been reviewed in past editions and discussed at several vintner roundtables. Mature tannins in the fruit are a requirement for premium red wines and therefore, are an important harvest consideration. With advances in maturity, phenolic compounds in the skins increase and those of the seeds slightly decrease. While these changes are important, it is qualitative not quantitative factors that are most significant regarding grape phenols. If fruit is harvested with immature, unpolymerised tannins, the resulting wine will be harsh and hard.
A simple strategy for tannin evaluation of grapes has been provided in previous editions. A random and representative sample of fruit is collected and the skins carefully removed from the pulp. The skins are sensorially evaluate for the presence or absence of hard, astringent tannins. It is imperative that the pulp be removed due to the confounding influence the decline in malic acid has on the perception of astringency. A portion of these skins are frozen and used as a reference for the evaluation of grapes at the next sample period. Tannins evoke a tactical response not a taste, therefore, evaluation is of the texture of the astringency. The textural changes which occur with time are perceived as softening resulting in enhanced suppleness or smoothness. The following is a list of descriptors which can be used to categorize grape tannin astringency. Naturally, these terms are the same as those applied to wines: hard, course, green, firm, chewy, dusty, supple, soft, fine and silky. One can conceptualize tannin evolution as undergoing changes analogous to the textural differences between wool, linen and silk. The more supple your desired style the more silk-like grape tannins required. If you are not evaluating tannin maturity, premium red wine quality and control of wine style is a matter of pure luck.
Grape Aroma Components. The intensity of aromas are another important wine quality issue. Since most of what we taste is related to what we smell, aromas and flavors are intimately linked. Aromas are secondary metabolites of grapes and are of two types: those that are free, volatile and, therefore, responsible for the perceived aroma of the fruit, and those that are conjugated or bound to sugars in a nonvolatile and flavorless form.
Our selective leaf removal research on White Reisling, Chardonnay and Cabernet Sauvignon in Virginia have demonstrated increases in conjugated (bound) aroma compounds as a result of treatment. The practical significance to the winemaker is that vineyard management practices can influence the pool of conjugated or potential aroma components.
Most of the aroma components of red grapes are located in the skins. The following is a procedure used to evaluate red grapes aroma:
1. Approximately 200 berries are lightly crushed and pressed in a hand press.
2. The skins are separated from the pulp and seeds and placed in approximately 200 ml of 15% ethanol which has been adjusted to pH 3.0 with tartaric acid.
3. The skins and alcohol are placed in an airtight jar for several days to a week. The alcohol solution is decanted and evaluated.
Smelling the solution will provide an indication of the odor and odor intensity. Red grapes should have an odor of fruit, cherries, etc. along with "notes" of pomace or tea from the tannins. If there is little varietal aroma or aroma intensity in the grape, these two features will be deficient in the resultant wine.
Complexity. Complexity denotes the harmony or marriage of characteristics. The most complex and therefore the most attractive flavors and aromas imply an intermediate stage in formation and degradation reactions. Excessive flavors or aromas such as too much oakiness, dominate herbaceous notes, etc. overbalance the impression, resulting in less complex, one dimensional products. This is a problem we must begin to address.
We must continue to work towards farming both for flavors and complexity. Viticultural considerations which may enhance subsequent wine complexity include utilizing multiples of the following:
2. training and trellis system (light exposure to the fruit, fruit to leaf ratio)
3. vine ages
4. soil profiles
5. clonal selections
6. maturity levels (beyond a minimum maturity)
II. Grape Maturity
One of the interesting discussions which occurred at the 4th International Symposium on Cool Climate Viticulture and Enology was on grape maturity. It has been suggested that maturity be evaluated in the same context that wine bottling decisions are reviewed. If bottling is not done properly, all previous efforts to instill and maintain quality are lost. If fruit maturity evaluation are not performed properly all the subsequent winemaking step to help assure quality are of limited value. Harvest decisions should be made based upon the intended goals. Table 1 list important red wine quality components, each of which are influenced by fruit maturity.
Table 1. Red Wine Quality Components
1. Color: hue, strength, purity, and stability
2. Intensity of aromas and flavors
- absence of assertive, one-dimensional 'notes' including reductive tones
- absence of one-dimensional aromas and flavors
- enhanced by varying terroir, clones, vine age and particularly by blending
4. Balance and harmony
- palate balance equation:
sweet taste = acid + bitter & astringent
5. Strength and suppleness of tannins
- requires adequate phenol maturity in the fruit (which is influenced by terrior and crop load)
- a major influence of balance
- clay soils and warm temperatures (mainly night time) tend to make more tannic wines
- requires gentle fruit handling throughout processing
6. Longevity of the wine
- is influenced by the alcohol, quantitative and qualitative aspects of phenol and pH
- wine pH should not be of concern unless it exceeds 3.8
An examination of the fruit in most Virginia vineyards indicates differences in ripeness. Lack of uniformity can occur as a result of variations among berries on a cluster, among clusters on a vine, among vines within a vineyard block and due to lack of block uniformity.
Asynchronous berry development has re-percussions for quality in that the proportion of berries with optima qualities (such as mature phenols and desirable aroma) are diluted by those that are inferior. Vines that produce the best quality wines are those with less variability (Long, 1987).
Even sunlight exposure is important in maximizing berry uniformity within a cluster. Additionally, the leaf area/fruit weight ratio may be an important consideration. In Cabernet Sauvignon, weak shoots (< 30 cm) produced berries with lower sugar, less color and low phenols. Fruit from 'normal' shoots (1.2 m) produced wines with desirable varietal aroma and flavor characteristics and were much less herbaceous (Long, 1987).
Variation within and among vines is often great. Therefore, determining harvest decisions requires considerable effort and care. Fine tuning to the best possible harvest regimes takes much attention, effort, cooperation and experience among vineyard and winery personnel. Since many things vary with maturity (polysaccharides, berry softness, juice turbidity, phenols, nitrogen compounds, aroma compounds, ammonia, proline, etc.) and many also relate directly to wine quality, it seems logical that several to many components would be necessary to define and determine optimum maturity. Maturity evaluation of red grapes, for example, should involve the following review:
1. taste assessment of the grape tannins and flavor
2. assessment of varietal aroma and aroma intensity
3. oBrix, acidity and pH
4. general fruit condition including berry softness
5. ability to ripen further
Maturity Sampling. Berry sampling is difficult because of the time required and variations in fruit chemistry. However, properly performed berry sampling can provide an accurate picture of the overall vineyard fruit chemistry. Berry samples are frequently higher in maturity than cluster samples due to the tendency to sample more mature, sun exposed berries. The problem is enhanced by small sample size. Reducing the sample size below 30 berries results in a significant increase in the variance. Jordan and Croser (1983) recommend collection of berries from the top, middle, and bottom of the cluster. Terminal berries on the rachis may be less mature than other berries. Berry sampling in various locations on the cluster may be significant in the case of larger clusters; but many sample by selecting berries only from the middle of the rachis, a technique that may be acceptable in the case of varieties with small clusters.
Additionally, in a berry sampling procedure the side of the cluster from which the berries are taken must be randomized. One should avoid selection of fruit from ends of rows or from isolated vines or those with obvious physiological or morphological differences. Significant variations in soil type, shading, and so on, may play an important role in vine growth and therefore fruit maturity. Ideally, sampling should be designated to reflect differences in soil type, topography, and vine growth. For consistency, some recommend that selected vines within each block be targeted for sampling.
Samples should be collected at approximately the same time period each day. Jordan and Croser (1983) have made the following sampling recommendations: (1) edge rows and the first two vines in a row should not be sampled; (2) samples should be collected from both sides of the vine; (3) for each row, estimate the proportion of shaded bunches and sample according to the proportion. This proportion may vary with the side of the row sampled because of variation in leaf cover.
Enhanced accuracy can be obtained by randomly sampling 10 to 20 clusters. The samples are pooled, crushed and pressed in a manner similar to the method to be used by the winery at harvest. Once again sampling must represent the entire plot. Such variations as soil, soil depth, vine age, and vigor, must be considered. Two practical problems associated with cluster sampling are depletion of crop and the possible difficulty of processing a large sample.
III. Department of Food Science and Technology Internship
In 1995 our department, in cooperation with the Williamsburg Winery, began a annual student internship. This program allows qualified students to gain practical skills while also participating in an applied research activity. This mutually beneficial experience serves the student, our department and certainly the industry. I want to publicly thank the Williamsburg Winery for their generous support. The first intern was Mr. Carleton Yoder who is completing his MS degree under my direction. He recently presented some of the results of his research at the 4th International Symposium on Cool Climate Viticulture and Enology, attended by over 700 people from 19 wine producing countries. A modified abstract of his presentation is given below: Effect of fruit zone leaf removal and shoot thinning on Vites vinifera L grape glycosides, Yoder, C., Zoecklein, B., Wolf, T. K., and Jasinski, Y., 1996:
Traditional measures of grape maturity such as oBrix, acidity and pH, may not be reliable indicators of potential wine quality, particularly in warm climates. Quantification of grape glycosides has been suggested as a possible alternative to 'index' potential grape and wine quality. Grape aroma compounds are present as free volatiles, which may contribute directly to odor or as bound conjugates (mainly glycosides), which are nonvolatile precursors. Grape glycosides are composed of aliphatic residues, monoterpenes, sesquiterpenes, norisoprenoids and shikimic acid metabolites. These represent, in large part, the potential flavor and aroma of a grape variety. The measure of glycoside, therefore, provides an estimation of the total pool of bound flavorants.
Specific vineyard management techniques have been shown to improve grape quality, and thus, potential wine quality. Despite the widespread use of selective leaf removal, however, not all studies have reported improvements in grape composition or wine quality.
Two separate studies (leaf removal of Chardonnay and Cabernet Sauvignon on multiple training systems; leaf removal and shoot thinning of Cabernet Sauvignon) were conducted in Virginia to evaluate the effect of vineyard management on these important secondary metabolites.
I. Selective leaf removal from the fruit zone of mature Chardonnay and Cabernet Sauvignon grapevines was evaluated for two seasons to determine the influence on grape glycosides. Two treatments were compared 1) removal of two to four leaves per shoot from around the fruit zone three weeks post-bloom and 2) no leaf removal. Chardonnay vines were grown on a Hudson River Umbrella (HRU), Cabernet Sauvignon on a mid-wire, HRU and Lyre systems. Leaf removal (LR) increased fruit zone porosity as measured by the percentage of sunlight penetration and point quadrat analysis. Yield components were generally unaffected by treatment as were fruit soluble solids, pH and titratable acidity at harvest. LR increased Chardonnay glycosides for five or six sampling periods while having no influence on total phenols, flavonoid and nonflavonoid phenols. Cabernet Sauvignon glycosides was greatest in fruit from leaf-pulled vines at four of six harvest dates. LR increased total phenols and anthocyanins, although not consistently.
II. In a separate study begun in 1995, shoot thinning, mechanical and hand fruit zone leaf removal of mature Cabernet Sauvignon grapevines were evaluated for their influence on non-colored glycosides. Four shoot densities [14.0, 18.9, 23.3, and 25.4 shoots per cordon meter (SPM)] were examines, each with no leaf removal (No LR), mechanical leaf removal (M LR) or mechanical plus hand leaf removal (M + H LR) imposed six weeks post-bloom. Shoot thinning (ST) reduced leaf area per vine while increasing fruit zone porosity as measured by the percentage of sunlight penetration and point quadrat analysis. LR increased fruit zone porosity. Fruit soluble solids, pH, and total phenols were generally increased with increased fruit exposure while titratable acidity and total anthocyanins were generally unaffected at harvest. Non-colored glycosides concentration was greatest in the most exposed treatment (14.4 SPM and M + H LR).
IV. VVA Annual Membership and Technical Meeting
The VVA's annual membership meeting and technical session will be held on August 26, 1996. The program will start at White Hall Vineyards and Winery and move to a second location after lunch. The two main aspects of the technical program will be (1) discussions about grape varieties that are suitable for locations where vinifera varieties are prone to winter injury, and (2) discussions and review of training and trellising system alternatives. See Tony's newsletter for details.