Vol.17, No. 1 January - February, 2002
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061
Web site: http://www.fst.vt.edu/Zoecklein
I. Wine Balance
Palate balance is a critical feature influencing wine quality, by impacting the harmony and integration of structural components. The major factors governing palate balance in dry wines are the quantity and "quality" of tannins, concentration of alcohol, and concentration and types of acidity. These components interrelate, influencing the perception of balance according to the following:
Sweet » acidity + bitterness and astringency
The perception of sweetness derived from alcohol, polysaccharides and sugar (when present), must be in balance with the sum of the perceptions of acidity, astringency, and bitterness. This relationship suggests that the lower the acidity, the more tannin a wine can support. The palate balance formula is functionally analogous to the suppleness index described by Peynaud (1984):
Suppleness index = alcohol (vol/vol) -
(titratable acidity + tannin)
Red wines are not considered supple unless the suppleness index is below 5.0 (note that the acidity calculation uses sulfuric acid equivalents vs. tartartic acid) . The palate balance relationship is important in both maturity assessments and processing.
Phenolic Components and Balance. Some vineyards harvest fruits from the side of the grapevine canopy at different time periods. Why? The effects of light and temperature on phenols, among other things, can be significant. During the summer, the sun is in the southern part of the sky. Vineyard row orientation and canopy management can influence both heat and light into the canopy. For example, fruit on the southern side of east-west rows receives more light and heat.
Heat and light interception can have an important influence on fruit chemistry as we and others have seen. Based solely on the side of the canopy, we have noted a 31% difference in skin tannin per berry. We have also noted differences in skin pigment components: a 13% difference in total skin color per berry, and a 43% difference in the concentration of small polymeric pigments (SPP) in the skins (small molecular weight tannins associated with anthocyanins). These changes have been noted in the absence of differences in degrees Brix.
With regard to balance, both the quantity and quality of phenols is important. Chemically, grape and wine tannins are composed of subgroups (flavon-3-ols). In simplistic terms, those tannins which contain fewer than five subgroups are predominantly bitter and contribute to tannin hardness. Those tannins which contain more than five are predominantly astringent and contribute to suppleness. Robichaud and Noble (1990) found that both bitterness and astringency increased with increased concentration of (+)-catechin (tannin building blocks), although the rate of bitterness increase was greater than that of astringency. On a per weight basis, both bitterness and astringency increase with increased degree of polymerization, but astringency increases at a greater rate. As polymerization increases, the number of possible hydrogen bonding sites increases, which would be expected to increase astringency. Bitterness is the result of access to membrane-bound receptors, and is likely limited by molecular size. Differences in the lipid solubility of small molecular weight phenols allows them to depolarize the taste receptor cells, thus increasing the perception of bitterness. As grape skins mature, the average number of subunits in the tannins increases, increasing tannin suppleness. This is an important sensory and maturity feature.
The difficulties in assessing bitterness and astringency of various phenolic compounds involve the fact that astringency masks bitterness. As wines age, tannin polymerization and, possibly, precipitation occurs. This, coupled with the reduction of tannin astringency resulting from protein fining agents, for example, may slowly unmask bitterness. Mature tannins in the fruit are a requirement for premium red wines and, therefore, are an important harvest consideration. If the skin contains non-polymerized, harsh tannins, so will the wine.
Acidity and balance. Although the acidic character of wine is due to hydrogen ion concentration, both pH and acidity play important roles in the total sensory perception of this stimulus. With equivalent levels of acidity, the order of perceived sourness of the common wine acids is malic, tartaric, citric, and lactic. Ethanol is effective in increasing the acid thresholds, and this increase is even more dramatic with the inclusion of sucrose. Phenols may also be active in increasing minimum detectable acid levels. The perception of acidity is a function of the palate balance equation. This relationship suggests that acidity is magnified by the phenolic elements (astringency and bitterness), and muted by sweet elements, including carbohydrates, alcohol, etc.
Alcohol and balance. We are gaining greater insights into what we consider optimism maturity. As an industry, we are much more focused on the relationships between physiological maturity and wine styles and quality. Frequently, that has resulted in fruit with a higher sugar per berry concentration coming to the winery, resulting in higher alcohol concentrations.
Theoretically, a given weight of fermentable sugar should yield 51.5% (by weight) ethanol, according to the Gay-Lussac relationship:
C6H12O6 --> 2 C2H5OH + 2 CO2
Thus, an initial 180 g glucose should theoretically produce 92 g ethanol (51%) and 88 g carbon dioxide, upon complete fermentation. The actual alcohol yield is generally less than the theoretical. As an estimate of potential alcohol, many winemakers have used the conversion factor of 0.55. However, this is frequently not valid.
The cooler the grape-growing region, the higher the conversion factor. In upper Monterey County (California), for example, winemakers may use figures as high as 0.62. Jones and Ough (1985) found that alcohol conversions ranged from 0.54 to 0.61. Differences were noted between regions and growing seasons, as well as slight variations between varieties. Another factor influencing the alcohol/° Brix ratio is fruit condition. With raisined berries, initial ° Brix readings are low. With more complete extraction during pressing and fermentation, additional fermentable sugar is liberated, yielding higher-than-expected final alcohol.
The winemaker must know the sugar to alcohol conversion ratio for the particular variety and region, in order to accurately predict alcohol, and make amelioration calculations and stylistic decisions. Alcohol/EBrix conversion is a major issue impacting winegrowing regions. For example, at 20° Brix, a Region 1 (UCD heat summation system) could provide 12.1% (v/v) alcohol, while a Region 5 with the same ° Brix would yield wine with 11.4%, all other factors being equal. Fermentation temperature is also an issue. For example, fermentation of reds at 75° F may provide an alcohol/° Brix ratio of 0.579, while whites fermented at 60° F may have a ratio of 0.601.
Alcohol provides a sense of sweetness. Thus, a wine with a high phenolic load is frequently in better balance with both a lower acidity and higher alcohol content, as the palate balance equation would suggest. Relatively small differences in the alcohol concentration can make a difference on the structure and aroma. Alcohol is not simply a structural component, but has a direct impact on the varietal aroma and aroma intensity. Too much alcohol provides a vodka-like character, reducing the perception of the varietal. Therefore, there is an additional reason beyond structural balance to attempt to regulate and control the alcohol concentration.
Some areas of the US wine industry are using relatively new technological advancements to control the alcohol concentration in their finished wine. These include RO and Spinning Cone technology, as well as the latest, OD or Osmotic Distillation.
Osmotic distillation (OD) is the popular term for a process that may more correctly be called isothermal membrane distillation. It is a process for concentration of solutions, by removing the volatile solvent across a non-wetted (i.e., the solvent passes in the gaseous phase) microporous membrane into a stripping solution. Most commonly, the volatile solvent is water. The microporous membrane must then be hydrophobic to prevent the solution from penetrating the pores. In the case of water as the solvent, the stripping solution is usually a concentrated salt brine, though sugar solutions have also been used. The most commonly used materials are polypropylene (PP) or polytetrafluoroethylene (PTFE), either in hollow fiber or flat sheet form.
The driving force for the vapor to pass across the membrane is provided by the difference in the vapor pressure at the surfaces of the two liquids contacting the membrane. For an isothermal process, and assuming that the heat of vaporization is conducted fairly quickly through the membrane, the vapor pressure at the surface of a solution is a function of the solute and its concentration.
Modification of Structural Balance. There are several methods commonly used to modify the palate balance of a wine, such as using protein and protein-like fining, wood fermentation of reds, bitartrate stabilization to lower the acidity, etc. We are conducting research using relatively new technologies, which have also been used to modify structural components, including microoxygenation and delestage (both have been reported in previous editions of the Vintner's Corner and Enology Notes, both available on-line at www.fst.vt.edu/zoecklein/index.html).
Another method of structural component integration is the use of lees. During sur lie storage, yeast components, such as cell wall polysaccharides and particularly mannoproteins, are released into the wine. These macromolecules can positively influence structural integration, phenols (including tannins), body, aroma, oxygen buffering and wine stability. The interest in lees utilization goes beyond barrel-fermented Chardonnay.
Yeast-derived macromolecules provide a sense of sweetness, as a result of binding with wood phenols and organic acids, aiding in the harmony of a wine's structural elements. The natural fining that occurs contributes to reducing the yellow tones in whites, and helps to protect against oxidation of certain fruit aroma compounds.
Consider utilization of light, 'clean lees,' to enhance structural integration. Some use lees to increase the complexity of tank-stored wines. If you have such an interest but are concerned with the potential of reductive tones, use lees that have been in barrels for two months or more. Such lees possess all of the desirable features, but are much less likely to cause reductive problems. Using 'barrel aged' lees is particularly important if you intend to store sur lie in tanks greater than about 1000 gallons. The low oxygen concentration at the bottom of such tanks can create problems. Naturally, a careful sensory evaluation of the lees should be conducted.
Proper utilization of lees many be an important stylistic tool for reds as well as white wines. After fermentation, red wines are frequently settled and barreled. Transfer of light lees into barrels is a tool some use to add structure and complexity to reds. A disadvantage of such a practice is the loss of some red color, although this is not a problem this season.
II. Winery Planning and Design Short Course Proceedings Available
Proceedings of the Winery Planning and Design Workshop conducted in July are available. The 104 page proceedings covers establishing a business plan, and winery design considerations, including gravity flow, winery tank selection, sanitation, etc. Send $45 payable to:
Dr. Bruce Zoecklein
Department of Food Science and Technology
Virginia Tech (0418)
Blacksburg, VA 24061.
Proceedings are used to support our enology graduate education efforts.
III. Sample Submission
When submitting samples to the Enology-Grape Chemistry Lab for supplemental analysis, please put the FULL name of your winery on the samples. With the recent growth of the industry, initials are not adequate for sample identification.