Enology Notes

Enology Notes #69, January 24, 2003

To: Regional Wine Producers

From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech

Subject: Wine Balance continued

The following is the second of a two part series on wine structure/texture balance. The Synergy of Wine Structure and Flavor is the theme for the enology portion of the Virginia Vineyards Association Winter meeting to be held February 13 - 15, 2003 at the Omni Hotel in Charlottesville, Virginia.

Details regarding the enology session are available in Enology Notes #68. Additional meeting information, including registration, can be found on the VVA website at www:virginiavineyardsassociation.com.

Alcohol and balance. We are gaining greater insights into what we consider optimal 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/° Brix 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. This will be discussed at the VVA meeting in February.

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, 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 for 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 may 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.

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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.vtwines.info or www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293
Email: bzoeckle@vt.edu