Wines Enology Grape Chemistry Lab at Virginia Tech

Enology Notes

Wine/Enology Notes # 87 March 24, 2004

To: Regional Wine Producers

From: Bruce Zoecklein, Head, Wine/Enology-Grape Group, Virginia Tech.

Subject: Winery Planning and Design Workshop, and American Society for Enology and Viticulture, Eastern Section, Technical Meeting and Symposium

Winery Planning and Design Workshop. A one-day Winery Planning and Design Workshop is scheduled for June 6, 2004, at Penn State, College Park, PA. This program is for those seriously interested in entering the commercial wine industry.

This event is being coordinated by Dr. Bruce Zoecklein, Head, Wine/Enology-Grape Chemistry Group, Virginia Tech, and Dr. Stephen Menke, Enology Extension Educator, Penn State, and is co-sponsored by Virginia Tech and Penn State. For details, see the Wine/Enology-Grape Chemistry Group website at

The workshop fee is $300, payable to Penn State. Send check to:

Mid-Atlantic Winery Planning and Design Workshop
c/o Stephen Menke
Adams County Extension Office
670 Old Harrisburg Road
Gettysburg, PA 17325-3404

American Society for Enology and Viticulture, Eastern Section, Technical Meeting and Symposium. In July 2004, the ASEV/ES annual meeting will once again convene in Virginia. The venue will be the Hotel Roanoke and Conference Center (, a short drive from spectacular scenery, the Blue Ridge Parkway, and some of Virginia’s finest wineries. Half-day Technical Sessions are scheduled for 14-16 July 2004, with the balance of each day focusing on a “Grapes, Wine and Environment” Symposium.

The underlying goal of the symposium is to explore how soils, climate (particularly temperature), and cultural practices affect fruit and wine composition and quality, especially in a warm, humid environment. Featured Symposium speakers will include Kees van Leeuwen, Erland Happ, Gregory Jones, Phil Freese, Andrew Reynolds, and Daniels Granes. For more information, see the Wine/Enology-Grape Chemistry Group website at

Important topics of discussion at our summer symposium, “Grapes, Wine and Environment” include the following.

Red Wine Textural Quality. What are the key natural processes that affect the formation and accumulation of phenols in the fruit? What vineyard management tools are available to influence phenols? What are the key natural processes that affect phenols in the wine and their modification during processing? How can we monitor the quantitative and qualitative changes in phenols in the fruit and the wine? What are appropriate concentrations and proportions of phenols for specific wine styles?

Important Environmental and Viticultural Conditions. Important environmental and viticultural conditions influencing grape phenols include macro-, meso- and microclimate, seasonal variation, maturity (pulp, skins, seeds, stems), berry size, and yield (leaf area per fruit weight ratio).

The following is adapted from Gladstones (1992). “High temperatures (35/30C day/night) during stage 1 of berry development cause irreversibly smaller berries. Because the skins contain a high concentration of so-called luxury or secondary metabolites, a change in the surface to volume ratio is highly important.

“An optimum mean ripening temperature for grapes is about 20C or slightly higher. This temperature is a little below the mean temperature of around 23-25C at which grape vines reach their greatest rate of photosynthesis and growth.

“Below that temperature growth rate is limited more by the rates of biochemical processes other than photosynthesis. As such, a surplus of sugars can accumulate which is available to be stored or transported to the fruit or to be used to produce luxury products not essential for plant growth, such as anthocyanins and aroma/flavor compounds.

“As the temperature increases above the optimum, the demand for sugar due to rapidly accelerating respiration, combined with no further increase in the rate of photosynthesis, leaves a smaller amount of sugar for growth and luxury products. This is why anthocyanins disappear at high growing temperatures.

“Phenol production is the result of enzymatic activity in the plant. The maximum rate of pigment synthesis depends on a good supply of sugar as the basic chemical substrate and energy source, together with favorable intermediate temperatures for maximum enzyme activity.”

Kliewer and Torres (1972) studied the effects of day/night temperature range on grape anthocyanins. They found a strong positive correlation between minimal temperature differences, and anthocyanin development and concentration.

Anthocyanin concentration in the skin is influenced by sun exposure, as are non-colored phenols. For example, flavonol concentration can be increased by a factor of 20 or more, due to solar exposure (Price et al., 1997). The increase in total phenols as a result of solar exposure is an important issue in defining the optimum degree of selective fruit zone leaf removal in a warm climate, such as Virginia’s.

Sensory Significance. What is the sensory significance of specific phenols? All phenols are both bitter and astringent. Bitterness is the result of access to membrane-bound receptors, and is likely limited by molecular size.

Generally, polymerization, or binding together, increases astringency to a point, due to an increased number of possible hydrogen binding sites. Monomeric flavonoids (relatively low molecular weight single flavonoid phenols) are primarily bitter; however, upon polymerization, their astringency increases more rapidly than their bitterness (Noble, 1998).

The relative proportion of bitter to astringent phenolic compounds is important with regard to textural quality. The ratio of bitterness to astringency helps to explain the concept of hard vs. soft tannins. Grapes yielding intense and balanced wines are also characterized as having a high anthocyanin to tannin ratio (Cheynier et al., 1998).

If there is insufficient color (anthocyanins), there will not be enough for bonding with non-colored phenols, such as tannins. There is never a disadvantage in getting the darkest color possible, but there is a notable disadvantage in having too much tannin.

The interactions of wine structural components, including phenols, are also an important feature governing wine texture as a function of the interaction with other structural elements, according to the following relationships (Zoecklein et al., 1995).

Suppleness Index: Alcohol - (Acidity + Tannin)

Palate Balance Equation

Organic acids

Skin, seed, and stem phenol

Barrel phenol
Enological tannins
Volatile phenols

Wine structural/textural components interact in certain relationships depicted here in what I call the palate balance equation.

This inverse relationship suggests that an increase in the perception on one side decreases the perception of components on the other. The converse is also true.

With this in mind, it is easy to understand how the specific components of wine mouthfeel interact, and make some important winemaking inferences.

The sweet elements in a dry red wine are derived from carbohydrates, polysaccharides, and mainly ethanol. The acid elements are grape-derived organic acids.

The phenolic elements include input from these components: skin, seeds, stems, plus winemaker intrusion, such as barrels and tannin additions.

Sweet <--> Acid + Phenolics (tannin intensity, astringency, bitterness and dry tannins)

The phenolic elements in this relationship include the perceptions derived from the following: tannin intensity, astringency, bitterness, and dry tannins in a relationship well described by Delteil (2003). Some correlations and associations of astringency and bitterness are provided below:

+ corr with grape and oak tannins
+ corr with acidity
0 corr with sugar
+ corr with VSCs (volatile sulfur compounds) and herbaceous compounds
- corr with alcohol up to 14%, + corr above 14%
+ corr with non-soluble solids

+ corr with ethanol
+ corr with grape and oak tannins, including immature seed tannins
+ corr with acid, specifically malic acid
+ corr with VSCs
+ corr with yeast in suspension
- corr with polysaccharides (reason for addition using high polysaccharide-producing yeast, yeast fining, and gums such as Gum Arabic)

Additionally, as viscosity and/or pH increase, the perception of astringency decreases, while bitterness is unchanged. Increasing the alcohol concentration increases the perception of bitterness, and reduces astringency. Astringency masks bitterness. This is an extremely important concept, as it relates to both aging and fining (see Zoecklein, 1988). It also helps to explain why even experienced sensory evaluators may have difficulty in distinguishing bitterness from astringency.

An understanding of the relationships among the various phenols and other wine components is essential to the understanding of textural quality of wines.


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Dr. Bruce Zoecklein
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:
Phone: (540) 231-5325
Fax: (540) 231-9293