Enology Notes

Enology Notes # 94, September 24, 2004

To: Regional Wine Producers

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

Subjects: Herbaceous Character in Red Wines; Environmental Taints; Upcoming Extension Program; New Edition of Winery Planning and Design Manual Available

1. Herbaceous Character in Red Wines.

   Occasionally, our regional red wines, including Cabernet Franc, have excessive herbaceousness, resulting in a reduction in fruit intensity, and detrimentally impacting palate structure and texture. This problem was highlighted by wine critic Tom Stevenson during his visit to Virginia (see his report posted at www.vtwines.info).

   With the relatively-high rainfall this season, there is an increased potential for herbaceousness. This sensory feature is mainly derived from pyrazines. Methoxypyrazines are nitrogen-containing compounds present in green plant tissue, including grape berries.

   There are two pathways of production, one dependent on grape maturity, climate, and fruit exposure, the other not. Combined concentrations in the fruit range from 0 to as much as 35 ng/L. The detection level for some of these compounds is remarkably low,

   1-2 ng/L.

   Methoxypyrazines are present in the free form, vs. bound glycosidically, etc. In the fruit, they decrease following veraison. The decrease is directly correlated to the decrease in malic acid concentration. Malic acid decreases at a faster rate during warm nighttime temperatures, as do methoxypyrazines. 

   High soil moisture delays fruit maturation and the reduction of methoxypyrazines. Increased sun exposure increases the rate of grape maturation and the reduction in methoxypyrazines. Therefore, there is a potential for a lower concentration on leaf-pulled vines and on the west side vs. east side of north-south vineyard rows. 

   Crop load also impacts these compounds, as expected. Excessive crop to leaf area can delay the rate of fruit development and, therefore, the breakdown of methoxypyrazines.

   Another factor impacting these compounds is the degree of uneven ripening. Because pyrazines are in higher concentrations in unripe fruit, the greater the degree of asynchronous ripening, the greater the problem. This stems from both unripe grape tissue and seeds. Immature seeds contain extractable pyrazines.

   These compounds are easily liberated into the juice. Therefore, the extent of cuvaison is not as an important as avoiding immature fruit and green plant tissues, such as leaves and stems.

   These compounds not only impact wine odor, but also structure/texture. Wine structural/textural balance can be depicted as follows:

   Sweet (body) D Acid + Phenols (tannin intensity, astringency, bitterness and dry tannins)

   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 important winemaking inferences.

   The sweet elements in a dry red wine impact body. They are derived from carbohydrates, polysaccharides and mainly ethanol. The acid elements are grape-derived organic acids. The phenolic elements include input from skin, seeds, and stems, plus winemaker intrusion, such as barrels and tannin additions. 

   The presence of methoxypyrazines increases the perception of tannin intensity, astringency, and dry tannins, thus magnifying the sense of acidity. As the perception of the acidity increases, the perception of the sweet elements (body or volume) goes down.  The result is a negative impact on structural/textural balance.

   We have studied the impact of interactions of pyrazine compounds (Hartmann et al., 2002) and have noted the binding impact of phenols. There is some evidence that certain phenols can bind these compounds, thus lowering their sensory impact. Oxidative polymerization occurring in young wines may be important. One of the benefits of microoxygenation appears to be the ability to bind various pyrazines, reducing their sensory impact, and resulting in more fruit aromas.  This is a justification for oxidative treatment of young red wines such as splash racking and the limited use of sulfur dioxide.

2. Environmental Taints.

   Until fairly recently, compounds such as TCA (2,4,6-trichloranisole), and to a lesser extent TeCA (2,3,4,6-tetrachloroanisole), were held mainly responsible for taint in wines. It was assumed that the ultimate problem stemmed from natural cork. This is why musty or wet cardboard-smelling wine is so frequently mistaken for cork taint.

   It has been understood for sometime that not all wine taints are cork-derived. Today, we know that there are a host of compounds, TCA included, which can cause taint and that do not necessarily come from cork. What is new is the understanding that taints can come from the environment in which wine is stored. As such, this is an important winery establishment and design issue.

   TBA (2,4,6-tribromoanisole) is a molecule which behaves like TCA and produces a similar effect, but is one of several source compounds responsible of environmental taint.

   The precursor of TBA is TBP (tribromophenol), which is widely used to treat wood and wood products. It is both a flame retardant and fungicide. Additionally, TBP can be produced by some algae, and it is also found in detergents containing bromine.  This extremely volatile compound has been found not only in wines, but on winery surfaces including barrels, plastics (including synthetic closures), natural corks, and wood structures, including walls, floors and ceilings. TBA released from treated wood circulates in the air and can cling to any surface. The perception threshold for TBA is as low as 4 ng/L, about the same as for TCA. This concentration is roughly equivalent to one sugar cube dissolved in the water contained in 100 Olympic-sized swimming pools.

   Wood treatments have become a serious environmental pollutant. The growing uses of recycled wood and plastics, which may have been treated with TBP or related compounds, may constitute a serious source of potential contamination in the future. 

   The extent of TBA or other-source environmental taint is unknown. However, several prestigious French and US wineries have undergone the significant expense of attempting to rid their cellars of this problem.

   Environmental taint is derived from the interaction of molds, halogens (chlorine and bromine), and phenols (such as wood products) and may result in either the direct perception of a taint or simply the loss of wine fruit and aroma intensity. The problem is frequently associated with chemically-treated wood, most notably wood which has been treated with TBP (tribromophenol).

   Sources of possible environmental taint include barrels, wood barrel racks, interior walls and ceilings, wood catwalks, ladders, cardboard, wood pallets, tap water, hoses, filter pads, fining agents, and polymeric tank liners. Environmental taint is a greater problem in new wineries, in humid cellars, and in air-conditioned cellars.

3. Upcoming Extension Program.

   The Wine/Enology-Grape Chemistry Group will offer a two-day juice and wine analysis short course January 11 and 12, 2005. This program will be a hands-on, practically oriented laboratory course. It will be conducted in the teaching laboratory of the Food Science and Technology Building at Virginia Tech. For further information, see Enology Notes #93.

   Enrollment is Limited and Restricted: The short course will be limited to a total of 14 participants. Only one person per bonded Virginia winery may register. Registration is for both days. Preference will be given to bonded winery representatives who register BEFORE MONDAY, NOVEMBER 15, 2004.

4. New Edition of Winery Planning and Design Manual Available.

   Edition #8 is 237 pages long and reviews a variety of subjects discussed during seven Winery Planning and Design short courses given throughout the country. Details are available on the web at www.vtwines.info.

   References

   Chatonnet, P., S. Bonnet, S. Boutou, and M. Labadie. 2004. Identification and responsibility of 2,4,6-tribromoanisole in musty, corked odors in wine. J. Agric. Food Chem. 52:1255-1262.

   Hartmann, P.J., H.M. McNair, and B.W. Zoecklein. 2002. Measurement of 3-alkyl-2-methoxypyrazine by headspace solid-phase microextraction in spiked model wines. Am. J. Enol. Vitic. 53:285-288.

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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: http://www.vtwines.info/
Phone: (540) 231-5325
Fax: (540) 231-9293
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