Enology Notes

Enology Notes #114, July 14, 2006

To: Regional Wine Producers

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

Subject: Herbaceous Character in Red Wines; Practical Monitoring and Management of Brettanomyces Workshop; Virginia Tech Enology Service Lab

I. Herbaceous Character in Red Wines. Occasionally, red wines have excessive herbaceousness, resulting in a reduction in fruit intensity, and detrimentally impacting palate structure and texture.

This sensory feature is mainly derived from a group of nitrogen-containing compounds, pyrazines, present in green plant tissues, including grapes. One important methoxypyrazine, IBMP (2-methoxy-3-isobutylpyrazine) imparts a vegetal aroma to Cabernet Sauvignon, Cabernet franc, and Sauvignon blanc, described as bell- or green pepper-like.

Concentrations in the fruit range from zero to 35 ng/L. To put the sensory threshold of these important components in perspective, 1 ng/L is 1 part per trillion. The detection level of IBMP is 2 ng/L in water, and about 15 ng/L in red wines. IBMP may contribute to leafy-type aromas, even in concentrations as low as 2 ng/L (Allen 2006).  

Several factors have confounded our understanding of methoxypyrazines, including their very low concentrations (and, therefore, difficulty in quantification) and their association with other compounds.

Odor synergism and antagonism also confound our understanding of methoxypyrazines in a matrix as complex as wine. Sulfur-containing compounds can complement the vegetative odor, and some have green-type odors themselves.

Methoxypyrazines not only contribute to odor but also impact palate balance (see Enology Notes #94).

Viticultural Factors Influencing Herbaceousness. There are two pathways to production of methoxypyrazines, one dependent on grape maturity, climate, and fruit exposure, the other not (Allen 1998). Important viticultural factors influencing wine herbaceousness include the following:

• Vegetative growth
• Soil moisture
• Leaf maturity
• Fruit exposure to light
• Crop load and rate of fruit maturation
• Uneven fruit ripening 

In the fruit, the major methoxypyrazine (IBMP) is formed early and breaks down following véraison. The level in ripe fruit is related to the prevailing weather conditions, which lead to the initial IBMP concentration. The breakdown is initially very rapid, then slows as fruit maturity increases (Roujou de Boubee 2004).

Does this drop represent photo-degradation? Not likely. It appears that the decrease in pyrazines is the result of temperature (Allen 2006). The decrease in concentration is directly correlated to the decrease in malic acid. Malic acid decreases at a faster rate during warm nighttime temperatures, as do methoxypyrazines like IBMP. 

Photosynthesizing green leaves, and the conditions that promote their persistence (such as high soil moisture), contribute to harsh green aromas/flavors in the fruit. As such, there appears to be a correlation between leaf maturity (progression towards colors expected at senescence) and reduction in berry green-fruit character (Delteil 2003, Roujou de Boubee 2004). Therefore, the timing of leaf senescence, and the associated changes in plant hormones, may be important with regard to green fruit aroma/flavor.

High soil moisture can increase vegetative growth, and 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 in leaf-pulled vines, and in vines grown on training systems that may promote more light exposure to the fruit.

Excessive crop-to-leaf area can delay the rate of fruit development. If this occurs,   the breakdown of methoxypyrazines would be impacted. In a study evaluating the impact of fruitful buds per vine, those properly balanced, but with higher bud counts, had fruit with lower concentrations of IBMP (Allen 2004).

Because pyrazines are in higher concentrations in unripe fruit, the greater the degree of asynchronous ripening, the greater the concentration in the resultant wine. The degree of uneven ripening is an important wine quality limiting factor (see Enology Notes #58 and 81).

Processing Factors Influencing Herbaceousness. Processing steps influencing herbaceous compounds include:

• MOG removal
• Stem separation
• Cap management
• Délestage
• Microoxygenation
• Thermovinification

All green grapevine tissues contain methoxypyrazines. The concentration of IBMP in basal leaves is reported to be very high, three to five times that found in the grape clusters. Therefore, leaves in the fermentor can be a source of herbal character.

Pyrazines, such as IBMP, are found in Cabernet Sauvignon stems (53%), seeds (31%), skins (15%), and flesh (1%). As such, green pepper-type character in some wines may be the result of stem contact. Many premium wine producers use post-destemming sorting of some red fruit varieties. This may be a critical step if destemmers leave a significant concentration of cap stem fragments (jacks) in the must. There are now commercially available post-destemmer sorting tables. During a Winemakers Roundtable meeting last fall, we presented wines that were made with and without jack stem removal. The sensory differences were dramatic.

Herbaceous compounds are also found in the fruit. In Cabernet Sauvignon, the skins contain about 72% of the fruit IBMP, and the seeds about 24% of the total (Roujou de Boubee, 2004). Compounds like IBMP are easily liberated into the juice. Therefore, cap management protocol may not be an important factor in controlling the liberation of these compounds from the fruit, depending upon the length of cuvaison. In some instances, however, press wine will have a higher concentration than free run wine.

The concentration of methoxypyrazines liberated from the seeds during fermentation depends on several factors, including seed maturity and uniformity of maturity. We have conducted a number of studies using délestage with seed deportation (see Enology Notes #8, 23, 69, 76, 78, 80, and 90). In many instances, there is less herbal character in the resultant wine. This may be the result of seed removal, oxygenation, or other factors.

We have conducted several studies using heat pre- or postfermentation (Mansfield and Zoecklein, 2003, McMahon et al., 1999). Thermal treatment may have an influence on the perception of herbaceousness, as a result of volatility and possible chemical changes, including oxidation.

Principal Component Analysis of Sensory Results for Microoxygenation Experiment. Sensory characteristics are represented by different axes. T=treated wine, C=control wine, numbers indicate wine station.

Source: Zoecklein et al. (2002), from the MS thesis of Patrick Sullivan

Wine Oxygenation and Methoxypyrazines. Microoxygenation has been shown to impact herbaceousness. The figure above (from Patrick Sullivan’s MS thesis) illustrates a principal component sensory evaluation of red wines, pre- and post-microoxygenation. The distance from the center, and the proximity of each data point to the vector line, indicates the strength of each sensory attribute. For example, control wine C3 had a strong vegetative aroma pre-treatment. Following microoxygenation, this same wine was perceived as having much more fruit intensity (T3).

The effect of microoxygenation on methoxypyrazines is not well understood. It appears that the reduction in the herbal character may not be the result of changes in methoxypyrazines, but changes in thios or sulfur-containing compounds, that help to reinforce the herbal or vegetative sensory perception.  Some thio compounds complement the odor of methoxypyrazines. Sulfur-containing compounds, unlike methoxypyrazines, are not stable. During microoxygenation, it is the oxidation of some sulfur-containing compounds that may result in the muting of the vegetal character of treated wines.  

Excessive herbal and vegetative character results in aromatically disbalanced wines. It is essential that winemakers carefully evaluate their young wines (at the proper temperature, not cellar temperature) to determine the aromatic profile. It is equally essential that premium winemakers understand the environmental, viticultural, and enological factors that produce and impact methoxypyrazines.

2. Practical Monitoring and Management of Brettanomyces Workshop. The Enology-Grape Chemistry Group is sponsoring a workshop, titled “Practical Monitoring and Management of Brettanomyces,” to be held at Horton Cellars, Monday, August 7, 2006, beginning at 1:00 PM.

This program will be lead by Dr. Bruce Zoecklein and Lisa Van de Water. Lisa Van de Water is a technical consultant for The Vinotec Group, located in Napa, New Zealand, Chile, and South Africa. She is the founder of Pacific Rim Oenological Services, and an internationally recognized expert in wine microbiology.

This practically-oriented program will review the following:

• Practical considerations in monitoring Brett:
  What is the importance of 4-ethylphenol analysis? What do genetic-based tests, like Scorpion, tell us?

• Practical considerations in managing Brett in the cellar:
  Is ozone the most effective sanitizer for Brett? How do you know the effectiveness of sanitization?

• Brettanomyces population growth, and factors impacting growth:
  What is the relationship between excessive DAP addition and Brett growth?

• The relationship between cell growth and metabolite formation:
  Is cell culturing the best way to monitor Brett?

• The effect of strains, and metabolites produced by them:
  Are all Brett strains the same?

• Relationships between cell populations and wine sensory impact
  
  
  
• Relationships between Brett metabolites and Brett sensory impact:
  What are the most important metabolites for producing Brett character? Are they the same for all wines? In the same concentrations?

Registration:  Seating is limited and pre-registration is required. Registration fee of $25 must be received by Friday, August 4, 2006. Mail check to Terry Rakestraw, Department of Food Science and Technology (0418), Virginia Tech, Blacksburg, Virginia 24061. Make checks out to Virginia Tech Foundation. Please put Brett on the memo line.

3. Virginia Tech Enology Service Lab.  The Enology-Grape Chemistry Group at Virginia Tech has established a fee-based, full-service enology laboratory. This lab provides chemical, physical, microbiological, and sensory analysis, including standardized reagents.

This service is available to wineries in any state, and to importers and wholesalers. The prices are competitive, with profits designed to help support extension activities of the Enology-Grape Chemistry Group.

The goal of this analysis program is to provide very rapid turn-around, with optimum precision and accuracy. Details regarding this service are available at www.vtwines.info.

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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
Cell phone: 540-998-9025
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