Enology Notes #132, July 24, 2007
To: Regional Wine Producers
From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech
Subject: Herbaceous Character in Red Wines – a Review; Viticultural Factors Influencing Herbaceousness; Processing Factors Influencing Herbaceousness; Wine Oxygenation and Methoxypyrazines; New Edition of Winery Planning and Design CD Available; French Wine Study Tour November 25-December 7, 2007; The VT Enology Analytical Services Laboratory Certification; Fermentable Nitrogen Analysis; Cabernet franc Meeting; Please Adjust Your Email Filters to Allow Enology Notes Through
1. Herbaceous Character in Red Wines – a Review. 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, which are present in green plant tissues, including grapes. One important methoxypyrazine, IBMP (2-methoxy-3-isobutylpyrazine), imparts a vegetal aroma at relatively low concentrations in the fruit, ranging from zero to 35 ng/L.
Several factors have confounded our understanding of methoxypyrazines, including their very low concentrations (and, therefore, difficulty in quantification) and their association with other compounds.
A concentration of 1 ng/L is 1 part per trillion. Allen (2006) put this in perspective in a presentation outlining the difficulty in monitoring methoxypyrazines: if the earth’s population is 6,493,359,729, then measuring IBMP would be roughly equivalent to measuring one part in 154 earth populations.
An additional confounding factor is that herbaceousness can be caused by methoxypyrazines, certain monomeric phenols, and polyunsaturated fatty acid derivatives. These include compounds such as hexanal and hexenal, which we have measured in relatively high concentrations in some regional wines.
IBMP (2-methoxy-3-isobutylpyrazine) imparts a vegetal aroma to Cabernet Sauvignon, Cabernet franc, and Sauvignon blanc, described as bell- or green pepper-like. 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). Too much of this character disbalances the wine and results in the overall loss of complexity. This is a notable problem with some Cabernet franc wines produced in our region.
In addition to the problems of low concentrations and association with other compounds, odor synergism and antagonism also confound our understanding of methoxypyrazines, in a matrix as complex as that of wine. Sulfur-containing compounds can complement the vegetative odor, and some have green-type odors themselves. This is why the oxidation of some thiols to disulfides, which occurs with splash racking and/or microoxygenation, can change the perception of herbaceousness.
Methoxypyrazines not only contribute to odor, but also impact palate balance (see Enology Notes #94).
2. Viticultural Factors Influencing Herbaceousness. There are two pathways to production of methoxypyrazines, one dependent on grape maturity, climate, and fruit exposure, but 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? It is 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 the persistence of odor-active compounds (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 pyrazines’ concentration in the resultant wine. The degree of uneven ripening is an important wine quality limiting factor (see Enology Notes #58 and 81).
- MOG (materials other than grapes) removal
- Stem separation
- Cap management
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 fermenter 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.
4. Wine Oxygenation and Methoxypyrazines. 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 in 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.
Source: Zoecklein et al. (2002), from the MS thesis of Patrick Sullivan
In evaluating the changes occurring with microoxygenation, we noted that there was a change in the perception of SLO or sulfur-like odors. This change was correlated with a lower perception of herbal character, as a result of the oxidation of thiols to disulfides:
2 R-S-H + ½ O2 → R-S-S-H + H20
Source: Zoecklein et al. (2002), from the MS thesis of Patrick Sullivan
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 at 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.
5. New Edition of Winery Planning and Design CD Available. For additional details, see Enology Notes #129.
6. French Wine Study Tour. For additional details, see Enology Notes #131.
8. Fermentable Nitrogen Analysis. For additional details, see Enology Notes #131. The importance of measuring yeast assimilable nitrogen (YAN) or fermentable nitrogen is discussed in various editions of Enology Notes on the website at www.vtwines.info; click Enology Notes Index.
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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
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