Ohio Grape-Wine Electronic Newsletter
Editor: Imed Dami, PhDAssistant Professor and Extension Viticulture Specialist
Department of Horticulture and Crop Science
Ohio Agricultural Research and Development Center
1680 Madison Avenue
Wooster, OH 44691-4096
Phone: (330)-263-3882
Fax: (330)-263-3887
E-mail: dami.1@osu.edu
www.oardc.ohio-state.edu/grapeweb/
22
October 2007
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Herbaceous Character in Red Wines
By Dr. Bruce Zoecklein, Enologist and Head, Enology-Grape Chemistry Group, Virginia Tech
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.
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.
3. Processing Factors Influencing Herbaceousness. Processing steps influencing herbaceous compounds include:
- MOG (materials other than grapes) 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 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. 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.
Deer Control Workshop in Ohio
Name of Event: Are Deer Eating Your Profits?
Date(s): Friday, November 2, 2007
Time: Start/Stop with AM and/or PM: Registration begins at 9:00 am. Workshop runs from 9:30 - 3:30 and includes lunch.
Program Description (1-2 sentences): Specialists in deer biology and control will provide valuable insight on how to protect your land from the increasing deer population. You’ll also hear from vendors on what is available on both a small and large scale for deer deterrent and exclusion. After lunch, you will tour the fields of OSU South Centers and view their deer fencing systems and learn the pros and cons of the many options available. Here’s your opportunity to spend a day with the experts and learn valuable ways to protect your crops and plants while increasing your yield by reducing damage from deer.
Intended participants/attendees: Farmers, gardeners, landowners
Location: OSU South Centers, 1864 Shyville Road, Piketon, OH 45661
Registration Details: Cost is $30.00 and includes lunch. Registration limited to first 75 paid attendees. Registration deadline is Mon. Oct. 29
Event Web Link: http://southcenters.osu.edu/hort/pdfs/Deer_workshop07.pdf
Contact Person: Name/Phone/Email: Julie Strawser, 740-289-2071 ext 223, strawser.35@osu.edu
Freeze Prevention Workshop in Missouri
By Eli Bergmeier, Viticulture Research Specialist, University of Missouri-Columbia
As you’re well aware, many grape producers in the Midwestern, eastern, and southern United States experienced widespread frost or freeze injury in early 2007. To aid producers in understanding what can be done to prevent losses to weather events of this type in the future, the Institute for Continental Climate Viticulture and Enology (ICCVE) at the University of Missouri will host a one and one-half day workshop entitled “Understanding and Preventing Freeze Damage in Vineyards” on December 5 and 6, 2007. The workshop location will be the Memorial Union on the University of Missouri campus in Columbia, Missouri.