Enology Notes

Enology Notes #101, April 20, 2005

To: Regional Wine Producers

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

Subjects: The Good Volatile Sulfur Compounds: Nitrogen, Maturity Evaluation, Green vs. Brown Juice, Yeast; American Society for Enology and Viticulture, Eastern Section, annual meeting; Wine Closure Roundtable Meeting

The Good Volatile Sulfur Compounds. We tend to think of volatile sulfur compounds (VSCs) in negative terms because they are often associated with ‘off’ odors, such as hydrogen sulfide and mercaptans. However, not all VSCs provide negative sensory attributes. Several grape varieties, and their subsequent wines, are positively impacted by VSCs. For example, volatile sulfur compounds are important to the nature of Riesling, Gewürztraminer, Manseng, Chenin blanc and Sauvignon blanc wines.

Complex organic compounds, with abbreviations such as 4-MMP, 4-MMPOH and 3-MH, are examples of VSCs that contribute positively. In wines, they provide odors described as box tree/bloom, citrus zest and grapefruit/passion fruit, respectively. These components are particularly important for the Sauvignon blanc character. As such, monitoring VSCs has increased our understanding of the impact of vineyard and wine management on these important aroma/flavor components.

Each VSC is produced from S-cysteine (a sulfur-containing amino acid) metabolism in the plant, and impacted by the following:

  • Soil and climate
  • Fruit maturity
  • Juice processing
  • Yeast species and strain
  • Malolactic fermentation
  • Aging

One of the reasons it is said that cool nighttime temperatures are important for Sauvignon blanc, is due to the production and retention of fruit volatiles, such as these VSCs. Ideal conditions reported in parts of the Loire Valley for Sauvignon blanc are 25-27°C day, and 10°C at night, from veraison to harvest.

Nitrogen. Some interesting work reported by Denis Dubourdieu (2004) suggests a relationship between fruit nitrogen and the production of VSCs in Sauvignon blanc. Too little fruit nitrogen substantially reduced the concentration of VSCs, resulting in wines which are described as less fruity and aromatic, sweaty and cheesy in character.

The level of fermentable N in Sauvignon blanc juice for optimizing aroma/flavor is reported to be between 175 and 300 mg/L (Dubourdieu, 2004).

The easiest way to measure fruit nitrogen (which is different from other plant tissue or soil N) is to conduct a Formol titration on the juice. In a study conducted several years ago, we demonstrated the ease and viability of this assay for determining fruit nitrogen status (Gump et al., 2002). This analysis can be used to determine if plant N is low and adjustments in soil N should be made.

Maturity Evaluation. Volatile sulfur compounds are not uniformly distributed in the fruit. The relative ratio in Sauvignon blanc skins and pulp, respectively, is as follows (Dubourdieu, 2004):

  • 4-MMP 7% and 92%
  • 4-MMPOH 10% and 80%
  • 3-MH 50% and 50%

Due to the distribution between skin and pulp, some prefer to taste Sauvignon blanc whole fruit for maturity evaluation, rather than simply evaluating juice aroma and aroma intensity. Additionally, a portion of each of these sulfur-containing compounds is in the free form, and a percentage bound to other components. When the whole berry is put into the mouth, some of the bound forms are believed to be released, making the fruit seem more intense in aroma/flavor than simply the juice. The obvious problem is the variation in maturity level among individual Sauvignon blanc berries.

Green vs. Brown Juice. In our industry trip to the Loire Valley, we visited a number of Sauvignon blanc producers who blanket their juice with carbon dioxide during pressing, helping to keep the juice green by minimizing oxidative degradation. There is an age-old debate regarding the merits of brown vs. green juice in the fermentor. As new research comes to light, we find that the dichotomy between green vs. brown relates not simply to style, but also variety.

The three VSCs listed above are easily oxidized. Therefore, protecting them from the impact of molecular oxygen makes sense, particularly in a variety such as Sauvignon blanc, which is so dependent on VSCs for varietal character. Maximizing the passion fruit notes requires the limitation of oxidative degradation. Indeed, this is a primary reason why some also add ascorbic acid to the juice. Ascorbic acid is a very good oxygen scalper, acting much more rapidly than sulfur dioxide.

Another very important sulfur-containing compound in grape juice is glutathione. This tripeptide is by far the most abundant source of reduced sulfur in the grape berry. It acts as an antioxidant and helps to stabilize 4-MMP, 4-MMPOH and 3-MH (Dubourdieu, 2004). Lees contain glutathione, one reason why lees storage keeps wines from oxidative degradation.

One very important fact to consider when managing 4-MMP, 4-MMPOH and 3-MH, is that the concentration of each is greatly lowered in the presence of copper. This makes sense; we add copper to bind with VSCs that negatively impact odor. Copper sulfate in the fermentor greatly reduces the concentration of desirable volatile sulfur compounds and limits the ability of glutathione to act as an oxygen buffer. This is a major concern with late season copper sulfate sprays.

Yeast. Yeast species and strains vary in their abilities to hydrolyze or release bound sulfur-containing compounds. In a study reported by de Barros Lopes (2004), commercial wine yeast strains differed by a factor of 20 in their release of free 4-MMP from the bound form. Thus, some yeasts are very effective in releasing these desirable VSCs.

Work by a number of research groups, including ours, suggests that it may be desirable to have the fermentation conducted by several yeasts, one for the basic fermentation, and one for increasing the aromatic nature of the wine, by hydrolysis of bound VSCs and other compounds. This may be the most effective means of maximizing the grape potential.

More to follow.

American Society for Enology and Viticulture, Eastern Section, annual meeting. The 2005 annual meeting of the American Society for Enology and Viticulture, Eastern Section, will be held July 13-16 in St. Louis, Missouri, at the Millennium Hotel. This spectacular facility, on the banks of the Mississippi, is within walking distance of the Arch and downtown cultural attractions.

The meeting will involve technical presentations, the Viticulture Consortium East research summit, a wine industry trade show, the annual banquet, a local wine industry tour, and a symposium.

This year’s symposium will involve viticultural and enological discussions, and sensory evaluations, on the Cutting Edge Varieties: Norton, Pinot Gris, Traminette, and the cold-hardy Frontenac and La Crescent. Speakers from the academic community, commercial growers, and winemakers will present practical information and extensive sensory evaluations. For information and registration, see the website at http://www.nysaes.cornell.edu/fst/asev/.

Wine Closure Roundtable Meeting. A wine closure roundtable meeting is scheduled for April 25 at Veritas Winery beginning at 1:00 p.m.. The primary purpose of this meeting will be to evaluate wines bottled with screwcaps, synthetic closures, and natural cork. The background information for this meeting has been provided in Enology Notes, #96 - 100. Wines will include products I brought back from New Zealand, and wines involved in our wine closure research trials.

There is limited seating, therefore this meeting is restricted to Virginia producers. If space is available after the deadline, registration will be opened to others.

Pre-registration will be required. Registration fee is $25; please make checks payable to Virginia Tech Foundation, and mail to:

Terry Rakestraw
Department of Food Science and Technology
Virginia Tech, Blacksburg, VA 24061

Please provide your email address so that we can provide registration confirmation.

Call Terry Rakestraw after April 20 to determine if space is available (540-231-6805).

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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