Enology Notes

Enology Notes #79, August 25, 2003

To: Regional Wine Producers

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

Subject: A Review of Sulfur-Containing Compounds; Yeast Metabolism and H2S Formation; Sulfur Compounds and Management; On-line Publications.

1. A Review of Sulfur-Containing Compounds.

This continues the discussion on volatile sulfur compounds (VSCs) from Enology Notes # 70 and 71 (on line at www.vtwines.info). Given the potential for fruit rots this growing season, there is increased potential for reductive tonesin the 2003 wines.

Problem VSCs have been around for 8000 years of winemaking and remain today. Nearly 100 VSCs have been reported in wines. Sulfur-containing compounds can add to varietal character and complexity, while some can significantly detract. It is essential that winemakers understand the nature of these compounds and how to effectively manage their impact.

Most think of VSCs only in terms of olfactory sensations. However, as Dominique Delteil noted during his presentation at our Red Wine Mouthfeel short course, sulfur-containing compounds can have an impact on palate components. Specifically, they can influence the perception of volume (body) and increase the sense of wine bitterness. (For a discussion on wine mouthfeel issues, see the Enology-Grape Chemistry Group website at http://www.vtwines.info/. Click extension, then on-line publications.)

The various forms of sulfur in wine are a function of transfer of electrons (Butzke, 1997). Compounds such as sulfides (like hydrogen sulfide or H2S) contain sulfur in its most reduced or electron-rich form. Compounds such as elemental sulfur (S) added to the vineyard are electron neutral, while the oxidized forms of sulfur, such as sulfites (like sulfur dioxide) and sulfates (like copper sulfate), are electron deficient. Oxidation is the loss of electrons, so these forms are electron poor.

A wine's oxidation state, known as its oxidation/reduction or redox potential, helps to determine this electron flow pattern. This has an impact on potential wine odor, due to the sensory differences among members of these forms.

We can pair the list of VSCs in wine down to a few key players, most with different sensory characteristics and thresholds (Park, 2000).

The central reduced sulfur compound is H2S, with a sensory threshold of 1ppb (part per billion).

The organic sulfur-containing compounds are characterized by having a sulfhydryl (SH) group. Ethyl mercaptan (EtSH) contributes the sensation of burnt rubber, and methyl mercaptan (MeSH) to odors described as rotten or cooked cabbage. Both have a sensory threshold of 1ppb.

The oxidized sulfides (mono- and di-) are the group of VSCs which have a milder character than either H2S or mercaptans.

Dimethyl sulfide (DMS) has the odor of canned corn, cooked cabbages or vegetables, and is the most abundant VSC. It increases with bottle age and contributes to bottle bouquet. It is one of the most important sulfides because it can affect overall wine quality either positively or negatively, depending upon its concentration and the balance with other wine odors. Optimum levels contribute to complexity and roundness, while excessive concentrations detract. Another oxidized sulfide, diethyl sulfide, can also be present in wine, and has a rubbery character. Disulfides are also found in wines, but the levels are usually below the sensory threshold.

2. Yeast Metabolism and H2S Formation.

Yeast metabolism can result in H2S formation, by reduction of elemental sulfur, break-down of sulfur-containing amino acids, or reduction of sulfur from inorganic sources and/or sulfites (Butske, 1997).

Yeast strains are extremely variable in their ability to form H2S. However, even with the same yeast, winemakers can get enormous differences, depending upon must chemistry, yeast preparation methods, etc.

H2S formation by yeast during fermentation occurs in two distinct phases (Butzke, 1997). Phase 1 occurs during exponential growth, and phase 2 at or near the end of fermentation. The mechanism for the formation of sulfides during each of these phases is different. This difference helps to explain why addition of fermentable nitrogen such as DAP sometime solves the problem of H2S formation, and at other times increases the problem.

During phase 1, nitrogen additions may help control H2S production if there is a nitrogen deficiency. At this time, the yeast cell is attempting to produce two sulfur-containing amino acids, cysteine and methionine. These two amino acids are essential for protein and peptide formation.

During the early stages of yeast growth, the cell brings in sulfates and sulfites to form these two sulfur-containing amino acids. However, if the available N is limited, these two amino acids cannot be produced, and reduced sulfur in the form of H2S is excreted from the cell instead of being incorporated into amino acid syntheses. In this case, nitrogen supplementation can be helpful.

The second peak of H2S formation is not directly related to N level. This occurs at the end of fermentation, and the cells would not have gotten this far if N were too low. Most of the H2S formed during phase 1 is removed as a result of the rapid production of carbon dioxide. However, far less CO2 is produced during the later stage of fermentation, which can result in high concentrations of H2S remaining in the wine.

For phase 2, there is a positive correlation between the available N and H2S formation--meaning the higher the N, the higher the H2S!

The practical point is this: the simple addition of DAP or other N-containing supplements without knowing the concentration of fermentable N is counterproductive. If there is a N deficiency, supplementation can have a desirable impact, but only for phase 1 formation.

Optimum fermentation management is needed to minimize the production of VSCs. For a review of fermentation considerations, see Enology Notes # 26, Vintners Corner Vol. 17, No. 4 and Factors Influencing Fermentation all available on-line.

Enology Notes # 70 lists some of the many factors influencing hydrogen sulfide production, which includes both too much nitrogen and too little nitrogen. Unfortunately, it appears that the most important N factor influencing H2S is the ratio of certain amino acids in the juice, rather than simply the total fermentable N. This helps explain why we can have such large variations in the H2S formation from one season to the next, even when all other factors appear to be the same.

With the current knowledge, the best approach is to test for fermentable N prior to fermentation. As has been outlined in this series, we have modified the Formol test for fermentable N, which can easily be run by all wineries, large or small. For details, see the Enology-Grape Chemistry website at http://www.vtwines.info/. Click extension, then on-line publications.

3. Sulfur Compounds and Management.

Elemental sulfur used as a fungicide has long been associated with the increased incidence of H2S formation in wine. It should be noted that grapes should not be sprayed with sulfur immediately prior to harvest. How much time should elapse between spray and harvest depends upon the environmental conditions, and the type, quantity and particle size of the sulfur. The safest recommendation is six weeks.

Some synthetic pesticides contain chemically-bound sulfur which has been reported to be the source of sulfides and mercaptans.

Despite the fact that it seems to be accepted that lees are responsible for the development of off sulfur-type odors in white wines (not necessarily true), most producers understand the benefits of storage sur lie (see Enology Notes # 23, 25, 26, 27, and 50).

For white wine production in barrels, reducing the turbidity of the must to 100-200 NTUs, reducing the amount of sulfur dioxide, and making sure the barrels are properly rinsed are important issues.

Because of the varied nature of VSCs, it is desirable to determine the general type of compound in sulfidy wines immediately post-fermentation. An aroma screen can be used to determine if the problem is H2S, mercaptans, or both (see Zoecklein et al., 1999).

Treatments can include sulfur dioxide addition at least 8 days post-fermentation, aeration, copper addition, and/or lees addition.

Yeast walls can essentially act as fining agents, which can be an asset in management of sulfur compounds. The main proteins of the yeast cell wall have the ability to form disulfide bridges with some volatile sulfur compounds (Lavigne-Cruege and Dubourdieu, 2001).

Racking and aeration of the wine, with the temporary removal of the lees, has the advantage of providing some elimination of sulfur-containing compounds. Keeping the lees separate, with frequent stirring, and adding them back to the wine has an added advantage. The lees can bind some VSCs, frequently eliminating the problem.

It is important to carefully screen wines for VSCs prior to bottling. Methyl mercaptans can form dimethyl disulfide, a compound that has an onion or cooked cabbage-like odor and a sensory threshold of 30 ppb. Disulfides can also be converted to ethyl and methyl mercaptans, depending on the redox potential and pH of the wine. This can occur post-bottling.

Mercaptans react with copper, while oxidized forms generally do not. This is the basis for the use of ascorbic acid in evaluating VSCs. This reducing agent has the ability to convert dimethyl disulfide back to methyl mercaptan, which can then be bound, with the addition of copper in the form of copper sulfate. For additional information, see Vintners Corner Vol. 18, No.1. available at http://www.vtwines.info/, click extension, then Vintners Corner. Or see Wine Analysis and Production (Zoecklein et al., 1999).

4. On-line Publications.

Dominque Delteils excellent presentation on Wine Mouthfeel has been posted. Additionally, a review of Maturity Sampling and Sample Evaluation and Factors Influencing Fermentation are also posted. These are available at http://www.vtwines.info/. Click extension, then on-line publications.

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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
Email: bzoeckle@vt.edu