Sulfides

Of Rotten Eggs, Burnt Rubber & Cooked Cabbage - A Review and Update on Sulfide Formation in Winemaking
Christian E. Butzke
Cooperative Extension Enologist
Department of Viticulture & Enology
University of California, Davis
April 1997

Following is a concise summary of our current knowledge, including a discussion on removal and prevention methods. Research conducted over the past five years by Roger Boulton’s group at UC Davis sheds new light on the relationship between sulfide formation, yeast performance and grape juice composition.

For more on sulfur read Waterhouse’s page as well

Major Sulfur Containing Compounds
H2S: Contains sulfur in its chemically most reduced, negatively charged (electron-loaded) form, S-II. On the other side of the spectrum of sulfur components with significance to winemaking are the oxidized forms, e.g. sulfur dioxide (sulfite S+IVO2) or copper sulfate (CuS+VIO4). The sensory properties of H2S are best described by the classic “rotten egg” character. Group sensory threshold is about 1 ppb.
Mercaptans: Sulfur compounds corresponding to the alcohols predominant in fermenting grape juice, but with a thiol (-SH) group instead of a hydroxy (-OH) group. Ethyl mercaptan possesses a “burnt rubber”, “garlic” or “skunk” character while methyl mercaptan reminds of “rotten or cooked cabbage”. Group sensory threshold of both in wine is about 1 ppb.
Disulfides: In the presence of air, the mercaptans can be oxidized to disulfides. For example, methyl mercaptan reacts to form dimethyl (di)sulfide, a compound displaying “onion” or “cooked cabbage”-like odors. Higher sensory threshold of around 30 ppb.

It should be noted that numerous other combinations of reduced sulfur groups and alcohols exist (over twenty volatile sulfur components have been identified in wine aroma). These various sulfur-containing substances can contribute qualitatively to the complexity of wine aroma if present in small amounts.

Sources of sulfur containing compounds

Summary: Sulfur is an essential element for wine yeast as part of vital organic cell components, such as the amino acids cysteine and methionine, and several metabolic coenzymes (vitamins). Yeast can utilize both inorganic (elemental sulfur, sulfate, sulfide, sulfite, thiosulfate) and organic sources of sulfur in grape juice. Yeast metabolic activity can form H2S by reduction of elemental sulfur, catabolism of cysteine or by reduction of sulfur from inorganic sources. Formation of reduced sulfur is dependent on yeast strains, original juice composition, fermentation temperature and rate. But even with the same yeast strain, enormous differences can be observed. Today, no commercial yeast strain can be consider superior to another. H2S production during fermentation happens in two distinct phases: 1. during the exponential growth phase of the yeast and 2. near the end of fermentation. The mechanisms for sulfide formation during both phases appear to be different.

It appears that reduced sulfur production is INEVITABLE, therefore yeast strains should be selected based on OTHER PRIORITIES.
Juice Nitrogen Status: Due to some misinterpretation of previous studies and temporary effects seen during actual fermentations, it has become common knowledge among winemakers that generous additions of yeast assimilable nitrogen, in form of diammonium phosphate (DAP), prevents H2S formation. This is, unfortunately, not true.
Early response of fermentations to DAP: The two sulfur-containing amino acids, methionine and cysteine, essential for the yeast, need either to be present in sufficient amounts in the must, or they need to be synthesized by the yeast. The yeast will produce sulfide from sulfates or sulfites (see below), but if no matching nitrogen-containing precursors to methionine and cysteine (o-acetyl homoserine and o-acetyl serine) are present, the sulfide has no place to go and is liberated into the must. However when the nitrogen coming from DAP can be used to form the precursors, the release of H2S will stop almost instantly.
The second peak of H2S formation during fermentation is not directly related to total nitrogen deficiencies. Our research shows that H2S levels after fermentation are actually positively correlated to total yeast assimilable nitrogen levels, i.e. the more nitrogen was present in the must the more sulfide was formed. Looking at the individual amino acids in must, stronger correlations were found between H2S induction and glutamic acid, alanine, and gamma-amino butyric acid (GABA) as well as weaker relationships with methionine, lysine, arginine and phenylalanine. The ratios of individual amino acids in a juice rather than total nitrogen level appear to play a leading role in the liberation and final concentration of H2S in wine.
Sulfites/Sulfates and Vitamin Deficiencies: Oxidized inorganic sulfur compounds such as or sulfites (made from sulfate by the yeast during fermentation) and especially sulfates themselves (naturally taken up by the grapevine) can form a significant source of sulfur for H2S liberation. Under certain conditions, e.g., when methionine is deficient in the must as well as the vitamins required for its synthesis, pantothenate and pyridoxine (B6), H2S will be released from sulfate or sulfites during the yeast’s exponential growth phase (see above). Metal ions also play a catalytic role in this complex interaction. High initial levels of added sulfur dioxide bind acetaldehyde which is normally reduced to form ethanol. If not enough acetaldehyde is present, other juice components such as sulfate may be reduced instead, resulting in H2S. SO2 can also convert H2S to elemental sulfur, which may later be reduced to H2S again if the wine was not carefully racked.
Fungicide/Pesticide Residues: Higher levels of residual sulfur on grapes coming into the winery can be a significant source of H2S. Modern viticultural practices and sufficient intervals between last application and harvest, elemental sulfur does not play a major role concerning the sulfide problem for winemakers. In addition, several synthetic pesticides containing sulfur groups have been reported as sources for off-odors related to sulfides and mercaptans.
Glutathione: The tripeptide glutathione (GSH) is known to be the most abundant source of reduced sulfur in grape berries, being nearly 20 times as abundant as the sulfur-containing amino acid cysteine. Controversial results suggest an either repressive or stimulating effect of GSH on H2S formation, but it is not easily taken up the yeast. Nearly half of the GSH is being oxidized to GSH disulfide during crushing. If GSH disulfide is present in significant quantities in wine, it may be a precursor for one of the undesirable volatile disulfide compounds above.
Removal of sulfides
Copper Sulfate: Copper sulfate removes H2S and mercaptans by binding the reduced sulfur as highly insoluble copper sulfide (CuS). Excess copper can contribute to protein hazes and it can catalyze undesirable oxidation reactions.
o There are no efficient copper removal techniques available to the American winemaker today. Reaching levels of residual copper below 0.2 mg/L at wine pH can be very difficult to achieve, requiring subsequent backblending with less severely treated wine.
o Copper sulfate alone will not remove disulfides from wine. It has been used in conjunction with SO2 and ascorbic acid, whereby sulfite cleaves the disulfide, resulting in two mercaptans which can then be bound by copper sulfate. The ascorbic acid acts as an antioxidant, keeping the mercaptans from being reoxidized. However this reaction is extremely slow at wine pH and will require months (!) to complete.
Aeration/Stripping: H2S can be partly removed by aerating the wine through splashing or open pump-overs. To avoid an unwanted oxidation, especially of white wine, the H2S may be blown off with inert gases such as nitrogen.
o However this may take significant volumes of gas to accomplish.
o In comparison, the carbon dioxide gas produced during a normal 22° Brix fermentation, equals over 55 times the volume of the wine.
o The “blowing-off” of H2S by both aeration or stripping with inert gases will certainly also cause a loss of equally volatile yet desirable aroma compounds.
o A greater danger using aeration lies in the potential oxidation of mercaptans to disulfides which, as mentioned above, cannot be fined with copper sulfate. The disulfides have higher sensory thresholds than the corresponding mercaptans, so the wine might smell clean when it is bottled. However, in the reductive environment of the bottle, disulfides may be slowly reduced back to mercaptans, leading to an unpleasant surprise when the cork is pulled.

This info. came form a reprint from the Proceedings of the 12th Annual Midwest Regional Grape and Wine Conference, Osage Beach, Missouri, January 19-21, 1997 (Editor: Dr. Murli Dharmadhikari, Research Professor & Enology Advisor, Dept. of Fruit Science, Southwest Missouri State University).