Hydrogen sulfide

From Brewing Forward

Hydrogen sulfide (H2S), or just "sulfide" is a microbe-derived off flavor. It is the most common of a group of fermentation products known as volatile sulfur compounds (VSCs). Sulfide aroma and flavor is often described as sulfurous like rotten eggs, "rhino farts", sewer, or volcanic gas. It is also sometimes called a "reductive" aroma because it is more likely to accumulate under low-oxygen conditions.[1]

Sulfide is one of the most common off flavors that occurs in wine and cider. It can also occur in beer and other fermented beverages.[2] In fact, a slight note of sulfide may be acceptable in some styles of lager. The recognition threshold of sulfide is about 1-2 µg/L (parts per billion) and even lower levels can play a role in aroma complexity, or mask desirable aromas.[3][4][5][6][7][8][9][10]

The odor threshold is well below the threshold for human toxicity, and therefore safety is generally not a concern.[11]

Sulfide should not be confused with sulfite or sulfate.

Formation

Yeast produce hydrogen sulfide naturally, as part of the production of certain amino acids. Specifically, sulfide is an essential metabolic intermediate in the biosynthesis of cysteine and methionine, which are necessary for protein synthesis and cellular metabolism.[12][13][14] Sulfide is also now recognized as having important functions in detoxification, population signalling, and extending life span in yeast.[14][15]

Sulfide is produced mainly by molecular reduction of sulfate or sulfite present in the juice or wort.[16][12][13][17][14][18][5] Sulfate is fairly ubiquitous, and sulfite is a common addition in wine and sometimes in beer. Sulfide is also formed from elemental sulfur, which is sometimes used as an antifungal treatment on grapes.[14][12][13] Utilization of these sulfur-containing compounds to produce amino acids occurs through a series of steps called the Sulfate Reduction Sequence (SRS).

Bacteria can also produce sulfide.[19][20]

Causes of Overproduction

  • Yeast strain is one of the main factors influencing the production of sulfide.[5][17][21][22] Some strains of yeast are biologically much more prone to over-producing sulfide.
  • Lack of adequate yeast nutrients is another main factor.[4][5][13][18][23][24] In order for the yeast to scavenge the sulfide and incorporate it into cysteine and methionine, the yeast need plenty of nitrogen and co-factors such as pantothenic acid to form the precursors for these sulfur-containing amino acids.[5][13][25][26][27] If there is not enough of the precursor, the yeast release the hydrogen sulfide into the wine or beer.
  • Any factors that increase nutrient demand may also lead to increased sulfide production. Pitching a inadequate amount of yeast or an unhealthy yeast culture may cause numerous fermentation-related problems.[28]
  • Addition or over-use of sulfite may increase or cause sulfide production, particularly with yeast strains used for beer.[13][29] This is because sulfite is the direct precursor to hydrogen sulfide in the SRS.
  • Elemental sulfur is frequently sprayed in the vineyard to fight powdery mildew, and residual sulfur on grapes has been observed to contribute to the formation of sulfide during fermentation by yeast, and the reappearance of VSCs after bottling.[14][12][13][1]
  • The presence of metals (e.g. copper) during fermentation can stimulate sulfide production.[30]

Timing of its Appearance

Maximum amounts of sulfide are liberated when the depletion of nitrogen occurs during the exponential growth phase. Conversely, when depletion of nitrogen occurs during the stationary phase, sulfide liberation is a lower amount and is short-lived.[13] (See Yeast for more about growth phases.)

While sulfide formation occurs mainly during primary fermentation, additional VSCs can be formed at later stages of production, particularly in wine. This phenomenon is rarely a problem in beer production. VSC formation in wine can be difficult to predict and is not necessarily related to sulfide issues during the primary fermentation.[18] The VSCs involved include mercaptans (AKA thiols or mono-sulfides or higher sulfides) and disulfides that have distinctive aromas such as skunky, rubbery, garlic, onion, or cabbage-like.[18][14][31] These compounds result from degradation of sulfur-containing compounds in the yeast lees, and chemically-bound sulfide may be released during aging or storage.[18][23][1] Sulfide formation has also been reported to occur in the bottle when naturally bottle carbonating cider with yeast.[32] Even VSCs that had apparently been removed may reappear if conditions in wine become more reductive (e.g. during barrel aging or in the bottle).[33][34]

There is not always correlation between total sulfide produced by yeast during fermentation and the sulfide concentration in the final wine/beer/etc.[21][35]

Prevention

It's far better to focus on minimizing the production of sulfide rather than allow it to occur and remove it later because prevention strategies are easier and less damaging/risky than the available options for removing sulfide if it lingers in the final beverage. For best results, a multi-faceted approach is needed.[18]

For wine and beer:

  • Sulfite - Reduced pre-fermentation sulfite usage has been shown to reduce formation of sulfide.[17][13][19] Brewers that use sulfite (e.g. low oxygen brewing) need to adequately aerate/oxygenate the wort to neutralize the residual sulfite when pitching. (See Sulfite) In presence of lees, it is recommended to wait at least two weeks before adding sulfite after fermentation.[36]
  • Aeration - Adding oxygen before pitching yeast is especially important in affecting nitrogen utilization, fermentation vigor, and hence sulfide stripping from the medium.[13][19] (See Aeration)
  • Vitamins - Vitamins should be supplemented, especially in wine. Supplementation is not strictly necessary in beer production, since wort typically contains adequate vitamins[37]. However it may still be a good practice. Deficiencies of vitamins that act as co-factors to SRS enzymes (pantothenic acid and pyridoxine) cause overproduction of sulfide even when adequate nitrogen is present.[13][19][25][26][27] (See Yeast Nutrition)
  • Nitrogen - Supplementing yeast-assimilable nitrogen (YAN) can help lower sulfide production, but only when there are also adequate co-factors (vitamins) present for the SRS.[13][38] Otherwise nitrogen supplementation may increase sulfide production.[27][25][39][5][35][21] There may also be some variability among yeast strains or species with regard to whether increasing nitrogen decreases sulfide formation.[21][40] Similar to vitamins, nitrogen supplementation in wort or beer is not always required, but still may be helpful under certain conditions.[41][28][citation needed] (See Yeast)
  • Yeast strain - Low sulfide-producing and/or low nitrogen-requirement yeast strains may be considered.[19] This is easier said than done since the propensity to over-produce sulfide is not well characterized for most yeast strains. However, Scott Labs and Renaissance Yeast have both bred some wine yeast strains specifically to reduce sulfide production.[42][43]
  • Pitch rate and yeast health - Pitch healthy yeast at a good pitch rate to decrease nutrient demand.[28][29] "Shocking" the yeast (rapid changes in growth conditions like temperature or pH) should be avoided.[19][44] Significant over-pitching may also cause excessive sulfide.[45] (See Yeast)
  • Fermentation temperature - Generally lower temperatures decrease sulfide liberation, although not necessarily because of decreased production.[27] However each strain has an optimum fermentation temperature to minimize its production, so lower temperature doesn't always mean lower sulfide production.[22] The fermentation temperature should be controlled within the suggested range of the yeast. (See Temperature control)
  • Fermentation duration - A shorter fermentation decreases the amount sulfide ultimately present.[38] This is probably because fermentation time is linked to aeration and nutrient supplementation. (See Yeast)
  • Yeast contact - The exact role of lees on sulfide formation has not been established. Aging on lees could be the cause of sulfur-like off flavors, but also the solution to removing them. Evidence of both the release of VSCs from lees and the removal of VSCs by lees has been widely reported. The conditions under which each phenomenon occurs is a very complex matter closely related with the yeast strain and other conditions.[46][18][47][48][49][50][51][52][3] For wine, many experts suggest that if the lees start to form these compounds, the wine should be immediately racked.[15][18][53] Some experts suggest that only the gross lees are problematic, while the light lees are beneficial.[44] For beer, the absorption of sulfide by yeast has been demonstrated, and home brewers do not report VSC production from aging on yeast.[3]

For wine only:

  • Sulfur Spray - Farmers using a sulfur spray should limit residual sulfur on fruit to 7 mg/kg or less (with less than 1 mg/kg being ideal). Stop spraying at least 5 weeks pre-harvest for the lowest risk of sulfide formation.[1][44]
  • Must Clarification - Winemakers can minimize the formation of excess sulfide production in white wines by either settling, centrifuging or filtering the must before fermentation, which removes high-density solids which might contain elemental sulfur.[30] In other words, must clarification will reduce the formation of VSCs. Be aware that fining must with bentonite can remove some nitrogen, which will need to be added back.

Screening

Before attempting to remove sulfur-like off aromas, it's important to know which compounds are present because they are removed with different methods.

Materials:

  • Tasting glasses
  • A measuring device capable of allotting about 50 mL, such as a graduated cylinder or beaker
  • Plastic wrap or watch glasses
  • 1% Copper sulfate (CuSO4) solution (If you can't obtain copper sulfate, you can clean several pennies in an acid solution like lemon juice or vinegar.[4])
  • Optional: Ascorbic acid and distilled water to make a 5% ascorbic acid solution. Mix 2.5 g ascorbic acid into 50 mL water.

Procedure:[54][55][56]

  1. Obtain two 50mL samples of wine in glasses. Label one "Control" and the other "Copper".
  2. To the glass marked "Copper", add 1 mL of the copper sulfate solution (approx 50 ppm - this is a strong excess of copper), or pennies.
  3. Cover both glasses with a watch glass or plastic wrap and swirl.
  4. Let glasses sit for around 15 minutes and then examine by smell. High concentrations of copper are toxic, do NOT taste experimental glass.

Determining the results:

  1. If the off odor is gone from the experimental glass, it is likely that only hydrogen sulfide and/or mercaptans are present.
  2. Otherwise, if the experimental glass is still stinky, this can mean that the odors are disulfides and/or dimethyl sulfide (DMS), neither of which react with copper.
  3. In the second case, you need to differentiate between disulfides and DMS. Create a third glass with a 50 mL sample labeled "Copper and AA". To this glass add 1 mL of 5% ascorbic acid solution 2 minutes before adding 1 mL copper sulfate solution. Follow the same evaluation procedure. If the sulfur-like off aroma is removed from this glass, it means disulfides are present.

Removal

While large amounts of sulfide may be produced during fermentation, much of this sulfide is usually volatilized (off-gassed) from the wine or beer along with CO2 during active fermentation.[18][3] Therefore, these removal methods should only be applied after fermentation is complete.

These removal procedures will cover the 3 types of VSCs that cause off-flavors: hydrogen sulfide, mercaptans, and disulfides. For best results, conduct a screening procedure before making interventions because some interventions make impair the removal of certain VSCs under certain circumstances.

Almost all of these methods have a potential negative effect, which is what makes prevention so important.

Yeast contact

Increased yeast contact after fermentation may help remove all types of VSCs. Wine may benefit from racking off the gross lees. See the Prevention section above for more discussion.

Oxidation of desirable compounds is the only potential negative effect of increased time in the primary fermentation vessel.

Sparging with inert gas

Hydrogen sulfide is highly volatile. If you have the appropriate equipment, it can be removed through "sparging" with inert gas (such as nitrogen or carbon dioxide). In other words, bubbling gas through the beer/wine/etc will carry off the hydrogen sulfide along with it.[1][15][45][36]

Perform this method only in a well-ventilated space. The specifics of a gas sparging setup are beyond the scope of this article, but you will need a gas cylinder, an appropriate regulator, gas tubing, and an "oxygen stone".

This approach is less effective against mercaptans and disulfides. It will also strip desirable aromatic compounds.

Oxygen exposure

Hydrogen sulfide is easily oxidized to elemental sulfur, which is insoluble and flavorless. If fermentation is still active, stirring it may help volatilize and/or gently remove sulfide with low risk of also oxidizing desirable compounds. If fermentation has completed, you can simply leave the beer/wine/etc in the fermenter and oxygen that enters the vessel will react with hydrogen sulfide.[1][4][18][36] Aeration (e.g. through splash racking) may also be used, particularly in wine with sulfite.

Oxygen exposure does not remove mercaptans or disulfides. Furthermore, most sources suggest that aeration adds a danger of forming mercaptans and/or converting mercaptans to disulfides.[57][citation needed] The disulfides have a higher taste threshold so they may seem to disappear, but they can potentially change back to mercaptans later under low-oxygen conditions such as a wine with sulfite.[58][citation needed] Oddly there is one conflicting study with regard to whether oxygen exposure increases the formation of disulfides.[59]

Oxygen exposure (including aging without the anti-oxidant protection of sulfite) can cause oxidation of desirable compounds, which negatively affects flavor.

Copper

This section is under construction.

Copper is a common tool used for the removal of both hydrogen sulfide and mercaptans.[18][4] Copper binds to these sulfur compounds to form odorless complexes, which precipitate to some degree. When copper is used in combination with ascorbic acid, disulfides can be removed as well (see below). Adding copper is fairly easy and inexpensive. Copper fining works in both wine and beer.[60]

Copper should not be added until the fermentation is complete and the amount of yeast material is reduced by racking. This is because yeast cells can bind with copper and reduce effectiveness, and because addition of copper during fermentation may promote sulfide production by yeast.[18] However some sources suggest that yeast may be helpful for removing the excess copper.[61] The wine/beer should be racked several days after adding copper.[18][62]

Despite its widespread use, copper has a lot of potential disadvantages and problems. The copper-sulfide complexes are challenging to remove from wine, and they can potentially release the sulfide later.[14][63][64][65][66] In addition, copper might catalyze the release of sulfide from sulfur containing amino acids like cysteine, although this is still speculative.[1] Copper also reacts with any other thiols in the wine. Therefore, if you are dealing with a wine variety rich in aromatic varietal thiols (e.g., Sauvignon blanc, rosés, and to a lesser extent Riesling and Gewürztraminer), the addition of copper sulfate can actually reduce the wine’s varietal aroma in addition to treating the reduced aroma.[67][57] Too much copper can cause a haze, referred to as "copper casse".[57] Risk of haze formation is greatly increased if copper is added immediately prior to bottling, without allowing adequate time for the wine to stabilize during bulk storage. Lastly, excess copper catalyzes oxidation reactions, which can accelerate staling.[57]

For those reasons, the best results are obtained by using the minimum amount of copper needed to remove the offensive sulfur compounds. To do this, a "bench trial" should be performed to experimentally determine the minimum amount. Also for those reasons, more "traditional" methods such as stirring with a copper pipe should be avoided because this practice may lead to undesirable effects.

Bench Trial
Concentrations of between 0.05 and 0.3 mg/L of copper are commonly needed.[18]

Fining trials should start with fairly small copper sulfate additions, starting at 0.05 ppm. To achieve the best approximation of treatment effects, allow the treated wines to sit approximately 12 hours and re-smell before making decisions about treatment. If the aroma screen indicates the present of methyl and ethyl mercaptan, aeration should not be attempted under any circumstances.[57]

The beer/wine should be left undisturbed for several days after treatment so the copper sulfide (a very fine black powder) can settle to the bottom of the container. Then it should be carefully racked off the residue.[62]

Bench trial procedure

Excess copper sulfate can be removed with bentonite, yeast hulls, or fresh lees additions.[55] Contrary to conventional wisdom, the majority of added copper remains in wine and is not readily removed by racking or filtration.[1][63] Bentonite fining and yeast hulls can help remove small amounts of copper between 0.1-0.2 mg/L.[36]

Copper citrate is recommended instead of copper sulfate since it is an "organic chelating agent" of copper meaning the copper does not totally go into the ionic form.[4] Consequently, it does not leave as much residual copper in the wine.[4] The manufacturer claims only about 50% goes into wine.

  • copper sulfate: 1g = 0.255mg copper
  • copper citrate: 1g = 0.350mg copper
  • kupzit: 50g = 1g copper citrate = .350g copper

Ascorbic acid

This section is under construction.

Adding ascorbic acid may aid the removal of disulfide compounds since these are not removed by copper.

If mercaptans have oxidized to form dimethyldisulfide (DMDS) and diethyldisulfide (DEDS), they must be converted to their parent mercaptan species prior to removal. Disulfides are first reduced with the addition of 50mg/L or more of ascorbic acid, immediately followed by an appropriate addition of copper sulfate. This reaction can be fairly slow, requiring as long as two months to reach equilibrium- obviously not something that you want to do right before bottling. It’s important to make sure that free SO2 levels are adequate before adding ascorbic acid, which can increase the potential for wine oxidation.[57]

Recent work has shown that dilution with brine can release metal-sulfide complexes such as copper sulfides (FrancoLuesma et al., 2014), and that this release is correlated with release during accelerated or normal aging. Interestingly, this release can also be triggered by ascorbic acid (Chen et al. 2016) – in other words, the commonly used “ascorbic acid test” may not be detecting disulfides as is commonly assumed, but instead may be detecting copper sulfide (or copper mercaptide) complexes![1]

If a wine containing methanethiol and ethanethiol is aerated to remove a suspected hydrogen sulfide fault, these can be oxidized to DMDS and DEDS (disulfides), which do not react with copper and therefore cannot be removed by copper fining. Removal of DMDS and DEDS requires the creation of reducing conditions, by the addition of ascorbic acid and SO2, in order to reduce these compounds back to the reactive species (methanethiol and ethanethiol), which may then be removed by treatment with copper.[30][15]

To do this, first ensure there is >30 mg/L free SO2 in the wine. For white wines, add 10 mg/L ascorbic acid and then another 10 mg/L ascorbic acid the following day. For red wines, add 2 mg/L ascorbic acid and then another 2 mg/L ascorbic acid the following day. Wait another 24 hours for the ascorbic acid to react with any free oxygen and to allow disulfides to be reduced back to mercaptans. A copper fining trial can then be performed on this treated wine to determine the appropriate copper addition rate to react with and remove the methanethiol and ethanethiol.[30]

0.25g/gal ascorbic acid will often help by converting disulfides to mercaptans. The process may take up to three weeks, after which the wine should be treated with copper.[68]

Generally, addition levels of 50 mg/L or more of ascorbic acid are used, and such additions usually are made several days prior to the addition of copper.[44]

Ascorbic acid sometimes protects the fruit and acts as an antioxidant, while at other times it can act as a proto-oxidant, or oxidative promoter. The two roles of ascorbic acid are mainly the result of concentration and the presence of adequate sulfur dioxide. When ascorbic acid is added to wine, it binds oxygen rapidly to form two reaction products, dehydroascorbate and hydrogen peroxide. If there is not enough ascorbic acid maintained to react with the oxygen, oxidative degradation, including coupled oxidation, can occur. If there is not adequate sulfur dioxide maintained to bind with the hydrogen peroxide formed by the ascorbic acid, wine oxidation can occur. The reaction between ascorbic acid and oxygen is much more rapid than with SO2.[29]

Oak tannins

This section is under construction.

The addition of tannins, especially ellagic (oak) tannins, has the ability to bind with mercaptans and form odorless complexes. These complexes are very stable over time and do not entail the risk of a later sulfur off-aroma appearance. EnartisTan SLI (ellagic tannin from untoasted American oak) and TanCoeur de Chêne (ellagic tannin from toasted French oak) are very effective in scavenging mercaptans and can successfully replace the addition of copper also before bottling.[36]

Mineral oil

This section is under construction.

Disulfides can be removed ad adding mineral oil to the wine and agitating it daily for several days. When the wine beneath the mineral oil layer smells clean, it should be racked, leaving the oil behind.[69] This may be used as a last resort for wine.

Science

This whole section is under construction.

See https://aem.asm.org/content/aem/61/2/461.full.pdf for the pathway of H2S formation. (fig 1)[13], or https://sfamjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.2003.01827.x for the full pathway including protein synthesis and other byproducts.[25] or here: https://academic.oup.com/femsyr/article/17/6/fox058/4056150 for an in-depth review of the biological pathways.[14]

To be utilized for production of H2S, SO2 simply diffuses into the cell, equilibrates at cytosolic pH as bisulfite (HSO3-) and sulfite (SO32-), and thereby can actually accumulate to 60-times its extracellular concentration.[17][13]

Glutathione is naturally present in grape juice (∼1.3 to 102 mg/L) and can also be synthesized by yeast through the Sulfate Assimilation Pathway. The addition of glutathione to grape juice has been observed to increase H2S production.[14] The mechanism is not yet fully understood but it is generally assumed that glutathione is first hydrolyzed to cysteine, which is then degraded by cysteine desulfhydrase to release H2S under nitrogen-limited conditions (Rauhut 2009).[14]

H2S has been demonstrated to react with (E)-2-hexenal in grape juice to form the fruity varietal thiols 3-mercapto-hexanol and 3-mercaptohexylacetate.[14] However, only tiny amounts of thiols (<1%) are produced through this pathway as (E)-2-hexenal is rapidly metabolised by yeast during fermentation (Schneider et al.2006; Subileau et al.2008; Harsch et al.2013).[14]

If conditions in wine become more reductive (e.g. during barrel aging or in the bottle), the disulfides can be reduced back to mercaptans resulting in a reappearance of sulfide aromas.[70][71] This phenomenon does not occur in beer because beer always becomes increasingly oxidized over time, and never more reductive since it generally doesn't contain anti-oxidants (sulfite).

Final wine concentration of glutathione was correlated with both total N and organic nitrogen.[38] (see http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.913.1799&rep=rep1&type=pdf)

Some sources suggest that sulfide may be a precursor to some other VSCs,[62][30] but there appears to be no scientific evidence supporting that claim, or the claim that prompt H2S removal is needed.

Some sources claim that adding sulfite after fermentation theoretically helps eliminate sulfide by reacting with it.[4][30][72] However, this appears generally not to be the case in practice; perhaps it is instead related to oxygen exposure.

Mercaptan: if H2S is not removed from the wine, it will react with ethanol or acetaldehyde to form a new, even nastier compound called ethyl mercaptan or ethanethiol (burnt rubber, garlic, mercaptan or ethanethiol (burnt rubber, garlic, cabbage).[4]
CH3-CH2OH + H2S → CH3-CH2-SH + H2O

Diethyl disulfide: if ethyl mercaptan is not eliminated, then two molecules of mercaptan can react to form another molecule, even more nasty
CH3CH2-SH + HS-CH2-CH3 → CH3-CH2-S-S-CH2-CH3 B.P. = +154°C (very non-volatile)

  • This molecule is impossible to eliminate from wine by normal means and has a very cheese-like aroma[4]

Reaction of sulfide with oxygen:[4] 2 H2S + O2 → 2 S + 2 H2O

Reaction of sulfide with sulfite:[4] 2 H2S + SO2•H2O → 3 S + 3 H2O

Reaction of sulfide with copper sulfate:[4][58] H2S + CuSO4 → CuS + H2SO4

Recent work suggests that copper complexes may serve as a latent source of free hydrogen sulfide (H2S) and other malodorous volatile thiols during wine storage.[1] Advanced methods for measuring this sulfide exist. (see https://www.ajevonline.org/content/68/1/91)

Sulfide has been shown to perform an important signaling function. H2S will arrest the respiration phase and signal the onset of fermentation. This allows the population of cells to coordinate metabolic activity. H2S is deliberately made and released to coordinate rapid fermentation onset. There could be a strong selective advantage to the production of H2S under certain environmental conditions.[15]

Re-pitching yeast may cause increased production of sulfide.[73]

Ascorbic Acid

The reversibly oxidized form of ascorbic acid (called dehydroascorbic acid) undergoes an irreversible change in aqueous solution above pH 4 at ordinary temperatures. The product of this change is a stronger acid than dehydroascorbic acid, and is a more powerful reducing agent than ascorbic acid itself. The rates of appearance of all of these manifestations of the irreversible change in dehydroascorbic acid exhibit the same dependence on the hydrogen ion concentration. They are also all independent of the presence of air or oxidizing agents. The irreversible change is therefore not an oxidation. It is also independent of the oxidizing agent used to form dehydroascorbic acid.[74]

Ascorbic acid can be made to take up the equivalent of at least 3 atoms of oxygen in the course of its oxidation, in three separate steps, depending on pH.[74]

Dehydroascorbic acid is restored practically quantitatively to ascorbic acid by H2S in acid solution. After its conversion to diketogulonic acid this property is lost. However, reduction becomes more rapid as pH increases.[74]

Surveys by different methods show that strains of yeast vary widely in their ability to produce H2S, thereby making the yeast strain the most important factor in determining H2S production (Rankine 1968a, Eschenbruch et al. 1978, Thornton and Bunker 1989, Thomas et al. 1993, Giudici and Kunkee 1994, Jiranek et al. 1995c, Spiropoulis et al. 2000, MendesFerreira et al. 2002). However, the characterisation of strains has proven technically problematic due to poor understanding of the regulation of sulfur metabolism in yeast. Recent studies by L. Bisson at UC Davis have suggested that no one method is suitable for determining the potential of a strain to produce H2S under winemaking conditions (Spiropoulis et al. 2000). This is largely due to the fact that the regulation of sulfur and nitrogen metabolism in yeast is complex (Mountain et al. 1991, Hinnenbusch 1992, Gasch et al. 2000, Marks et al. 2003).[75]

Amino Acids

The higher sulfide compounds are believed to largely generate from the degradation of the S-containing amino acids. Spiking wines with methionine, cysteine or the cysteine-containing tripeptide glutathione leads to the formation of these compounds in juices and wines. Some of these compounds appear to continue to increase during storage of the wine after yeast activity has ceased, suggesting that there are precursor forms present in wine that, as the reductive conditions of the wine change, generate S-volatiles. In beer, for example, dimethyl sulfide can form from reduction of Dimethyl sulfoxide derived from S-methylmethionine. This pathway has not been shown to exist in wine, where dimethyl sulfide is believed to come from the degradation of cysteine, glutathione, methionine or S-adenosyl–L-methionine. Some higher sulfides may also come from degradation of S-containing pesticides, but this is a rare occurrence and more often these characters are derived from catabolism of S-amino acids and their derivatives, glutathione and S-adenosyl-L-methionine.[15] Mercaptans and thioalcohols can also be found in wine. These components likewise are believed to derive from degradation products of S-containing amino acids and their derivatives or from the interaction of H2S with acetaldehyde, which forms the reactive 1,1-ethanedithiol, and other reactive components in wine. Some of these products are reactive themselves, leading to even more diverse S-volatiles. The chemical reactivity of these compounds in combination with a host of potential reactants in wine and very low thresholds of detection, has made it challenging to delineate the true pathways by which they are formed.[15]

Cysteine is commonly referenced as contributing to increased formation for sulfide, but in practice its effect is negligible since cysteine concentration in must is typically very low.[76][18][16][12][14][13] It is likely cysteine is being enzymatically catabolized to ammonium, pyruvate, and H2S directly (2, 46).[13] Indeed it is -- aspartate aminotransferase deaminates cysteine to give mercaptopyruvate, which in a subsequent step catalyzed by MST liberates H2S and pyruvate.[77] However, other in vivo studies suggested that deletion of yeast CYS4 or CSY3 did not reduce the production of H2S.[14] Grape juice usually contains plenty of sulfate (∼160 to 700 mg/L) but very low concentrations of cysteine and methionine (<20 mg/L).[14][19] Although cysteine-hydrolyzing enzymes which release H2S have been described (2, 46), cysteine is not important to excessive H2S production during winemaking because of its scarcity in grape juice (3, 18).[13][14] while a link between nitrogen depletion and H2S liberation continues to be reported (17, 27, 28, 40, 47), the direct involvement of cysteine has not been demonstrated.[13]

Adding cysteine increases sulfide and inhibits sulfite formation. Added methionine inhibits both sulfide and sulfite formation.[16] Conflicting research: The addition of 20 mg l−1 methionine to grape musts has no great effect on the production of hydrogen sulphide.[12] At low methionine concentrations, DAP impact on H2S is minimal, and at low ammonium concentrations the effect of methionine addition is likewise minimal.[24] Methionine addition has a veriable effect on H2S production, and the varies by yeast strain.[78] Irrespective of the availability of an average nitrogen source, insufficient regulatory methionine derivatives are formed. The result is not only a derepression of the SRS (5–7), and thus increased H2S formation, but also a reduced incorporation of H2S into methionine[13] Methionine repressed the cysteine-induced increase in the H2S production but had no effect on the formation of SO2.[79]

The most potent amino acid suppressants of H2S liberation are typically those which support high growth rates, i.e., serine, glutamine, ammonium, aspartate, arginine, and asparagine, or amino acids which act as direct precursors for O-acetylserine or O-acetylhomoserine synthesis, i.e., serine and aspartate.[13]

Linear regression of published data (Fig. 1 and 2) showed a general negative correlation between juice arginine concentrations and total H2S formation as well as residual H2S concentration in wine.[5]

See Also

  • Randy Mosher - article about VSCs in beer.
  • UC Davis - scientific discussion of the variety of VSCs and their formation.


Potential Sources

References

  1. ↑ a b c d e f g h i j Jastrzembski, J., and Sacks, G. "Sulfur Residues and Post-bottling Formation of Hydrogen Sulfide." Research News from Cornell’s Viticulture and Enology Program Research Focus 2016-3a.
  2. ↑ Smith, B. "Sulfur and Rotten Egg Aromas in Beer – Off Flavors in Home Brewing." BeerSmith™ Home Brewing Blog. 2018.
  3. ↑ a b c d Oka K, et al."Decrease in Hydrogen Sulfide Content during the Final Stage of Beer Fermentation Due to Involvement of Yeast and Not Carbon Dioxide Gas Purging." Journal of Bioscience and Bioengineering. Vol. 106, No. 3, 253–257. 2008.
  4. ↑ a b c d e f g h i j k l m n Kaiser, K. "Controlling Reductive Wine Aromas." Brock University CCOVI lecture series. 1 Feb 2010.
  5. ↑ a b c d e f g Butzke, CE and Park, SK. "Impact of Fermentation Rate Changes on Potential Hydrogen Sulfide Concentrations in Wine." J. Microbiol. Biotechnol. 2011. 21(5). pp. 519–524
  6. ↑ "Hydrogen Sulfide, Hazards." Occupational Safety and Health Administration (OSHA). Accessed March 2020.
  7. ↑ "Hydrogen Sulfide1 Acute Exposure Guideline Levels." Acute Exposure Guideline Levels for Selected Airborne Chemicals. Volume 9. National Research Council (US) Committee on Acute Exposure Guideline Levels. Washington (DC): National Academies Press (US); 2010.
  8. ↑ "ToxGuide™ for Hydrogen Sulfide H2S" Agency for Toxic Substances and Disease Registry (ATSDR). December 2016.
  9. ↑ Odor perception and physiological response. The Science of Smell Part 1. Iowa State University. May 2004.
  10. ↑ "hydrogen sulphide." aroxa. Accessed March 2020.
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  12. ↑ a b c d e f Moreira, N., et al. "Volatile sulphur compounds in wine related to yeast metabolism and nitrogen composition of grape musts." Anal. Chim. Acta 2002. 458:157-167.
  13. ↑ a b c d e f g h i j k l m n o p q r s t Jiranek, V., et al. "Regulation of Hydrogen Sulfide Liberation in Wine-Producing Saccharomyces cerevisiae Strains by Assimilable Nitrogen." Applied and Environmental Microbiology. Vol. 61, No. 2. Feb. 1995. pp. 461–467.
  14. ↑ a b c d e f g h i j k l m n o p Huang, CW., et al. "Hydrogen sulfide and its roles in Saccharomyces cerevisiae in a winemaking context." FEMS Yeast Research. Volume 17, Issue 6, September 2017
  15. ↑ a b c d e f g "Off Characters." Viticulture and Enology. University of California, Davis. Accessed online March 2020.
  16. ↑ a b c Giudici, P. and Kunkee, R.E. "The effect of nitrogen deficiency and sulfur-containing amino acids on the reduction of sulfate to hydrogen sulfide by wine yeasts." Am. J. Enol. Vitic. 1994. 45:107-12.
  17. ↑ a b c d Hallinan, CP, et al. "Differential utilisation of sulfur compounds for H2S liberation by nitrogen-starved wine yeasts." Australian Journal of Grape and Wine Research1999. 5, pp. 82-90.
  18. ↑ a b c d e f g h i j k l m n o Osborne, James. "Development of sulfur off-odors post-fermentation." Oregon Wine Research Institute. 2013.
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  20. ↑ Bokulich. NA, and Bamforth, CW. "The Microbiology of Malting and Brewing." Microbiology and Molecular Biology Reviews. May 2013. 77 (2) 157-172.
  21. ↑ a b c d Ugliano, Maurizio, et al. "Effect of Nitrogen Supplementation and Saccharomyces Species on Hydrogen Sulfide and Other Volatile Sulfur Compounds in Shiraz Fermentation and Wine." J. Agric. Food Chem. 2009, 57, 11, 4948-4955.
  22. ↑ a b Kim YR, et al. "Effects of Yeast Strains and Fermentation Temperatures in Production of Hydrogen Sulfide During Beer Fermentation." Korean J. Food Sci. Technol. 2008. Vol. 40, No. 2, pp. 238-242.
  23. ↑ a b Rauhut, D. "Yeasts – production of sulfur compounds" in Wine Microbiology and Biotechnology. ed. G.H Fleet. Harwood Academic Publishers, Switzerland. 1993. p. 183-223.
  24. ↑ a b Spiropoulos, A., et al. "Characterization of Hydrogen Sulfide Formation in Commercial and Natural Wine Isolates of Saccharomyces." Am. J. Enol. Vitic. 2000. 51:233-248.
  25. ↑ a b c d Wang, XD, et al. "Fermentative activity and production of volatile compounds by Saccharomyces grown in synthetic grape juice media deficient in assimilable nitrogen and/or pantothenic acid." Journal of Applied Microbiology. 2003. 94, pp. 349-359.
  26. ↑ a b Tokuyama, Takashi, et al. "Hydrogen Sulfide Evolution Due to Pantothenic Acid Deficiency in the Yeast Requiring this Vitamin, with Special Reference to the Effect of Adenosine Triphosphate on Yeast Cysteine Desulfhydrase." J. Gen. Appl. Microbiol. 19, 1973. pp. 439-466
  27. ↑ a b c d Bohlscheid, Jeffri C, et al. "Interactive Effects of Selected Nutrients and Fermentation Temperature on H2S Production by Wine Strains of Saccharomyces." Journal of Food Quality. 2011. 34 pp. 51-55.
  28. ↑ a b c Ferreira, Inês M., and Guido, Luís F. "Impact of Wort Amino Acids on Beer Flavour: A Review." Fermentation. 2018, 4, 23.
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  32. ↑ "Preventing sulfide when bottle conditioning." homebrewtalk.com. 2019.
  33. ↑ Ferreira, Vicente, et al. "Elusive Chemistry of Hydrogen Sulfide and Mercaptans in Wine." J. Agric. Food Chem. 2018, 66, 10, pp. 2237-2246.
  34. ↑ Vela, Eduardo, et al. "Micro-oxygenation does not eliminate hydrogen sulfide and mercaptans from wine; it simply shifts redox and complex-related equilibria to reversible oxidized species and complexed forms." Food Chemistry. Volume 243, 15 March 2018. pp. 222-230.
  35. ↑ a b Ugliano, Maurizio, et al. "Occurrence of hydrogen sulfide in wine and in fermentation: influence of yeast strain and supplementation of yeast available nitrogen." Journal of Industrial Microbiology & Biotechnology. volume 38. 2011. pp. 423-429.
  36. ↑ a b c d e "Prevention and Treatment of Reductive Aromas." Enartis News. Accessed online March 2020.
  37. ↑ Briggs, DE, et al. Malting and Brewing Science: Malt and Sweet Wort. Second edition. Kluwer Academic Plenum Publishers. p 92.
  38. ↑ a b c Park, SK, et al. "Formation of Hydrogen Sulfide and Glutathione During Fermentation of White Grape Musts." Am J Enol Vitic. January 2000 51: 91-97.
  39. ↑ Kumar, GR, et al. "Survey of Hydrogen Sulfide Production in Wine Strains of Saccharomyces cerevisiae." Am J Enol Vitic. September 2010 61: 365-371.
  40. ↑ Mendes-Ferreira, A., et al. "Survey of Hydrogen Sulphide Production by Wine Yeasts." Journal of food protection. 65(6):1033-7. 2002.
  41. ↑ Duan, WD., et al. "A Parallel Analysis of H2S and SO2 Formation by Brewing Yeast in Response to Sulfur-Containing Amino Acids and Ammonium Ions." Journal of the American Society of Brewing Chemists. January 2004. 62(1):35-41.
  42. ↑ "Understanding the Production of Hydrogen Sulfide (H2S) During Fermentation." Scott Laboratories. scottlabs.com Accessed March 2020.
  43. ↑ "10 g Fresco Cider Yeast - H2S Preventing." The Beverage People. Product page. Accessed March 2020.
  44. ↑ a b c d Osborne, James. "Sulfides and Mercaptans in Wine." Oregon State University Wine and Grape Research and Extension Newsletter. October 2007.
  45. ↑ a b "Fart Smell." ProBrewer Message Board. probrewer.com. 2015.
  46. ↑ Pérez-Serradilla, JA and Luque de Castro MD. "Role of lees in wine production: A review." Food Chemistry. Volume 111, Issue 2, 15 November 2008, pp. 447-456.
  47. ↑ Bautista, R., et al. "Effect of the contact with fermentation-lees or commercial-lees on the volatile composition of white wines." European Food Research and Technology 2007 volume 224, pp 405-413.
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  49. ↑ Palacios, S., et al. "Evidence For Sulfur Volatile Products Adsorption by Yeast Lees." Am J Enol Vitic. January 1997 48: 525-526/
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  51. ↑ Bueno, JE, et al. "Effect of a Short Contact Time With Lees on Volatile Composition of Airen and Macabeo Wines." Biotechnology Letters. volume 28, pp. 1007-1011. 2006.
  52. ↑ Kraft, DN. "Impact of Lees Content, Nitrogen, and Elemental Sulfur on Volatile Sulfur Compound Formation in Vitis vinifera L. cv. ‘Pinot noir’ wine." Master's Thesis, 2015.
  53. ↑ Rager, A. "H2S Hydrogen Sulfide." Bacchus & Barleycorn, Ltd. Wine Making & Home Brewing Supplies. April 2012.
  54. ↑ "Copper Sulfate Trials." MoreWine! Accessed online April 2020.
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  56. ↑ "Diagnostic Test for Reductive Wine Characters (Cu/Cd Test)." The Australian Wine Research Institute (AWRI). Accessed online April 2020.
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  58. ↑ a b Zoecklein, Bruce. "Factors Impacting Volatile Sulfur-Like Off Aromas in Wine and Winery Options." Wine/Enology Grape Chemistry Group at Virginia Tech. July 2007.
  59. ↑ McCord, Jeff. "Application of toasted oak and micro-oxygenation to ageing of cabernet sauvignon wines." 2003.
  60. ↑ Walker, MD. "The influence of metal ions on concentrations of flavour‐active sulphur compounds measured in beer using dynamic headspace sampling." J. Sci. Food Agric. 1995. 67:1 25-28.
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  64. ↑ Ugliano, Maurizio, et al. "Evolution of 3-Mercaptohexanol, Hydrogen Sulfide, and Methyl Mercaptan during Bottle Storage of Sauvignon blanc Wines. Effect of Glutathione, Copper, Oxygen Exposure, and Closure-Derived Oxygen." J. Agric. Food Chem. 2011, 59, 6, 2564-2572
  65. ↑ Viviers, MZ., et al. "Effects of Five Metals on the Evolution of Hydrogen Sulfide, Methanethiol, and Dimethyl Sulfide during Anaerobic Storage of Chardonnay and Shiraz Wines." J. Agric. Food Chem. 2013. 61, 50, pp. 12385-12396.
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  68. ↑ "Ascorbic Acid, USP, Powder (Vitamin C)." Product description. Presque Isle Wine Cellars. Accessed online March 2020.
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  71. ↑ Vela, Eduardo, et al. "Micro-oxygenation does not eliminate hydrogen sulfide and mercaptans from wine; it simply shifts redox and complex-related equilibria to reversible oxidized species and complexed forms." Food Chemistry. Volume 243, 15 March 2018. pp. 222-230.
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  78. ↑ Boudreau, TF., et al. "Hydrogen sulphide production during cider fermentation is moderated by pre‐fermentation methionine addition." J. Inst. Brew. 2017. 123. pp. 553-561.
  79. ↑ Duan, W. et al. "A Parallel Analysis of H2S and SO2 Formation by Brewing Yeast in Response to Sulfur-Containing Amino Acids and Ammonium Ions." Journal of the American Society of Brewing Chemists. 62:1, 35-41. 2004.
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