Hydrogen sulfide

From Brewing Forward

(This article is in progress)

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 sulfite compounds (VSCs). Sulfide aroma and flavor is often described as sulfurous like rotten eggs, "rhino farts", burnt match, volcanic gas, or simply "sulfur". It is also sometimes called "reduced" or "reductive" aroma.

Sulfide is one of the most common off flavors that presents in wine and cider. It can also occur in beer and other fermented beverages.[1] In fact, a slight note of sulfide may be acceptable in some styles of lager (beer). The aroma/flavor threshold of H2S is within 0.0005-1.5 mg/L,[2][3][4][5][6][7][8]

Sulfide is a precursor to other VSCs, notably mercaptans and disulfides. Mercaptan sensory threshold in wine is 0.00002-0.002 mg/L.[3] In 2000, the “Guinness Book of World Records” lists ethanethiol (another VSC) as the “smelliest substance in existence” (at 0.0028 mg/L).[3] However not all VSCs are bad; some sulfur compounds are desirable and important for wine character.[9] Hydropolysulfides such as H2S2 and H2S3 have been shown to contribute to the flint and mineral odor in wine.[10]

Hydrogen sulfide is toxic is large amounts, however the odor threshold is well below the threshold for toxicity and therefore toxicity is not a concern.[11]

Sulfide should not be confused with sulfite or sulfate. They are all different compounds.

Sulfide Formation

Yeast produce sulfide naturally, as part of the production of certain amino acids.

Specifically, sulfide is an essential metabolic intermediate in the biosynthesis of the amino acids cysteine and methionine, which are necessary for protein synthesis and cellular metabolism.[12][13][10] Sulfide is also now recognized as having important functions in detoxification, population signalling, and extending life span in yeast.[10]

Sulfide is produced mainly by molecular reduction of sulfate or sulfite present in the juice or wort.[14][12][13][15][10][16][4] 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[10][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.

Causes of Overproduction

Yeast strain is one of the main factors influencing the production of sulfide.[4][15][17][18] Some strains of yeast are biologically much more prone to over-producing sulfide.

Lack of adequate yeast nutrients is another main factor.[3][4][13][16][19][20] 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.[4][13][21][22][23] If there is not enough of the precursor, the yeast release the hydrogen sulfide into the wine or beer.

Elemental sulfur is frequently sprayed in the vineyard to fight grapevine powdery mildew, and the residual sulfur on grape has been observed to contribute to the formation of sulfide during fermentation by yeast.[10][12][13]

Timing of its Appearance

Maximum amounts of sulfide is 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]

While sulfide formation occurs mainly during primary fermentation, additional VSCs can be formed at later stages. Their formation can be difficult to predict and is not necessarily related to sulfide issues during the primary fermentation.[16] The VSCs involved include mercaptans and disulfides that have distinctive aromas such as skunky, rubbery, garlic, onion, or cabbage-like.[16][10][24] These compounds often result from degradation of sulfur-containing compounds in the yeast lees or from the re-release of chemically-bound sulfide during aging.[16][19] Sulfide can be formed when naturally bottle carbonating.[25]

There is not always correlation between total sulfide produced by yeast during fermentation and the sulfide concentration in the final wine.[17][26]

Prevention

An ounce of prevention is worth a pound of cure. For best results, a multi-factorial approach is needed to reduce the sulfide level in the final wine.

  • Low sulfite-producing and low sulfide-producing yeast strains can be considered. Scott Labs has bred some strains specifically to reduce sulfide production.[27]
  • Reduced sulfite usage has been shown to reduce liberation of sulfide.[15] Beer brewers using sulfite should adequately aerate/oxygenate to neutralize the residual sulfite when pitching (see low oxygen brewing).
  • Aeration is especially important in affecting nitrogen utilization, fermentation vigor, and hence sulfide stripping from the medium.[13]
  • Vitamins should be supplemented. Deficiencies of vitamins that act as co-factors to SRS enzymes (e.g. pantothenic acid) cause a methionine shortage and hence overproduction of sulfide even when adequate nitrogen is present.[13][21][22][23]
  • Nitrogen supplementation can help lower sulfide production, but only when there are also adequate co-factors present for the SRS.[13][28] Otherwise nitrogen supplementation may increase sulfide production.[23][21][29][4][26][17] There may also be some variability among yeast strains or species with regard to whether increasing nitrogen decreases sulfide formation.[17][30]
  • Generally higher temperatures increase sulfide liberation, although not necessarily because of increased production.[23] Lower temperature doesn't necessarily mean lower sulfide production; each strain has an optimum fermentation temperature to minimize its production. [18]
  • A lengthier fermentation increases sulfide in the final wine.[28] This is probably because fermentation time is linked to aeration and nutrient supplementation.
  • The exact role of lees on sulfide formation has not been established. Aging on lees could be the cause of the problem but also the solution; evidence of both the release of VSCs from lees and the removal of VSCs by wine lees has been widely reported. The conditions under which each phenomenon occurs is a very complex matter closely related with the yeast strain and the winemaking conditions.[31][16][32][33][34][35][36][37]

Nutrition Strategy

"Nutrition" in this context refers to sources of yeast-assimilable nitrogen (YAN), necessary vitamins, and certain trace minerals. YAN is the amount of nitrogen from the combination of ammonium plus Free Amino Nitrogen (FAN), in the form of amino acids.

Measuring YAN
We can measure YAN via with a few reagents and a pH meter:

YAN Target
The optimal YAN level for wine is generally 250-350ppm (nitrogen).[3][38][39]

Nutrient tables:

Too much YAN (>350mg/L) can induce an overpopulation of yeast, which will increase stress conditions and produce undesirable characteristics such as off-flavors or stuck fermentation.[40] To ferment 1g/L of sugar, yeast need 1mg/L of YAN. (1°Brix = 10g/L sugar) For good population growth, a minimum of 150mg/L YAN is needed.

YAN target depends heavily on yeast strain and fermentation conditions (e.g. initial sugar, temperature, fermentation aeration).[39][38]

  • YAN requirement for clean/fruity flavour has only been determined in Chardonnay: low YAN juices gave more complex aromas whereas moderate YAN gave cleaner and more fruity aromas in young wines.[39]
  • Large additions of inorganic nitrogen (DAP) can increase risk of ester taint (ethyl acetate) formation.[39]

DAP is 21% nitrogen by weight. Adding DAP during active fermentation will help the yeast remove existing sulfide within a few hours, but only if the ABV is under 7%.[3]

Equal proportions of ammonium to amino nitrogen and moderate initial concentrations of DAP (100 to 150 mg N/l) result in the lowest sulfide formation after peak fermentation.[4]

Vitamins and Trace Elements

Thiamine
Used as a co-enzyme for fermentation. It stimulates yeast growth, speeds up fermentation, and decreases undesirable fermentation byproducts, notably acetaldehyde.[41]
Biotin
Biotin is the most important vitamin for yeast (Fig.2) It is involved in almost all enzyme reactions that create the compounds yeast are made of: proteins, DNA, carbohydrates, and fatty acids. Biotin deficiency results in slow yeast growth and stuck fermentations.[42]
Zinc
It is needed in the micro molar (10-3M) range in wort. Zinc is important in the cell cycle (reproduction), and is a cofactor for alcohol dehydrogenase, the enzyme responsible for alcohol production. Other metal ions can not substitute in place of zinc. Supplementation of zinc into brewers worts generally has the effect of speeding up fermentation, as well as preventing stuck fermentations.[42]

Typical vitamin requirements for yeast include biotin, nicotinic acid, vitamin B, and pantothenic acid.[42]

Essential vitamins: 250 µg/l Ca-pantothenate, 250 µg/l thiamin a HCl, 25 µg/l pyridoxine, 2 µg/l biotin.[4]

0.2 mg/L folic acid, 200 mg/L myo-inositol, 4 mg/L pyridoxine, 4 mg/L nicotinic acid, 1 mg/L thiamin, 0.4 mg/L riboflavin and 0.250 mg/L pantothenic acid.[43]

Staggered Nutrients

  • Higher initial juice/must YAN values increase fermentation rate and heat production.[39]
  • DAP can be added in divided doses to give a more moderate rate of fermentation.[39]
  • Higher initial juice/must YAN values or DAP additions can increase the risk of residual YAN in finished wines.[39]

Amino acids are brought into the yeast cell through transport across the cell membrane. The presence of alcohol and ammonium ions (i.e., DAP) inhibit amino acids from being brought into the cell. This is why winemakers are advised NOT to add DAP at inoculation or at the beginning of fermentation, as yeast can actively absorb organic nitrogen in the juice (aqueous) environment.[44] Once alcohol concentrations begin to increase, as a result of primary fermentation progression, transport of amino acids from the wine into the yeast cell will be inhibited. Therefore, the primary source of nitrogen will then come from inorganic sources, such as DAP.[44][citation needed] Higher concentrations of the inorganic component of YAN can lead to a high initial biomass of yeast. This is a problem because the rapid increase in yeast populations can lead to starvation by the majority of the yeast by mid- to late-fermentation, especially if there is not enough nutrition to fulfill all of the yeast during fermentation. Yeast starvation leads to yeast stress, and one of the stress responses by yeast is the production and release of hydrogen sulfide. Therefore, having a high YAN at the start of fermentation may cause hydrogen sulfide issues in the wine by the time fermentation is complete.[44]

Yeast don’t need all the nutrients at the same time:

  • During the growth phase, yeast need vitamins, minerals and nitrogen. The presence of alcohol and/or ammonium ions inhibits transport of amino acids through cell membranes and reduces their consumption.[40]
  • To optimize their absorption and efficiency, amino acids should be added at inoculation, before ammonium ions. At this stage, yeast can assimilate amino acids to build ‘healthy’ cells which are resistant to stress conditions and produce aromas.[40]
  • At 1/3 of sugar depletion, yeast start to become stressed and the assimilation of nitrogen is lower. To complete fermentation and increase their alcohol resistance, they need fast and easy nutrients to absorb (ammonium ions) and survival factors (sterols and unsaturated fatty acids) with oxygen.[40]
  • In case of strong nitrogen deficiency, must needs to be corrected by an addition of ammonium ions 24-48 hours after inoculation (after the addition of amino acids).[40]
  • The nutrient additions should be split between inoculation and no later than 1/3 sugar depletion.[40]
  • Late nutrient additions are ineffective for yeast activity and can promote development of spoilage organisms, appearance of off-flavors and formation of biogenic amines.[40]

Nutrient Products

Removal

Hydrogen sulfide should be removed promptly because it becomes more difficult to remove the longer it stays in the wine.[45]

While large amounts of H2S may be produced during fermentation, much of this H2S is usually volatilized (off-gassed) from the wine or beer along with CO2 during active fermentation.[16] After fermentation, aeration and splashing may dissipate any residual H2S.[16] The simplest way sometimes is to aerate since H2S has a low B.P. of only -60.7°C[3] If H2S aromas persist, then it may be necessary to treat the wine with copper.[16] The decrease in H2S content during the late stage of beer fermentation, after yeast growth, is said to be mainly attributed to CO2 purging.[2]

A simple method of removing H2S is to add enough 1 percent copper sulfate solution to produce about 0.1 ppm of copper in the wine. Then the wine should be stirred thoroughly, and after a few hours, the wine should be carefully smelled. Table 1 can be used to determine how much of the 1 percent copper sulfate solution is needed for a 0.1 ppm treatment. One treatment is often enough, but a second or even a third treatment may be necessary for difficult cases. The wine should be left undisturbed for several days after this treatment so the copper sulfide (a very fine black powder) can settle to the bottom of the container. Then the wine should be carefully racked off the copper sulfide residue.[45]

The H2S decrease at the end of fermentation was mainly through uptake by yeast, when the CO2-purging effect was very small.[2] A study shows that as the number of suspended yeast cells increased, there was a higher rate of decrease in H2S, and that increased yeast contact after fermentation removes H2S faster.[2]

Frequently a sulfiting eliminates H2S.[3] Aeration combined with sulfite often gives the best results.[3][45]

Copper ions combine with H2S and mercaptans to form complexes with no offensive smell. After treatment with copper, the wine can then be racked off the lees.[16][45] Bench trials MUST be conducted to determine the appropriate dose.[16] Concentrations of between 0.05 and 0.3 mg/L of copper are commonly added.[16] Copper should not be added to the wine until the fermentation is complete and the amount of yeast material is reduced by racking.[16] Yeast cells can bind with copper and reduce effectiveness.[16] Also, addition of copper during fermentation may promote H2S production by yeast.[16]

Penny test? Bench trial?[3]

Disulfides are not removed by copper.[16] If you aerate wine to remove sulfide aromas, you may oxidize mercaptans present to disulfides.[16] Initially, you will notice a loss of the offensive mercaptan aromas as disulfides have a much higher sensory threshold than mercaptans and may not be detected even with the disulfides still present.[16] Copper sulfate can react with hydrogen sulfide and slowly with mercaptans (several days), but not diethyldisulfide.[3]

Wines with microoxygenation have a significantly lower level of both methyl and ethyl mercaptan.[46] "When conditions in the wine become more reductive (during barrel aging or in the bottle) the disulfides can be reduced back to mercaptans resulting in a reappearance of sulfide aromas.[47][48] Sulfide aromas may also reappear even after a copper treatment initially seemed to remove them; this is due to the presence of disulfides that were not removed by copper being reduced back to mercaptans. Since disulfides are difficult to remove from wine, the best approach is taking early preventative measures to minimize the production of H2S during fermentation and the formation of mercaptans. These measures include providing sufficient yeast nutrients for a healthy fermentation, using low H2S producing yeast strains, early removal of wine from heavy lees, and monitoring wine lees for sulfur off-odors during barrel aging."[16]

Copper(II)-citrate (Cu2C6H6O7) 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.[3] Consequently, it does not leave as much residual copper in the wine.[3] The manufacturer claims only about 50% goes into wine.[3]

  • copper sulfate: 1g = 0.255mg copper[3]
  • copper citrate: 1g = 0.350mg copper[3]
  • kupzit: 50g = 1g copper citrate = .350g copper[3]

Silver chloride is very effective for removing sulfide, mercaptans, and diethylsulfide.[3] It leaves no silver in the wine.[3]

Excess copper may increase H2S formation over time.[49] after addition of copper(II) to wines containing sulfide, the presence of residual copper is unavoidable and remains active in mediating reactions.[50]

Growing evidence suggests that copper treatment leads to increased sulfide formation during bottle storage (Ugliano et al.2011; Viviers et al.2013). One of the proposed mechanisms is that sulfide reacts with copper to form copper sulfide complexes, which may then release sulfide under anaerobic storage conditions.[10]

Ascorbic acid

Science

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.[21] or here: https://academic.oup.com/femsyr/article/17/6/fox058/4056150 for an in-depth review of the biological pathways.[10]

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.[15][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.[10] 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).[10]

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.[10] 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).[10]

Final wine concentration of glutathione was correlated with both total N and organic nitrogen.[28] (see http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.913.1799&rep=rep1&type=pdf)

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).[3]
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[3]

Reaction of sulfide with oxygen:[3] 2 H2S + O2 → 2 S + 2 H2O

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

Reaction of sulfide with copper sulfate:[3] H2S + CuSO4 → CuS + H2SO4 Not quite sure about this.

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. Advanced methods for measuring this sulfide exist. (see https://www.ajevonline.org/content/68/1/91)

Amino Acids

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.[16][14][12][10][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.[51] However, other in vivo studies suggested that deletion of yeast CYS4 or CSY3 did not reduce the production of H2S.[10] Grape juice usually contains plenty of sulfate (∼160 to 700 mg/L) but very low concentrations of cysteine and methionine (<20 mg/L).[10] 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][10] 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.[14] 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.[20] 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.[52]

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.[4]

Sources yet to be included


tox

amino acids in apple juice https://onlinelibrary.wiley.com/doi/full/10.1002/jib.519


AA


References

  1. ↑ Smith, B. "Sulfur and Rotten Egg Aromas in Beer – Off Flavors in Home Brewing." BeerSmith™ Home Brewing Blog. 2018.
  2. ↑ 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.
  3. ↑ a b c d e f g h i j k l m n o p q r s t u v w x Kaiser, K. "Controlling Reductive Wine Aromas." Brock University CCOVI lecture series. 1 Feb 2010.
  4. ↑ a b c d e f g h i 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
  5. ↑ "Hydrogen Sulfide, Hazards." Occupational Safety and Health Administration (OSHA). Accessed March 2020.
  6. ↑ https://www.ncbi.nlm.nih.gov/books/NBK208170/
  7. ↑ https://www.atsdr.cdc.gov/toxguides/toxguide-114.pdf
  8. ↑ http://www.extension.iastate.edu/Publications/PM1963A.pdf
  9. ↑ Swiegers, JH and Pretorius, IS. "Modulation of volatile sulfur compounds by wine yeast." Applied Microbiology and Biotechnology vol 74, 2007. pp.954-960.
  10. ↑ a b c d e f g h i j k l m n o p q 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
  11. ↑ Guidotti, TL. "Hydrogen Sulfide: Advances in Understanding Human Toxicity." International Journal of Toxicology. 2010. 29(6) pp. 569-581.
  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 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 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.
  15. ↑ 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.
  16. ↑ a b c d e f g h i j k l m n o p q r s t Osborne, James. "Development of sulfur off-odors post-fermentation." Oregon Wine Research Institute. 2013.
  17. ↑ 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.
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  20. ↑ 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.
  21. ↑ 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.
  22. ↑ 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
  23. ↑ 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.
  24. ↑ https://www.etslabs.com/library/31
  25. ↑ "Preventing sulfide when bottle conditioning." homebrewtalk.com. 2019.
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  27. ↑ https://scottlab.com/h2s-production
  28. ↑ 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.
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  30. ↑ Mendes-Ferreira, A., et al. "Survey of Hydrogen Sulphide Production by Wine Yeasts." Journal of food protection. 65(6):1033-7. 2002.
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  34. ↑ 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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  37. ↑ 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.
  38. ↑ a b "Approximate YAN Contribution for the Important Yeast Nutrients."
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  41. ↑ https://pennsylvaniawine.com/wp-content/uploads/2017/04/Yeast-Nutrition.pdf
  42. ↑ a b c White, C. "Yeast Nutrients Make Fermentations Better."
  43. ↑ Bohlscheid, JC., et al. "The influence of nitrogen and biotin interactions on the performance of Saccharomyces in alcoholic fermentations." Journal of Applied Microbiology. 102 2007. 390-400.
  44. ↑ a b c Gardner, DM. "Starting your fermentation right: nutrient supplementation." Penn State Extension Wine & Grapes U. 2016.
  45. ↑ a b c d "Removing H2S (Hydrogen Sulfide)." The Vintner's Vault. Accessed online March 2020.
  46. ↑ McCord, Jeff. "Application of toasted oak and micro-oxygenation to ageing of cabernet sauvignon wines." 2003.
  47. ↑ Ferreira, Vicente, et al. "Elusive Chemistry of Hydrogen Sulfide and Mercaptans in Wine." J. Agric. Food Chem. 2018, 66, 10, pp. 2237-2246.
  48. ↑ 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.
  49. ↑ 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
  50. ↑ Clark, AC, et al. "Copper(II) addition to white wines containing hydrogen sulfide: residual copper concentration and activity." Australian Journal of Grape and Wine Research. 2015. 21: 30-39.
  51. ↑ Kabil, O. and Banerjee, R. "Redox Biochemistry of Hydrogen Sulfide." The Journal of Biological Chemistry. Vol. 285, No. 29, pp. 21903-21907. July 16, 2010.
  52. ↑ 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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