Editing Hydrogen sulfide

[[Category:off flavors]] [[Category:brewing chemistry]]
[[File:1024px-Tavurvur volcano edit.jpg|thumb|Volcanic gas]]
Hydrogen sulfide (H<sub>2</sub>S), 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.<ref name="Jastrzembski"/>

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]].<ref>Smith B. [http://beersmith.com/blog/2018/11/05/sulfur-and-rotten-egg-aromas-in-beer-off-flavors-in-home-brewing/ Sulfur and rotten egg aromas in beer – off flavors in home brewing.] BeerSmith Brewing Blog. Published 2018. Accessed July 2020.</ref> 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.<ref name="oka">Oka K, Hayashi T, Matsumoto N, Yanase H. [https://pdfs.semanticscholar.org/4c99/c9be2c401ada550eb779b865ab695c8df000.pdf Decrease in hydrogen sulfide content during the final stage of beer fermentation due to involvement of yeast and not carbon dioxide gas purging.] ''J Biosci Bioeng.'' 2008;106(3):253–257.</ref><ref name="Kaiser">Kaiser KJ. [https://brocku.ca/ccovi/wp-content/uploads/sites/125/Karl-Kaiser-Controlling-reductive-red-wine-aroma-2010-.pdf Controlling reductive wine aromas.] CCOVI lecture series presented at: Brock University; Feb 1, 2010; St. Catharines, Ontario. Accessed July 2020.</ref><ref name="Butzke"/><ref>[https://www.osha.gov/SLTC/hydrogensulfide/hazards.html Hydrogen sulfide hazards.] Occupational Safety and Health Administration (OSHA). Accessed March 2020.</ref><ref>[https://www.ncbi.nlm.nih.gov/books/NBK208170/ Hydrogen sulfide acute exposure guideline levels.] In: National Research Council (US) Committee on Acute Exposure Guideline Levels. ''Acute Exposure Guideline Levels for Selected Airborne Chemicals.'' 9th ed. Washington (DC): National Academies Press (US); 2010.</ref><ref>[https://www.atsdr.cdc.gov/toxguides/toxguide-114.pdf ToxGuide&trade; for hydrogen sulfide H<sub>2</sub>S.] Agency for Toxic Substances and Disease Registry (ATSDR). Published December 2016. Accessed July 2020.</ref><ref>[http://www.extension.iastate.edu/Publications/PM1963A.pdf Odor perception and physiological response.] The Science of Smell Part 1. Iowa State University. Published May 2004. Accessed July 2020.</ref>

The odor threshold is well below the threshold for human toxicity, and therefore safety is generally not a concern.<ref>Guidotti TL. [https://journals.sagepub.com/doi/pdf/10.1177/1091581810384882 Hydrogen sulfide: advances in understanding human toxicity.] ''Int J Toxicol.'' 2010;29(6):569–581.</ref>

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.<ref name="Moreira">Moreira N, Mendes F, Pereira O, Guedes de Pinho P, Hogg T, Vasconcelos I. [https://repositorio.ucp.pt/bitstream/10400.14/6812/4/Volatile%20sulphur%20compounds.pdf Volatile sulphur compounds in wine related to yeast metabolism and nitrogen composition of grape musts.] ''Anal Chim Acta.'' 2002;458(1):157–167.</ref><ref name="Jiranek">Jiranek V, Langridge P, Henschke PA. [https://aem.asm.org/content/aem/61/2/461.full.pdf Regulation of hydrogen sulfide liberation in wine-producing Saccharomyces cerevisiae strains by assimilable nitrogen.] ''Appl Environ Microbiol.'' 1995;61(2):461–467.</ref><ref name="Huang"/> Sulfide is also now recognized as having important functions in detoxification, population signalling, and extending life span in yeast.<ref name="Huang">Huang CW, Walker ME, Fedrizzi B, Gardner RC, Jiranek V. [https://academic.oup.com/femsyr/article/17/6/fox058/4056150 Hydrogen sulfide and its roles in Saccharomyces cerevisiae in a winemaking context.] ''FEMS Yeast Res.'' 2017;17(6).</ref><ref name="Off">[https://wineserver.ucdavis.edu/industry-info/enology/fermentation-management-guides/wine-fermentation/characters Off Characters.] University of California Davis, Viticulture and Enology. Accessed online March 2020.</ref><ref name="Muller">Müller N, Rauhut D. [https://www.mdpi.com/2311-5637/4/3/62/pdf Recent developments on the origin and nature of reductive sulfurous off-odours in wine.] ''Fermentation (Basel).'' 2018;4(3):62.</ref>

Sulfide is produced mainly by molecular reduction of [[sulfate]] or [[sulfite]] present in the juice or [[wort]].<ref name="Giudici">Giudici P, Kunkee RE. [https://www.ajevonline.org/content/45/1/107.short 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(1):107-112.</ref><ref name="Moreira"/><ref name="Jiranek"/><ref name="Hallinan">Hallinan CP, Saul DJ, Jiranek V. [https://www.researchgate.net/profile/Vladimir_Jiranek/publication/229784618_Differential_utilisation_of_sulfur_compounds_for_H2S_liberation_by_nitrogen_starved_yeasts/links/5ae31890a6fdcc9139a11277/Differential-utilisation-of-sulfur-compounds-for-H2S-liberation-by-nitrogen-starved-yeasts.pdf Differential utilisation of sulfur compounds for H<sub>2</sub>S liberation by nitrogen-starved wine yeasts.] ''Aust J Grape Wine Res.'' 1999;5:82–90.</ref><ref name="Huang"/><ref name="Osborne">Osborne J. [http://blogs.oregonstate.edu/owri/2013/12/10/development-sulfur-odors-post-fermentation/ Development of sulfur off-odors post-fermentation.] Oregon Wine Research Institute. Published 2013. Accessed 2020.</ref><ref name="Butzke">Butzke CE, Park SK. [https://pdfs.semanticscholar.org/5ee8/503db38eb8e0b117b62e499c21218eb0ffd7.pdf Impact of fermentation rate changes on potential hydrogen sulfide concentrations in wine.] ''J Microbiol Biotechnol.'' 2011;21(5):519–524.</ref> Sulfate is fairly ubiquitous, and sulfite is a common addition in wine and sometimes in beer.<ref name="Huang"/><ref name="Jiranek2002"/> Sulfide is also formed from elemental sulfur, which is sometimes used as an antifungal treatment on grapes.<ref name="Huang"/><ref name="Moreira"/><ref name="Jiranek"/> 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.<ref name="Jiranek2002">Jiranek V. [https://web.archive.org/web/20231020181935/https://www.infowine.com/intranet/libretti/libretto7316-01-1.pdf Causes of hydrogen sulfide formation in winemaking.] ''vinidea.net - Wine Internet Technical Journal.'' 2002;3.</ref><ref>Bokulich NA, Bamforth CW. [https://mmbr.asm.org/content/77/2/157 The microbiology of malting and brewing.] ''Microbiol Rev.'' 2013;77(2):157–172.</ref><ref name=fix>Fix G. ''Principles of Brewing Science.'' 2nd ed. Brewers Publications; 1999.</ref>

===Causes of Overproduction===
*Yeast strain is one of the main factors influencing the production of sulfide.<ref name="Butzke"/><ref name="Hallinan"/><ref name="UglianoM"/><ref name="Kim"/> Some strains of yeast are biologically much more prone to over-producing sulfide.
*Lack of adequate yeast nutrients is another main factor.<ref name="Kaiser"/><ref name="Butzke"/><ref name="Jiranek"/><ref name="Osborne"/><ref name="Rauhut">Rauhut D. 
Yeasts – production of sulfur compounds. In: Fleet GH, ed. ''Wine Microbiology and Biotechnology.'' Switzerland: Harwood Academic Publishers; 1993:183–223.</ref><ref name="Spiropoulos">Spiropoulos A, Tanaka J, Flerianos I, Bisson LF. [https://www.ajevonline.org/content/51/3/233.short Characterization of hydrogen sulfide formation in commercial and natural wine isolates of ''Saccharomyces''.] ''Am J Enol Vitic.'' 2000;51(3):233–248.</ref> 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.<ref name="Butzke"/><ref name="Jiranek"/><ref name="Wang"/><ref name="Tokuyama"/><ref name="Bohlscheid"/> 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 an inadequate amount of yeast or an unhealthy yeast culture may cause numerous fermentation-related problems.<ref name="Ferreira"/>
*The presence of metals (e.g. copper) during fermentation can stimulate sulfide production.<ref name="awri"/><ref name="Tamayo">Tamayo C, Ubeda J, Briones A. [https://www.nrcresearchpress.com/doi/abs/10.1139/w99-010 Relationship between H2S-producing strains of wine yeast and different fermentation conditions.] ''Can J Microbiol.'' 1999;45(4):343–346.</ref>
*Addition or over-use of [[sulfite]] may increase or cause sulfide production, particularly with yeast strains used for beer.<ref name="Jiranek"/><ref name="Zoecklein">Zoecklein B. [https://www.enology.fst.vt.edu/EN/133.html Enology notes #133.] Wine/Enology Grape Chemistry Group at Virginia Tech. Published 2007. Accessed 2020.</ref> This is because sulfite is the direct precursor to hydrogen sulfide in the SRS.
Wine only:
*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.<ref name="Huang"/><ref name="Moreira"/><ref name="Jiranek"/><ref name="Jastrzembski"/><ref name="Muller"/>

===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.<ref name="Jiranek"/> (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.<ref name="Muller"/> 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.<ref name="Osborne"/> 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.<ref name="Osborne"/><ref name="Huang"/><ref>[https://www.etslabs.com/library/31 Volatile sulfides: detection and prevention.] ETS Laboratories. Accessed March 2020.</ref> These compounds result from degradation of sulfur-containing compounds in the yeast [[lees]], and chemically-bound sulfide may be released during aging or storage.<ref name="Osborne"/><ref name="Rauhut"/><ref name="Jastrzembski">Jastrzembski J, Sacks G. [https://grapesandwine.cals.cornell.edu/sites/grapesandwine.cals.cornell.edu/files/shared/Research%20Focus%202016-3a.pdf Sulfur residues and post-bottling formation of hydrogen sulfide.] Research News from Cornell's Viticulture and Enology Program Research Focus. 2016;3a.</ref> Sulfide formation has also been reported to occur in the bottle when naturally bottle carbonating with yeast.<ref>[https://www.homebrewtalk.com/forum/threads/preventing-sulfide-when-bottle-conditioning.665501/ Preventing sulfide when bottle conditioning.] HomebrewTalk forum. Published 2019. Accessed July 2020.</ref><ref>[https://www.homebrewtalk.com/forum/threads/sulfur-after-bottling-what-can-be-done.677439/ "Sulfur after bottling...what can be done?"] HomebrewTalk forum. Published 2020. Accessed July 2020.</ref> Even VSCs that had apparently been removed may reappear if conditions in wine become more [[reductive aroma|reductive]] (e.g. during barrel aging or in the bottle).<ref name="Elusive"/><ref name="Vela">Vela E, Hernandez-Orte P, Franco-Luesma E, Ferreira V. [https://www.sciencedirect.com/science/article/pii/S0308814617315984 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 Chem.'' 2018;243:222–230.</ref>

There is not always correlation between total sulfide produced by yeast during fermentation and the sulfide concentration in the final wine/beer/etc.<ref name="UglianoM">Ugliano M, Fedrizzi B, Siebert T, et al. [https://pubs.acs.org/doi/abs/10.1021/jf8037693 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.</ref><ref name="Ugliano"/>

==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.<ref name="Osborne"/>

For wine and beer:
* '''Sulfite''' -  The presence of [[sulfite]] at the beginning of fermentation has been shown to cause formation of H<sub>2</sub>S.<ref name=Hallinan/><ref name=Jiranek/><ref name=Jiranek2002/><ref>[https://brewbrothers.freeforums.net/thread/682/biab-deoxygenated-strike-water BIAB and deoxygenated strike water.] The Biergarten website. April 2022. Accessed April 5, 2022.</ref> Brewers that use sulfite in the wort (i.e. [[low oxygen brewing|low oxygen brewers]]) need to adequately aerate/oxygenate the wort to neutralize the residual sulfite when pitching. For wine stabilization, it is recommended to wait at least two weeks before adding sulfite after fermentation ends, particularly when the yeast is still present.<ref name=Enartis>[https://www.enartis.com/wp-content/uploads/2019/12/Prevention-and-Treatment-of-Reductive-Aromas.pdf Prevention and treatment of reductive aromas.] Enartis News. Accessed March 2020.</ref> Because there is such a high risk of H<sub>2</sub>S production when adding sulfite soon after fermentation completes, we do not recommend adding sulfite to beer.
* '''Aeration''' - Adding oxygen before pitching yeast is especially important in affecting nitrogen utilization and fermentation vigor, which increases the amount of sulfide stripping from the wine or beer.<ref name=Jiranek/><ref name=Jiranek2002/> In wine production, aerating during fermentation is also helpful.<ref>Bekker MZ, Day MP, Holt H, Wilkes E, Smith PA. [https://onlinelibrary.wiley.com/doi/abs/10.1111/ajgw.12172 Effect of oxygen exposure during fermentation on volatile sulfur compounds in Shiraz wine and a comparison of strategies for remediation of reductive character.] ''Aust J Grape Wine Res.'' 2016;22(1):24–35.</ref> (See [[Aeration]])
* '''Vitamins''' - Vitamins should be supplemented in wine. Supplementation is not strictly necessary in beer production since [[wort]] typically contains adequate vitamins,<ref>Briggs DE, Hough JS, Stevens R, Young TW. [https://books.google.com/books?id=bHuCdG5VSmUC&pg=PA92&lpg=PA92&source=bl&ots=8c_VpU3Fs4&sig=ACfU3U1fgQ3aPJpEANWLRXjbv580IWc1Zw&hl=en&sa=X&ved=2ahUKEwiq96Ouqp_oAhUGVa0KHf3bDJoQ6AEwA3oECAcQAQ#v=onepage&f=false ''Malting and Brewing Science: Malt and Sweet Wort.''] 2nd ed. Kluwer Academic Plenum Publishers; 1981:92.</ref><ref name=fix/><ref name=bsp>Briggs DE, Boulton CA, Brookes PA, Stevens R. [[Library|''Brewing Science and Practice.'']] Woodhead Publishing Limited and CRC Press LLC; 2004.</ref> although it is potentially helpful. Deficiencies of [[pantothenic acid]] and [[pyridoxine]] (co-factors to SRS enzymes) may cause overproduction of H<sub>2</sub>S — even when adequate [[nitrogen]] is present.<ref name=Jiranek/><ref name=Jiranek2002/><ref name=Wang>Wang XD, Bohlscheid JC, Edwards CG. [https://sfamjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.2003.01827.x Fermentative activity and production of volatile compounds by ''Saccharomyces'' grown in synthetic grape juice media deficient in assimilable nitrogen and/or pantothenic acid.] ''J Appl Microbiol.'' 2003;94(3):349–359.</ref><ref name="Tokuyama">Tokuyama T, Kuraishi H, Aida K, Uemura T. [https://www.jstage.jst.go.jp/article/jgam1955/19/6/19_6_439/_pdf 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.'' 1973;19:439–466.</ref><ref name="Bohlscheid"/> (See [[yeast#Nutrition|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.<ref name="Jiranek"/><ref name="Park">Park SK, Boulton RB, Noble AC. [https://www.ajevonline.org/content/51/2/91.short Formation of hydrogen sulfide and glutathione during fermentation of white grape musts.] ''Am J Enol Vitic.'' 2000;51(2):91–97.</ref> Otherwise nitrogen supplementation may ''increase'' sulfide production.<ref name="Bohlscheid"/><ref name="Wang"/><ref>Kumar GR, Ramakrishnan V, Bisson LF. [https://www.ajevonline.org/content/61/3/365.short Survey of hydrogen sulfide production in wine strains of ''Saccharomyces cerevisiae''.] ''Am J Enol Vitic.'' 2010;61(3):365–371.</ref><ref name="Butzke"/><ref name="Ugliano">Ugliano M, Kolouchova R, Henschke PA. [https://link.springer.com/article/10.1007/s10295-010-0786-6 Occurrence of hydrogen sulfide in wine and in fermentation: influence of yeast strain and supplementation of yeast available nitrogen.] ''J Ind Microbiol Biotechnol.'' 2011;38:423–429.</ref><ref name="UglianoM"/> There may also be some variability among yeast strains or species with regard to whether increasing nitrogen decreases sulfide formation.<ref name="UglianoM"/><ref>Mendes-Ferreira A, Mendes-Faia A, Leão C. [https://www.researchgate.net/publication/11282471_Survey_of_Hydrogen_Sulphide_Production_by_Wine_Yeasts Survey of hydrogen sulphide production by wine yeasts.] ''J Food Prot.'' 2002;65(6):1033–1037.</ref> Similar to vitamins, nitrogen supplementation in [[wort]] or beer is not always required, but still may be helpful under certain conditions.<ref name="Duan">Duan WD, Roddick FA, Higgins VJ, Rogers PJ. [https://www.researchgate.net/publication/281600914_A_Parallel_Analysis_of_H2S_and_SO2_Formation_by_Brewing_Yeast_in_Response_to_Sulfur-Containing_Amino_Acids_and_Ammonium_Ions A parallel analysis of H2S and SO2 formation by brewing yeast in response to sulfur-containing amino acids and ammonium ions.] ''J Am Soc Brew Chem.'' 2004;62(1):35–41.</ref><ref name="Ferreira">Ferreira IM, Guido LF. [https://www.mdpi.com/2311-5637/4/2/23/pdf Impact of wort amino acids on beer flavour: a review.] ''Fermentation (Basel).'' 2018;4(23).</ref> (See [[yeast#Nutrition|Yeast]])
* '''Yeast strain''' - Low sulfide-producing and/or low nitrogen-requirement yeast strains may be considered.<ref name="Jiranek2002"/> Unfortunately it is not very well known which strains are high or low producers of H<sub>2</sub>S. Scott Labs and Renaissance Yeast have both bred some wine yeast strains specifically to reduce sulfide production.<ref>[https://scottlab.com/preventing-and-treating-sulfur-off-odors-in-cider Preventing and treating sulfur off-odors in cider.] Scott Laboratories website. 2023. Accessed online May 2024.</ref><ref>[https://www.thebeveragepeople.com/products/yeast/10-g-fresco-cider-wine-yeast.html 10 g fresco cider yeast - H2S preventing.] The Beverage People. Accessed March 2020.</ref>
* '''Pitch rate and yeast health''' - Pitch healthy yeast at a good pitch rate to decrease nutrient demand.<ref name="Ferreira"/><ref name="Zoecklein"/> "Shocking" the yeast (rapid changes in growth conditions like temperature or [[pH]]) should be avoided.<ref name="Jiranek2002"/><ref name="OsborneJ">Osborne JP. [https://owri.oregonstate.edu/sites/agsci/files/owri/05_vitnewsletteroct07webversion.pdf Sulfides and mercaptans in wine.] Oregon State University Wine and Grape Research and Extension Newsletter. Published October 2007.</ref> Significant over-pitching may also cause excessive sulfide.<ref name="fart"/><ref name=fix/> Even re-pitching yeast may cause increased production of sulfide.<ref>Thorne RSW. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1968.tb03167.x Some observations on yeast mutation during continuous fermentation.] ''J Inst Brew.'' 1968;74:516–524.</ref> (See [[Yeast]])
* '''Fermentation temperature''' - Generally lower temperatures decrease sulfide liberation, although not necessarily because of decreased production.<ref name="Bohlscheid">Bohlscheid JC,  Osborne JP, Ross CF, Edwards CG. [https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1745-4557.2010.00365.x Interactive effects of selected nutrients and fermentation temperature on H<sub>2</sub>S production by wine strains of ''Saccharomyces''.] ''J Food Qual.'' 2011;34:51–55.</ref> However each strain has an optimum fermentation temperature to minimize its production, so lower temperature doesn't always mean lower sulfide production.<ref name="Kim">Kim YR, Moon ST, Park SK. [http://ocean.kisti.re.kr/downfile/volume/ksfst1/SPGHB5/2008/v40n2/SPGHB5_2008_v40n2_238.pdf Effects of yeast strains and fermentation temperatures in production of hydrogen sulfide during beer fermentation.] ''Korean J Food Sci Technol.'' 2008;40(2):238–242.</ref> 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.<ref name="Park"/> 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.<ref>Pérez-Serradilla JA, Luque de Castro MD. [https://www.sciencedirect.com/science/article/pii/S0308814608004494 "Role of lees in wine production: A review."] ''Food Chem.'' 2008;111(2):447–456.</ref><ref name="Osborne"/><ref>Bautista R, Fernández E, Falqué E. [https://link.springer.com/article/10.1007/s00217-006-0336-7 Effect of the contact with fermentation-lees or commercial-lees on the volatile composition of white wines.] ''Eur Food Res Technol.'' 2007;224:405–413.</ref><ref name="Tamayo"/><ref>Palacios S, Vasserot Y, Maujean A. [https://www.ajevonline.org/content/48/4/525 Evidence for sulfur volatile products adsorption by yeast lees.] ''Am J Enol Vitic.'' 1997;48(4):525–526.</ref><ref>Vasserot Y, Steinmetz V, Jeandet P. [https://www.researchgate.net/profile/Philippe_Jeandet/publication/10733971_Study_of_thiol_consumption_by_yeast_lees/links/557ecf0d08ae26eada8f4699/Study-of-thiol-consumption-by-yeast-lees.pdf Study of thiol consumption by yeast lees.] ''Antonie van Leeuwenhoek.'' 2003;83;201-207.</ref><ref>Bueno JE, Peinado RA, Medina M, Moreno J. [https://link.springer.com/article/10.1007/s10529-006-9038-2 Effect of a short contact time with lees on volatile composition of Airen and Macabeo wines.] ''Biotechnol Lett.'' 2006;28:1007-1011.</ref><ref>Kraft DN. [https://ir.library.oregonstate.edu/downloads/f4752k49f Impact of lees content, nitrogen, and elemental sulfur on volatile sulfur compound formation in ''Vitis vinifera'' L. cv. 'Pinot noir' wine.] Master's Thesis, Oregon State University. Published 2015.</ref><ref name="oka"/> For wine, many experts suggest that if the lees start to form these compounds, the wine should be immediately racked.<ref name="Off"/><ref name="Osborne"/><ref>Rager A. [https://web.archive.org/web/20200805105452/https://www.bacchus-barleycorn.com/catalog/article_info.php?articles_id=120 "H2S hydrogen sulfide."] Bacchus & Barleycorn, Ltd. Published April 2012.</ref> Some experts suggest that only the gross lees are problematic, while the light lees are beneficial.<ref name="OsborneJ"/> For beer, the absorption of sulfide by yeast has been demonstrated, and home brewers do not report VSC production from aging on yeast.<ref name="oka"/>

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.<ref name="Jastrzembski"/><ref name="OsborneJ"/>
* '''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.<ref name="awri">[https://www.awri.com.au/industry_support/winemaking_resources/sensory_assessment/recognition-of-wine-faults-and-taints/wine_faults/ Wine flavours, faults and taints.] The Australian Wine Research Institute (AWRI). Accessed March 2020.</ref> 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. Any sample checked for reduced aromas must be clear for a valid test.<ref name="Kaiser"/>

'''Materials:'''<br />
* Tasting glasses
* A measuring device capable of allotting about 50 mL, such as a graduated cylinder or beaker
* A small syringe or pipette to measure 1 mL volumes
* Plastic wrap or watch glasses
* 1% [[Copper sulfate]] (CuSO<sub>4</sub>) solution (If you can't obtain copper sulfate, you can clean several U.S. pennies in an acid solution like lemon juice or vinegar.<ref name="Kaiser"/>)
* Optional: [[Ascorbic acid]] and distilled water to make a 5% ascorbic acid solution. Mix 2.5 g ascorbic acid into 50 mL water.

'''Procedure:'''<ref name="morewine">[https://www.morebeer.com/images/file.php?file_id=1309 Copper sulfate trials.] MoreWine! Accessed April 2020.</ref><ref name="Gardner"/><ref>[https://www.awri.com.au/industry_support/winemaking_resources/sensory_assessment/diagnostic_test/ Diagnostic test for reductive wine characters (Cu/Cd test).] The Australian Wine Research Institute (AWRI). Accessed April 2020.</ref><br />
# Obtain two 50mL samples of wine in glasses. Label one "Control" and the other "Copper".
# 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.
# Cover both glasses with a watch glass or plastic wrap and swirl.
# Let glasses sit for around 15 minutes and then examine by smell. Do NOT taste experimental glass.

'''Determining the results:'''<br />
# If the off odor is gone from the experimental glass, it is likely that only hydrogen sulfide and/or [[mercaptans]] are present.
# 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.
# 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 5 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. Ascorbic acid only works in a sample with adequate sulfite levels (30ppm Free SO<sub>2</sub> or higher).

==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 CO<sub>2</sub> during active fermentation.<ref name="Osborne"/><ref name="oka"/> 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|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.<ref name="Jastrzembski"/><ref name="Off"/><ref name="fart">[https://discussions.probrewer.com/showthread.php?49714-Fart-Smell "Fart Smell."] ProBrewer Message Board. Published 2015–2016.</ref><ref name="Enartis"/>

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.<ref name="Jastrzembski"/><ref name="Kaiser"/><ref name="Osborne"/><ref name="Enartis"/> [[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.<ref name="Mansfield"/><ref name="awri"/> 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.<ref name="Zoecklein2">Zoecklein B. [https://www.enology.fst.vt.edu/downloads/SLOFactorsFinal.pdf Factors impacting volatile sulfur-like off aromas in wine and winery options.] Wine/Enology Grape Chemistry Group at Virginia Tech. Published July 2007.</ref><ref name="Elusive">Ferreira V, Franco-Luesma E, Vela E, López R, Hernández-Orte P. [https://pubs.acs.org/doi/abs/10.1021/acs.jafc.7b02427 Elusive chemistry of hydrogen sulfide and mercaptans in wine.] ''J Agric Food Chem.'' 2018;66(10):2237–2246.</ref><ref name="Vela"/> This phenomenon does not occur in beer because beer always becomes increasingly oxidized over time.

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

===Copper===
[[Copper]] is a common tool used for the removal of both hydrogen sulfide and mercaptans.<ref name="Osborne"/><ref name="Kaiser"/><ref name="Off"/> Copper binds to these sulfur compounds to form odorless complexes, which precipitate to some degree.<ref name="Off"/><ref name="Muller"/> When copper is used in combination with [[ascorbic acid]] and [[sulfite]], disulfides can be removed as well (see below). Adding copper is fairly easy and inexpensive. Copper fining works in both wine and beer.<ref>Walker MD. [https://onlinelibrary.wiley.com/doi/abs/10.1002/jsfa.2740670105 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.</ref>

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.<ref name="Osborne"/> However some yeast may be helpful for removing the excess copper.<ref>Viviers M, Smith M, Wilkes E, Smith P, Johnson D. [https://winesvinesanalytics.com/features/article/132047/Role-of-trace-metals-in-wine-reduction Role of trace metals in wine 'reduction'.] ''Wines & Vines.'' 2014;May.</ref> 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.<ref>[https://web.archive.org/web/20200930204619/http://www.thevintnervault.com/index.php?p=w_m_tips&id=5781 Removing H2S (hydrogen sulfide).] The Vintner's Vault. Accessed March 2020.</ref><ref name="Gardner"/><ref name="Osborne"/>

Despite its widespread use, copper usage has a lot of potential disadvantages. The copper-sulfide complexes are challenging to remove from wine/beer, and they can potentially release the sulfide or mercaptans later.<ref name="Huang"/><ref name="Clark"/><ref>Ugliano M, Kwiatkowski M, Vidal S, et al. [https://pubs.acs.org/doi/abs/10.1021/jf1043585 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.</ref><ref>Viviers MZ, Smith ME, Wilkes E, Smith P. [https://pubs.acs.org/doi/abs/10.1021/jf403422x 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):12385–12396.</ref><ref>Bekker MZ, Kreitman GY, Jeffery DW, Danilewicz JC. [https://pubs.acs.org/doi/abs/10.1021/acs.jafc.8b04690 Liberation of hydrogen sulfide from dicysteinyl polysulfanes in model wine."] ''J Agric Food Chem.'' 2018;66(51):13483–13491.</ref><ref name="Muller"/> Copper might catalyze the release of sulfide from sulfur-containing amino acids.<ref name="Jastrzembski"/> Copper also reacts with any other thiols in the beer/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 can reduce the wine's varietal aroma.<ref>Gardner D. [https://www.dgwinemaking.com/winemakers/wine-flaw-focus-hydrogen-sulfide-and-reduction/ Wine flaw focus: hydrogen sulfide and "reduction".] Denise Gardner Winemaking. Published October 19, 2018. Accessed March 2020.</ref><ref name="Mansfield"/> In beer, thiol-containing hop compounds may be affected. Too much copper can cause a haze, referred to as "copper casse".<ref name="Mansfield"/><ref name="Muller"/> Risk of haze formation is greatly increased if copper is added immediately prior to packaging, without allowing adequate time for the beer/wine to stabilize during bulk storage. Lastly, excess copper catalyzes oxidation reactions, which can accelerate staling.<ref name="Mansfield"/><ref>Clark AC, Wilkes EN, Scollary GR. [https://onlinelibrary.wiley.com/doi/abs/10.1111/ajgw.12159 Chemistry of copper in white wine: a review.] ''Aust J Grape Wine Res.'' 2015;21(3):339–350.</ref> It's even possible that copper additions may actually increase the amount of VSCs in the final wine!<ref name="Muller"/>

For those reasons, the best results are obtained by using the minimum amount of copper needed to remove the offensive VSCs.<ref name="Muller"/> To do this, a "bench trial" should be performed to determine the minimum effective amount. Old-fashioned methods such as stirring with a copper pipe should be avoided because that practice may lead to excessively high levels of copper. If a bench trial is too complicated, you may add copper sulfate directly to the wine in incremental amounts (0.05-0.1ppm at a time). However that may end up being more work in the long run.

'''Bench trial materials:'''
* Glasses, vials or flasks, 50 mL or larger (although closer to 50 mL is preferred)
* A measuring device capable of allotting about 50-100 mL, such as a graduated cylinder or beaker
* A small syringe or pipette to measure 0.1 mL volumes
* Plastic wrap, stoppers, or watch glasses
* Distilled water
* 1% [[copper sulfate]] solution

'''Bench trial procedure:'''<ref name="Gardner"/><ref name="Gusmer">[https://www.gusmerenterprises.com/wp-content/uploads/2015/05/Sulfide-Detection-Kit-Instructions1.pdf Sulfide detection kit instructions.] Gusmer Enterprises, Inc. Accessed April 2020.</ref><ref name="morewine"/>
# Create a '''0.01%''' copper sulfate solution by adding 1 mL of the '''1%''' copper sulfate solution to a beaker or graduated cylinder and topping it up to 100 mL with water.
# Put 50 mL samples into four glasses.
# Label the following glasses: (1) control, (2) 0.1 ppm copper, (3) 0.2 ppm copper, and (4) 0.5 ppm copper.
# Using a '''0.01%''' copper sulfate solution, add the following increments to each glass:
#: 0.0 mL to glass 1 = no copper addition
#: 0.2 mL to glass 2 = 0.1 mg/L copper addition
#: 0.4 mL to glass 3 = 0.2 mg/L copper addition
#: 1.0 mL to glass 4 = 0.5 mg/L copper addition
# Cap each glass and wait 12-24 hours.
# Sniff (smell only!) each glass to determine when the offensive aroma is gone. This is the concentration of copper sulfate that will need to be added to the beer/wine. (0.04 mL of '''1%''' copper sulfate per liter of beer/wine gives 0.1 mg/L copper.)

Notes:
* Try to minimize [[headspace]] in the sample glasses, as air that can cause variation through oxidation.<ref name="Gusmer"/>
* Feel free to alter the concentrations evaluated and/or conduct additional trials. As little as 0.05 mg/L may be needed,<ref name="Osborne"/><ref name="Mansfield"/> and the maximum copper should not exceed 6 mg/L.<ref name="Gusmer"/>

Small amounts of excess copper sulfate (between 0.1-0.2 mg/L) can be removed with bentonite, yeast hulls, or fresh lees additions.<ref name="Gardner">Gardner D. [https://extension.psu.edu/sulfur-based-off-flavors-in-wine Sulfur-based off-flavors in wine.] Pennsylvania State University. Published Feb 14, 2020</ref><ref name="Enartis"/> However, the majority of added copper remains in wine and is not readily removed by racking or filtration.<ref name="Jastrzembski"/><ref name="Clark">Clark AC, Grant-Preece P, Cleghorn N, Scollary GR. [https://onlinelibrary.wiley.com/doi/abs/10.1111/ajgw.12114 Copper(II) addition to white wines containing hydrogen sulfide: residual copper concentration and activity.] ''Aust J Grape Wine Res.'' 2015;21(1):30–39.</ref>

'''Other copper products:'''<br />
[[Copper citrate]] may be a good alternative to copper sulfate because supposedly copper citrate does not totally go into the ionic form, and therefore does not leave as much residual copper in the wine.<ref name="Kaiser"/>

Kupzit® contains 2% copper citrate. For easy dosage and handling, it is coated onto a mineral carrier material, a particularly pure, high-quality granulated bentonite.<ref>[https://erbsloeh.com/englisch/download/54/datenblatter/3279/kupzit-technical-data-sheet.pdf Kupzit®.] Erbslöh Geisenheim GmbH. Accessed May 2020.</ref>
* 1g copper sulfate pentahydrate contains 0.255g copper
* 1g copper citrate hemipentahydrate contains 0.353g copper
* 50g Kupzit contains 1g copper citrate, which contains 0.353g copper


Another alternative product is Reduless®, which is a proprietary fining product from Lallemand that can "naturally enhance roundness while treating sulfur problems and reducing phenol related defects".<ref>[https://scottlab.com/reduless-reduless Reduless.] Scott Laboratories. Accessed April 2020.</ref> However, their claim that it removes DMS is pretty unlikely. Being proprietary, they don't really disclose what's in it, but it includes bentonite, inactivated yeast, and "natural elements which are rich in copper". Reduless® may also be easier to handle compared to copper sulfate solution.<br />
Suggested usage is 0.10-0.15 g/L, which adds no more than 0.02 mg/L copper (a low amount). If using this product, the copper sulfate bench trial described above isn't helpful since there's not a dosage conversion to determine how much Reduless would be needed to achieve the same effect.

===Ascorbic acid===
[[Ascorbic acid]] enables the removal of disulfide compounds by converting them to mercaptans, which allows copper to bind with them.<ref name="Mansfield"/><ref name="Off"/><ref name="awri"/>

First, it's important to make sure that free SO<sub>2</sub> levels are  at least 30 mg/L before adding ascorbic acid. Otherwise ascorbic acid will not help, and it will potentially lead to oxidation.<ref name="Mansfield">Mansfield AK. [https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/0/7265/files/2016/11/SulfurOffOdor-1vp1vm4.pdf Kicking up a stink: treatment for sulfur off-odors.] ''Cellar Dweller.'' Cornell University. Published April 2010.</ref><ref name="Gusmer"/><ref name="awri"/> (See [[Sulfite]] and [[SO2 testing]])

Ascorbic acid in the range of 20-100 mg/L may be used, depending on how strong the off odor is and how well it seemed to respond in the screening evaluation above.<ref name="Gusmer"/> Many sources suggest starting with around 50 mg/L or higher.<ref name="Mansfield"/><ref name="OsborneJ"/><ref name="PI"/> However, the well-respected Australian Wine Research Institute (AWRI) recommends adding much lower levels: For white wines, add 10 mg/L ascorbic acid and then another 10 mg/L the following day. For red wines, add 2 mg/L ascorbic acid and then another 2 mg/L the following day.<ref name="awri"/> However those recommendations are for commercial wine, which may have lower levels of disulfides compared to a problematic home brew.

To add ascorbic acid, simply calculate the amount based on your target level, dissolve it in a bit of water, and then add to your wine or beer.<br />
Here's an example calculation to target 50 mg/L for 20 L of wine:<br />
50 * 20 ÷ 1000 = 1 gram of ascorbic acid<br />
For lower levels or smaller volumes, it may be wise to make a stock solution as described in the screening procedure so that measurements will be more accurate.

After the ascorbic acid addition, we need to give the chemical reactions enough time to occur before conducting a copper bench trial (or adding copper to the batch). Unfortunately this process can take days to months, and recommendations for how long to wait are inconsistent.<ref name="Mansfield"/> The AWRI suggests to wait only 24 hours after the second addition of ascorbic acid.<ref name="awri"/> Other sources suggest to wait about 4-5 days,<ref name="Gusmer"/><ref name="OsborneJ"/> or as long as several weeks.<ref name="PI">[https://www.piwine.com/ascorbic-acid-usp-powder-vitamin-c.html Ascorbic acid, USP, powder (vitamin C).] Presque Isle Wine Cellars. Accessed March 2020.</ref> If the subsequent copper bench trial is unable to fully remove the odor with reasonable amounts of copper, consider adding more ascorbic acid, or giving it more time.

Ascorbic acid can trigger the release of sulfides and mercaptans from the copper.<ref>Chen Y, Jastrzembski JA, Sacks GL. [https://www.ajevonline.org/content/68/1/91.short Copper-complexed hydrogen sulfide in wine: measurement by gas detection tubes and comparison of release approaches.] ''Am J Enol Vitic.'' 2017;68(1):91–99.</ref><ref name="Jastrzembski"/> Therefore, the lowest effective amount of ascorbic acid may be preferred, so as not to interfere with the action of copper. However, given the slow speed of reaction, and the fact that it doesn't improve the odor by itself, bench trials to determine the lowest effective amount are not practical. The other potential downside of using ascorbic acid is that it can lead to faster oxidation if the level of sulfite is too low.

Note that there are many sulfide compounds that do not respond to fining with ascorbic acid and copper. Examples of these include [[diethyl sulfide]] (DES) and [[dimethyl sulfide]] (DMS). Unfortunately, there is no good or predictable way for us to remove these compounds.<ref name="Gusmer"/>

===Other measures===
'''Tannins''' - Enartis (a company that produces winemaking products) claims that the addition of tannins, especially ellagic (oak) tannins, has the ability to bind with mercaptans and form odorless complexes. These complexes are supposedly very stable over time and do not entail the risk of a later sulfur off-aroma appearance.<ref name="Enartis"/> However, we can't find any scientific research to support this. In fact, one study showed that adding oak does not seem to affect the levels of VSCs, with or without microxygenation (along with adequate sulfite levels).<ref>McCord J. [https://pdfs.semanticscholar.org/c9d6/21a697015d17a4f4c3ff84203912035194c4.pdf Application of toasted oak and micro-oxygenation to ageing of cabernet sauvignon wines.] StaVin Inc. Published 2003.</ref> There has been some anecdotal success with 0.1-0.2 g/L chestnut tannins for removing H<sub>2</sub>S, but not other VSCs.<ref>[https://www.winemakingtalk.com/threads/sulfide-aroma-sulfur-rotten-egg-prevention-and-removal.71238/#post-751940 Sulfide aroma (sulfur, rotten egg) - prevention and removal.] Wine Making Talk. Published April 2020.</ref>

'''Mineral Oil''' - As crazy as it sounds, disulfides can be removed by adding mineral oil to the wine and agitating it daily for several days. This works because the disulfides are more soluble in oil compared to the wine. When the wine beneath the mineral oil layer smells clean, it should be racked, leaving the oil behind.<ref>Jackisch P. ''Modern Winemaking.'' Cornell University Press; 1985:95.</ref> An additional racking may be needed to fully remove the oil. This may be useful as a last resort for removing disulfides from wine, but for beer it's probably not a great idea.

'''Silver Chloride''' - This additive is effective for removal of hydrogen sulfide, mercaptans, AND disulfides, but this compound has limited availability, it is relatively expensive, and there is very little information about its use.<ref name="Muller"/><ref name="Kaiser"/>

==Science==
'''Biology and sulfide formation'''<br />
For a review of the biological production pathways, see these articles:
* Wang XD, Bohlscheid JC, Edwards CG. [https://sfamjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.2003.01827.x Fermentative activity and production of volatile compounds by Saccharomyces grown in synthetic grape juice media deficient in assimilable nitrogen and/or pantothenic acid.] ''J Appl Microbiol.'' 2003;94(3):349–359.
* Huang CW, Walker ME, Fedrizzi B, Gardner RC, Jiranek V. [https://academic.oup.com/femsyr/article/17/6/fox058/4056150 Hydrogen sulfide and its roles in Saccharomyces cerevisiae in a winemaking context.] ''FEMS Yeast Res.'' 2017;17(6).

'''Sulfite'''<br />
To be utilized for production of H<sub>2</sub>S, SO<sub>2</sub> simply diffuses into the cell, equilibrates at cytosolic pH as bisulfite (HSO<sub>3</sub><sup>-</sup>) and sulfite (SO<sub>3</sub><sup>2-</sup>), and thereby can actually accumulate to 60-times its extracellular concentration.<ref name="Hallinan"/><ref name="Jiranek"/>

'''Myth busting'''<br />
Some sources claim that adding sulfite after fermentation theoretically helps eliminate sulfide by reacting with it.<ref name="Kaiser"/><ref name="awri"/><ref>Heunisch GW. [https://pubs.acs.org/doi/abs/10.1021/ic50172a033 Stoichiometry of the reaction of sulfites with hydrogen sulfide ion.] ''Inorg Chem.'' 1977;16(6);1411–1413.</ref> However, this appears generally not to be the case in practice, otherwise sulfide would basically never be a problem in wine. Perhaps it is instead related to oxygen exposure. Other sources indicate that H<sub>2</sub>S may simply exist in equilibrium with products of a sulfite + sulfide reaction, which also serve as a latent reserve of H<sub>2</sub>S that would release as sulfite declines during aging &mdash; this is arguably even worse than not helping.

Certain sources ([https://beerandbrewing.com/dictionary/6KDwm8vWwW/ Beer & Brewing]) claim that high levels of sulfate in brewing water may lead to increased sulfide production. This isn't supported by any modern science, nor the most modern textbook they reference (''Handbook of Brewing'', 2006).<ref>Priest FG, Stewart GG. ''Handbook of Brewing.'' 2nd ed. Taylor & Francis Group, LLC.; 2006:308.</ref> Jiranek (1992) showed that fermentations containing as much as 4800ppm sulfate (50mM) produce negligible amounts of sulfide.<ref name=thesis/>

'''Chemistry'''<ref name="Kaiser"/><ref name="Zoecklein2"/><br />
Reaction of sulfide with oxygen:<br />
'''2 H<sub>2</sub>S + O<sub>2</sub> &rarr; 2 S + 2 H<sub>2</sub>O'''

Reaction of sulfide with copper sulfate:<br />
'''H<sub>2</sub>S + CuSO<sub>4</sub> &rarr; CuS + H<sub>2</sub>SO<sub>4</sub>'''

There is still a great deal of uncertainty with regard to the formation and chemical reactions of both sulfur compounds and copper compounds. For a more thorough explanation of why this is the case, refer to the article by [https://www.mdpi.com/2311-5637/4/3/62/pdf Müller and Rauhut (2018)] if you dare.

'''Signalling'''<br />
Sulfide has been shown to perform an important signaling function. H<sub>2</sub>S will arrest the respiration phase and signal the onset of fermentation. This allows the population of cells to coordinate metabolic activity. H<sub>2</sub>S is deliberately made and released to coordinate rapid fermentation onset. There could be a strong selective advantage to the production of H<sub>2</sub>S under certain environmental conditions.<ref name="Off"/>

'''Mercaptans and disulfides'''<br />
The higher sulfide compounds (e.g. mercaptans and disulfides) are believed to largely generate from the degradation of the sulfur-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 VSCs. In beer, for example, dimethyl sulfide (DMS) can form from reduction of Dimethyl sulfoxide derived from S-methylmethionine (SMM). 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 sulfur-containing pesticides, but this is a rare occurrence and more often these compounds are derived from catabolism (break down) of sulfur-amino acids and their derivatives, glutathione and S-adenosyl-L-methionine.<ref name="Off"/>
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 H<sub>2</sub>S 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 VSCs. 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.<ref name="Off"/><ref name="Muller"/>

'''Cysteine'''<br />
The amino acid cysteine is commonly referenced as contributing to increased formation of hydrogen sulfide, but in practice its effect is negligible since cysteine concentration in must is typically very low.<ref name=thesis>Jiranek V. [https://digital.library.adelaide.edu.au/dspace/bitstream/2440/21637/2/02whole.pdf Hydrogen sulfide formation in ''Saccharomyces cerevisiae'' and its regulation by assimilable nitrogen.] Doctoral thesis for University of Adelaide. Published August 1992.</ref><ref name="Osborne"/><ref name="Giudici"/><ref name="Moreira"/><ref name="Huang"/><ref name="Jiranek"/><ref name="Huang"/><ref name="Jiranek2002"/> It's believed that aspartate aminotransferase deaminates cysteine to give mercaptopyruvate, which in a subsequent step catalyzed by MST liberates H<sub>2</sub>S and pyruvate.<ref>Kabil O, Banerjee R. [https://www.jbc.org/content/285/29/21903.full.pdf Redox biochemistry of hydrogen sulfide.] ''J Biol Chem.'' 2010;285(29):21903–21907.</ref> However, that hasn't been fully confirmed by other sources.<ref name="Huang"/>

'''Methionine'''<br />
Methionine addition has a variable effect on hydrogen sulfide (and sulfite) production, and the effect also varies by yeast strain.<ref>Boudreau TF IV, Peck GM, Ma S, et al. [https://onlinelibrary.wiley.com/doi/full/10.1002/jib.449 Hydrogen sulphide production during cider fermentation is moderated by pre-fermentation methionine addition.] ''J Inst Brew.'' 2017;123(4);553–561.</ref><ref name="Giudici"/><ref name="Moreira"/><ref name="Duan"/><ref name="Spiropoulos"/>

==See also==
* [http://allaboutbeer.com/article/sulfur-in-beer/ Article about VSCs in beer] by Randy Mosher.
* [https://wineserver.ucdavis.edu/industry-info/enology/fermentation-management-guides/wine-fermentation/characters Scientific discussion of the variety of VSCs and their formation] from UC Davis.
* [https://www.awri.com.au/industry_support/winemaking_resources/calculators/copper_sulfate/ Copper sulfate calculator] from the AWRI.

==References==