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update to diacetyl
| Hydrogen sulfide (rotten egg) || Chalcogen hydride gas <ref>[https://en.wikipedia.org/wiki/Hydrogen_sulfide "Hydrogen sulfide". Wikipedia. Retrieved 08/29/2020.]</ref> || || 4 µg/l in beer <ref>[https://www.aroxa.com/beer/beer-flavour-standard/hydrogen-sulphide/ "Hydrogen sulphide". Aroxa. Retrieved 08/29/2020.]</ref> || Produced in high amounts by ''B. custersianus'' and ''B. naardenensis'' <ref name="colomer_2020_genome" />.
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| Diacetyl/butanedione (butter) || Vicinal diketone <ref>[https://en.wikipedia.org/wiki/Diacetyl "Diacetyl". Wikipedia website. Retrieved 08/29/2020.]</ref> || || 10–40 µg/l 0.1–0.2 ppm in lager and 0.1–0.4 ppm in beer ales, although flavor thresholds as low as 17 ppb and 14–61 ppb have been reported <refname="krogerus_2013">[https://www.aroxaresearchgate.comnet/beerpublication/beer259331290_125th_Anniversary_Review_Diacetyl_and_its_control_during_brewery_fermentation Krogerus, K. and Gibson, B.R. (2013), 125th Anniversary Review: Diacetyl and its control during brewery fermentation. J. Inst. Brew., 119: 86-flavour-standard97. https:/2-3-butanedione/ "2,3-butanedione"doi. Aroxa websiteorg/10. Retrieved 081002/29/2020jib.84.]</ref> . || Produced by all strains and all species of ''Brettanomyces'' except ''B. naardenensis''; the amount that it is produced varies widely and not much is known about what determines diacetyl levels produced from ''Brettanomyces'' <ref name="colomer_2020_genome" />. ''Brettanomyces'' generally produce very low amounts of diacetyl (0.019 - 0.048 mg/L). It is hypothesized that ''Brettanomyces'' can reduce diacetyl during its maturation phase similar to ''Saccharomyces'' species, but this has not been investigated that we are aware of <ref>[https://onlinelibrary.wiley.com/doi/full/10.1002/jib.381 Michel, M., Meier‐Dörnberg, T., Jacob, F., Methner, F. ‐J., Wagner, R. S., and Hutzler, M. (2016) Review: Pure non‐Saccharomyces starter cultures for beer fermentation with a focus on secondary metabolites and practical applications. J. Inst. Brew., 122: 569– 587. doi: 10.1002/jib.381.]</ref>.
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