Hydrogen Sulfide, H2S, is a gas that has the distinctive smell of rotten eggs, which easily can overshadow the flavors of fresh malt and hops. Hydrogen sulfide is highly volatile and has a low flavor threshold, measured in parts per billion. Although high levels of hydrogen sulfide are notably offensive to the palate and nose, trace amounts are considered a traditional and accepted characteristic for a few beer styles, particularly English pale ales from Burton-on-Trent.
Hydrogen sulfide is produced by the normal metabolism of yeast, whereby sulfate ions are taken into the cell and reduced for processing into the amino acids cysteine and methionine. Low nitrogen levels will limit the rate of this reaction, leaving excess hydrogen sulfide to be excreted into the beer. Yeast stress and autolysis will likely occur in parallel, resulting in a complex profile of off-flavors. In addition, hydrogen sulfide may combine with carbonyl compounds to produce even more undesirable off-flavors such as the pungent vegetal and rubbery aromas of mercaptan. Vigorous fermentations, on the other hand, will lead to lower levels of hydrogen sulfide.
Levels of hydrogen sulfide can be controlled by ensuring that nitrogen levels are adequate during fermentation and that some oxygen is available in solution at the outset of fermentation, limiting the reducing condition that promotes hydrogen sulfide formation.