Impact of Hydrogen Sulfide in Brewing

Understand the role of yeast in controlling H2S to craft better beer.

Sulfur metabolism is of special relevance for beer quality during the entire brewing process. Humans have evolved to be very sensitive to sulfur compounds, which are associated with toxic or rotten foods. Nowadays we can determine food quality by other means, however, these molecules still play an important role in beer flavor and quality.

There are a wide range of sulfur compounds in beer. Hydrogen sulfide (H2S) is of particular interest because its presence is related to the yeast strain and metabolism. H2S is a very small molecule, known in the brewing industry since the end of the 19th century. It is highly volatile with a very low flavor threshold level reminiscent of rotten eggs. H2S can impact the beer flavor profile directly or can mask other flavor compounds present in the beer.

Although small amounts of this compound can be acceptable, or even desirable, in excess it can lead to unpleasant off-flavors, usually described as rotten eggs. For this reason, H2S production by the yeast should be limited as much as possible during fermentation, which requires a detailed understanding of the metabolism and nutritional requirements of specific yeast strains.

Sulfur is an important element required for all living organisms, particularly as a component of the amino acids cysteine and methionine as well as a component of vital co-factors. The brewing yeasts, as well as many other microorganisms, can metabolize sulfur compounds by different pathways. It is generally recognized that the major pathway for H2S formation in yeast is the sulfate reduction sequence (SRS) pathway. The activity of SRS pathway enzymes is strain-dependent and influenced by environmental conditions, which is described in detail in a dedicated white paper from Lallemand Brewing.

Elevated levels of H2S may also result from conditions that stress the yeast or contribute to its premature autolysis. On top of that, hydrogen sulfide is also a very reactive molecule that may combine with carbonyl compounds to produce other more stable off-flavors such as the pungent vegetal, rubbery or sewer-like aromas of mercaptans. To avoid problems with H2S in your beer, choose a yeast strain that produces less H2S and make sure to understand its nutrient requirements. Ensure a vigorous fermentation to drive off volatile H2S through CO2 stripping and ensure that healthy yeast is present in the beer at the end of fermentation to reabsorb the remaining H2S Strain choice is particularly important for lager strains, which are less vigorous and therefore do not efficiently remove H2S through CO2 stripping. Vigorous fermentations are achieved by pitching a sufficient amount of healthy yeast into a nutrient-rich wort and fermenting at an appropriate temperature for that strain. Reabsorption of H2S by the yeast after fermentation is promoted by allowing longer maturation times, especially for lager strains.

The best defense against H2S is to ensure that it is never produced in the first place. At Lallemand Brewing, we are providing brewers with useful tools that will limit the production of H2S and help the brewer deal with it. As you might have read in our previous newsletter, through a recent partnership with Renaissance Biosciences (Vancouver, Canada), Lallemand Brewing is now able to produce novel hybrid yeast strains that are unable to produce H2S and eliminate the risk of this common off-flavor from the brewery. Our global technical team is always available and willing to share tips and recommendations regarding traditional methods to prevent or reduce H2S in beer. We are constantly working on the characterization of our yeasts to be able to provide more and more information regarding their specific nutritional and environmental requirements.

We invite you to take a look at Lallemand Brewing’s recent technical paper on this topic and please feel free to contact your local rep for any additional information you might need.