Flavor Focus: Hydrogen Sulfide

Hydrogen Sulfide (H₂S)

Perceived As: Boiled Eggs, Rotten Eggs, Rubbery, Volcanic Gas/Hot Springs, Hair Perm, Swamp Gas

Flavor Threshold: 5-20 ppb

Few groups of aroma compounds have shaped life, and impacted our senses quite like sulfur compounds.  Whether it’s the unmistakable odor of skunk spray or hard boiled eggs, the pungency of garlic or onions, the enticing aroma of fresh brewed coffee or tropical fruits- sulfur compounds evoke some of the strongest sensory responses.  There’s a good reason for that.  Human beings have evolved to recognize sulfur-containing molecules as indicators of both danger and opportunity- helping us avoid spoiled food or toxic environments, while also identifying nutrient-rich foods.   As a result, we are highly attuned to detecting sulfurs.  They are often perceptible at levels in the parts per billion, or even parts per trillion range, which means we are 1000s of times more sensitive to these than some other flavor active compounds that are relevant in beer. 

Much like sulfur compounds found throughout the natural world, sulfur compounds in beer can range from highly desirable flavor contributors to overwhelming off-flavors.  Some sulfur compounds toe the line in between these two extremes, adding layers of depth and complexity when present at appropriate levels, but become a distraction when present in excess.  Today’s spotlight will focus on one of these sulfur compounds, hydrogen sulfide, which is also known as H₂S. 

What is Hydrogen Sulfide?

For context, hydrogen sulfide is a sulfur compound that naturally occurs across of wide range of environments, including crude petroleum and natural gas deposits.  It also appears as a result of the bacterial breakdown of organic matter in places like sewers and swamps.  In beer, the main source of hydrogen sulfide is its natural production by brewer’s yeast in fermentation.  It may also arise from other sources, including bacterial or wild yeast contamination.   In the Cicerone Certification Program, hydrogen sulfide is one of the off flavors that may be tested on the Advanced Cicerone or Master Cicerone tasting exams.

How is Hydrogen Sulfide Perceived?

Because H₂S is found naturally in a variety of environments, many tasters may recognize its characteristic aroma from everyday encounters.  This may add to a greater number of associations, but like any off flavor, it is important to understand common industry terminology, and use that as a reference point to relate this back to your peers in a sensory setting.  The most common descriptors for hydrogen sulfide are boiled or rotten eggs.  Secondary descriptors of environmental origins like swamp gas or hot springs may resonate with those who have experienced this distinct aroma in those settings.  Like many similar sulfur compounds, it may present as having a rubbery quality.   One unique descriptor for hydrogen sulfide is “hair perm”.  This makes sense as volatile sulfur compounds are released as part of the chemical reaction that occurs during a hair perm treatment. 

Since hydrogen sulfide is detectable at extremely low concentrations, a distant sniff, followed by a few quick sniffs are typically more effective than one long sniff.

As we’ve mentioned, H₂S can be an acceptable, or even welcomed part of the flavor profile of certain beer styles.  It can add depth or complexity to a number of pale lagers, and can be a signature background note in certain English bitters or pale ales.  Recognizing where the line falls between appropriate levels and a distracting or unpleasant flaw is crucial in settings ranging from internal sensory panels to beer judging tables. 

How Is Hydrogen Sulfide Produced?

In the brewing industry, hydrogen sulfide is produced by yeast as a natural byproduct of fermentation.  It is released as part of a complex pathway that leads to the synthesis of sulfur containing amino acids, which are needed for cell growth and replication.  The primary reason that H₂S is released is to prevent the buildup of hydrogen ions inside the yeast cell.  This buildup creates an acidic environment that can cause stress and ultimately, cell death. 

Fermentation is a dynamic process, and H₂S levels can ebb and flow as sulfur metabolism and yeast cell requirements vary throughout the course of a normal fermentation.  The amount of H₂S produced is partly influenced by the yeast strain, with some strains—particularly lager strains—naturally producing higher levels.

The choices that brewers make throughout the brewing process play a pivotal role on H₂S levels in finished beer.  Generally speaking, fermentation conditions that favor yeast stress and restrict yeast growth will lead to higher levels of H₂S. 

• Low FAN levels, and insufficient levels of vitamins and nutrients, like zinc, that are critical to the sulfur metabolism pathways, can lead to higher levels of H₂S.  This needs to be taken into consideration during recipe development when producing beers with higher levels of adjuncts. 
• Fermentation temperatures that are either too low or too high for the yeast strain used
• Low yeast pitch rates that lead to greater metabolic stress
• Under aeration at the start of fermentation
• The introduction of oxygen at the end of fermentation or during beer transfer
• Using yeast that has undergone a high number of repitches, or repitching yeast that is in poor condition

Despite a long-standing association between Burton-on Trent’s sulfate-rich water, and the historic “Burton snatch”, a number of peer reviewed scientific articles show little correlation between high sulfate levels in wort leading to increase H₂S levels in beer.  The prevailing thought is that the signature aroma of these beers likely has more to do with yeast genetics, nutrient availability and brewing process decisions than a simple increase in the available sulfate.   

Finally, hydrogen sulfide can also be produced by contamination with a range of spoilage organisms.  Two of particular concern are Pectinatus sp. and Megesphaera sp., which are very difficult to detect and produce high levels of H₂S.  H₂S can also be produced to varying degrees by wild yeast strains. 

How is Hydrogen Sulfide Controlled or Prevented?

As mentioned earlier, H₂S levels commonly rise and fall throughout the course of a healthy fermentation.  Maintaining acceptable levels in the final beer comes from a combination of controlling fermentation inputs and environmental factors, as well as adhering to process best practices that will help remove H₂S that has naturally formed.  

During periods of vigorous fermentation, large amounts of H₂S can be “scrubbed” from the beer by CO₂.   When given the appropriate contact time, healthy yeast will reabsorb hydrogen sulfide, which means that following proper fermentation, lagering or maturation protocols can help reduce the levels in finished beer.

Because yeast genetics plays a significant role in the amount of H2S produced, understanding the yeast strain being employed and its nutritional requirements is of crucial importance when managing H₂S levels.  Today, there are even yeast strains on the market that have been selected or engineered to produce lower levels of H₂S.  Regardless of the yeast strain chosen, providing the ideal nutrients, FAN levels, and fermentation parameters such as temperature, pitch rate, and aeration are critical.  Following yeast handling and repitching best practices to ensure optimum yeast heath can also help mitigate the presence of H₂S.  Following proper cleaning, sanitation and micro testing protocols can ensure that H₂S is not produced at higher levels by spoilage microorganisms.    

In addition to preventing H₂S formation, brewers can employ several techniques later in the brewing process to reduce any hydrogen sulfide that remains.  CO₂ purging is a process where CO₂  is bubbled through the beer in the maturation tank.  It can mimic the H₂S “scrubbing” effect that occurs during a vigorous fermentation, but may also remove other positive volatile aroma compounds. 

Copper treatment has been shown as one of the most effective methods for removing H₂S from beer.   Historically speaking, beer came into frequent contact with copper throughout the brewing process, as brewhouse vessels and piping were commonly constructed from copper. As the industry has largely transitioned to stainless steel, the opportunities for copper contact have been diminished.  This has led some brewers to reintroduce copper through several different avenues, including the application of copper sulfate, pumping beer through copper tubing, or treatment with copper anodes.  The copper binds to the hydrogen sulfide and is precipitated out before the finished beer is packaged.  However, these methods should be used with caution because elevated copper levels in the beer can cause astringent or metallic flavors, as well as lead to quicker staling reactions.