The presence of hydrogen sulfide (H2S) in beer imparts a unique smell of rotten eggs. In certain beer styles, a small amount of hydrogen sulfide contributes to the distinctive flavor. However, if beer contains excessive hydrogen sulfide, it can result in a pungent rotten egg taste that is undesirable for the flavor of the beer.
What Is Hydrogen Sulfide?
Hydrogen sulfide (H2S) is a gas that emits a distinct rotten egg smell, which can overpower the aroma of fresh malt and hops. Hydrogen sulfide is highly volatile and has a low flavor threshold (0.9-1.5 ppb can be detected). While high levels of hydrogen sulfide can significantly affect taste and smell, a small amount is considered a traditional and characteristic element in some beer styles. For example, British Pale Ayre from Burton-on-Trent has a strong bitterness and a dry aftertaste, which are enhanced by a small amount of hydrogen sulfide. However, in other beer styles, the smell of rotten eggs caused by hydrogen sulfide is considered undesirable and indicative of defects.
Where Does The Sulfur In Beer Come From?
Some sulfur compounds in beer originate from raw materials. During the process of malt manufacturing and wort preparation, volatile sulfides (primarily hydrogen sulfide) may also form. However, most of these sulfides evaporate during wort boiling and are therefore removed from the beer.
When fermenting beer, many yeast strains (especially lagers) emit a rotten egg smell. The primary source of this odor is hydrogen sulfide gas, which is produced as a by-product when yeast processes sulfur during active fermentation. Sulfur itself comes from various sources, including roasted malt, as sulfur is produced during roasting or kilning of malt.
In addition, hops often contain sulfur-containing compounds and aromatic hydrocarbons, and certain water sources have high sulfur content. Yeast itself may also contain sulfur, and specific yeast strains (such as many lagers) produce higher levels of sulfur-containing gases during fermentation.
How Is Hydrogen Sulfide Formed In Beer?
Compared to ale, lager beer requires a longer fermentation time, more yeast cells, and lower temperatures. The smell of rotten eggs (hydrogen sulfide) can be detected within one or two days of the start of fermentation. But how does this odor come about? Let’s find out together.
Yeast’s Decomposition Of Sulfur-Containing Amino Acids
Most of the hydrogen sulfide in beer is derived from yeast’s assimilation of sulfur-containing amino acids (such as cysteine), sulfate, and sulfite. Additionally, hydrogen sulfide is produced when yeast’s synthesis of methionine is inhibited.
Yeast Uses Sulfate To Form
After sulfate enters yeast cells, it undergoes activation by adenosine triphosphate under the catalysis of ATP-sulfatase. Through a series of enzymatic reactions, sulfate is converted into sulfite. Sulfite is an intermediate product that forms hydrogen sulfide, which is further reduced by sulfite reductase. Methionine inhibits ATP-sulfatase and sulfite, thereby limiting sulfate decomposition. Furthermore, pantothenic acid also inhibits sulfite reductase.
Decomposition Of Cysteine
Most of the hydrogen sulfide in wort comes from yeast’s assimilation of sulfate, while the amount of cysteine is relatively small. Cysteine is catalyzed and decomposed into hydrogen sulfide by yeast’s cysteine desulfurylase. Methionine inhibits cysteine desulfurylase, affecting cysteine decomposition. Since most cysteine in the wort decomposes during boiling, leaving only a minimal amount in the cold wort, cysteine decomposition is not the main source of hydrogen sulfide.
What Factors Affect The Formation Of Hydrogen Sulfide?
The Influence Of The Type Of Yeast Strain
The amount of hydrogen sulfide produced varies among different yeast strains, with bottom-fermenting yeast producing significantly more than top-fermenting yeast. Mutations can be utilized to breed yeast strains that produce less hydrogen sulfide. Additionally, the amount of hydrogen sulfide produced by yeast is related to its metabolic activity. Higher yeast metabolic activity results in increased hydrogen sulfide production.
Pantothenic acid can inhibit hydrogen sulfide formation both directly by suppressing sulfite reductase and indirectly as a cofactor for methionine biosynthesis. Therefore, wort should contain sufficient pantothenic acid to support yeast growth and methionine biosynthesis. Generally, wort has adequate pantothenic acid content. However, during wort preparation and mashing stages, excessive protein degradation should be avoided. Methionine in the wort inhibits ATP-sulfatase, limiting sulfate utilization. Additionally, methionine inhibits sulfite reductase and cysteine desulfhydrylase.
Threonine, glycine, and some other amino acids can inhibit methionine synthesis, resulting in increased production of hydrogen sulfide. Cysteine stimulates hydrogen sulfide production as a substrate for cysteine desulfurylase and induces ATP-sulfatase, promoting sulfate utilization and hydrogen sulfide formation. Even if the amino acid composition in the wort remains constant, methionine is quickly consumed during fermentation, leaving a relatively large amount of other amino acids that inhibit methionine synthesis and stimulate hydrogen sulfide formation. Additionally, metal ions also impact hydrogen sulfide formation, with copper and zinc ions generally increasing its production.
Effects Of Fermentation
During the initial stage of fermentation, hydrogen sulfide can be continuously produced due to the presence of methionine. Consequently, the largest amount of hydrogen sulfide is generated when the initial fermentation of wort is completed. As fermentation progresses, other amino acids are consumed, resulting in a decrease in sulfide production rate.
What Measures Can Be Taken To Reduce The Hydrogen Sulfide Content?
Most of the sulfur-containing amino acids in wort originate from malt. Using auxiliary materials to partially replace malt can reduce the hydrogen sulfide content in beer.
In the past, it was believed that brewing beer using copper wort boiling pots and pipes yielded better taste. It was proven that copper ions in the wort indeed lower the H2S content in the brewed beer. However, the negative impact of copper ions on beer flavor stability is significant, so excessive copper ions in the wort are undesirable.
Using completely separated cold and hot coagulum in the wort results in reduced sulfide content and helps to minimize the generation of hydrogen sulfide during fermentation.
Slow fermentation at low temperatures or with a low inoculum can reduce the amount of H2S produced.
At the end of fermentation, adding antioxidants (such as sulfite) or using a sterilized wine storage container at 50°C can lead to increased generation of H2S.
Activating yeast with phosphoric acid can eliminate most of the contaminating bacteria but can also increase the permeability of yeast cell walls. This facilitates sulfate entry into yeast cells, promoting H2S production.
During the storage period, a significant amount of volatile H2S can be eliminated with the emission of CO2. Generally, the final H2S content in beer can be controlled within the range of 0-10μg/L. However, wort contaminated with bacteria (coliforms, Zymomonas, etc.) can result in high levels of hydrogen sulfide in the beer.
Beer sterilization, especially prolonged sterilization time, greatly increases H2S content, sometimes exceeding the concentration after beer filtration. Sterilized beer may initially exhibit an immature taste due to H2S, but over time, the H2S concentration gradually diminishes to pre-sterilization levels or lower.