Beer Odor Guide: Hydrogen Sulfide

The presence of hydrogen sulfide (H2S) in beer will give the beer a unique smell of rotten eggs. In some beer styles, a small amount of hydrogen sulfide will form a unique flavor in the beer. But, if beer contains more hydrogen sulfide, the beer may produce a pungent rotten egg flavor, which is not conducive to the flavor of the beer.

What Is Hydrogen Sulfide?

Hydrogen sulfide (H2S) is a gas with a unique smell of rotten eggs, which can easily mask the smell of fresh malt and hops. Hydrogen sulfide is highly volatile and has a low flavor threshold (0.9-1.5 ppb can be tasted). Although high levels of hydrogen sulfide have a significant effect on taste and smell, a small amount of hydrogen sulfide is considered to be a tradition and characteristic of some beer styles. For example, the British Pale Ayre from Burton-on-Trent, this beer has a strong bitterness and a dry aftertaste and is enhanced by a small amount of hydrogen sulfide. In other styles of beer, the smell of rotten eggs caused by hydrogen sulfide is considered bad and defective.

Where Does The Sulfur In Beer Come From?

The sulfur compounds in beer are partly derived from raw materials. Some volatile sulfides (mainly hydrogen sulfide) may also be formed in the process of malt manufacturing and wort preparation. But, most of these sulfides are evaporated during the boiling of the wort, and thus removed from the beer.

Many yeast strains (especially lagers) often give off the smell of rotten eggs when fermenting beer. The most important source of rotten egg odor is hydrogen sulfide gas, which is usually produced as a by-product of yeast processing sulfur during active fermentation. Sulfur itself comes from a variety of sources, including roasting malt, because some sulfur is produced when roasting or roasting malt.

In addition, hops often contain sulfur-containing compounds and aromatic hydrocarbons, and certain water bodies contain high sulfur content. Yeast itself may also contain some sulfur, and certain yeast strains (such as many lager beers) produce higher levels of sulfur-containing gases during fermentation.

How Is Hydrogen Sulfide Formed In Beer?

Compared to malt wine, Lager beer requires a longer time, more yeast cells, and requires a lower temperature for fermentation. Also, you can smell rotten eggs (hydrogen sulfide) within one or two days of the start of fermentation. But how does the smell of rotten eggs come about? Let us find out together.

• Yeast's Decomposition Of Sulfur-Containing Amino Acids

Most of the hydrogen sulfide in beer comes from yeast’s assimilation of sulfur-containing amino acids (cysteine), sulfate, and sulfite. In addition, hydrogen sulfide is also produced when the synthesis of methionine by yeast is inhibited.

• Yeast Uses Sulfate To Form

After the sulfate enters human yeast cells, it is first activated by adenosine triphosphate under the catalysis of ATP-sulfatase, and after a series of enzymatic reactions, it becomes sulfite. Sulfite is an intermediate product that forms hydrogen sulfide, which is further reduced by sulfite reductase to form hydrogen sulfide. Methionine has an inhibitory effect on ATP-sulfatase and sulfite, thereby limiting the decomposition of sulfate. In addition, pantothenic acid also has an inhibitory effect on sulfite reductase.

• Decomposition Of Cysteine

Most of the hydrogen sulfide in wort comes from the assimilation of sulfate by yeast, and the amount of cysteine is relatively small. Cysteine is catalyzed and decomposed into hydrogen sulfide by cysteine desulfurylase of yeast. Methionine has an inhibitory effect on cysteine desulfurylase, thereby affecting the decomposition of cysteine. Because most of the cysteine in wort is decomposed when the wort is boiled, and the amount left in the cold wort is very small, the decomposition of cysteine 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 by different types of yeast strains is different, and the amount of hydrogen sulfide produced by the bottom-fermenting yeast is much greater than that of the top-fermenting yeast. In terms of production, mutations can be used to breed yeast strains that formless hydrogen sulfide. At the same time, in the metabolic process of yeast strains, the amount of hydrogen sulfide produced is related to the metabolic activity of yeast. The higher the metabolic activity of yeast, the greater the production of hydrogen sulfide.

• Wort Components

Pantothenic acid can inhibit the generation of hydrogen sulfide from two aspects. First, it directly inhibits sulfite reductase. Secondly, it is a cofactor for methionine biosynthesis, so it can also indirectly inhibit the production of hydrogen sulfide. So, the wort should contain enough pantothenic acid to ensure the needs of yeast growth and the biosynthesis of methionine. But generally speaking, the content of pantothenic acid in wort is enough. The wort preparation and mashing stages should strictly prevent excessive protein degradation. Because the methionine in the wort inhibits ATP-sulfatase, it can limit the use of sulfate. Moreover, methionine can also inhibit sulfite reductase and cysteine desulfhydrylase.

Threonine, glycine, and some other amino acids can inhibit the synthesis of methionine, so their presence will cause more hydrogen sulfide to be produced. Cysteine can stimulate the production of hydrogen sulfide as the substrate of cysteine desulfurylase, and it can induce ATP-sulfatase, so it increases the use of sulfate and promotes the production of hydrogen sulfide. Even if the amino acid composition in the wort is constant, methionine will be quickly consumed during fermentation, and a relatively large amount of other amino acids will be left to inhibit the synthesis of methionine and stimulate the formation of hydrogen sulfide. Also, metal ions affect the formation of hydrogen sulfide. It is generally believed that copper and zinc ions increase the formation of hydrogen sulfide.

• Effects Of Fermentation

In the initial stage of fermentation, hydrogen sulfide can be produced continuously due to the presence of methionine. So, when the first fermentation of wort is completed, the amount of hydrogen sulfide generated is the largest. When the fermentation continues, other amino acids are also consumed, and the rate of sulfide production will decrease.

What Measures Can Be Taken To Reduce The Hydrogen Sulfide Content?

• Most of the sulfur-containing amino acids in wort come from malt, and auxiliary materials can be used to replace part of the malt to reduce the hydrogen sulfide content in beer.

• In the past, people thought that using copper wort boiling pots and pipes to brew beer tasted good. The practice has proved that the wort contains copper ions, and the H2S content in the brewed beer is indeed low. But, the harm of copper ions to beer flavor stability is also serious, and people still do not want to contain too many copper ions in the wort.

• The wort with completely separated cold and hot coagulum has reduced sulfide content and can reduce the generation of hydrogen sulfide during fermentation.

• Slow fermentation at low temperature or low inoculum can reduce the amount of H2S produced.

• At the end of fermentation, adding antioxidants (such as sulfite) or using a wine storage container sterilized at 50°C can easily cause more H2S to be generated.

• Activating yeast with phosphoric acid can eliminate most of the contaminating bacteria, but it is also easy to change the permeability of the yeast cell wall. This will make it easier for sulfate to enter human cells and be used, and promote the production of H2S.

• During the storage period, with the emission of a large amount of CO2, most of the volatile H2S can be eliminated. Generally speaking, the final content of H2S in beer can be controlled within the range of 0-10μg/L. But, wort contaminated with bacteria (coliforms, Zymomonas, etc.) will cause the beer to contain a lot of hydrogen sulfide.

• Sterilization of beer, especially prolonging the sterilization time, will greatly increase H2S, even more than 1 times the H2S content after filtering the beer so that the sterilized beer will have an immature beer taste. But, after a time, the H2S concentration will gradually disappear to the concentration before sterilization or lower.