Hydrogen sulfide is one of the main sulfides in beer, which has a great influence on the flavor of beer, and its flavor threshold is also the lowest (5PPm). The influence of hydrogen sulfide on the flavor of beer has a dual effect, that is, when it is present in trace amounts, it can constitute a special style of craft beer flavor, and excessive amounts will show an unfavorable taste of beer.
Sulfide in beer is partly derived from raw materials, and some volatile sulfides (mainly hydrogen sulfide) may also be formed in the process of malting and wort preparation, but most of these sulfides are in the process of wort boiling. removed by evaporation. The non-volatile organic sulfur compounds contained in wort, including sulfur-containing amino acids, biotin, thiamine and sulfur-containing proteins, peptides, etc., have a total content of about 50 mg/L.
Most of the volatile sulfur compounds in beer are formed during fermentation. The metabolism of sulfide by yeast first utilizes organic sulfide. In the presence of metabolizable organic sulfide, sulfate ions are rarely utilized, because the intake of inorganic sulfur requires energy and nitrogen sources.
(1) Decomposition of sulfur-containing amino acids by yeast
Most of the hydrogen sulfide in beer mainly comes from the assimilation of sulfur-containing amino acids (cysteine) and sulfate and sulfite by yeast, and the intermediate product when yeast synthesis of methionine is inhibited.
⑵. Yeast use sulfate to form
After sulfate enters into yeast cells, it is first activated by adenosine triphosphate under the catalysis of ATP-sulfatase, and becomes sulfite after a series of enzymatic reactions. Sulfite is an intermediate product, which is further reduced by sulfite reductase to form hydrogen sulfide. Methionine inhibits both ATP-sulfatase and sulfite, thereby limiting the breakdown of sulfate. Pantothenic acid also inhibits sulfite reductase.
⑶. The decomposition of cysteine forms H2S
Most of the hydrogen sulfide in the 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 desulfurase of yeast, and methionine has inhibitory effect on cysteine desulfurase, thereby affecting the decomposition of cysteine. Because most of the cysteine in the wort is cracked when the wort is boiled, the amount left in the cold wort is very small, so the decomposition of the cysteine is not the main source of hydrogen sulfide.
⑴. The influence of yeast species
The hydrogen sulfide yield of different yeasts is also different. The hydrogen sulfide yield of the yeast below is much higher than that of the yeast above. In terms of production, strains with less hydrogen sulfide can be formed by mutation selection. At the same time, in the process of yeast metabolism, the yield of hydrogen sulfide is parallel to the metabolic activity of yeast. The higher the yeast growth rate, the higher the yield of hydrogen sulfide.
⑵. 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 in the biosynthesis of methionine, so it can also indirectly inhibit the generation of hydrogen sulfide. Therefore, wort should contain enough pantothenic acid to ensure the growth of yeast and the biosynthesis of methionine. But generally speaking, the content of pantothenic acid in wort is sufficient.
In the stage of malting and mashing, excessive protein decomposition should be strictly prevented, because methionine in wort can inhibit ATP-sulfatase, so it can limit the utilization of sulfate, and methionine can also inhibit sulfite reductase and cysteine desulfurization enzymes.
Threonine, glycine and some other amino acids can inhibit the synthesis of methionine, so their presence will lead to the production of more hydrogen sulfide.
As the substrate for cysteine dethiolase, cysteine can stimulate the generation of hydrogen sulfide, and it can induce ATP-sulfatase, so it increases the utilization of sulfate and promotes the generation of hydrogen sulfide.
Even if the amino acid composition in the wort is constant, methionine will be quickly consumed during fermentation, leaving a relatively large amount of other amino acids, which will inhibit the synthesis of methionine and stimulate the formation of hydrogen sulfide.
Metal ions also have an effect on the formation of hydrogen sulfide, and it is generally believed that copper and zinc ions increase the formation of hydrogen sulfide.
(3) The effect of fermentation
In the early stage of fermentation, the generation of hydrogen sulfide can be inhibited due to the presence of methionine. When the wort is fermented until the residual sugar is 50Bx, the generation of hydrogen sulfide is the highest. As fermentation continues and other amino acids are consumed, the rate of sulfide formation decreases again. .
⑴. Most of the sulfur-containing amino acids in wort come from malt.
If auxiliary raw materials are used to replace part of malt, the content of hydrogen sulfide in beer can be reduced.
⑵. In the past, people believed that the use of copper wort boiling pots and pipes would produce good beer flavor. Practice has proved that the wort contains copper ions, and the H2S content in the prepared beer is indeed low. However, the harm of copper ions to the stability of beer flavor is also serious, and people are still reluctant to contain too much copper ions in wort.
(3) The wort with the complete separation of cold and hot coagulation, the sulfide content is reduced, and the generation of hydrogen sulfide can be reduced during fermentation.
⑷. Slow fermentation at low temperature or low inoculum can reduce the generation of H2S.
⑸. High sparkling wine added during wine storage often contains more hydrogen sulfide.
⑹. When the fermentation is completed, adding antioxidants such as sulfites, or using a wine storage container sterilized at 50 °C will easily cause more H2S to be generated.
⑺. Activating yeast mud with phosphoric acid can eliminate most of the polluting bacteria, but it is also easy to change the permeability of the yeast cell wall, so that the sulfate can easily enter the cells and be used, and promote the generation of H2S.
⑻. During the wine storage period, most of the volatile H2S can be eliminated along with the emission of a large amount of CO2. Generally speaking, the final content of H2S in beer can be controlled in the range of 0~100g/L. However, wort contaminated with miscellaneous bacteria (coliforms, zymomonas, etc.) will make beer contain a large amount of hydrogen sulfide.
⑼. After beer is sterilized, especially prolonging the sterilization time, H2S will increase a lot, and even double the H2S content after ultrafiltration, so that the sterilized beer will have an immature beer taste. But after a period of placement, the H2S concentration will gradually disappear to the concentration before sterilization or lower.