Flavor from yeast and beer fermentation

Yeast cells are not the final product in beer production, but are indeed important to the quality of the final product beer. The most important thing in the selection of yeast production strains is the speed of fermenting wort and the degree of fermentation of wort. In addition to forming alcohol and carbon dioxide during fermentation, brewer's yeast also produces a certain number and type of by-products. These by-products include: higher alcohol , aldehydes and ketones, esters, diacetyl, sulfur compounds, fatty acids and organic acids, these by-products greatly affect the taste and smell of beer. These characteristics are first determined by the enzymatic activity of yeast cells, and are related to the type and concentration of yeast cells. Different yeast strains produce different amounts and types of by-products. Therefore, in order to breed brewer's yeast and carry out the fermentation process correctly, it is necessary to have a certain understanding of the properties of some strains. At the same time, it must be noted that yeast strains have various traits, some of which are constantly changing, which will cause changes in fermentation indicators.

The concept of good yeast should be a yeast strain with high fermentation degree, good cohesion, effectively removing diacetyl, and giving beer ideal physical and chemical properties and perfect taste.

Higher alcohols

The vast majority of higher alcohols in beer are formed during the vigorous main fermentation, and the fermentation pathways to generate higher alcohols mainly include degradative and anabolic pathways.

The sensory thresholds of various higher alcohols in beer are related to the type of beer and are affected by the composition of all flavor substances in beer. Studies have shown that higher alcohols have an additive effect when mixed with other flavor components. Amyl alcohol combined with phenethyl alcohol becomes the most influential factor on beer flavor among higher alcohols. Generally speaking, the content of isoamyl alcohol is the highest among the higher alcohols. For beer with high isoamyl alcohol content, drinkers should reduce the amount of alcohol relatively.

Aldehydes and Ketones

Acetaldehyde is a normal precursor of ethanol fermentation and is formed by pyruvate irreversibly catalyzed by pyruvate decarboxylase. Acetaldehyde and diacetyl are two important carbonyl compounds in beer, and they have an important influence on the flavor of beer. In the final stage of fermentation, acetaldehyde is reduced to ethanol by the enzymatic reaction of alcohol dehydrogenase, and a small part of acetaldehyde is oxidized to acetic acid. How much acetaldehyde is reduced and how much is oxidized is governed by the alcohol concentration during fermentation. The content of acetaldehyde varies during fermentation and storage. A large amount of acetaldehyde is formed in the early stage of the main fermentation. When the fermentation degree is between 35% and 60% in the following fermentation, the acetaldehyde content is the highest, and sometimes two high peaks can be generated, and then very Fall fast.

Acetaldehyde affects the maturation of beer taste. When the content of acetaldehyde in beer exceeds the threshold, it gives an unpleasant rough feeling, and if the content is too high, there is a spicy rotten grass smell. However, changing the content of acetaldehyde in beer cannot achieve the effect of improving beer flavor as expected. It has been studied that when acetaldehyde coexists with diacetyl and hydrogen sulfide, it constitutes the inherent green flavor of tender beer. The main ketone in beer is acetone, but the content is very small and has no effect on the flavor. It is probably the precursor of isopropanol, which comes from acetoacetic acid.

Esters

Most of the esters in beer are formed during fermentation, but some are formed during the maturity of beer. Although the content of esters in beer is small, it has a great impact on the flavor of beer. Its formation is related to the lipid metabolism of yeast. related.

The key compound that forms esters during fermentation is acyl-CoA. Coenzyme A exists in yeast, and esters are formed by the infiltration of fatty acids into yeast cells. A part of the formed ester is returned to the fermentation broth through the cell membrane, and the other part is adsorbed by the yeast and retained in the cell body. Esters are more important to beer flavor than higher alcohols, and have an important impact on wine flavor. But like other flavor components in beer, too high a level of esters will give the beer a fruity or candy-like flavor, but too much will give the beer a bitter taste.

Diacetyl

Diacetyl is an important substance that affects the maturity of beer, and its content is the main basis for evaluating whether the beer is mature. Typically, the threshold for diacetyl in bottom-fermented beer is 0.1-0.2 mg/kg, while in top-fermented beer or stout, the threshold is 0.1 mg/kg or 0.4 mg/kg. When the diacetyl content in the beer reaches its threshold, a sour burnt malt taste can be identified, and when the content exceeds the threshold too high, a typical diacetyl taste, like sour rice, can be identified.

Sulfide

Sulfides mainly include hydrogen sulfide, mercaptans, dimethyl sulfide, sulfur-containing proteins, thioalanine, methionine and peptides. Part of the sulfur compounds in beer comes from raw materials. For example, when hops are dried, in order to enhance their preservation, sulfur fumigation is often used; some volatile sulfur compounds may also be produced in the process of malting and wort preparation. The vast majority of sulfur compounds in finished beer are formed during fermentation. First, brewer's yeast metabolizes sulfate to form hydrogen sulfide, which further reacts with nitrogen metabolites and carbohydrate metabolites in the fermentation broth to form volatile sulfur-containing compounds. Hydrogen sulfide and dimethyl sulfide have the greatest relationship with beer flavor. Hydrogen sulfide is prone to sunlight odor when exposed to sunlight. The formation of hydrogen sulfide in beer is mainly due to the assimilation of cysteine, sulfate and sulfite by yeast and the intermediate product generated when yeast synthesis of methionine is inhibited. Beer needs a moderate content of dimethyl sulfide, but if the content is too much, the flavor of the beer will be abnormal. Generally, if it exceeds 100ug/L, the flavor of the beer will be deteriorated. The production of dimethyl sulfide comes from the production of raw malt during the germination process on the one hand, and the metabolism of yeast on the other hand.

Fatty Acids and Organic Acids

As yeast metabolites, fatty acids are mainly formed simultaneously with corresponding higher alcohols in the first 3-4 days of fermentation, and the precursors are all corresponding aldehydes. The fatty acid content of the upper fermented beer is 1/3 higher than that of the lower fermented beer, and the content of fatty acids in the beer varies with different raw materials, strains and fermentation conditions.

The organic acids in beer include lactic acid, succinic acid, citric acid, malic acid, isocitric acid and related keto acids, etc. Most of these organic acids have a certain impact on the flavor of beer, some exist in malt mash, and some are yeast cells. normal metabolites.