Industrial regulation of esters in beer fermentation
During the fermentation process, Saccharomyces cerevisiae produces a large number of flavor active substances, which include six kinds: organic acids, higher alcohols, hydroxyl compounds, sulfur-containing molecules, phenolic components and volatile esters.
Although volatile ester is only a trace component of these flavor active substances, it is the most important flavor active component.
In the beer industry, volatile esters are concerned because they can produce fruit flavor, candy flavor and perfume flavor, and are contributors to the main flavor of beer.
Esters generally have a lower flavor threshold in beer.
Because most of the esters are produced by Saccharomyces cerevisiae during fermentation and their concentrations fluctuate around their respective thresholds, small changes in these secondary metabolites will have a great impact on the sensory quality of beer.
Therefore, understanding the mechanism of ester formation is to better control the ester level of the final product and is also one of the goals of industrialization.
The biochemical background of ester synthesis has been deeply studied. It has been found that the rate of ester formation depends on three factors: the concentration of two co-substrates (Acetyl CoA and alcohol), Enzyme activity (Including its synthesis and hydrolysis activity).
Therefore, all factors that affect substrate concentration or enzyme activity may affect ester formation.
There are two kinds of flavor active esters in beer, one is acetic acid ester and the other is ethyl ester.
Among these two types of esters, ethyl acetate of acetic acid ester has received the most extensive attention because its concentration is higher and easier to detect, and its synthetic gene was also reported to be found first.
Relatively speaking, so far people know little about the production of ethyl esters, only know that they have a pleasant apple-like smell.
Recent advances have identified several genes that synthesize them and described their biochemical pathways.
These active esters are formed in cells by fermenting yeast. Because they are fat soluble, ethyl esters such as medium chain fatty acids (
MCFA ethyl ester)
Can spread through the cytoplasmic membrane into the fermentation medium.
Acetic esters can spread rapidly and completely in the culture medium, and the transfer of ethyl ester to the fermentation medium depends on their composition.
With the increase of fatty acid chain length, its diffusion rate decreases sharply.
The distribution of MCFA ethyl ester between yeast and beer is affected by the type of yeast. Lager yeast (
Saccharomyces pastorianus)More MCFA ethyl ester is retained in the cell.
Moreover, the distribution of MCFA ethyl ester between yeast and beer depends on the temperature. At lower temperatures, more esters are retained in yeast cells, and vice versa.
Ester synthesis requires two substances: alcohols and carboxylic acids.
Esters can be formed by chemical reactions, but the esterification reaction is too slow to form a sufficient number of esters in beer.
In 1962, researchers discovered that the esters were carried out through acyltransferases (Or ester synthase)Catalyzes the formation of intracellular processes. The reaction requires acyl-CoA (CoA)The energy provided by the thioester bond of the auxiliary substrate.
The most abundant acyl CoA is acetyl CoA, which can not only be formed by oxidative decarboxylation of pyruvate, but also be activated directly by acetic acid under the action of ATP.
Most of the CoA is formed by the oxidative decarboxylation of pyruvate, most of the other acetyl CoA by acyl CoA synthetase (Fatty acid metabolism)
The catalytic effect of the free CoA acyl formation.
The problems encountered by the modern beer industry and need to be urgently solved are: ① high concentration fermentation (
Resulting in disproportionate ethyl acetate and isoamyl acetate); (2) Application of large-scale cone-bottom fermenter (The formation of Ester is reduced); (3) production of low alcohol beer (Deficiency of flavor substances).
There are many parameters that affect the synthesis of beer Ester, and there are a large number of literatures studying the parameters that affect the flavor Level of beer ester.
Factors affecting ester synthesis can be divided into three categories: yeast characteristics, Wort composition and fermentation conditions (See Table 6-1).
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