How to control diacetyl content in beer brewing？

Diacetyl is an important control index in beer production. If the content of diacetyl in beer exceeds the threshold value, it will bring an unpleasant taste of sour rice to beer. Therefore, this problem must be taken seriously in the beer production process, and its content must be reduced to the specified range. Generally, it can be controlled from the following aspects.

★Yeast strain selection

Diacetyl is formed from alpha-acetolactate. Alpha-acetolactate is an intermediate product of yeast synthesizing valine and oxidizing ketoisovaleric acid. When wort lacks valine or valine is consumed, excess alpha-acetolactate will be formed. Lactic acid is converted into diacetyl through oxidative decarboxylation reaction. This reaction is carried out under non-enzymatic catalysis outside yeast cells, which is a relatively slow chemical change process that has nothing to do with yeast. Under the action of yeast reductase, diacetyl can be reduced to produce 2,3-butanediol which has no adverse effect on beer. Therefore, yeast species play an important role in the control of diacetyl. Different yeast species have different abilities to reduce diacetyl due to different properties.

★Yeast Algebra and Yeast Preservation

For a given yeast, the efficiency of reducing diacetyl depends on the metabolic state of the yeast. When the fermentation is vigorous, the amount of diacetyl produced is more than the reduction; the older the yeast is, the worse its ability to reduce diacetyl is. Long-storage or high-temperature yeast, whose metabolic rate is reduced, cannot reduce diacetyl as efficiently as healthy yeast. In view of the above reasons, the yeast to be recovered should be stored at low temperature and put into use as soon as possible. In addition, the yeast should be expanded in time to ensure the quality of the yeast.

★Yeast added

During fermentation, the amount of yeast added is related to the nature of the yeast. Such as yeast with strong suspending power, the reducing ability is also strong. Therefore, stirring during fermentation can reduce a large amount of diacetyl, mainly because ventilation stirring can accelerate the non-enzymatic decomposition of α-acetolactate, and at the same time, it can also accelerate the synthesis of α-acetolactate, thereby reducing the content of diacetyl. Using higher temperature to inoculate more yeast is conducive to the early formation and early reduction of diacetyl, which reduces the content of diacetyl in beer. Generally, the amount of yeast inoculated is 15~20×106/ml.

★ Wort ingredients

1. Increase the content of α-amino nitrogen

When the content of α-amino nitrogen is high, the formation of α-acetolactic acid can be controlled, thereby reducing the formation of diacetyl; when the content of α-amino nitrogen is insufficient, the assimilation of nitrogen and nutrients in the wort is lacking, and the assimilation power of yeast at the end of fermentation is reduced. , resulting in the diacetyl can not be reduced, so that the diacetyl content in beer increased. If too much auxiliary raw materials or malt with poor solubility are used, the α-amino nitrogen in the brewed wort is low, which is unfavorable for the production and reduction of diacetyl, and affects the content of diacetyl in beer.

★Increase the content of valine

The content of valine in wort is also related to the formation of diacetyl. Generally, the content of valine should be increased, and the metabolic reaction of valine synthesis from pyruvate should be inhibited through feedback action.

★ Oxygenation of wort

If the oxygenation of the wort is insufficient, the fermentation will not be vigorous, the fermentation degree will be low, and the diacetyl content in the beer will be high. Properly increase the oxygenation amount, more live yeast cells will be generated, so that more α-acetolactic acid can be formed in the fermentation liquid in the early stage of fermentation. When the fermentation temperature is high, diacetyl is rapidly reduced and further decomposed into 2 with a higher threshold. 3-Butanediol. If there is too much oxygenation, the yeast will grow too fast, causing the yeast to be separated before the α-acetolactate is broken down, which will lead to an increase in the diacetyl content.

Beer brewing process chart

★ Brewing process control

1. Fermentation temperature

Both the non-enzymatic decomposition of α-acetolactate and the enzymatic reduction of diacetyl are temperature-dependent. The higher the temperature, the faster the reaction. Therefore, if the main fermentation temperature is increased, or the fermentation temperature is maintained at a higher temperature before the end of the main fermentation, the diacetyl is reduced quickly, and the post-fermentation time of the beer is greatly shortened.

2. Add high sparkling wine

When the diacetyl content of the fermented beer cannot be reduced normally, try adding 20~30% high-bubble wine, and use the yeast of the high-bubble wine to reduce the diacetyl.

3. CO2 washing

Scrubbing with high-purity CO2 gas produced by fermentation removes volatile diacetyl. Although CO2 washing can reduce the diacetyl content, it resuspends the gradually deposited yeast, condensate, etc., which may affect the filtration of beer.

4. Beer storage and filtration

Inhalation of oxygen during beer storage and filtration accelerates the decomposition of α-acetolactate into diacetyl. Therefore, when beer is filtered, efforts are made to reduce the inhalation of oxygen, and the content of diacetyl can be controlled under certain conditions. The general process is as follows:

4.1 Convert diacetyl to hydroxybutanone and butanediol.

4.2 Diacetyl reacts with sulfite to form a non-volatile complex to remove the odor caused by diacetyl.

4.3 Reduces diacetyl content by preventing the conversion of acetolactate to diacetyl.

beer manufacturing equipment

5. Beer packaging

On the one hand, it is necessary to reduce the oxygen content, and on the other hand, it is necessary to control the temperature and time of pasteurization, because with the increase of pasteurization temperature, it is easier to decompose acetolactate into diacetyl under aerobic and higher temperature conditions. . Practice has proved that instantaneous sterilization produces less diacetyl than long-term low-temperature sterilization.

6. Prevent bacterial contamination

Large amounts of diacetyl and acetolactate are produced if the wort or additive yeast is infected with lactic acid bacteria or tetrad. Wort or yeast added to infect other miscellaneous bacteria. Although these bacteria do not produce diacetyl by themselves, they can seriously affect the activity, state and other metabolism of yeast, and may also increase the content of diacetyl. Therefore, it is necessary to control the hygiene of the brewing process. And environmental sanitation, avoid wort or add yeast process contamination.