The fermentation process is actually a yeast metabolism process. To obtain high-quality beer, you must first have a yeast strain with vigorous vitality, strong itself, good performance, and beneficial flavor. The performance of yeast changes due to factors such as fermentation process conditions and appearance environment. Yeast senescence, death and autolysis will inevitably occur. How to reduce yeast autolysis and prolong the service life of yeast is the fundamental basis for ensuring stable beer quality.
One, Causes of yeast autolysis

The cytosol of yeast cells contains more intracellular proteolytic enzymes. Under normal process conditions, the yeast is strong and the yeast intracellular proteolytic enzymes will not leak out. When the process environment deteriorates and the yeast senescence or death, the intracellular proteolytic enzymes will leak out, and act on the protein structure of the yeast cell wall, causing the yeast cells to rupture, and the yeast autolysis will occur, commonly known as "yeast internal consumption" . After yeast autolysis, some substances in the cytoplasmic solution, such as polysaccharides, amino acids, proteins, peptides, nucleotides, and a little salt, enter the beer in large quantities, causing the total nitrogen, ɑ-amino nitrogen, ph value, conductivity and other indicators in the beer to occur Changes will have an impact on beer flavor and colloidal stability.

Yeast autolysis is inevitable during beer fermentation, but the degree and speed of autolysis are different. Our goal is not to prevent yeast autolysis, but to control the degree of yeast autolysis and delay the process of yeast senescence and death.

Two, Factors affecting yeast autolysis

1. Yeast

Due to the poor performance of yeast itself, it is manifested as senescence, mutation, and low yeast activity. It is easy to die and self-dissolve when the process conditions change.

2. Wort composition

The composition of wort nutrients is unreasonable, leading to malnutrition of yeast, especially lack of ɑ-amino nitrogen, fermentable sugar, vitamins, auxin, etc. Too high zinc content in wort will also accelerate yeast autolysis.

3.Yeast added

The addition of yeast is too high, causing some nutrients in the wort to be consumed in a short time, causing the yeast to be in a state of poor nutrition and "internal consumption" in the subsequent process. Too high a dosage will result in little new yeast production, which will also cause yeast senescence and autolysis.

4. Yeast use algebra

Yeast has a high algebraic number, especially the continuous use of yeast without washing, bringing aging and dead yeast into the next batch of fermentation broth, causing the physiological function of the yeast to decline and autolysis.

5. Fermentation process conditions

Improper control of fermentation process conditions, such as temperature, pressure, and pH, promotes yeast mutation. For example, the initial temperature of a full tank of wort is high, the high temperature of the fermentation process lasts for a long time, the temperature and pressure fluctuate widely, the temperature of the cone is poor, the temperature is high, and the pH value is high.

6. Yeast recovery time

Yeast recovery time, method, pressure, yeast storage conditions, etc. have an important impact on yeast quality. Yeast recovery is not timely, the recovery method is improper, the pressure difference during recovery is too large, and the pressure is too fast, which will cause the yeast cell to rupture and store it Long wine time and high cone temperature can cause the yeast performance to decline. Yeast mud can not be separated for a long time and stay in the fermentation broth will also cause the yeast cells to autolyze. China International Beer Net. From this, it can be seen that yeast is collected too late and stored for a long time, and it is easy to auto-dissolve, which can be judged by the change of yeast mud pH value. If the ph value of the yeast puree is significantly higher than that of mature beer, it means that the yeast has been autolyzed. When the ph value of the yeast mud is higher than the beer ph value by more than 0.5, it is determined that the yeast has been autolyzed.

7. Microbial contamination

Once contaminated with harmful bacteria, changes in its metabolites and pH value will seriously affect yeast activity, leading to yeast autolysis. After yeast autolysis, the polysaccharides, amino acids, proteins, nucleic acids, nucleotides, a little salt and other substances in the cell juice solution will enter the beer. Autolysin can also degrade its own cell proteins and produce a series of nitrogen-containing compounds, thereby changing the ratio of solids in beer, leading to a decrease in beer colloidal stability and flavor stability.

Three,The influence of yeast autolysis on beer flavor

1.Produce beer "yeast flavor". After yeast autolysis, a large amount of intracellular substances enter the beer, which makes the beer produce "yeast flavor". Its representative level is ethyl decanoate, and when the content exceeds 1.5×10-6 (1.5ppm), it can make people feel very uncomfortable.

2. Beer is bitter and astringent. The amino acids released by the yeast autolysis have many taste substances. When entering the beer, the bitterness of the beer will increase. Commonly known as "bitter yeast", the astringency will also increase and reduce the refreshing taste of the beer.

3. Produce diacetyl taste. After yeast autolysis, the diacetyl and its precursor ɑ-acetolactate will enter the beer, and acetyl decarboxylase will generate diacetyl, which will inevitably cause the diacetyl content in beer to increase or even exceed the standard, resulting in unqualified products. , Loss of re-drinking.

4.Affect the stability of beer

After yeast autolysis, autolysase decomposes its own protein to produce a series of large, medium and small molecular nitrogen-containing substances, which are difficult to be removed during filtration, which affects the mixing degree of beer and causes the beer to lose gloss and precipitate. The change of the ph value will also cause the beer colloid to lose balance and decrease its stability.

5. Foam that affects beer

The nucleic acid substances in the yeast solution increase the total acid content in the beer. On the other hand, after the proteolytic enzyme is released into the beer, it hydrolyzes the foaming protein in the beer, thereby destroying the foaming and persistence of the beer foam.

6. Affect the acidity of beer

Yeast increases the total acid content in beer from nucleic acid substances in the solution. On the other hand, protein substances react with naoh during the acid-base titration and denature, resulting in increased consumption of naoh titration, and the calculation results increase the total acid content. The release of amino acids and proteins after yeast autolysis will change the pH value of beer. Although amino acids and proteins increase the ph value of beer due to the higher isoelectric point, beer itself has a buffering capacity, so the ph value of beer does not change much and is not linear.

7. Impact on beer filtration performance

The macromolecules formed by the autolysis of beer will block the pore size of the media in beer filtration. Autolyzed yeast will increase the viscosity of beer, causing filtration difficulties and increased soil consumption, thereby increasing filtration costs. Yeast autolysis is one of the important reasons affecting beer filtration speed.

four，Measures to prevent yeast autolysis

1. Choose good strains

Yeast strains are the decisive factor affecting the autolysis of beer yeast. The types and quantities of flavor substances produced by different yeast strains are very different. Under the same fermentation conditions, some yeast strains will produce higher numbers than other strains. Times the flavor substances. The amount of yeast inoculation also has a certain impact on the amount of flavor substances produced. When the amount of yeast inoculation is increased, the amount of yeast reproduction will be reduced, and the amount of flavor substances produced will also decrease accordingly; when the amount of inoculation is insufficient, the amount of yeast reproduction Will increase and produce more flavors. Therefore, a reasonable selection of yeast species is the most effective way to fundamentally control yeast. Bacteria is the prerequisite for the production of high-quality beer, and it determines the flavor characteristics of the product. Only by comprehensively, objectively and truly analyzing the characteristics of the strains, according to the characteristics of the strains, adapting to the physiological characteristics of the yeast strains to the greatest extent, strengthening the use and management of the yeast during the whole production process, and providing a reliable guarantee for stable product quality, can brewing be made. Produce high-fresh beer. Yeasts with weak vitality are difficult to survive under high temperature, which will inevitably lead to increased mortality and promote yeast autolysis. Therefore, the selected yeast should meet the following requirements: (1) No harmful bacteria in beer; (2) Mortality rate ≤5%; (3) Fast onset, high yeast value-added peak; (4) Complete fermentation, fast sugar reduction, and fermentation High degree; (5) Good fermentation performance, fast diacetyl reduction; (6) Clean appearance, good cohesiveness, and thick muddy yeast mud; (7) Ensure beer aroma, taste and no peculiar smell. Yeast has uniform morphology, uniform size, smooth and cellular cell wall, transparent and uniform cell juice, colony diameter ≥5nm, milky white, smooth surface, and a few wrinkles, which can be provoked by the inoculation needle.

2. Yeast addition and algebra control

The amount of yeast added is affected by factors such as wort composition, wort concentration, temperature, oxygenation, etc. The small amount of yeast added, slow onset, not only easy to contaminate bacteria, but also cause a series of quality defects, such as slow value addition. Yeast is prone to premature aging, which will eventually lead to abnormal flavor and fragrance. However, if the amount of yeast is too high, it will cause a small number of new cells in the yeast, and it is easy to lack nutrients in the later period, which also causes premature aging and mutation of yeast. China International Beer Net The number of yeast in a full tank of wort should be subject to microscopic inspection. The concentration of yeast cells in a full tank during production is preferably 12-15×106 cells/ml. The data shows that the performance of yeast begins to decline after 5 generations, and it increases with the increase of the generation number (after 5 generations). As the number of generations used by yeast increases, some characteristics of the yeast itself will be degraded, such as the slowing down of sugar reduction rate and reduction Decreased acetylation capacity. Some aging yeast will die and autolyze. As far as the open-air fermentation tank is concerned, if the yeast is used continuously without washing, it will inevitably bring the aging and dead yeast into the next batch of fermentation broth, accelerate the yeast autolysis, and affect the taste of beer. . In order to ensure the normal activity of yeast, it is advisable to use 4 to 5 generations. From: haicent.com, if the recovery of washing and rejuvenation, the use of algebra can be appropriately extended. The quality requirements for the recovered yeast: the recovered yeast mud must be white in color, without peculiar smell, without sour taste, and viscous in appearance. The yeast cells must have uniform shape and size, fullness, small vacuoles, thin cell walls, inconspicuous contents, and no abnormal cells. The test results of the miscellaneous and harmful bacteria in the fermentation broth are a sign to evaluate the degree of contamination of the yeast in the tank. The principle of "using yeast aseptically" should be followed. Tanks with unqualified microorganisms cannot be used as passage yeast. The principle of “check first, use later” should be adhered to. The recovered yeast should be tested before being added and used. The death rate of the yeast should be less than 5%, and the pH value should not be higher than 5. If the pH value is high, it means that the yeast has autolysis. , Such yeast mud should not be recycled. The number of yeast recovery generations should be controlled within 5 generations to ensure the strength and good activity of the yeast, maintain the good flavor of the beer, and reduce the probability of the yeast being contaminated by bacteria.

The timing of yeast recovery must be grasped. Practice has proved that when the appearance of the fermentation broth drops to 3.5 ops, start sealing and holding pressure (0.08mpa～0.10mpa), and it is the best time to start recovering the yeast after the pressure is increased for 2 to 3 days. At this time, the yeast is mostly active. High, vigorously fermented and strong. The advantages of this kind of recovery are: because the yeast cells do not go through the stagnation period, they directly enter the rapid growth period, so they start to grow quickly, the fermentation is vigorous, and the sugar reduction and diacetyl reduction speed are fast, and the beer freshness can be more stable.

Correctly choose to use recycled yeast. During the fermentation process, the sedimentation of the yeast has a gradient, the lower layer is mostly senescent and dead cells, and is mixed with a large amount of condensed solids, etc., and the middle layer is the most vigorous, strong, and fermented cell that reproduces during the vigorous fermentation period. For the yeast with high strength, the upper layer is lighter yeast, mixed with impurities such as hop resin, and the quality is worse than the middle layer. Therefore, when selecting yeast, the method of "pinch the head and remove the tail, take the middle" method should be adopted.

3. Prepare nutrient-rich wort with reasonable composition

Wort is the basic nutrient solution for yeast propagation and fermentation, and also the main source of nitrogen and carbon sources. Therefore, the quality of wort is directly related to yeast performance and fermentation, and is also the main factor affecting yeast senescence, death, and autolysis. The components related to yeast in wort are mainly composed of free amino acids, fermentable carbohydrates, zn2+ concentration and wheat Juice lipids are determined. The expansion stage of yeast requires a large amount of value-added, and a rich supply of nutrients, including amino acids, inorganic salts, and auxins. The most important thing is amino acids. The lack of nutrients in the wort will lead to poor development of the yeast. The value-added yeast cells will become smaller or even form abnormalities, which will affect their normal physiological performance. When the wort lacks nutrients during the expansion process, you can add yeast nutrients or Yeast extract to remedy. When the wort extracted from large-scale production is used for expansion, long-term high-temperature sterilization should be avoided as much as possible, so as not to damage the nutrients of the wort. The purpose of yeast inoculation and oxygenation is to provide conditions for the synthesis of lipid compounds for aerobic metabolism. Practice has proved that yeast under aerobic conditions can still maintain life even under starvation, and vice versa, it is very easy to die and autolyze. The oxygenation method of wort is cooling and oxygenation. In principle, oxygenation is batch by batch. Whether the last batch is aerated or not depends on the situation. As long as the wort does not overflow, the wort oxygen content is 8mg/l～10mg/ l. At the same time, it should be noted that the dissolved oxygen content of wort of the same concentration is related to wort temperature, wort movement and wort air pressure contact area. The dissolved oxygen of the wort is directly proportional to the concentration and temperature of the wort. The oxygenation should not be excessive to avoid nutritional deficiency caused by the process of value-added. But it should not be too low, otherwise, the rise is slow, the fermentation time is long, the yeast is sensitive when the temperature is lowered, the sugar reduction stops, the post-fermentation is poor, and the final quality of beer is difficult to guarantee.

4. Strict fermentation process conditions

Temperature is the main factor affecting yeast performance. The drastic changes in temperature during the fermentation process can cause mutations in the yeast, and the temperature accelerates the death and autolysis of the yeast through ethanol toxicity. The optimum growth temperature of yeast is 28℃～30℃. The yeast expansion process is gradually reduced from high temperature (25℃～30℃) to the inoculation temperature (6℃～8℃), so the expansion process and fermentation process should be carefully operated, and the temperature and pressure should be adjusted slowly to give the yeast a self The process of adjustment and self-adaptation.

5. Strengthen yeast management

Yeast quality management is an important part of beer quality management. The key points are: yeast recovery time, yeast storage time, temperature, storage method, etc. The storage principles of recovered yeast are: (1) short-term storage; (2) single-species storage. Yeast that settles after recovery is not suitable for production; (3) low-temperature storage. Yeast recovery should be carried out under isobaric conditions to prevent yeast cells from rupturing due to pressure changes. The temperature of yeast recovery should not be high. Yeast cohesion is poor at high temperature and the recovery amount is small. The recovery temperature is preferably 0℃～5℃. The storage temperature of yeast is 0℃～3℃, and the storage time should not exceed 3 days.

6. Strengthen hygienic management to ensure thoroughbred fermentation

In all stages of beer production, once the bacteria are immersed, it will cause the liquor to become turbid, rancidity and other phenomena. Beer fermentation should be carried out in a relatively "pure" environment. Any invasion of bacteria will affect the normal progress of fermentation. In particular, the fermentation broth contaminated with wild yeast will significantly increase the content of higher alcohols in beer. Therefore, the detection of harmful microorganisms in beer should be strengthened. Typical harmful bacteria are: Tetragenococcus, acetic acid bacteria, pectinbacteria and macrococcus, etc. Acetic acid bacteria bring sour taste to beer. Tetragenococcus bring beer to beer The unpleasant diacetyl odor, pectin bacillus and macrococcus give beer a sewer odor. It is necessary to strictly control the cleaning and sterilization of the production process to ensure that the cleaning and sterilization process is carried out, and the production can be carried out after the microbiological control points are qualified. It is necessary to ensure that the sterilization process is strictly followed to provide a good environment for beer brewing. Each pot of wort must be sterilized. After the fermenter is cleaned and sterilized, sterile air should be used to prepare the pressure to 0.01mpa～0.02mpa to ensure a certain positive pressure in the fermenter to prevent air from entering and bringing in bacteria. Before entering the wort, sterilize it with the prepared disinfectant again. The requirement for cold wort bacteria is 0/ml, anaerobic bacteria is 0/100ml; fermentation broth bacteria is less than 5/ml, anaerobic bacteria is less than 3/100ml.

Achieve pure-bred expansion and pure-bred fermentation, from the beginning of yeast expansion to the end of fermentation to prevent contamination by bacteria. Hygienic testing and management should be strengthened to prevent the nutritional competition between bacteria and yeast and the toxic side effects of metabolites on yeast Lead to autolysis of yeast.