Overview: Brewer's yeast is very important in beer brewing. Due to the influence of many environmental factors in the brewing process, some yeasts will die and even autolysis will occur. When more than 5% of brewer's yeast is autolyzed in beer, the beer will produce obvious yeast taste, bitterness and astringency, along with other quality problems. Autolyzed yeasts are senescent dead yeasts. In actual brewing, it is impossible to completely prevent the autolysis of yeast cells, and can only control the limit of yeast cell autolysis. Let's talk about the influence of environmental factors on the death and autolysis of yeast.

1. Wort

Under certain conditions such as wort aeration and culture temperature, the nutrients of wort are very important for the metabolism of beer yeast. The unreasonable nutrients such as α-amino nitrogen, fermentable sugars, PH value, inorganic ions and auxins in the wort will lead to lack of nutrition, slow metabolism, and senescence of yeast, which will cause the death of yeast and the possibility of autolysis .

For example, the condensed solids of the wort are not completely precipitated, and the fermentation will inevitably hinder the reproduction and metabolism of the yeast, reduce the activity of the yeast, and cause the yeast cells to age, die and autolyze.

2．Inorganic ion

The content of zinc ions in wort shall not be less than 0.25 mg/L, but not more than 2 mg/L. If the zinc ion content is insufficient, the activity of intracellular enzymes such as alcohol dehydrogenase will decrease significantly, causing the yeast to proliferate slowly and the fermentation speed; if the zinc ion concentration is too high, it can promote the growth and metabolism of the yeast, but the yeast is very susceptible to senescence and autolysis . When the copper ion content in wort is higher than 0.1ppm, it is easy to mutagenize yeast and inhibit enzyme activity. Heavy metal ions such as lead ions, divalent tin ions, and hexavalent chromium ions are toxic to yeast and will inactivate beer yeast. When the iron ion content in the wort is higher than 1.0mg/L. It will also make yeast premature aging, increasing the probability of death and autolysis.

Nitrite ions are strongly toxic to yeast cells and make beer yeast inactive. Fluoride ions and nitrate ions can also change yeast genetics and inhibit fermentation and yeast growth. Silica ions combine with protein to form silica turbidity, which forms micelles during fermentation, which are adsorbed on the surface of yeast and reduce the metabolic capacity of yeast. When the available phosphorus content in the wort is insufficient, the whole life process of beer yeast will be adversely affected.

3．Dissolved oxygen

When the dissolved oxygen in the wort is insufficient, the proliferation rate of brewer's yeast decreases, and the number of newly-added robust brewer's yeast decreases, which is easy to cause the senescence and death of yeast cells. When storing yeast in Hansen tanks or beer yeast storage tanks, exposure of yeast to oxygen may aggravate the death and autolysis of yeast.

4．Fermentation conditions

The temperature of the fermentation broth fluctuates from high to low, the reduction period heats up and the pressure rises too late. Will promote the degradation of beer yeast and increase mortality. After fermentation and diacetyl reduction, the beer yeast at the bottom of the cone should be discharged in time. If the discharge is not clean, it will be difficult to cool the bottom and the temperature will rise, which will easily cause the death and autolysis of the yeast.

Yeast mud is directly used to grow yeast in other cone-shaped tanks without washing (in traditional fermentation), which will inevitably lead to some aging and dead yeast and increase the chance of autolysis.

5．Temperature

Under low temperature conditions, yeast can also autolyse, but the speed of autolysis is slow. As the temperature rises, the autolysis of beer yeast cells increases. Increasing the fermentation temperature can speed up the metabolism of yeast and shorten the fermentation time, but it is easy to increase the amount of harmful by-products and accelerate the senescence of beer yeast.

The design of the cone-shaped tank and the yeast storage tank is improper, that is, there is no cooling system or heat preservation device at the bottom, and the temperature of the beer yeast deposited on the bottom is 3-6°C higher than that on the top, which increases the mortality of the yeast.

6．Pressure

If the sealing pressure of the fermentation broth is too high, some yeast cells will die. When the yeast is recovered and added, the pressure difference changes too much, which will cause the yeast cell wall to burst and increase the mortality of the yeast.

7．Yeast added

If the number of full yeast in the breeding tank or fermentation tank is too high (more than 28.00X 106/ml), the α-amino nitrogen in the wort will be quickly assimilated, which will cause relatively few generations of yeast reproduction, that is, the new yeast concentration is too high. Low, lack of nutrition in the later stage, yeast is easy to age and die. If autolyzed yeast is re-inoculated and used, it will cause a vicious circle.

8．Miscellaneous

The immersion of wild yeast and miscellaneous bacteria in the fermentation broth will inevitably damage the brewer's yeast, showing a higher mortality rate.

9．Hops

The high concentration of hops added to wort will also affect yeast activity.

10．Enzyme

When adding enzyme preparations to the fermentation broth, the number of yeast used should not be too high, because enzyme preparations will promote the lysis of yeast cells, leading to increased mortality. Therefore, the use of enzyme preparations must be cautious, to ensure that the addition amount is accurate, the addition time is reasonable, and the quality of the enzyme preparation should be high.

Saccharomyces cerevisiae can secrete a variety of autolysing enzymes, the most suitable pH of these autolysing enzymes is 6-8. The pH of the fermentation broth is too high or too low, which is not conducive to the normal growth and fermentation of yeast.

11．Washing time

After the breeding tank, cone-shaped tank, and storage tank are sterilized with chlorine dioxide or hydrogen peroxide, the chlorine ion concentration will exceed 100mg/l if the rinse time is too short (within 15 minutes), which can make the brewer’s yeast premature aging, so after sterilization Rinse with water for more than 40 minutes.