Yeast is commonly known as the soul of beer production. It achieves different styles and flavors of beer, and directly affects the normal production of fermentation and the quality of beer. Therefore, the quality, performance and living environment of yeast itself are the decisive factors of beer flavor. , So yeast management is a very important task.
Today we will share with you the content of yeast recovery and preservation commonly used in beer production.
In order to ensure that the activity of yeast is not affected by the outside world, or to minimize harmful effects, the recovery and storage of yeast should follow the principle of "short-term storage, low-temperature storage, single strain storage, and timely access".
1. Yeast recovery time
When the main fermentation is completed, a large amount of yeast settles on the bottom of the fermentor cone. At this time, the yeast basically loses the ability to ferment and reduce the diacetyl of the original fermentation broth, and it should be recovered in time. If the recovery time is too early, the dead yeast or senescent yeast that settled first, as well as condensate, hops and other residues will be recovered; if the recovery time is too late, the yeast that settles at the bottom of the fermenter cone will accumulate for a long time. High temperature (5°C and above), high pressure, and alcohol will accelerate aging, degradation, death and autolysis, so the quality of the recovered yeast is poor.
2. Yeast recovery quality requirements
1) The recovered yeast mud must be white, viscous, no sour taste, and no peculiar smell; the cells should be uniform in size, regular in shape, thick in cell walls, and uniform in cytoplasm without abnormalities.
2) The mortality rate of recovered yeast mud should be less than 3%, and should not exceed 10% at most.
3) In order to ensure the strong yeast and good activity and reduce the probability of contamination by bacteria, the algebraic number of the recovered yeast should be controlled within 3 generations.
4) When recovering yeast, follow the principle of "pinch the head and remove the tail and take the middle" according to the law of yeast sedimentation.
3. Yeast recovery method
1) Manual recycling
Yeast settled at the bottom of the fermenter cone is roughly divided into three layers. The bottom yeast that is discharged first is weak and dead cells, mixed with a large amount of sediment impurities, and can be used as feed or discarded; the middle layer is new cells with strong fermenting power, accounting for about 75%, and should be taken out separately and reserved for future use ; There are many light cells in the upper layer, mixed with residues such as protein and hops. After separation, they can be used as feed or discarded.
2) Centrifuge recycling
Using the relative density of yeast and fermentation broth to separate the wine and yeast with a centrifuge. This method is convenient to operate, but the disadvantage is that the centrifuge equipment is not easy to sterilize and the centrifugal process is easy to absorb and contact oxygen, which leads to an increase in yeast mortality.
4. Yeast preservation
For the yeast recovered in time, if there is no storage condition, it should be inoculated into cold wort as soon as possible; if there is storage condition, it should be processed: that is, it should be fully, quickly and evenly cooled until its metabolic activity is slow to the lowest limit temperature ( Usually controlled at 2~4℃) storage. Recovered yeast cooling method: plate heat exchanger or yeast storage tank jacket cooling; plate heat exchanger cooling can make the yeast cool fully, quickly and evenly, and the yeast is softly cooled; jacket cooling will cause uneven cooling; yeast storage time after cooling The shorter the better.
5. Yeast recovery
The total amount of yeast recovered is related to the original wort concentration, wort oxygen content and yeast proliferation; it is not proportional to the amount of yeast inoculation. The higher the concentration of the original wort, the more assimilable nitrogen it contains, the more favorable it is for yeast proliferation; the higher the oxygen content, the more vigorous the yeast proliferation and the more its recovery. The higher the amount of yeast inoculation, the less the proliferation of new yeast under the limited wort nutrition condition, the less the yeast recovery rate will be.
6. Precautions for yeast recovery
1) In the process of recycling and storage, sanitation must be done, and it is strictly prohibited to contaminate the bacteria.
2) Avoid contact with oxygen as much as possible, because oxygen contact will accelerate the yeast's aerobic respiration, thereby accelerating the consumption of yeast's own glycogen reserves.