The Selection of Yeast
Yeast control and requirements for brewing equipment. Yeast is the fundamental element in beer production, and the quality of the beer is closely related to the quality of the yeast.
The recovered yeast paste should be white in color, without any other flavors or sour taste, and have a sticky appearance. The detection results of miscellaneous bacteria and harmful microorganisms in the fermentation brew should be used to assess the contamination level of yeast in the tank. According to the principle of “sterile use of yeast,” tanks with inadequate microorganisms cannot be used as flowing yeast. The principle of “check first, use later” must be followed. The recovered yeast should be tested before adding and using it. The mortality rate of yeast should be less than 5%, and the pH value should not exceed 5. The yeast recovery cycle should be controlled within 5 generations to ensure good yeast activity and excellent beer flavor.
The Timing of Yeast Recovery
The method has shown that when the apparent sugar content of the fermentation brew drops to 3.5 OP, the tank is sealed and maintained at a pressure of 0.08~0.10 Mpa. After 2-3 days of pressure increase, the yeast is recovered. At this time, the yeast is most active, fermenting vigorously, and robust.
The advantage of this type of recovery is that yeast cells do not go through a period of stasis but enter the growth phase directly. This leads to rapid multiplication, vigorous fermentation, and a fast reduction in diacetyl, ensuring stable beer quality.
Yeast Control During Fermentation
In the fermentation process, the yeast sedimentation forms layers. The lower layer mainly contains aged and dead cells mixed with a lot of cold coagulation, etc. The middle layer is in the thriving reproduction period of fermentation, with vigorous yeast and high fermentation power. The upper layer consists of relatively light yeast mixed with hop resin and other impurities, and its quality is lower than that of the middle layer. Therefore, when selecting yeast, the “head and tail, take the middle” approach should be adopted.
The yeast count in a full tank should be controlled between (1.5~2.0) x 10^7/ml, and the inoculation amount of yeast should be kept at 0.8%. If the amount of yeast inoculation is insufficient (less than 1.0 x 10^7/ml), the yeast cell growth time will be increased, and the fermentation cycle will be prolonged, which is detrimental to the yeast’s development and prone to microbial contamination. On the other hand, if the inoculation amount is too high, the reproduction of new yeast cells will be hindered, leading to maturation and aging and affecting the recovery quality of yeast.