Beer fermentation is a very complex biochemical change process. The yeast mud used for beer production is generally stored in a yeast basin and preserved in ice water at 0.5~2℃. After adding yeast to the cooled wort, beer fermentation begins. During the entire fermentation process, the yeast first undergoes aerobic respiration and then anaerobic fermentation. These two stages are interconnected and together constitute a complex beer fermentation process. Under the action of the enzyme system contained in beer yeast, the main change is that the fermentable sugar in the wort is fermented and decomposed into alcohol and CO₂, and a series of decomposition by-products are generated, such as alcohols, aldehydes, acids, esters, ketones, and sulfides. Yeast, a tiny organism, is not only the key driving force of beer fermentation but also the soul that gives beer a unique flavor and taste. Mastering the correct yeast addition method can brew high-quality and unique beer. This article will introduce in detail the brewing knowledge of yeast addition and recovery summarized by Tiantai Beer Equipment to help dear brewers better master this technology and improve the quality and taste of beer.
Yeast addition method
(1) Dry addition method – put an appropriate amount of cooled wort in the yeast inoculator, then pour off the clean water on top of the washed and preserved yeast mud, measure the required amount of yeast (0.6%~0.8%), add it to the inoculator, mix the wort and yeast evenly, and then use sterile compressed air to send the yeast to the fermentation tank to mix with the wort to be fermented.
(2) Wet addition method – add part of the wort at 10℃~15℃ in the yeast inoculator, and then add the required yeast, and maintain this temperature for 10~12 hours to allow the yeast to fully restore its activity. After budding and reproduction, use sterile compressed air to press the yeast into the fermentation tank and fully mix it with the cold wort.
(3) Split method – this method is mostly used to cultivate and expand the first generation of yeast when the yeast supply is insufficient at the production site. The method is as follows: add all the yeast to the breeding tank at one time, add wort to the full amount, and after breeding for 20~24 hours, divide it into two tanks and add wort to each tank to the full amount. The temperature of the wort added by the split method should be equal to or slightly higher than the temperature of the original fermentation liquid to prevent the inhibition of yeast activity. In order to prevent contamination by foreign bacteria, the split time should be carried out after the yeast has reached a certain density [(5~10)×1000000 cells/ml]. Generally speaking, the wet addition method is more conducive to shortening the yeast adaptation period in the early stage of fermentation than the dry addition method, so that the yeast can enter the logarithmic growth period faster.
(4) Continuous addition method – ventilation and yeast addition can be carried out in one device. In order to make the yeast evenly distributed in the fermentation tank, the yeast should be added evenly during the entire wort inflow process. The amount of yeast added is controlled by a variable frequency pump. The following parameters are mainly controlled in production: ① The expected number of yeast cells. ② The amount of air/ml of wort.
(5) Additional method – The additional method refers to adding wort to the fermentation tank that has started fermentation. Due to the entry of fresh wort, the yeast will be activated immediately and fermentation will proceed directly. The additional method can save the fermentation time. However, it should be noted that the temperature of the additional wort should be roughly the same as the temperature of the young beer in the fermentation, otherwise, the cold stimulation of the yeast will cause the fermentation to be slow or completely terminated. The advantages of this method are: shortening the adaptation period of the yeast and accelerating the fermentation speed. The specific operation method is: to take out part of the young beer from the fermentation tank in the high foaming period and pump it into other fermentation tanks to continue conventional fermentation; at the same time, pump the cooled wort of the same temperature (8.5℃) into the fermentation tank in the high foaming period, and the amount of wort in the fermentation tank remains constant. The unfermented wort is mixed with the fermented young beer in the fermentation tank in the high foaming period, and the yeast proliferates and continues to remain in the proliferation stage. This method is often used in traditional fermentation processes. The key is to strongly ventilate the cold wort to ensure yeast proliferation. It should be noted that insufficient ventilation will prolong the fermentation time.
Yeast addition amount
The amount of yeast added varies depending on the yeast activity, wort concentration, and fermentation process. In principle, it is based on the ability to quickly start the fermentation. It is not advisable to add too much or too little. If too little is added, the fermentation will be slow, the yeast proliferation time will be prolonged, and it is easy to cause bacterial infection and prolong the fermentation time; if too much is added, it is easy to cause yeast degradation and autolysis, giving the beer an unpleasant yeast taste. Generally speaking, the higher the wort concentration, the lower the fermentation temperature, the higher the yeast generation, and the higher the addition amount. According to this principle, the yeast addition amount of 10~12°P wort in my country is generally 0.4%~0.6% (0.3%~0.4% for 7~9°P wort, 0.5%~0.7% for 13~15°P wort), and the cell concentration in the wort after yeast addition is (1.5~1.8)×10^7 cells/mL. In recent years, some breweries have increased the amount of yeast added to 0.6%~0.8% to increase the fermentation speed, speed up equipment turnover, and reduce the diacetyl content in beer, without causing adverse consequences.
Yeast recovery
The yeast that settles at the bottom of the fermentation tank can be roughly divided into three layers. The upper layer is mostly light yeast cells, mainly composed of the fallen bubble cap and the yeast cells that settle last, mixed with protein and hop resin precipitates and other impurities. After separation, it can be used as feed or for other comprehensive utilization. The middle layer is the core yeast, which is composed of strong and fermentative yeast cells, accounting for 65%~70%. It should be taken out separately and reserved for the next batch of yeast. The color is lighter. The lower layer is weak cells and dead cells, which are composed of particles that settle first, such as hop resin, coagulant particles, etc., mixed with a large amount of sediment impurities, which can be used as feed or discarded.
Recovery principle:
In the late stage of conical tank fermentation, the yeast mud deposited at the bottom of the cone is usually subjected to a pressure of 0.19~0.24MPa. In order to protect the yeast, the yeast mud should be discharged under pressure. If the yeast is discharged under normal pressure, the yeast cells will often be damaged or even ruptured due to the sudden drop in pressure, increasing the yeast mortality rate. In addition, due to the sudden drop in pressure, a large amount of carbon dioxide in the yeast mud will escape, which will produce a large amount of foam, often making the white yeast mud appear brown. The yeast mud discharged when the residual sugar drops to 3.6~3.8°P or before the second cooling is the most active. The recovered yeast should be clean and free of impurities, free of foreign bacteria under microscopic examination, with neat cells and normal morphology, and methylene blue staining of less than 5%.
Yeast recovery method:
(1) Manual recovery – yeast recovery usually first discharges the bottom yeast and sediment and discards them; then discharges the middle layer of high-quality yeast mud for production. After the yeast is recovered in a yeast storage tank with a reserve pressure of 0.1~0.15MPa, the pressure of the yeast storage tank is slowly reduced to normal pressure, and sterile air is introduced from the bottom of the tank for aeration and stirring. The yeast is stored at 2℃~3℃; the upper layer of yeast is not recycled.
The operating procedure for yeast recovery is as follows: 1. Clean the yeast storage tank and connect it to the cooled wort for diluting the yeast (accounting for about 10%~20% of the yeast slurry). 2. Clean and sterilize the yeast recovery pipeline. 3. Observe through the sight glass that the yeast in the pipeline for the first 2~5 minutes is discharged into the sewer, and then open the inlet valve to allow the yeast to enter the yeast tank. 4. The yeast tank is pressurized with sterile air to 0.1~0.15MPa. When the yeast enters, the pressure is kept consistent with the constant pressure state of the fermentation tank through the pressure regulating valve. 5. After the yeast recovery is completed, use water to push the yeast in the pipeline into the yeast tank. 6. Clean the yeast recovery pipeline with water.
(2) Use a centrifuge to recover yeast – the relative density of yeast and fermentation liquid is different, so a centrifuge can be used to separate the liquor and yeast. There are many types of centrifuges, and the centrifuge for separating yeast mostly uses a self-opening disc centrifuge. In order to maintain the yeast concentration in the wine when serving, this method is used. Part of the wine is not centrifuged and is directly mixed with other centrifuged wine to keep the yeast concentration at (5~10)×10^6 cells per milliliter. The yeast after centrifugation is processed in a storage tank. This method is easy to operate, but the centrifuge and its facilities are not easy to sterilize. In addition, during the centrifugation process, the wine is easy to absorb oxygen (0.2mg/L), the wine temperature rises by 0.4℃~0.8℃, the yeast temperature rises to above 10℃, and the yeast mortality rate increases significantly. This is the biggest disadvantage of centrifuge yeast recovery.
Processing and storage methods of recovered yeast:
In modern times, yeast recovery is mostly directly recovered in yeast recovery tanks. The capacity of the tank can accommodate 2~3 batches of yeast of the same generation. The recovery tank is equipped with a stirrer and cooling facilities. After yeast recovery, it can be washed or not (large cans of yeast mud generally do not need to be washed. Some processes require the yeast to be washed with clean cold water of appropriate hardness to remove impurities. If a small amount of contaminating bacteria appears, the yeast can be washed with acid). After the yeast recovery is completed, it should be cooled immediately to a temperature of 5.0℃, and then indirectly cooled to reduce the temperature to 2℃. While the yeast is cooling, the yeast is circulated. At this temperature, the yeast is basically dormant and no longer warms up. In order to mix the yeast evenly, it can be circulated once a day, 5 to 10 minutes each time. While cooling the recovered yeast, the yeast tank should be immediately depressurized slowly to prevent the overflow of foam.
Activation of recovered yeast
The recovered yeast can be used for inoculation again after only simple treatment. Although the operation is practical and convenient, the dosage is not easy to accurately grasp. In addition, during the fermentation process, the yeast is in an environment with high pressure (air pressure, hydraulic pressure), anaerobic (high CO₂ content) and high alcohol content, and its performance is inhibited. Therefore, after the yeast is recovered, before inoculation, it is necessary to take measures to restore the activity of the yeast. After fermentation, the yeast is recovered in a special device for activation. This device has functions such as central and eccentric stirring, oxygenation and degassing, CIP cleaning, cooling, and an acid wash of the yeast.
Technical points of yeast activation:
(1) If the yeast is to be used within the next few days after recovery, it should be cooled immediately to a temperature of 5.0℃ and ventilated immediately to remove CO2 and promote yeast metabolism.
(2) If the yeast is not used immediately after recovery, it should be slowly depressurized and stored at 2℃. Otherwise, the yeast performance will decline. After the recovered yeast is stored, it should be closed and cooled for 4~8h before inoculation, and the circulation pipeline should be opened to circulate the yeast for 10min and stop for 50min. While the yeast is circulating, it should be ventilated intermittently, ventilating and oxygenating for 10s and stopping for 120s. This can volatilize CO2 and alcohol, improve the external environment of the yeast, gradually restore the yeast performance, enter the oxygen consumption state, and the yeast can reproduce immediately after inoculation.
(3) After the recovered yeast is stored, if you want to use part of the recovered yeast, you can circulate the yeast for 30 minutes 4 hours before inoculation, stop for 30 minutes, continue cooling, and then add the required part of the yeast directly to the wort. The remaining yeast in the yeast tank should not be stored for a long time. It is best to inoculate it within 15 hours, otherwise the yeast will autolyze.
Advantages of yeast activation:
Pressurized recovery ensures the quality of yeast recovery; carbon dioxide can be basically removed; the yeast is further activated so that it can enter the fermentation state as early as possible before adding; the possibility of yeast pickling is provided; the quality of the activated yeast is uniform. After the yeast is activated, the delay period after inoculation is reduced, and the wort can be quickly fermented, thereby shortening the fermentation time, improving the utilization rate of the fermentation tank, and indirectly increasing the output.
By understanding the above yeast addition related content, I believe you can better control the fermentation process of beer and improve the quality and taste of beer. At Tiantai, we are always committed to providing you with the most advanced and practical technology and equipment support.
