What is bottom fermentation of beer?

The fermentation room is required to be clean and hygienic, with good heat insulation to reduce air-conditioning loss, and the temperature is generally controlled at 5℃~6℃. The wall and floor of the fermentation room should be easy to clean, and the floor should maintain a certain slope to facilitate drainage. The ceiling is waterproof and mold proof. The pre-fermentation adopts an open or closed fermentation container. The fermentation container has two types, round or square, and is mostly made of metal, wood or concrete.

• 1. Characteristics of pre-(main) fermentation

• 2. Traditional pre-(main) fermentation process

• 3.Phenomena and requirements of the pre-(main) fermentation process

• 4. The control of the fermentation process




Characteristics of pre-(main) fermentation




Pre-fermentation is also called main fermentation, referred to as pre-fermentation. The pre-fermentation stage is the stage of yeast reproduction. Yeast utilizes sugars in wort, α-amino nitrogen, and dissolved oxygen in wort to obtain bioenergy through respiration, and yeast cells multiply in large numbers. At this stage, the rate of hypoglycemic reduction in the fermentation broth was slow, the α-amino nitrogen was rapidly assimilated, and the yeast cell density gradually increased. With the progress of yeast cell respiration, the dissolved oxygen in the wort is quickly consumed, and the fermentation broth enters the fermentation stage. At this time, the speed of reducing sugar in the fermented liquid is accelerated, and the heat released will increase the temperature of the wine, which needs to be cooled. After reaching a certain degree of fermentation, the yeast begins to coagulate and precipitate, the density of the yeast suspended in the wine liquid gradually decreases, the speed of sugar reduction slows down, and the liquid surface forms a bubble cap. After skimming off the bubble cap, the wine can be served in the post-fermentation storage tank to start post-fermentation. Before drinking, the yeast should be recovered, and the yeast can be used for production after being processed.

Traditional pre-(main) fermentation process




Now the traditional 12°P wort low temperature fermentation process is briefly described as follows:

 

(1) Cool the wort to the inoculation temperature of about 9℃. After part of the wort flows into the yeast breeding tank, add the required yeast, the amount of which is about 1% of the wort volume. Yeast can also be added to the cold wort in the pipeline with a dosing pump. With this way of adding yeast, the yeast is fully mixed with the wort, and the wort rises faster.

 

(2) The sterile air passes through the special oxygenation equipment to make the oxygen dispersed finely, and fully mixes with the wort in the pipeline to make the oxygen evenly dispersed in the wort. Wort ventilation and wort cooling are carried out simultaneously. The dissolved oxygen content of the inoculated wort should be controlled at about 8 mg/L.

 

(3) After adding yeast, add wort to the breeding tank until the tank is full. The wort liquid level should be 30cm away from the upper mouth of the breeding pond to prevent overflow.

 

(4) After about 20 hours of yeast propagation, a layer of white foam will form on the surface of the wort. At this time, the operation of tank inversion is performed, and the fermented liquid is pumped from the breeding tank into the fermentation tank. Through the tank inversion, impurities such as dead yeast cells, protein coagulation and hop resin that settle at the bottom of the breeding tank can be separated.

 

(5) After the tank is poured, all the dissolved oxygen in the wort has been consumed by the yeast, and the yeast starts to perform anaerobic fermentation. After that, the temperature of the fermented liquid and the situation of reducing sugar should be checked regularly.

 

(6) After about 2 to 3 days of fermentation, the temperature of the fermentation broth rises to the specified maximum fermentation temperature, and the fermentation enters a vigorous period. this issue

The speed of sugar reduction is fast, the apparent sugar content drops by 1.5% to 2.0% every day, and the temperature rises gradually. The ice water (about 2℃) should be turned on in time, and the temperature should be controlled for 2 to 3 days according to the process requirements.

Phenomena and requirements of the pre-(main) fermentation process




• According to the superficial phenomena of previous (main) fermentation, preceding (main) fermentation can be divided into 3 stages, and concrete situation is described as follows: (1) low bubble period. After 15-20 hours of inoculation, white foam appeared around the fermentation tank and expanded to the middle of the tank. At this point, the fermented liquid tank is introduced into the main fermentation tank. After pouring into the pool for 4~5 hours, the foam gradually thickens, white and dense, and swarms into the middle from all sides. This stage lasts for 2 to 3 days, that is, it enters the high foaming stage. During this period, the temperature rises by 0.9℃ to 1.0℃ per day, and the blood sugar drops by an average of 1°P per day, without artificial cooling. (2) High bubble period. In the vigorous period of fermentation, after 2-3 days of fermentation, the foam can reach 25-30cm, and the product temperature can reach as high as 9℃. Pay attention to cooling. The surface of the wine gradually turns brownish yellow due to the precipitation of hop resin and protein-tannin complexes. The high bubble period generally lasts for 2 to 3 days, and the daily blood sugar reduction is about 1.5°P. (3) Foaming period. After 5 days of fermentation, the fermentation power gradually weakened, the carbon dioxide bubbles decreased, the foam retracted, the precipitates in the wine increased, and the foam changed from brown to brown. The bubble drop period generally lasts about 2 days. When the sugar content drops to 4.0~4.2°P, the temperature can be lowered, and the wine serving operation can be carried out when it reaches 5℃. Before drinking, you should pay attention to one problem, that is, the foam retracts and forms a layer of bubble cover on the liquid surface. It is composed of foam, protein-polyphenol compound, hop resin, yeast cells and other impurities, which should be skimmed in time , to prevent sinking into the wine.

The control of the fermentation process




During the pre-(main) fermentation period, the elements of technical control are temperature, sugar content and time, and the three restrict each other and complement each other. If the fermentation temperature is low, the sugar content will drop slowly, and the fermentation time will be long; on the contrary, if the fermentation temperature is high, the sugar content will drop quickly, and the fermentation time will be short. To control the three elements, factors such as beer variety, yeast strain and wort components must also be taken into consideration. Its purpose is to reach the required fermentation degree in the shortest time and obtain ideal metabolites. (1) Temperature control The inoculation temperature of the following fermentation is generally controlled at 5℃ to 9℃. The maximum temperature of low-temperature fermentation is controlled at 7.5℃ ~ 9.0℃; the maximum temperature of high-temperature fermentation is controlled at 10℃~13℃. The temperature at the end of fermentation is generally controlled at 4℃~5℃. It is required to lower the temperature to make the yeast coagulate and precipitate, and only keep a certain concentration of yeast [(5~10)×10-6 cells/ml] in the wine liquid to facilitate post-fermentation and diacetyl reduction. The so-called high and low temperatures are relative terms, and there is no clear boundary. In fact, they are all lower than the fermentation temperature above. For beer fermentation, when the temperature is lower than 13℃, there is no significant difference in the metabolic process of yeast, and high-quality beer can be brewed. Low temperature is conducive to reducing the formation of esters, higher alcohols, hydrogen sulfide and dimethyl sulfide, and the formation of α-acetyl hydroxybutyric acid is also reduced, thereby reducing the content of diacetyl and making beer taste better Some, and the foam is also better, which is more suitable for the fermentation of light beer; while the temperature is high, the fermentation time is short, and the utilization rate of the equipment is high, which is economically reasonable, but too high fermentation temperature will affect the taste and quality of beer. Often have some influence. Continue to lower the temperature to 0℃~-1℃, which is convenient for low-temperature storage, so as to facilitate the clarification and carbon dioxide saturation of the wine, otherwise the wine storage period will be prolonged. (2) Concentration control Under certain yeast strains and wort composition conditions, the concentration control is controlled by adjusting the fermentation temperature and fermentation time. If the fermentation is vigorous and the sugar drop is fast, it is necessary to appropriately lower the maximum fermentation temperature and shorten the holding time of the highest temperature; otherwise, it is necessary to prolong the holding time of the maximum temperature or adopt a slow cooling method to promote the drop of sugar. (3) Time control When the wort composition, yeast activity and fermentation degree requirements are determined, the fermentation time mainly depends on the temperature. The higher the fermentation temperature, the shorter the fermentation time, and vice versa. The pre-(main) fermentation time of the following fermentation is generally 7 to 10 days. Low-temperature and slow-fermented wine has soft and mellow flavor, fine and long-lasting foam, and relatively good quality, but the utilization rate of equipment is low.