Main fermentation, also known as primary fermentation or simply fermentation, is the primary stage of beer fermentation. Its main purpose is to ferment wort, which is primarily composed of maltose, with beer yeast to produce alcohol, CO2, and various by-products that form the main components of beer. Taking the traditional low-temperature fermentation of 12% wort as an example, the key points of the process are described as follows:
The wort is cooled to an inoculation temperature of about 6°C. After a portion of the wort is transferred to the fermentation tank, the required yeast (0.5% of the wort) is added, followed by the addition of the cooled wort to ensure even mixing.
The wort is aerated using specialized oxygenation equipment to evenly and densely disperse sterile air throughout the wort. The dissolved oxygen content in the wort after inoculation should reach approximately 8 mg/L.
Once the wort reaches full capacity (with a liquid level 30cm away from the top), the yeast enters the fermentation period. After 16-20 hours, a white foam will form on the surface of the wort. At this point, the wort is transferred from the fermentation tank to the pouring tank, allowing for separation of dead yeast cells and coagulated proteins that settle at the bottom of the fermentation tank.
With the depletion of dissolved oxygen by the yeast, anaerobic fermentation begins. The temperature of the fermentation mixture and the hypoglycemic effect should be regularly monitored.
After 2-3 days of fermentation, the temperature of the fermentation mixture approaches its maximum, marking the vigorous fermentation period. During this phase, the rate of sugar reduction is fast, with an apparent sugar content drop of 1.5%-2.0% per day. Cooling water should be turned on as needed (around 2°C), and the temperature should be controlled according to the process requirements for an additional 2-3 days.
Subsequently, the cooling capacity is gradually increased, leading to a gradual decrease in fermentation temperature and a slowing down of the sugar reduction rate. The decrease in fermentation temperature should be coordinated with the progress of hypoglycemia, aiming to control the temperature at 4.0-4.5°C and the sugar content at 4.0%-4.2% after the main fermentation stage.
On the last day of the main fermentation, the temperature should be lowered sharply to encourage most of the yeast to settle at the bottom of the tank. The mixture is then transferred to the conditioning tank for secondary fermentation. Only (5-10) × 106 CFU/ml yeast cells are retained in the fermentation mixture for the secondary fermentation and diacetyl reduction processes.
The settled yeast in the middle layer is recovered, screened, washed, and stored at 2-4°C for inoculation in the next batch.