Yeast is added to the cold wort, which contains oxygen, fermentable sugars, and various nutrients. The yeast quickly consumes the oxygen along with part of the nutrients, such as phosphorus, sodium, magnesium, and zinc. Fermentation temperature is maintained at 4–9°C for bottom-fermenting yeasts and 15–20°C for top-fermenting yeasts, usually lasting 5–9 days. In theory, the yeast population approximately triples through repeated doubling, ultimately becoming about seven times larger than the amount initially added.

When reproduction is complete, it means that all the oxygen has been consumed. At this point, the yeast begins to develop in an anaerobic environment, producing alcohol and CO2. The CO2 bubbles are collected, purified, and either reused or released into the atmosphere. The collection and reuse of CO2 involves a very high cost in industrial brewing.