Once the beer wort has been boiled, it’s time for the most valuable player to come to field: the yeast. Brewers would joke that they don’t make beer, yeast does it, they only make sure it feels good. Yeast is actually a bunch of microorganisms, which are also responsible for the essential processes of making bread and wine. In matter of fact, top-fermenting yeast is the same species as the one involved in making bread. It’s called Saccharomyces cerevisiae. Basically, these little heroes feed on sugars, releasing alcohol (C2H5OH) and carbon dioxide (CO2).
In order for them to breed and do their job, they need certain vital conditions: sterile environment, optimal temperature and enough nutrition. The sterile part is essential for preventing other sugar-loving organisms from the environment to come in and settle. They may hinder the normal existence of yeast or may affect the taste of the final beer, more often in undesirable ways. Unfortunately our usual surroundings are plentiful of such organisms, thriving at warm temperature and moist environment. That’s why it’s important to clean thoroughly the tools and vessels used for making beer and properly use food-grade disinfectants. It’s also important to cool the boiled wort quickly enough to room temperature in order to minimize the chance of contamination. Cooling can be done by chilling the boiling pot by rinsing the outside with cold water or by submerging inside a spiral metal tube with cold water running through it (a chiller).
The optimal fermentation temperature would depend on the type of yeast, but would usually be between 15 – 25 °C for top-fermenting (ale) yeasts and 10 – 20 °C for the bottom-fermenting (lager) ones. It is recommended for the wort to get enough air inside before planting the yeast. This can be done by vigorously shaking the fermentation vessel.
When using dry yeast, it’s good to rehydrate it by pouring it in a small amount of lukewarm water that you boiled beforehand, before pitching it into the fermenter. Like bread yeast, it would start forming bubbles and foam shortly after being re-hydrated, showing signs of life.
Once the yeast has been pitched into the fermentation vessel it’s covered or closed with an air-lock. This way the carbon dioxide, generously produced by the yeast can go out, but no surrounding air can come in. This is the beginning of the primary fermentation – within few hours the first bubbles will show up and the air lock will start bubbling more and more intensively. This lasts for couple of days, after which the process gradually calms down. Usually it takes 7-10 days for primary fermentation to finish.