4. Fermentation
At 23 °C, the hopped wort is the perfect food at the perfect temperature for the yeast (Saccharomyces cerevisae). But the several million cells per millilitre are still not enough to make our heavenly brew.
The yeast breathes. It needs oxygen just as we do. The oxygenation of the wort allows the yeast to breathe and multiply. When all the oxygen has been used up, the yeast cell employs a survival mechanism: fermentation. During this process the yeast uses the sugars produced during the brewing to turn them into alcohol and carbon dioxide.
The carbon dioxide escapes through the foam trap. The intensity of the degassing gives us an idea of how the fermenting is going. We can measure the progress of the fermentation by the decrease in density (attenuation). When the yeast has nothing left to eat, it slows down its activity, and gradually goes to sleep.
The beer will be cooled down to around 2 °C, and the yeast will be put into hibernation to stop it from indulging in cannibalism. This would give a beer a nasty taste and would not be ethical. This is the waiting period, during which the beer will become clearer and more refined
