When cooled and clarified, the wort is then sent to the fermentation vats, a process which is triggered by a "starter yeast" (pure culture of yeast) which converts the sugars into alcohol and CO2 and determines the aromatic character of the beer. At the end of the fermentation which lasts, depending on the type of beer, from 3 to 5 days, a liquid is obtained which we can now call beer.

Because the fermentation takes place at a temperature of between 18 and 32°C (64.4 and 89.6°F) and because the yeast rises to the surface of the fermented beer, we speak of top fermentation.

The yeast is a primary constituent. It is this that makes the story of Chimay beer. In fact, the yeast was selected by Father Theodore in 1948. His work was to isolate the yeast cells and cultivate them, make some "micro-brews" and then, of course, to evaluate the flavour and brewing qualities. When he had isolated the best strain, Father Theodore propagated it selectively.