At a certain moment in the brewing process, the wort must be cooled into shallow cooling vessels. The brewer throws open vented windows, turns on fans and leaves the liquid overnight to cool. The brew then has maximum contact with the surrounding air so the wild yeasts and other microbial flora can graft on the wort. The spontaneous fermentation can start. The sugars present in the wort can now be turned into alcohol and carbon dioxide. This fermentation of wort only takes place in a small area around Brussels and only from October until April, when outside temperatures remain under 15°C.