Top and spontaneous fermentation
This combination is rather rare. Vicaris Tripel Geuze is an example in which this combination is used. Most Belgian special beers ferment during 5 to 6 days at quite a high temperature (between 18°C and 22°C). The yeast then rises to the top of the tank and forms a thick layer on the beer. Because of the rising yeast, these beers are known as ‘top fermented’ or ‘high fermented beers’. It is not the temperature that makes the yeast rise, this is due to the used type of yeast culture. The yeast culture also plays an important role in determining the beer’s aroma.
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 a 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.