Yeast is a single-celled fungus that converts glucose into alcohol and carbon dioxide. Yeast is therefore very important for brewing beer. Three different yeast variants can be distinguished in the brewing process, spontaneous fermentation, high fermentation and low fermentation. With spontaneous fermentation, the beers are stored in open barrels and the beer spontaneously fermentes. Because it happens in a natural way and the process is difficult to control, the beers of spontaneous fermentation are generally complex beers. Beers of high fermentation or top fermentation are beers where the fermentation takes place at a reasonably high temperature, namely between fifteen and twenty degrees Celsius. In addition, the yeast floats on the brew, hence the name high fermentation. The last variant that remains are the low fermentation or bottom fermentation beers. Bottom fermentation takes place at somewhat lower temperatures, namely between four and 12 degrees Celsius. In addition, the yeast sinks to the bottom during the fermentation process, hence the name low fermentation or bottom fermentation.