Yeast triggers the process of fermentation and decomposes maltose into alcohol and carbonic acid. At the end of the fermentation process, top- fermented yeast swims to the surface of the "green beer" whereas bottom-fermenting yeast settles to the bottom of the tanks. We therefore differentiate between top-fermentated beer (e.g. white beer) and bottom-fermented beers (e.g. Pilsner, Export).