No, not all yeast is created equal for making beer! Two common strains of yeast are used to ferment beer. Saccharomyces carlsbergensis, also known as bottom yeast, sinks to the bottom of the fermentation vat. Strains of bottom yeast ferment best between temperatures of 42.8 to 53.6°F (6–12°C) and take eight to fourteen days to produce a lager beer. Saccharomyces cerevisiae, also known as top-fermenting yeast, is distributed throughout wort and is carried to the top of the fermenting vat by carbon dioxide (CO2). Top-fermenting yeast ferments at a higher temperature—57.2 to 73.4°F (14–23°C) over only five to seven days—and produces ales, porter, and stout beers.