They are unicellular organisms, of the family of fungus, and are responsible for the fermentation of wort. The beer fermentation begins once the yeasts are mixed with wort. In a first phase, yeasts feed with the oxygen, but once finished the oxygen, they begin to metabolize sugars, which are in the wort, to produce ethanol and carbon dioxide (CO₂). So, it is thanks to the yeast that must acquire the volume of alcohol and gas of the beer.