From Captain of Industry Wiki
Fermentation tanks use wild yeast to turn Corn Mash or Sugar into Ethanol. Saccharomyces Cerevisiae take in all the Oxygen to reproduce, then eat the simple sugar molecules and excrete Ethanol and belch Carbon Dioxide.
Employs microorganisms to convert sugars Into other useful substances such as ethanol. The microorganisms employed here are not getting any salary (they just love sugar).