The enzymes excreted by yeast that actually do the work of fermentation. While this is so technical that most books do not even bother to mention zymase, it has yet another claim to fame. It is the root of the word Zymurgy which ends nearly all english language dictionaries.
Zymase is an enzyme complex that catalyze glycolysis, the fermentation of sugar into ethanol and carbon dioxide. As the conversion takes place, the reaction will gradually slow down. They occur naturally in yeasts.