The anaerobic breakdown of glucose and other organic fuels to obtain energy. Fermentations are thought to be the oldest energy-yielding catabolic pathways. Various types of fermentation are known, which differ in their substrate and their final product or products. The best known fermentation is *glycolysis
, in which glucose is broken down to pyruvic acid. *Alcoholic fermentation
in yeast yields ethanol and carbon dioxide as end products. Other fermentations in microorganisms and bacteria lead to such end products as butyric acid and acetone.