Anaerobic metabolism converting sugars into acids, gases, or alcohol. Life extracting energy without oxygen.
types
ethanol fermentation: glucose -> ethanol + CO2. Yeast (Saccharomyces cerevisiae), basis of beer, wine, bread
lactic acid fermentation: glucose -> lactate. Muscle cells under oxygen debt, yogurt, sauerkraut, kimchi
acetic acid fermentation: ethanol -> acetic acid. Vinegar production by Acetobacter
butyric acid fermentation: producing butyrate, crucial for gut health, performed by Clostridium
biochemistry
Glycolysis produces pyruvate and 2 ATP. Without oxygen, pyruvate is reduced to regenerate NAD+, allowing glycolysis to continue. Far less efficient than aerobic respiration (2 ATP vs ~36 ATP per glucose).
applications
food preservation: acidic environments inhibit pathogen growth
biofuel: ethanol from cellulose and sugarcane
industrial chemistry: organic acids, solvents, bioplastics
gut microbiome: microbial fermentation of dietary fiber produces short-chain fatty acids
connections
Paired with combustion as two modes of energy release -- slow biological vs rapid chemical. polymerization of fermentation products yields bioplastics.