fermentation

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.