ferment

Fermentation is a metabolic process in which microorganisms or enzymes convert organic substrates, especially sugars, into smaller molecules such as alcohols, acids, gases, and flavor compounds, often under anaerobic conditions. In microbial fermentation, organisms such as yeasts and various bacteria drive the reaction, regenerating NAD+ to sustain glycolysis. The end products and pathways vary with the organism and substrate: alcoholic fermentation yields ethanol and carbon dioxide; lactic acid fermentation produces lactic acid; acetic fermentation oxidizes alcohols to acetic acid; and other routes produce gases, esters, or diacetyl.

Fermentation has been used for food preservation and flavor development for thousands of years. Common food and beverage examples include bread, beer, wine, yogurt, sauerkraut, kimchi, miso, and tempeh. In industry, fermentation is used to produce a wide range of products, such as antibiotics, enzymes, amino acids, organic acids, fuels, and solvents, often at large scale in controlled bioreactors with monitored temperature, pH, and sterility.

The term ferment comes from Latin fermentum meaning a yeast, yeast starter, or leaven; the science of fermentation was advanced in the 19th century by Louis Pasteur, who demonstrated that fermentation is driven by living microorganisms. Fermentation remains central to food science and industrial biotechnology, and continues to be optimized through microbial selection, genetic engineering, and process control.