fermentation

Fermentation is a metabolic process that converts sugars and other carbohydrates into simpler compounds, typically under anaerobic conditions, by microorganisms such as yeasts, bacteria, and mold. In the absence of oxygen, cells regenerate NAD+ from NADH through fermentation, enabling glycolysis to continue and yield small amounts of ATP. The most common end products are alcohols, organic acids, and carbon dioxide.

The term encompasses several distinct pathways. Alcoholic fermentation converts sugars to ethanol and CO2, primarily driven by yeasts such as Saccharomyces cerevisiae and is central to beer, wine, and many spirits. Lactic acid fermentation reduces pyruvate to lactic acid and occurs in lactic acid bacteria and some vertebrate tissues; it is fundamental to yogurt, cheese, kimchi, sauerkraut, and sourdough. Acetic acid fermentation oxidizes ethanol to acetic acid, generating vinegar via Acetobacter species.

Fermentation has long been used for food preservation, flavor development, and leavening. In industry, controlled fermentation under defined conditions is used to produce alcohols, organic acids, enzymes, antibiotics, biofuels, and other commodities. Fermentation can be driven by pure starter cultures or occur via natural or wild microbiota, though standardization and safety considerations are important in modern production.

Historically, observations of fermentation date to ancient civilizations, and Louis Pasteur’s experiments in the 19th century helped establish microbial causation. Today, fermentation remains a key area of study in biochemistry and industrial biotechnology, with ongoing work in metabolic engineering and sustainable production.