Fermentation is an anaerobic metabolic process in which organisms convert organic substrates, such as carbohydrates, into energy and by-products like acids, alcohols and gases.
Biochemical pathways and significance
During fermentation, cells metabolize sugars through glycolysis to produce pyruvate, generating a small amount of ATP and reducing NAD+ to NADH. Because there is no oxygen or other terminal electron acceptor to oxidize NADH, the cell must regenerate NAD+ by reducing pyruvate or its derivatives to various end products. In lactic acid fermentation, pyruvate is reduced to lactate, while in alcoholic fermentation pyruvate is decarboxylated to acetaldehyde and then reduced to ethanol. Other forms include mixed-acid, butanediol and butyric fermentations seen in enteric and clostridial bacteria. Fermentation yields far less ATP per molecule of glucose than aerobic respiration but allows energy extraction under anaerobic conditions. It also produces a diverse array of organic acids, solvents and gases that can influence the environment and be used industrially. Many microbes use fermentation not only for energy but also to survive in niches where oxygen is scarce, such as deep soil, animal intestines or anaerobic reactors.
Microbial fermentations in practice
Fermentative metabolism underpins many food and industrial processes. Lactic acid bacteria such as Lactobacillus and Streptococcus convert sugars in milk to lactic acid, creating yogurt and cheese, and acidify vegetables to produce sauerkraut and kimchi. Yeasts like Saccharomyces cerevisiae ferment glucose to ethanol and carbon dioxide, a reaction harnessed in brewing beer, making wine and leavening bread. Clostridium acetobutylicum performs acetone– fermentatacetone-butanol fermentationion, historically used to manufacture solvents. In the laboratory, microbiologists identify bacteria by the pattern of fermentation end products using tests such as the mixed-acid methyl red test and the Voges–Proskauer test. Fermentation also occurs in human muscle cells durinVoges-Proskauerrcise when oxygen supply is limited, leading to temporary lactate accumulation. In ecosystems, fermentative microbes break down organic matter in sediments, wetlands and the guts of herbivores, producing methane and other gases that contribute to biogeochemical cycles.
Although fermentation generates little energy compared with oxidative phosphorylation, it is a versatile strategy that enables organisms to thrive in anaerobic niches and provides products that humans have utilized for millennia.
Related Terms: Anaerobic respiration, Glycolysis, Lactic acid bacteria, Yeast, Mixed-acid fermentation