Lactic acid fermentation is a metabolic process that occurs when oxygen is not available, allowing cells to continue producing energy. In this process, pyruvate, which is generated from glycolysis, is reduced by NADH to form lactic acid (or lactate) and regenerate NAD⁺. This regeneration of NAD⁺ is crucial because it enables glycolysis to continue, producing a small yield of ATP—specifically, 2 ATP molecules per glucose molecule.

During lactic acid fermentation, the typical aerobic pathways, such as pyruvate oxidation, the Krebs cycle, and the electron transport chain, cannot take place due to the absence of oxygen. Instead, glycolysis remains functional, allowing for some energy production even under anaerobic conditions. This process is particularly important in human muscle cells during intense exercise when oxygen levels are low. The accumulation of lactic acid in muscles can lead to fatigue, and while it allows for temporary energy production, it cannot sustain muscle activity for long periods. Once oxygen becomes available, aerobic respiration resumes, allowing for more efficient ATP production.

Additionally, lactic acid fermentation is not limited to human cells; it also occurs in certain bacteria. This fermentation process is responsible for the sour taste of yogurt, as these bacteria convert lactose into lactic acid. Thus, lactic acid fermentation plays a vital role in both human physiology and food production.