Fermentation is a metabolic process in which organisms convert carbohydrates, such as starch or sugar, into lactic acid or alcohol. Another name for fermentation is anaerobic respiration because the process is done in the absence of oxygen (“an” = not, “aero” = air or atmosphere). Compared to aerobic respiration, anaerobic respiration produces much less ATP. ATP stands for adenosine triphosphate and is the energy source used by cells.
Ethanol fermentation is the type of fermentation that produces alcohol. It is done by yeast and some strains of bacteria. During ethanol fermentation, pyruvate from glucose metabolism is broken into ethanol and carbon dioxide. Because ethanol fermentation produces alcohol, it is used to produce beer and wine. The carbon dioxide produced by ethanol fermentation is advantageous in the making of breads.
During lactic acid fermentation, six-carbon carbohydrates, such as the pyruvate molecules from glycolysis or lactose, are converted into cellular energy (ATP) and lactic acid. Lactic acid occurs within muscle cells during intense intervals of activity when energy is needed at a faster rate than oxygen can be supplied. The lactic acid produced is the “burn” that athletes feel after an intense workout.