Fermentation occurs in cells when there is a lack of oxygen. Oxygen is used in the process of making energy required for cellular function. The two types of fermentation are lactic acid fermentation and alcohol fermentation.
Animals and certain types of bacteria may use lactic acid fermentation to produce energy. Muscle cells use lactic acid fermentation during exercise when there is not enough oxygen present to provide the increased energy needed for muscle contractions. A waste product of this process is lactic acid. Lactic acid must be removed from the body by the liver. Alcohol fermentation also occurs without the use of oxygen. In addition to energy, alcohol fermentation results in a waste product called ethanol.