Fermentation is a chemical process that derives energy from the breakdown of organic compounds. Different types of fermentation can occur, including homolactic, heterolactic and alcoholic fermentation. The occurrence of each process is based on several factors, such the availability of oxygen and the type of organism using the process. Despite the variety of these different fermentation pathways, the reactant used for each process is a simple sugar that can easily be broken down to form the desired end products.

Homolactic fermentation in bacteria results in the formation of four molecules of lactic acid from one molecule each of the reactants, which are lactose and water. This type of fermentation takes place in anaerobic, or oxygen-deficient, environments and is responsible for the sour taste of most yogurts.

Homolactic fermentation also occurs in muscle cells and results in the formation of two molecules of lactic acid from the breakdown of the reactant. Glucose is the simple sugar reactant used in this type of homolactic fermentation, instead of lactose used in the case of bacteria. This process of energy production is employed by muscle cells when oxygen levels are low, such as during periods of extreme exercise. The lactic acid end product is also one of the factors responsible for muscle soreness after exercise.

Heterolactic fermentation, like the homolactic process in muscle cells, uses glucose as the reactant and occurs anaerobically. The products from this pathway, however, are one molecule of lactic acid, one molecule of ethanol and one molecule of carbon dioxide.

Alcohol, or ethanol, fermentation is used by yeast and some bacteria as a means of energy production from the breakdown of the simple sugar glucose, resulting in the formation of ethanol and carbon dioxide. This process is used commercially in bread and alcohol production.