Fermentation is a natural process that converts sugars into simpler compounds to produce substances that can be used in making chemical energy. It is an anaerobic process performed by a cell to generate chemical energy (e.g. ATP) from pyruvate (a product of glycolysis) but without going through the citric acid cycle and the electron transport chain system as cellular respiration does. Fermentation occurs in yeast cells, bacteria, and in the muscles of animals. Key features of fermentation include:

• Anaerobic: Fermentation does not use oxygen.
• Glycolysis: Fermentation begins with glycolysis, which is the breaking down of glucose into pyruvate.
• End products: The end products of fermentation depend on the type of fermentation. For example, alcoholic fermentation produces ethanol and carbon dioxide, while lactic acid fermentation produces lactic acid.

Fermentation is an important process in making bread, wine, cheese, soy sauce, and other foods and beverages. In particular, yeasts ferment the sugars in the dough, releasing CO2 in the process, which helps the bread to rise. As for wines and other liquors, yeasts are added to the fruit juice (e.g. grape juice), and the yeasts ferment the sugar in the juice. In addition, fermentation occurs in the muscles of animals, including humans, when there is a high energy demand while the oxygen supply becomes limited, such as during strenuous exercise.