Fermentation is the process where glucose is converted into usable energy (ATP) in the absence of oxygen. Once glucose is converted into pyruvate during the process of glycolysis, fermentation has two possible pathways when oxygen is not available. Processes that take place without oxygen are called anaerobic processes.
Lactic Acid Fermentation
In lactic acid fermentation, pyruvate is converted into lactic acid while NADH is oxidized into NAD+.
In alcohol fermentation, pyruvate is converted into carbon dioxide and ethyl alcohol while NADH is oxidized into NAD+.