When oxygen is not present or if an organism is not able to undergo aerobic respiration, pyruvate will undergo a process called fermentation. Fermentation does not require oxygen and is therefore anaerobic. Fermentation will replenish NAD+ from the NADH + H+ produced in glycolysis.

Can fermentation occur with oxygen?

Aerobic fermentation occurs in the presence of oxygen. It usually occurs at the beginning of the fermentation process. Aerobic fermentation is usually a shorter and more intense process than anaerobic fermentation.

Where does fermentation occur without oxygen?

Lactic acid fermentation: Lactic acid fermentation is common in muscle cells that have run out of oxygen.