Fermentation is an anaerobic process of using sugar, without oxygen.
Lactic Acid Fermentation
During lactic acid fermentation—when muscles have run out of oxygen—glucose is broken down into two pyruvate molecules during glycolysis, producing two ATP and oxidizing two NAD+ molecules into NADH.
To regenerate NAD+, the NADH molecules directly reduce pyruvate into lactate, with no release of carbon dioxide and no more ATP production. In fact, to produce the same amount of ATP as aerobic respiration, the cell must consume much larger amounts of sugar.
The NAD+ regenerated from the oxidation of NADH is fed back into the sixth step of glycolysis, where ATP is harvested by the breakdown of glucose. In humans, when oxygen is scarce, the muscle cells can produce ATP by this process during strenuous exercise. The excess lactate that is built up, is then transported to the liver by the blood, where it will be converted back to pyruvate molecules.