What is the principle of fermentation?

Fermentation is based on the principle of anaerobic respiration, in which energy is derived from the breakdown of carbohydrates in the absence of oxygen. 

The first step of fermentation involves partially oxidizing glucose to pyruvate by glycolysis. Pyruvate is then converted to alcohol or lactic acid. At the same time, NAD+ is regenerated. NAD+ participates in glycolysis to synthesize more ATP molecules.