Fermentation is a metabolic process by which organic molecules, mainly carbohydrates such as starch or sugar, are converted into acids, gases, or alcohol in the absence of oxygen. It is an anaerobic process that occurs in yeast cells, bacteria, and also in the muscles of animals. The breakdown of organic substances by organisms to release energy in the absence of oxygen is called fermentation. Fermentation is widely used in various applications, including the production of food and beverages such as bread, beer, wine, and yogurt, biofuel production, and the preservation of certain foods such as pickles. The study of fermentation is called "zymology".

There are different types of fermentation, including:

• Alcohol Fermentation: Pyruvate, the end product of glycolysis, is broken down into alcohol and carbon dioxide. Yeast performs this process in the absence of oxygen, and alcoholic fermentation is an anaerobic process. Ethanol fermentation is mainly used for the preparation of alcoholic beverages like beer and the production of ethanol fuel.

• Lactic Acid Fermentation: In this process, starch or sugar is converted into lactic acid by yeast strains and bacteria. During exercise, energy expenditure is faster than the oxygen supplied to the muscle cells. This results in the formation of lactic acid and painful muscles.

• Acetic Acid Fermentation: Starch and sugar present in grains and fruits ferment into vinegar and condiments, such as apple cider vinegar.

Fermentation serves the industry in various ways, including the production of alcoholic beverages, biofuel, and pharmaceuticals.