Fermentation is conducted in the absence of air because in the presence of air (aerobic conditions), enzymes in the yeast produce carbon dioxide and water instead of ethanol.
Fermentation is a chemical process by which molecules such as glucose are broken down anaerobically. More broadly, fermentation is the foaming that occurs during the manufacture of wine and beer, a process at least 10,000 years old.
The frothing results from the evolution of carbon dioxide gas, though this was not recognized until the 17th century. French chemist and microbiologist Louis Pasteur in the 19th century used the term fermentation in a narrow sense to describe the changes brought about by yeasts and other microorganisms growing in the absence of air (anaerobically); he also recognized that ethyl alcohol and carbon dioxide are not the only products of fermentation.
Industrial fermentation processes begin with suitable microorganisms and specified conditions, such as careful adjustment of nutrient concentration.
The products are of many types: alcohol, glycerol, and carbon dioxide from yeast fermentation of various sugars; butyl alcohol, acetone, lactic acid, monosodium glutamate, and acetic acid from various bacteria; and citric acid, gluconic acid, and small amounts of antibiotics, vitamin B12, and riboflavin (vitamin B2) from mold fermentation.
Ethyl alcohol produced via the fermentation of starch or sugar is an important source of liquid biofuel.
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