Fermentation

Fermentation, the breakdown of carbohydrates by enzyme action under anaerobic conditions. Probably the most important type of fermentation commercially is alcoholic fermentation, in which the action of zymase secreted by yeast converts simple sugars, such as glucose and fructose, into ethanol and carbon dioxide. Many other kinds of fermentation occur naturally. For example, lactase, an enzyme produced by bacteria usually found in milk, causes the milk to sour by changing lactose (milk sugar) into lactic acid. Butanoic acid is formed when butter becomes rancid, and ethanoic (acetic) acid is produced when wine turns to vinegar.

Fermentation takes place when the levels of oxygen available for respiration are low. So, for example, in the muscles of the human body, when plenty of oxygen is available glucose is broken down to carbon dioxide and water. But when oxygen levels are low anaerobic respiration or fermentation takes place, leading to the formation of lactic acid instead. Similarly, when yeast cells are deprived of oxygen they convert glucose into ethanol and carbon dioxide whereas in well-oxygenated conditions they would produce water and carbon dioxide.

People have made use of fermentation reactions from very early times. Alcoholic drinks such as beer or wine have been part of human social interactions for thousands of years. Further, the boiling in the brewing process and the alcohol produced killed many of the pathogens common in untreated water. Using fermentation to raise bread, and to make various milk products (such as acidophilus milk, Bulgarian milk, yoghurt, and cheese) are also old fermentation technologies which are still widely used today. Glycerine, propanone, butanol, and butyric acid are now also produced on a large commercial scale by special fermentation processes.

The action of certain bacteria on undigested carbohydrates causes fermentation in the human intestine. As a result, gases such as hydrogen sulphide and carbon dioxide may form in amounts large enough to cause distention and pain. Acids such as lactic acid and ethanoic acid may also form in the intestines of infants, causing diarrhoea.