Fermentation is a metabolic process where microorganisms, such as bacteria, yeast, or fungi, break down sugars and convert them into other substances like acids, gases, or alcohol. This anaerobic process occurs in the absence of oxygen. The process has been utilized by humans for thousands of years, primarily for food and drink production.
The Biological Imperative
Cells undergo fermentation to generate energy in environments lacking oxygen. In the absence of oxygen, aerobic respiration, which is highly efficient at producing adenosine triphosphate (ATP), cannot occur. Fermentation provides a way for cells to continue producing a small amount of ATP through glycolysis. Glycolysis, the initial stage of glucose breakdown, produces ATP and also generates NADH from NAD+.
The regeneration of NAD+ from NADH is central to fermentation. Glycolysis requires NAD+ to continue functioning, and without oxygen to accept electrons in the electron transport chain, NADH would accumulate, halting ATP production. By converting NADH back to NAD+, fermentation ensures that glycolysis can proceed continuously, allowing the cell to produce a limited but steady supply of ATP.
Key Pathways and Products
Fermentation pathways vary depending on the organism and the specific end products formed. Two common types are alcoholic fermentation and lactic acid fermentation. In alcoholic fermentation, sugars like glucose are converted into ethanol and carbon dioxide. This process involves glycolysis breaking down glucose into two pyruvate molecules, which are then further processed into acetaldehyde and subsequently ethanol, regenerating NAD+ in the process.
Lactic acid fermentation, conversely, converts glucose into lactic acid. This type of fermentation occurs in some bacteria, such as those found in yogurt, and also in animal muscle cells during intense exercise when oxygen supply is limited. Here, pyruvate directly accepts electrons from NADH, converting it into lactate (the deprotonated form of lactic acid) and regenerating NAD+. While homofermentative bacteria primarily produce lactic acid, some heterofermentative bacteria can also yield carbon dioxide and ethanol in addition to lactic acid.
Applications in Food and Industry
The products of fermentation have been widely harnessed by humans for various practical applications, especially in food production. Alcoholic fermentation, for instance, is used to create alcoholic beverages like beer and wine, where yeast converts sugars into ethanol. The carbon dioxide byproduct is also used to leaven bread, causing the dough to rise and giving it a light texture.
Lactic acid fermentation is central to producing many fermented foods, contributing to preservation, flavor, and texture. This includes dairy products such as yogurt and cheese, where bacteria convert lactose into lactic acid, causing milk proteins to coagulate. Fermented vegetables like sauerkraut, kimchi, and pickles also rely on lactic acid bacteria for their distinctive sour taste and extended shelf life. Beyond food, fermentation technology is used in various industries, including the production of biofuels like bioethanol and in the pharmaceutical sector for synthesizing antibiotics and other compounds.