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Fermentation is the process of deriving energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. This is in contrast to cellular respiration, where electrons are donated to an exogenous electron acceptor, such as oxygen, via an electron transport chain. Fermentation does not necessarily have to be carried out in an anaerobic environment, however. For example, even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as sugars are readily available for consumption.
Sugars are the common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, and hydrogen. However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone. Yeast carries out fermentation in the production of ethanol in beers, wines and other alcoholic drinks, along with the production of large quantities of carbon dioxide. Fermentation occurs in mammalian muscle during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid.
Pasteur performed careful research and concluded, "I am of the opinion that alcoholic fermentation never occurs without simultaneous organization, development and multiplication of cells.... If asked, in what consists the chemical act whereby the sugar is decomposed ... I am completely ignorant of it."
- See also: glycolysis
The reaction of fermentation differs according to the sugar being used and the product produced. In the reaction shown below, the sugar will be glucose (C6H12O6), the simplest sugar, and the product will be ethanol (2C2H5OH). This is one of the fermentation reactions carried out by yeast, and used in food production.
- Sugar (glucose or fructose) → alcohol (ethanol) + carbon dioxide + energy (ATP)
The actual biochemical pathway the reaction takes varies depending on the sugars involved, but the most common involves part of the glycolysis pathway, which is shared with the early stages of aerobic respiration in most organisms. The later stages of the pathway vary considerably depending on the final product.
 Energy source in anaerobic conditions
Fermentation is thought to have been the primary means of energy production in earlier organisms before oxygen was at high concentration in the atmosphere, and thus would represent a more ancient form of energy production in cells.
Fermentation products contain chemical energy (they are not fully oxidized) but are considered waste products, since they cannot be metabolized further without the use of oxygen (or other more highly-oxidized electron acceptors). A consequence is that the production of adenosine triphosphate (ATP) by fermentation is less efficient than oxidative phosphorylation, whereby pyruvate is fully oxidized to carbon dioxide. Fermentation produces 4 ATP molecules per molecule of glucose, compared to 38 by aerobic respiration: 8 are produced from FADH2, and 30 are produced from NADH, for a total of 38. Since 2 ATP molecules are used in glycolysis, the net yield for fermentation is 2 ATP versus 36 ATP from aerobic respiration.
Aerobic glycolysis is a method employed by muscle cells for the production of lower-intensity energy over a longer period of time when oxygen is plentiful. Under low-oxygen conditions, however, vertebrates use the less-efficient but faster anaerobic glycolysis to produce ATP. The speed at which ATP is produced is about 100 times that of oxidative phosphorylation. While fermentation is helpful during short, intense periods of exertion, it is not sustained over extended periods in complex aerobic organisms. In humans, for example, lactic acid fermentation provides energy for a period ranging from 30 seconds to 2 minutes.
The final step of fermentation, the conversion of pyruvate to fermentation end-products, does not produce energy. However, it is critical for an anaerobic cell, since it regenerates nicotinamide adenine dinucleotide (NAD+), which is required for glycolysis. This is important for normal cellular function, as glycolysis is a primary source of ATP production in anaerobic conditions.
Products produced by fermentation are actually waste products produced during the reduction of pyruvate to regenerate NAD+ in the absence of oxygen. In general, bacteria produce acids: Vinegar (acetic acid) is the direct result of bacterial metabolism (Bacteria need oxygen to convert the alcohol to acetic acid). In milk, the acid coagulates the casein, producing curds. In pickling, the acid preserves the food from pathogenic and putrefactive bacteria.
- Ethanol fermentation (performed by yeast and some types of bacteria) breaks the pyruvate down into ethanol and carbon dioxide. It is important in bread-making, brewing, and wine-making. Usually only one of the products is desired; in bread-making, the alcohol is baked out, and, in alcohol production, the carbon dioxide is released into the atmosphere or used for carbonating the beverage. When the ferment has a high concentration of pectin, minute quantities of methanol can be produced.
- Lactic acid fermentation breaks down the pyruvate into lactic acid. It occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some bacteria and some fungi. It is this type of bacteria that converts lactose into lactic acid in yogurt, giving it its sour taste.
- Hydrogen gas is produced in many types of fermentation (mixed acid fermentation, butyric acid fermentation, caproate fermentation, butanol fermentation, glyoxylate fermentation), as a way to regenerate NAD+ and FAD from NADH and FADH2. Electrons are transferred to ferredoxin, which in turn is oxidized by hydrogenase, producing H2. Hydrogen gas is a substrate for methanogens and sulfate reducers, which keep the concentration of hydrogen sufficiently low to allow the production of such an energy-rich compound. 
 Enzymology and Zymology
Zymology is the scientific term for the study of the chemical process of fermentation, and includes fermentation by yeast, bacteria, and other lower species. Zymology includes the study of enzymes that facilitate the chemical process of fermentation, as well as engineering issues of yeast selection, yeast physiology, and the practical issues of brewing. Enzymology, the study of enzymes, should not be confused with zymology, as the modern study of the enzyme encompasses enzymes from a multitude of sources, and studies enzymes in many terms, as well as studying enzymes from biochemical pathways including fermentation and beyond. Historically, zymologists were the first enzymologists, and since enzymology has expanded beyond the study of yeast fermentation, zymology is infrequently regarded as an archaic word for enzymology.
 See also
- Fermentation (wine)
- Fermentation (food)
- Industrial fermentation
- Fermentation lock
- Ethanol fermentation
- ^ a b Klein, Donald W.; Prescott, Lansing M.; Harley, John (2005). Microbiology. New York: McGraw-Hill. ISBN 0-07-255678-1.
- ^ Dickinson, J. R. (1999). Carbon metabolism. In: The Metabolism and Molecular Physiology of Saccharomyces cerevisiae, ed. J. R. Dickinson and M. Schweizer, Philadelphia, PA: Taylor & Francis.
- ^ Voet and Voet (1995) Biochemistry, 2nd ed., John Wiley & Sons, Inc., New York, NY
- ^ Madigan, Martinko, Brock Biology of Microorganisms, 11th ed
 External links
- The chemical logic behind fermentation and respiration
- Inline disintegration to reduce fermentation time and improve the yield