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Fermentation

form a froth on top of the fermentation mixture.]] Fermentation is a metabolic process that consumes sugar in the absence of oxygen. The products are organic acids, gases, or alcohol. It occurs in yeast and bacteria, and also in oxygen-starved muscle cells, as in the case of lactic acid fermentation. The science of fermentation is known as zymology. In microorganisms, fermentation is the primary means of producing ATP by the degradation of organic nutrients anaerobically. Humans have used fermentation to produce drinks and beverages since the Neolithic age. For example, fermentation is used for preservation in a process that produces lactic acid as found in such sour foods as pickled cucumbers, kimchi and yogurt (see fermentation in food processing), as well as for producing alcoholic beverages such as wine (see fermentation in winemaking) and beer. Fermentation can even occur within the stomachs of animals, including humans.

Biochemical overview

Fermentation turns NADH and pyruvate produced in glycolysis into NAD+ and an organic product (which varies depending on the type of fermentation; see examples below). In the presence of O2, NADH and pyruvate are used to generate ATP in respiration. This is called oxidative phosphorylation, and it generates much more ATP than glycolysis alone. For that reason, fermentation is rarely utilized when oxygen is available. The exception being obligate anaerobes, which cannot tolerate oxygen. The first step, Embden-Meyerof-Parnas glycolysis, is common to many fermentation pathways: :C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 CH3COCO2− + 2 NADH + 2 ATP + 2 H2O + 2H+ Pyruvate is CH3COCO2−. Pi is inorganic phosphate. Two ADP molecules and two Pi are converted to two ATP and two water molecules via substrate-level phosphorylation. Two molecules of NAD+ are also reduced to NADH. In oxidative phosphorylation, the energy for ATP formation is derived from an electrochemical proton gradient generated across the inner mitochondrial membrane (or, in the case of bacteria, the plasma membrane) via an electron transport chain. Glycolysis has substrate-level phosphorylation (ATP generated directly at the point of reaction).

Definitions

Fermentation simply means the production of alcohol: grains and fruits are fermented to produce beer and wine. If a food soured, one might say it was 'off' or fermented. Here are some definitions of fermentation. They range to informal, general usage to more scientific definitions.
  1. Preservation methods for food via microorganisms (general use).
  2. Any process that produces alcoholic beverages or acidic dairy products (general use).
  3. Any large-scale microbial process occurring with or without air (common definition used in industry).
  4. Any energy-releasing metabolic process that takes place only under anaerobic conditions (becoming more scientific).
  5. Any metabolic process that releases energy from a sugar or other organic molecule, does not require oxygen or an electron transport system, and uses an organic molecule as the final electron acceptor (most scientific).

Examples

Fermentation is a process which does not necessarily have to be carried out in an environment. For example, even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to aerobic respiration, as long as sugars are readily available for consumption (a phenomenon known as the Crabtree effect). The antibiotic activity of hops also inhibits aerobic metabolism in yeast . Fermentation reacts NADH with an endogenous, organic electron acceptor. Usually this is pyruvate formed from the sugar during the glycolysis step. During fermentation, pyruvate is metabolized to various compounds through several processes: Sugars are the most common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, carbon dioxide, and hydrogen gas (H2). However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone. Although yeast carries out the fermentation in the production of ethanol in beers, wines, and other alcoholic drinks, this is not the only possible agent: bacteria carry out the fermentation in the production of xanthan gum, while mammalian muscle carries out the fermentation that occurs during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid.

Chemistry

and most known fermentation types in eucaryotic cell. Numbers in circles indicate counts of carbon atoms in molecules, C6 is glucose C6H12O6, C1 carbon dioxide CO2. Mitochondrial outer membrane is omitted.]] 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.

Ethanol fermentation

The chemical equation below shows the alcoholic fermentation of glucose, whose chemical formula is C6H12O6.Life, the science of biology. Purves, William Kirkwood. Sadava, David. Orians, Gordon H. 7th Edition. Macmillan Publishers. 2004. . pp. 139–140 One glucose molecule is converted into two ethanol molecules and two carbon dioxide molecules: :C6H12O6 → 2 C2H5OH + 2 CO2 C2H5OH is the chemical formula for ethanol. Before fermentation takes place, one glucose molecule is broken down into two pyruvate molecules. This is known as glycolysis.

Lactic acid fermentation

Homolactic fermentation (producing only lactic acid) is the simplest type of fermentation. The pyruvate from glycolysisIntroductory Botany: plants, people, and the Environment. Berg, Linda R. Cengage Learning, 2007. . p. 86 undergoes a simple redox reaction, forming lactic acid.AP Biology. Anestis, Mark. 2nd Edition. McGraw-Hill Professional. 2006. . p. 61A dictionary of applied chemistry, Volume 3. Thorpe, Sir Thomas Edward. Longmans, Green and Co., 1922. p.159 It is unique because it is one of the only respiration processes to not produce a gas as a byproduct. Overall, one molecule of glucose (or any six-carbon sugar) is converted to two molecules of lactic acid: C6H12O6 → 2 CH3CHOHCOOHIt occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some kinds of bacteria (such as lactobacilli) and some fungi. It is this type of bacteria that converts lactose into lactic acid in yogurt, giving it its sour taste. These lactic acid bacteria can carry out either homolactic fermentation, where the end-product is mostly lactic acid, or Heterolactic fermentation, where some lactate is further metabolized and results in ethanol and carbon dioxide (via the phosphoketolase pathway), acetate, or other metabolic products, e.g.: C6H12O6 → CH3CHOHCOOH + C2H5OH + CO2If lactose is fermented (as in yogurts and cheeses), it is first converted into glucose and galactose (both six-carbon sugars with the same atomic formula): C12H22O11 + H2O → 2 C6H12O6 Heterolactic fermentation is in a sense intermediate between lactic acid fermentation, and other types, e.g. alcoholic fermentation (see below). The reasons to go further and convert lactic acid into anything else are:
  • The acidity of lactic acid impedes biological processes; this can be beneficial to the fermenting organism as it drives out competitors that are unadapted to the acidity; as a result, the food will have a longer shelf life (part of the reason foods are purposely fermented in the first place); however, beyond a certain point, the acidity starts affecting the organism that produces it.
  • The high concentration of lactic acid (the final product of fermentation) drives the equilibrium backwards ( Le Chatelier's principle), decreasing the rate at which fermentation can occur, and slowing down growth.
  • Ethanol, into which lactic acid can be easily converted, is volatile and will readily escape, allowing the reaction to proceed easily. CO2 is also produced, but it is only weakly acidic, and even more volatile than ethanol.
  • Acetic acid (another conversion product) is acidic, and not as volatile as ethanol; however, in the presence of limited oxygen, its creation from lactic acid releases additional energy. It is a lighter molecule than lactic acid, that forms fewer hydrogen bonds with its surroundings (due to having fewer groups that can form such bonds), thus is more volatile and will also allow the reaction to move forward more quickly.
  • If propionic acid, butyric acid, and longer monocarboxylic acids are produced (see mixed acid fermentation), the amount of acidity produced per glucose consumed will decrease, as with ethanol, allowing faster growth.

Aerobic respiration

In aerobic respiration, the pyruvate produced by glycolysis is oxidized completely, generating additional ATP and NADH in the citric acid cycle and by oxidative phosphorylation. However, this can occur only in the presence of oxygen. Oxygen is toxic to organisms that are obligate anaerobes, and is not required by facultative anaerobic organisms. In the absence of oxygen, one of the fermentation pathways occurs in order to regenerate NAD+; lactic acid fermentation is one of these pathways.

Hydrogen gas production in fermentation

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+ from NADH. 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 low and favor the production of such an energy-rich compound,{{cite book | last1 = Madigan | first1 = Michael T. | last2 = Martinko | first2 = John M. | last3 = Parker | first3 = Jack | year = 1996 | title = Brock biology of microorganisms | edition = 8th | publisher = Prentice Hall | isbn = 978-0-13-520875-5 | url = http://cwx.prenhall.com/bookbind/pubbooks/brock/ }} but hydrogen gas at a fairly high concentration can nevertheless be formed, as in flatus. As an example of mixed acid fermentation, bacteria such as Clostridium pasteurianum ferment glucose producing butyrate, acetate, carbon dioxide and hydrogen gas:{{Cite journal | last1 = Thauer | first1 = R.K. | last2 = Jungermann | first2 = K. | last3 = Decker | first3 = K. | year = 1977 | title = Energy conservation in chemotrophic anaerobic bacteria | journal = Bacteriological Reviews | volume = 41 | issue = 1 | pages = 100–80 | issn = 0005-3678 | pmid=860983 | pmc=413997 }} The reaction leading to acetate is: C6H12O6 + 4 H2O → 2 CH3COO− + 2 HCO3− + 4 H+ + 4 H2 Glucose could theoretically be converted into just CO2 and H2, but the global reaction releases little energy.

Methane gas production in fermentation

Acetic acid can also undergo a dismutation reaction to produce methane and carbon dioxide:{{Cite journal | last = Ferry | first = J.G. | year = 1992 | title = Methane from acetate | journal = Journal of Bacteriology | volume = 174 | issue = 17 | pages = 5489–5495 | url = http://jb.asm.org/content/174/17/5489.abstract | accessdate = 2011-11-05 | pmid = 1512186 | pmc = 206491 }}{{Cite book | last = Vogels | first = G.D. |author2=Keltjens J.T. |author3=Van Der Drift C. | year = 1988 | chapter = Biochemistry of methane production | editor = Zehnder A.J.B. | title = Biology of anaerobic microorganisms | publisher = Wiley | place = New York | pages = 707–770 }} CH3COO− + H+ → CH4 + CO2       ΔG° = -36 kJ/reaction This disproportionation reaction is catalysed by methanogen archaea in their fermentative metabolism. One electron is transferred from the carbonyl function ( e− donor) of the carboxylic group to the methyl group ( e− acceptor) of acetic acid to respectively produce CO2 and methane gas.

History of human use

The use of fermentation, particularly for beverages, has existed since the Neolithic and has been documented dating from 7000–6600 BCE in Jiahu, China, 5000 BCE in India, Ayurveda mentions many Medicated Wines, 6000 BCE in Georgia, 3150 BCE in ancient Egypt, 3000 BCE in Babylon, 2000 BCE in pre-Hispanic Mexico, and 1500 BC in Sudan.Dirar, H., (1993), The Indigenous Fermented Foods of the Sudan: A Study in African Food and Nutrition, CAB International, UK Fermented foods have a religious significance in Judaism and Christianity. The Baltic god Rugutis was worshiped as the agent of fermentation.Rūgutis. Mitologijos enciklopedija, 2 tomas. Vilnius. Vaga. 1999. 293 p. The first solid evidence of the living nature of yeast appeared between 1837 and 1838 when three publications appeared by C. Cagniard de la Tour, T. Swann, and F. Kuetzing, each of whom independently concluded as a result of microscopic investigations that yeast is a living organism that reproduces by budding. It is perhaps because wine, beer, and bread were each basic foods in Europe that most of the early studies on fermentation were done on yeasts, with which they were made. Soon, bacteria were also discovered; the term was first used in English in the late 1840s, but it did not come into general use until the 1870s, and then largely in connection with the new germ theory of disease. A brief history of fermentation, East and West. Soyinfocenter.com. Retrieved on 2011-01-04. Louis Pasteur (1822–1895), during the 1850s and 1860s, showed that fermentation is initiated by living organisms in a series of investigations. In 1857, Pasteur showed that lactic acid fermentation is caused by living organisms. Accomplishments of Louis Pasteur. Fjcollazo.com (2005-12-30). Retrieved on 2011-01-04. In 1860, he demonstrated that bacteria cause souring in milk, a process formerly thought to be merely a chemical change, and his work in identifying the role of microorganisms in food spoilage led to the process of pasteurization. HowStuffWorks "Louis Pasteur". Science.howstuffworks.com (2009-07-01). Retrieved on 2011-01-04. In 1877, working to improve the French brewing industry, Pasteur published his famous paper on fermentation, "Etudes sur la Bière", which was translated into English in 1879 as "Studies on fermentation".Louis Pasteur (1879) Studies on fermentation: The diseases of beer, their causes, and the means of preventing them. Macmillan Publishers. He defined fermentation (incorrectly) as "Life without air",Modern History Sourcebook: Louis Pasteur (1822–1895): Physiological theory of fermentation, 1879. Translated by F. Faulkner, D.C. Robb. but correctly showed that specific types of microorganisms cause specific types of fermentations and specific end-products. Although showing fermentation to be the result of the action of living microorganisms was a breakthrough, it did not explain the basic nature of the fermentation process, or prove that it is caused by the microorganisms that appear to be always present. Many scientists, including Pasteur, had unsuccessfully attempted to extract the fermentation enzyme from yeast. Success came in 1897 when the German chemist Eduard Buechner ground up yeast, extracted a juice from them, then found to his amazement that this "dead" liquid would ferment a sugar solution, forming carbon dioxide and alcohol much like living yeasts. New beer in an old bottle: Eduard Buchner and the Growth of Biochemical Knowledge. Cornish-Bowden, Athel. Universitat de Valencia. 1997. . p. 25. Buechner's results are considered to mark the birth of biochemistry. The "unorganized ferments" behaved just like the organized ones. From that time on, the term enzyme came to be applied to all ferments. It was then understood that fermentation is caused by enzymes that are produced by microorganisms. The enigma of ferment: from the philosopher's stone to the first biochemical Nobel prize. Lagerkvist, Ulf. World Scientific Publishers. 2005. . p. 7. In 1907, Buechner won the Nobel Prize in chemistry for his work.A treasury of world science, Volume 1962, Part 1. Runes, Dagobert David. Philosophical Library Publishers. 1962. p. 109. Advances in microbiology and fermentation technology have continued steadily up until the present. For example, in the late 1970s, it was discovered that microorganisms could be mutated with physical and chemical treatments to be higher-yielding, faster-growing, tolerant of less oxygen, and able to use a more concentrated medium. Strain selection and hybridization developed as well, affecting most modern food fermentations. Other approaches to advancing the fermentation industry has been done by companies such as BioTork, a biotechnology company that naturally evolves microorganisms to improve fermentation processes. This approach differs from the more popular genetic modification, which has become the current industry standard.

Etymology

The word "ferment" is derived from the Latin verb fervere, which means to boil. It is thought to have been first used in the late 14th century in alchemy, but only in a broad sense. It was not used in the modern scientific sense until around 1600.

See also

References

External links

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This article based upon the http://en.wikipedia.org/wiki/Fermentation, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
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