Fermentation (biochemistry)

Fermentation in progress: scum formed by CO₂ gas bubbles and fermenting material.

See also "Fermentation" for particular usages of the fermentation process

Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound.^[1] In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen, via an electron transport chain. Fermentation is important in anaerobic conditions when there is no oxidative phosphorylation to maintain the production of ATP (adenosine triphosphate) by glycolysis. During fermentation, pyruvate is metabolised to various different compounds. Homolactic fermentation is the production of lactic acid from pyruvate; alcoholic fermentation is the conversion of pyruvate into ethanol and carbon dioxide; and heterolactic fermentation is the production of lactic acid as well as other acids and alcohols. Fermentation does not necessarily have to be carried out in an anaerobic environment. 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 (a phenomenon known as the Crabtree effect).^[2]

Sugars are the most common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, lactose, 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.^[3]

Chemistry

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.

The chemical equation below shows the fermentation of glucose, whose chemical formula is C₆H₁₂O₆.^[4] 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.^[4][5]

Lactic acid fermentation

Lactic acid fermentation is the simplest type of fermentation. In essence, it is a redox reaction. In anaerobic conditions, the cell’s primary mechanism of ATP production is glycolysis. Glycolysis reduces – transfers electrons to – NAD⁺, forming NADH. However, there is only a limited supply of NAD⁺ available in a cell. For glycolysis to continue, NADH must be oxidized – have electrons taken away – to regenerate the NAD⁺. This is usually done through an electron transport chain in a process called oxidative phosphorylation; however, this mechanism is not available without oxygen.^[6]

Instead, the NADH donates its extra electrons to the pyruvate molecules formed during glycolysis. Since the NADH has lost electrons, NAD⁺ regenerates and is again available for glycolysis. Lactic acid, for which this process is named, is formed by the reduction of pyruvate.^[6]

In heterolactic acid fermentation, one molecule of pyruvate is converted to lactate; the other is converted to ethanol and carbon dioxide. In homolactic acid fermentation, both molecules of pyruvate are converted to lactate. Homolactic acid fermentation is unique because it is one of the only respiration processes to not produce a gas as a byproduct.

Homolactic fermentation breaks down the pyruvate into lactate. It 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 be classed as homofermentative, where the end-product is mostly lactate, or heterofermentative, where some lactate is further metabolized and results in carbon dioxide, acetate, or other metabolic products.

The process of lactic acid fermentation using glucose is summarized below.^[7] In homolactic fermentation, one molecule of glucose is converted to two molecules of lactic acid:

C₆H₁₂O₆ → 2 CH₃CHOHCOOH.

or one molecule of lactose and one molecule of water make four molecules of lactate (as in some yogurts and cheeses):

C₁₂H₂₂O₁₁ + H₂O → 4 CH₃CHOHCOOH.

In heterolactic fermentation, the reaction proceeds as follows, with one molecule of glucose being converted to one molecule of lactic acid, one molecule of ethanol, and one molecule of carbon dioxide:^[7]

C₆H₁₂O₆ → CH₃CHOHCOOH + C₂H₅OH + CO₂

Before lactic acid fermentation can occur, the molecule of glucose must be split into two molecules of pyruvate. This process is called glycolysis.^[8]

Glycolysis

Main article: Glycolysis

To extract chemical energy from glucose, the glucose molecule must be split into two molecules of pyruvate.^[8] This process also generates four molecules of adenosine triphosphate (ATP) but only a net gain of two because two were used initially and two molecules of NADH.^[7]

C₆H₁₂O₆ + 2 ADP + 2 P_i + 2 NAD⁺ → 2 CH₃COCOO⁻ + 2 ATP + 2 NADH + 2 H₂O + 2H⁺

The chemical formula of pyruvate is CH₃COCOO⁻. P_i stands for the inorganic phosphate. As shown by the reaction equation, glycolysis causes the reduction of two molecules of NAD⁺ to NADH.^[7] Two ADP molecules are also converted to two ATP and two water molecules via substrate-level phosphorylation.

Aerobic respiration

In aerobic respiration, the pyruvate produced by glycolysis is further 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 are 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.^[7]

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.^[4] 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.^[9]

Example of fermentation reaction of glucose producing acetic acid, carbon dioxide and hydrogen gas:^[10]

C₆H₁₂O₆ + 2 H₂O → 2 CH₃COOH + 2 CO₂ + 4 H₂       ΔG° = -136 kJ/reaction

Acetic acid can, in its turn, be converted into CO₂ and H₂, but the global final reaction releases 5 time less energy than the former one:

C₆H₁₂O₆ + 6 H₂O → 6 CO₂ + 12 H₂       ΔG° = -26 kJ/reaction

Methane gas production in fermentation

Acetic acid can also undergo a dismutation reaction to produce methane and carbon dioxide:^[11][12]

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

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. The word yeast, it should be noted, is cognate with the Sanskrit word meaning boiling.^[13] 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.^[14]

Louis Pasteur (1822–1895), during the 1850s and 1860s, showed that fermentation is initiated by living organisms in a series of investigations.^[6] In 1857, Pasteur showed that lactic acid fermentation is caused by living organisms.^[15] 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.^[16] 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".^[17] He defined fermentation (incorrectly) as "Life without air",^[18] 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 attempted unsuccessfully to extract the [fermentation enzyme from yeast.^[18] 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.^[19] 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.^[20] In 1907, Buechner won the Nobel Prize in chemistry for his work.^[21]

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.^[22] Strain selection and hybridization developed as well, affecting most modern food fermentations.

Etymology

The word fermentation is derived from the Latin verb fervere, which means to boil (same root as effervescence). It is thought to have been first used in the late fourteenth century in alchemy, but only in a broad sense. It was not used in the modern scientific sense until around 1600.^[23]

See also

• Acetone-butanol-ethanol fermentation
• Dark fermentation
• Fermentation (wine)
• Fermentation (food)
• Fermentative hydrogen production
• Industrial fermentation
• Fermentation lock
• Fed-batch
• Chemostat
• Ethanol fermentation
• Non-fermenter
• Photofermentation

References

1. ^ Klein, Donald W.; Lansing M.; Harley, John (2004). Microbiology (6th ed.). New York: McGraw-Hill. ISBN 978-0072556780. http://highered.mcgraw-hill.com/sites/0072556781/information_center_view0/. 
2. ^ Dickinson, J. R. (1999). "Carbon metabolism". In J. R. Dickinson and M. Schweizer. The metabolism and molecular physiology of Saccharomyces cerevisiae. Philadelphia, PA: Taylor & Francis. ISBN 978-0748407316. 
3. ^ Voet, Donald & Voet, Judith G. (1995). Biochemistry (2nd ed.). New York, NY: John Wiley & Sons. ISBN 978-0471586517. 
4. ^ ^{a b c} Life, the science of biology. Purves, William Kirkwood. Sadava, David. Orians, Gordon H. 7th Edition. Macmillan Publishers. 2004. ISBN 9780716798569. pp. 139–140
5. ^ Stryer, Lubert (1975). Biochemistry. W. H. Freeman and Company. ISBN 0-7167-0174-X. 
6. ^ ^{a b c} A dictionary of applied chemistry, Volume 3. Thorpe, Sir Thomas Edward. Longmans, Green and Co., 1922. p.159
7. ^ ^{a b c d e} AP Biology. Anestis, Mark. 2nd Edition. McGraw-Hill Professional. 2006. ISBN 9780071476300. p. 61
8. ^ ^{a b} Introductory Botany: plants, people, and the Environment. Berg, Linda R. Cengage Learning, 2007. ISBN 9780534466695. p. 86
9. ^ Madigan, Michael T.; Martinko, John M.; Parker, Jack (1996). Brock biology of microorganisms (8th ed.). Prentice Hall. ISBN 978-0135208755. http://cwx.prenhall.com/bookbind/pubbooks/brock/. 
10. ^ Thauer, R.K.; Jungermann, K.; Decker, K. (1977). "Energy conservation in chemotrophic anaerobic bacteria". Bacteriological Reviews 41 (1): 100–180. ISSN 0005-3678. http://www.ncbi.nlm.nih.gov/pubmed/860983. Retrieved 2011-11-05. 
11. ^ Ferry, J.G. (1992). "Methane from acetate". Journal of Bacteriology 174 (17): 5489–5495. http://jb.asm.org/content/174/17/5489.abstract. Retrieved 2011-11-05. 
12. ^ Vogels, G.D.; Keltjens J.T., Van Der Drift C. (1988). "Biochemistry of methane production". In Zehnder A.J.B.. Biology of anaerobic microorganisms. New York: Wiley. pp. 707–770. 
13. ^ Sanskrit: yásati "(it) boils" Yeast in dictionary.com
14. ^ A brief history of fermentation, East and West. Soyinfocenter.com. Retrieved on 2011-01-04.
15. ^ Accomplishments of Louis Pasteur. Fjcollazo.com (2005-12-30). Retrieved on 2011-01-04.
16. ^ HowStuffWorks "Louis Pasteur". Science.howstuffworks.com (2009-07-01). Retrieved on 2011-01-04.
17. ^ Louis Pasteur (1879) Studies on fermentation: The diseases of beer, their causes, and the means of preventing them. Macmillan Publishers.
18. ^ ^{a b} Modern History Sourcebook: Louis Pasteur (1822–1895): Physiological theory of fermentation, 1879. Translated by F. Faulkner, D.C. Robb.
19. ^ New beer in an old bottle: Eduard Buchner and the Growth of Biochemical Knowledge. Cornish-Bowden, Athel. Universitat de Valencia. 1997. ISBN 9788437033280. p. 25.
20. ^ The enigma of ferment: from the philospher’s stone to the first biochemical Nobel prize. Lagerkvist, Ulf. World Scientific Publishers. 2005. ISBN 9789812564214. p. 7.
21. ^ A treasury of world science, Volume 1962, Part 1. Runes, Dagobert David. Philosophical Library Publishers. 1962. p. 109.
22. ^ Wang, H. L.; Swain, E. W.; Hesseltine, C. W. (1980). "Phytase of molds used in oriental food fermentation". Journal of Food Science 45: 1262. doi:10.1111/j.1365-2621.1980.tb06534.x. 
23. ^ Fermentation | Define fermentation at Dictionary.com. Dictionary.reference.com. Retrieved on 2011-01-04.

External links

• Works of Louis Pasteur Pasteur Brewing.
• The chemical logic behind fermentation and respiration