Ethanol fermentation

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(1) A glucose molecule is broken down via glycolysis, yielding two pyruvate molecules. The energy released by this exothermic reactions is used to phosphorylate two ADP molecules, yielding two ATP molecules, and to reduce two molecules of NAD+ to NADH. (2) The two pyruvate molecules are broken down, yielding two acetaldehyde molecule and giving off two molecules of carbon dioxide. (3) The two molecules of NADH reduce the two acetaldehyde molecules to two molecules of ethanol; this converts NADH back into NAD+. Ethanol fermentation-1.svg
(1) A glucose molecule is broken down via glycolysis, yielding two pyruvate molecules. The energy released by this exothermic reactions is used to phosphorylate two ADP molecules, yielding two ATP molecules, and to reduce two molecules of NAD+ to NADH. (2) The two pyruvate molecules are broken down, yielding two acetaldehyde molecule and giving off two molecules of carbon dioxide. (3) The two molecules of NADH reduce the two acetaldehyde molecules to two molecules of ethanol; this converts NADH back into NAD+.

Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. It also takes place in some species of fish (including goldfish and carp) where (along with lactic acid fermentation) it provides energy when oxygen is scarce. [1]

Contents

Ethanol fermentation is the basis for alcoholic beverages, ethanol fuel and bread dough rising.

Biochemical process of fermentation of sucrose

A laboratory vessel being used for the fermentation of straw Gaering.png
A laboratory vessel being used for the fermentation of straw
Fermentation of sucrose by yeast Yeast fermentation.jpg
Fermentation of sucrose by yeast

The chemical equations below summarize the fermentation of sucrose (C12H22O11) into ethanol (C2H5OH). Alcoholic fermentation converts one mole of glucose into two moles of ethanol and two moles of carbon dioxide, producing two moles of ATP in the process.

C6H12O6 + 2 ADP + 2 Pi → 2 C2H5OH + 2 CO2 + 2 ATP

Sucrose is a sugar composed of a glucose linked to a fructose. In the first step of alcoholic fermentation, the enzyme invertase cleaves the glycosidic linkage between the glucose and fructose molecules.

Next, each glucose molecule is broken down into two pyruvate molecules in a process known as glycolysis. [2] Glycolysis is summarized by the equation:

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

CH3COCOO− is pyruvate, and Pi is inorganic phosphate. Finally, pyruvate is converted to ethanol and CO2 in two steps, regenerating oxidized NAD+ needed for glycolysis:

1. CH3COCOO− + H+ → CH3CHO + CO2

catalyzed by pyruvate decarboxylase

2. CH3CHO + NADH + H+ → C2H5OH + NAD+

This reaction is catalyzed by alcohol dehydrogenase (ADH1 in baker's yeast). [3]

As shown by the reaction equation, glycolysis causes the reduction of two molecules of NAD+ to NADH. Two ADP molecules are also converted to two ATP and two water molecules via substrate-level phosphorylation.

Fermentation of sugar to ethanol and CO2 can also be done by Zymomonas mobilis , however the path is slightly different since formation of pyruvate does not happen by glycolysis but instead by the Entner–Doudoroff pathway. Other microorganisms can produce ethanol from sugars by fermentation but often only as a side product. Examples are [4]

Effect of oxygen

Fermentation does not require oxygen. If oxygen is present, some species of yeast (e.g., Kluyveromyces lactis or Kluyveromyces lipolytica ) will oxidize pyruvate completely to carbon dioxide and water in a process called cellular respiration, hence these species of yeast will produce ethanol only in an anaerobic environment (not cellular respiration). This phenomenon is known as the Pasteur effect.

However, many yeasts such as the commonly used baker's yeast Saccharomyces cerevisiae or fission yeast Schizosaccharomyces pombe under certain conditions, ferment rather than respire even in the presence of oxygen. In wine making this is known as the counter-Pasteur effect. These yeasts will produce ethanol even under aerobic conditions, if they are provided with the right kind of nutrition. During batch fermentation, the rate of ethanol production per milligram of cell protein is maximal for a brief period early in this process and declines progressively as ethanol accumulates in the surrounding broth. Studies demonstrate that the removal of this accumulated ethanol does not immediately restore fermentative activity, and they provide evidence that the decline in metabolic rate is due to physiological changes (including possible ethanol damage) rather than to the presence of ethanol. Several potential causes for the decline in fermentative activity have been investigated. Viability remained at or above 90%, internal pH remained near neutrality, and the specific activities of the glycolytic and alcohologenic enzymes (measured in vitro) remained high throughout batch fermentation. None of these factors appears to be causally related to the fall in fermentative activity during batch fermentation.

Bread baking

The formation of carbon dioxide - a byproduct of ethanol fermentation - causes bread to rise. Masa fermentando.JPG
The formation of carbon dioxide – a byproduct of ethanol fermentation – causes bread to rise.

Ethanol fermentation causes bread dough to rise. Yeast organisms consume sugars in the dough and produce ethanol and carbon dioxide as waste products. The carbon dioxide forms bubbles in the dough, expanding it to a foam. Less than 2% ethanol remains after baking. [5] [6]

In a contemporary advancement, a group in Germany has been doing the opposite and converting stale bread into ethanol. [7]

Alcoholic beverages

Primary fermentation cellar, Budweiser Brewery, Fort Collins, Colorado Primary fermentation cellar, Budweiser Brewery.jpg
Primary fermentation cellar, Budweiser Brewery, Fort Collins, Colorado

Ethanol contained in alcoholic beverages is produced by means of fermentation induced by yeast.

In all cases, fermentation must take place in a vessel that allows carbon dioxide to escape but prevents outside air from coming in. This is to reduce risk of contamination of the brew by unwanted bacteria or mold and because a buildup of carbon dioxide creates a risk the vessel will rupture or fail, possibly causing injury or property damage.[ citation needed ]

Feedstocks for fuel production

Yeast fermentation of various carbohydrate products is also used to produce the ethanol that is added to gasoline.

The dominant ethanol feedstock in warmer regions is sugarcane. [8] In temperate regions, corn or sugar beets are used. [8] [9]

In the United States, the main feedstock for the production of ethanol is currently corn. [8] Approximately 2.8 gallons of ethanol are produced from one bushel of corn (0.42 liter per kilogram). While much of the corn turns into ethanol, some of the corn also yields by-products such as DDGS (distillers dried grains with solubles) that can be used as feed for livestock. A bushel of corn produces about 18 pounds of DDGS (320 kilograms of DDGS per metric ton of maize). [10] Although most of the fermentation plants have been built in corn-producing regions, sorghum is also an important feedstock for ethanol production in the Plains states. Pearl millet is showing promise as an ethanol feedstock for the southeastern U.S. and the potential of duckweed is being studied. [11]

In some parts of Europe, particularly France and Italy, grapes have become a de facto feedstock for fuel ethanol by the distillation of surplus wine. [12] Surplus sugary drinks may also be used. [13] In Japan, it has been proposed to use rice normally made into sake as an ethanol source. [14]

Cassava as ethanol feedstock

Ethanol can be made from mineral oil or from sugars or starches. Starches are cheapest. The starchy crop with highest energy content per acre is cassava, which grows in tropical countries.

Thailand already had a large cassava industry in the 1990s, for use as cattle feed and as a cheap admixture to wheat flour. Nigeria and Ghana are already establishing cassava-to-ethanol plants. Production of ethanol from cassava is currently economically feasible when crude oil prices are above US$120 per barrel.

New varieties of cassava are being developed, so the future situation remains uncertain. Currently, cassava can yield between 25 and 40 tonnes per hectare (with irrigation and fertilizer), [15] and from a tonne of cassava roots, circa 200 liters of ethanol can be produced (assuming cassava with 22% starch content). A liter of ethanol contains circa 21.46 [16] MJ of energy. The overall energy efficiency of cassava-root to ethanol conversion is circa 32%.

The yeast used for processing cassava is Endomycopsis fibuligera, sometimes used together with bacterium Zymomonas mobilis.

Byproducts of fermentation

Ethanol fermentation produces unharvested byproducts such as heat, carbon dioxide, food for livestock, water, methanol, fuels, fertilizer and alcohols. [17] The cereal unfermented solid residues from the fermentation process, which can be used as livestock feed or in the production of biogas, are referred to as Distillers grains and sold as WDG, Wet Distiller's grains, and DDGS, Dried Distiller's Grains with Solubles, respectively.

Microbes used in ethanol fermentation

See also

Related Research Articles

<span class="mw-page-title-main">Glycolysis</span> Series of interconnected biochemical reactions

Glycolysis is the metabolic pathway that converts glucose into pyruvate and, in most organisms, occurs in the liquid part of cells. The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by enzymes.

<span class="mw-page-title-main">Cellular respiration</span> Process to convert glucose to ATP in cells

Cellular respiration is the process by which biological fuels are oxidized in the presence of an inorganic electron acceptor, such as oxygen, to drive the bulk production of adenosine triphosphate (ATP), which contains energy. Cellular respiration may be described as a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into ATP, and then release waste products.

Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O2) are available. Anaerobic glycolysis is an effective means of energy production only during short, intense exercise, providing energy for a period ranging from 10 seconds to 2 minutes. This is much faster than aerobic metabolism. The anaerobic glycolysis (lactic acid) system is dominant from about 10–30 seconds during a maximal effort. It replenishes very quickly over this period and produces 2 ATP molecules per glucose molecule, or about 5% of glucose's energy potential (38 ATP molecules). The speed at which ATP is produced is about 100 times that of oxidative phosphorylation.

<span class="mw-page-title-main">Lactic acid fermentation</span> Series of interconnected biochemical reactions

Lactic acid fermentation is a metabolic process by which glucose or other six-carbon sugars are converted into cellular energy and the metabolite lactate, which is lactic acid in solution. It is an anaerobic fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells.

Digestion is the breakdown of carbohydrates to yield an energy-rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: aerobic and anaerobic. In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.

<span class="mw-page-title-main">Ethanol fuel</span> Type of biofuel

Ethanol fuel is fuel containing ethyl alcohol, the same type of alcohol as found in alcoholic beverages. It is most often used as a motor fuel, mainly as a biofuel additive for gasoline.

<span class="mw-page-title-main">Malolactic fermentation</span> Process in winemaking

Malolactic conversion is a process in winemaking in which tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid. Malolactic fermentation is most often performed as a secondary fermentation shortly after the end of the primary fermentation, but can sometimes run concurrently with it. The process is standard for most red wine production and common for some white grape varieties such as Chardonnay, where it can impart a "buttery" flavor from diacetyl, a byproduct of the reaction.

Cellulosic ethanol is ethanol produced from cellulose rather than from the plant's seeds or fruit. It can be produced from grasses, wood, algae, or other plants. It is generally discussed for use as a biofuel. The carbon dioxide that plants absorb as they grow offsets some of the carbon dioxide emitted when ethanol made from them is burned, so cellulosic ethanol fuel has the potential to have a lower carbon footprint than fossil fuels.

<span class="mw-page-title-main">Kilju</span> Finnish home made alcoholic beverage

Kilju is the Finnish word for home made alcoholic beverage typically made of sugar, yeast, and water.

Acidogenesis is the second stage in the four stages of anaerobic digestion:

<span class="mw-page-title-main">Mixed acid fermentation</span> Biochemical conversion of six-carbon sugars into acids in bacteria

In biochemistry, mixed acid fermentation is the metabolic process by which a six-carbon sugar is converted into a complex and variable mixture of acids. It is an anaerobic (non-oxygen-requiring) fermentation reaction that is common in bacteria. It is characteristic for members of the Enterobacteriaceae, a large family of Gram-negative bacteria that includes E. coli.

<span class="mw-page-title-main">Fermentation</span> Metabolic process producing energy in the absence of oxygen

Fermentation is a metabolic process that produces chemical changes in organic substances through the action of enzymes. In biochemistry, it is broadly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.

<span class="mw-page-title-main">Fermentation in food processing</span> Converting carbohydrates to alcohol or acids using anaerobic microorganisms

In food processing, fermentation is the conversion of carbohydrates to alcohol or organic acids using microorganisms—yeasts or bacteria—under anaerobic (oxygen-free) conditions. Fermentation usually implies that the action of microorganisms is desired. The science of fermentation is known as zymology or zymurgy.

Zymomonas mobilis is a Gram negative, facultative anaerobic, non-sporulating, polarly-flagellated, rod-shaped bacterium. It is the only species found in the genus Zymomonas. It has notable bioethanol-producing capabilities, which surpass yeast in some aspects. It was originally isolated from alcoholic beverages like the African palm wine, the Mexican pulque, and also as a contaminant of cider and beer in European countries.

The Pasteur effect describes how available oxygen inhibits ethanol fermentation, driving yeast to switch toward aerobic respiration for increased generation of the energy carrier adenosine triphosphate (ATP). More generally, in the medical literature, the Pasteur effect refers to how the cellular presence of oxygen causes in cells a decrease in the rate of glycolysis and also a suppression of lactate accumulation. The effect occurs in animal tissues, as well as in microorganisms belonging to the fungal kingdom.

Cellular waste products are formed as a by-product of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP. One example of cellular respiration creating cellular waste products are aerobic respiration and anaerobic respiration.

<span class="mw-page-title-main">Sugars in wine</span>

Sugars in wine are at the heart of what makes winemaking possible. During the process of fermentation, sugars from wine grapes are broken down and converted by yeast into alcohol (ethanol) and carbon dioxide. Grapes accumulate sugars as they grow on the grapevine through the translocation of sucrose molecules that are produced by photosynthesis from the leaves. During ripening the sucrose molecules are hydrolyzed (separated) by the enzyme invertase into glucose and fructose. By the time of harvest, between 15 and 25% of the grape will be composed of simple sugars. Both glucose and fructose are six-carbon sugars but three-, four-, five- and seven-carbon sugars are also present in the grape. Not all sugars are fermentable, with sugars like the five-carbon arabinose, rhamnose and xylose still being present in the wine after fermentation. Very high sugar content will effectively kill the yeast once a certain (high) alcohol content is reached. For these reasons, no wine is ever fermented completely "dry". Sugar's role in dictating the final alcohol content of the wine sometimes encourages winemakers to add sugar during winemaking in a process known as chaptalization solely in order to boost the alcohol content – chaptalization does not increase the sweetness of a wine.

<span class="mw-page-title-main">Tryptophol</span> Chemical compound

Tryptophol is an aromatic alcohol that induces sleep in humans. It is found in wine as a secondary product of ethanol fermentation. It was first described by Felix Ehrlich in 1912. It is also produced by the trypanosomal parasite in sleeping sickness.

<span class="mw-page-title-main">Yeast in winemaking</span> Yeasts used for alcoholic fermentation of wine

The role of yeast in winemaking is the most important element that distinguishes wine from fruit juice. In the absence of oxygen, yeast converts the sugars of the fruit into alcohol and carbon dioxide through the process of fermentation. The more sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to dryness. Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with dessert wines. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile filtering the wine to remove the yeast or fortification with brandy or neutral spirits to kill off the yeast cells. If fermentation is unintentionally stopped, such as when the yeasts become exhausted of available nutrients and the wine has not yet reached dryness, this is considered a stuck fermentation.

Aerobic fermentation or aerobic glycolysis is a metabolic process by which cells metabolize sugars via fermentation in the presence of oxygen and occurs through the repression of normal respiratory metabolism. Preference of aerobic fermentation over aerobic respiration is referred to as the Crabtree effect in yeast, and is part of the Warburg effect in tumor cells. While aerobic fermentation does not produce adenosine triphosphate (ATP) in high yield, it allows proliferating cells to convert nutrients such as glucose and glutamine more efficiently into biomass by avoiding unnecessary catabolic oxidation of such nutrients into carbon dioxide, preserving carbon-carbon bonds and promoting anabolism.

References

  1. ↑ Aren van Waarde; G. Van den Thillart; Maria Verhagen (1993). "Ethanol Formation and pH-Regulation in Fish". Surviving Hypoxia. pp. 157−70. hdl:11370/3196a88e-a978-4293-8f6f-cd6876d8c428. ISBN   978-0849342264.
  2. ↑ Stryer, Lubert (1975). Biochemistry . W. H. Freeman and Co. ISBN   978-0716701743.[ page needed ]
  3. ↑ Raj SB, Ramaswamy S, Plapp BV (2014). "Yeast alcohol dehydrogenase structure and catalysis". Biochemistry. 53 (36): 5791–6503. doi:10.1021/bi5006442. PMC   4165444 . PMID   25157460.
  4. ↑ Müller, Volker (2001). "Bacterial Fermentation" (PDF). eLS. John Wiley & Sons, Ltd. doi:10.1038/npg.els.0001415. ISBN   978-0470015902. Archived from the original (PDF) on 2014-09-08. Retrieved 2014-09-08.
  5. ↑ Logan, BK; Distefano, S (1997). "Ethanol content of various foods and soft drinks and their potential for interference with a breath-alcohol test". Journal of Analytical Toxicology. 22 (3): 181–83. doi:10.1093/jat/22.3.181. PMID   9602932.
  6. ↑ "The Alcohol Content of Bread". Canadian Medical Association Journal. 16 (11): 1394–95. November 1926. PMC   1709087 . PMID   20316063.
  7. ↑ "Wie aus altem Brot Alkohol wird". Deutschlandfunk Nova. 2 March 2024. Retrieved 7 March 2024.
  8. 1 2 3 James Jacobs, Ag Economist. "Ethanol from Sugar". United States Department of Agriculture. Archived from the original on 2007-09-10. Retrieved 2007-09-04.
  9. ↑ "Economic Feasibility of Ethanol Production from Sugar in the United States" (PDF). United States Department of Agriculture. July 2006. Archived from the original (PDF) on 2007-08-15. Retrieved 2007-09-04.
  10. ↑ "Ethanol Biorefinery Locations". Renewable Fuels Association. Archived from the original on 30 April 2007. Retrieved 21 May 2007.
  11. ↑ "Tiny super-plant can clean up hog farms and be used for ethanol production". projects.ncsu.edu. Archived from the original on July 18, 2020. Retrieved 2018-01-18.
  12. ↑ Caroline Wyatt (2006-08-10). "Draining France's 'wine lake'". BBC News . Retrieved 2007-05-21.
  13. ↑ Capone, John (21 November 2017). "That unsold bottle of Merlot is probably winding up in your gas tank". Quartz . Retrieved 21 November 2017.
  14. ↑ Japan Plans Its Own Green Fuel by Steve Inskeep. NPR Morning Edition, May 15, 2007
  15. ↑ "Agro2: Ethanol From Cassava". Archived from the original on 2016-05-19. Retrieved 2010-08-25.
  16. ↑ Pimentel, D. (Ed.) (1980). CRC Handbook of energy utilization in agriculture. (Boca Raton: CRC Press)
  17. ↑ Lynn Ellen Doxon (2001). The Alcohol Fuel Handbook . InfinityPublishing.com. ISBN   978-0-7414-0646-0.[ page needed ]
  18. ↑ Gil, C.; Gómez-Cordovés, C. (1986). "Tryptophol content of young wines made from Tempranillo, Garnacha, Viura and Airén grapes". Food Chemistry. 22: 59–65. doi:10.1016/0308-8146(86)90009-9.
  19. ↑ Szlavko, Clara M (1973). "Tryptophol, Tyrosol and Phenylethanol-The Aromatic Higher Alcohols in Beer". Journal of the Institute of Brewing. 79 (4): 283–88. doi: 10.1002/j.2050-0416.1973.tb03541.x .
  20. ↑ Ribéreau-Gayon, P.; Sapis, J. C. (2019). "On the presence in wine of tyrosol, tryptophol, phenylethyl alcohol and gamma-butyrolactone, secondary products of alcoholic fermentation". Comptes Rendus de l'Académie des Sciences, Série D. 261 (8): 1915–16. PMID   4954284. (Article in French)
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