Ethanol fermentation, also calledalcoholic fermentation, is abiological process which convertssugars such asglucose,fructose, andsucrose intocellular energy, producingethanol andcarbon dioxide as by-products. Becauseyeasts perform this conversion in the absence ofoxygen, alcoholicfermentation is considered ananaerobic process. It also takes place in some species of fish (includinggoldfish andcarp) where (along with lactic acid fermentation) it provides energy when oxygen is scarce.[1]
Ethanol fermentation is the basis foralcoholic beverages,ethanol fuel andbread dough rising.
Thechemical equations below summarize the fermentation of sucrose (C12H22O11) into ethanol (C2H5OH). Alcoholic fermentation converts onemole ofglucose into two moles of ethanol and two moles of carbon dioxide, producing two moles ofATP in the process.
Sucrose is a sugar composed of a glucose linked to a fructose. In the first step of alcoholic fermentation, the enzymeinvertase cleaves theglycosidic linkage between the glucose and fructose molecules.
Next, each glucose molecule is broken down into twopyruvate molecules in a process known asglycolysis.[2] Glycolysis is summarized by the equation:
CH3COCOO− is pyruvate, and Pi is inorganicphosphate. Finally, pyruvate is converted to ethanol and CO2 in two steps, regenerating oxidized NAD+ needed for glycolysis:
catalyzed bypyruvate decarboxylase
This reaction is catalyzed byalcohol dehydrogenase (ADH1 in baker's yeast).[3]
As shown by the reaction equation, glycolysis causes the reduction of two molecules ofNAD+ toNADH. TwoADP molecules are also converted to two ATP and two water molecules viasubstrate-level phosphorylation.
Fermentation of sugar to ethanol and CO2 can also be done byZymomonas mobilis, however the path is slightly different since formation of pyruvate does not happen by glycolysis but instead by theEntner–Doudoroff pathway.Othermicroorganisms can produce ethanol from sugars by fermentation but often only as a side product. Examples are[4]
Fermentation does not require oxygen. If oxygen is present, some species of yeast (e.g.,Kluyveromyces lactis orKluyveromyces lipolytica) will oxidizepyruvate completely to carbon dioxide and water in a process calledcellular respiration, hence these species of yeast will produce ethanol only in an anaerobic environment (not cellular respiration). This phenomenon is known as thePasteur effect.
However, many yeasts such as the commonly used baker's yeastSaccharomyces cerevisiae or fission yeastSchizosaccharomyces 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 underaerobic 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.
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]
Ethanol contained in alcoholic beverages is produced by means of fermentation induced by yeast.Liquors are distilled fromgrains,fruits,vegetables, orsugar that have already gone through alcoholic fermentation.
Alcohol products:
In all cases, fermentation must take place in a vessel (e.g. afermentation lock) that allowscarbon dioxide to escape and prevents outside air from coming in. This is because letting in outside air could contaminate the brew due to risk ofbacteria ormold, and a buildup of carbon dioxide could cause the vessel to rupture.[citation needed]
Yeast fermentation of various carbohydrate products is also used to produce the ethanol that is added togasoline.
The dominant ethanol feedstock in warmer regions issugarcane.[8] In temperate regions,corn orsugar 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 asDDGS (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 ofduckweed is being studied.[11]
In some parts of Europe, particularly France and Italy,grapes have become ade facto feedstock for fuel ethanol by thedistillation of surpluswine.[12] Surplus sugary drinks may also be used.[13] In Japan, it has been proposed to use rice normally made intosake as an ethanol source.[14]
Ethanol can be made frompetroleum or from sugars or starches. Starches are cheapest. The starchy crop with highest energy content per acre iscassava, 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 overallenergy efficiency of cassava-root to ethanol conversion is circa 32%.
The yeast used for processing cassava isEndomycopsis fibuligera, sometimes used together with bacteriumZymomonas mobilis.
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 ofbiogas, are referred to asDistillers grains and sold as WDG,Wet Distiller's grains, and DDGS,Dried Distiller's Grains with Solubles, respectively.
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