Ethanol fermentation

Ethanol fermentation, also calledalcoholic fermentation, is abiological processwhich convertssugarssuch asglucose,fructose, andsucroseintocellular energy, producingethanolandcarbon dioxideas by-products. Becauseyeastsperform this conversion in the absence ofoxygen, alcoholicfermentationis considered ananaerobicprocess. It also takes place in some species of fish (includinggoldfishandcarp) where (along with lactic acid fermentation) it provides energy when oxygen is scarce.
Ethanol fermentation is the basis foralcoholic beverages,ethanol fuelandbreaddough rising.
Thechemical equationsbelow summarize the fermentation of sucrose (C12H22O11) into ethanol (C2H5OH). Alcoholic fermentation converts onemoleofglucoseinto two moles of ethanol and two moles of carbon dioxide, producing two moles ofATPin the process.
Sucroseis a sugar composed of a glucose linked to a fructose. In the first step of alcoholic fermentation, the enzymeinvertasecleaves theglycosidic linkagebetween the glucose and fructose molecules.
Next, each glucose molecule is broken down into twopyruvatemolecules in a process known asglycolysis.Glycolysis is summarized by the equation:
CH3COCOOis pyruvate, and Piis inorganicphosphate.  Finally, pyruvate is converted to ethanol and CO2in two steps, regenerating oxidized NAD+ needed for glycolysis:
catalyzed bypyruvate decarboxylase
This reaction is catalyzed byalcohol dehydrogenase(ADH1 in baker's yeast).
As shown by the reaction equation, glycolysis causes the reduction of two molecules ofNADtoNADH. TwoADPmolecules are also converted to two ATP and two water molecules viasubstrate-level phosphorylation.
Fermentation of sugar to ethanol and CO2can 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.
Othermicroorganismscan produce ethanol from sugars by fermentation but often only as a side product. Examples are
Fermentation does not require oxygen. If oxygen is present, some species of yeast (e.g.,Kluyveromyces lactisorKluyveromyces lipolytica) will oxidizepyruvatecompletely 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 cerevisiaeor fission yeastSchizosaccharomyces pombeunder 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 underaerobicconditions, 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.
In a contemporary advancement, a group in Germany has been doing the opposite and converting stale bread into ethanol.
Ethanol contained inalcoholic beveragesis produced by means of fermentation induced by yeast.Liquorsare distilled fromgrains,fruits,vegetables, orsugarthat have already gone through alcoholic fermentation.
Alcohol products:
In all cases, fermentation must take place in a vessel (e.g. afermentation lock) that allowscarbon dioxideto escape and prevents outside air from coming in. This is because letting in outside air could contaminate the brew due to risk ofbacteriaormold, and  a buildup of carbon dioxide could cause the vessel to rupture.
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.In temperate regions,cornorsugar beetsare used.
In the United States, the main feedstock for the production of ethanol is currently corn.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).Although most of the fermentation plants have been built in corn-producing regions,sorghumis also an important feedstock for ethanol production in the Plains states.Pearl milletis showing promise as an ethanol feedstock for the southeastern U.S. and the potential ofduckweedis being studied.
In some parts of Europe, particularly France and Italy,grapeshave become ade factofeedstock for fuel ethanol by thedistillationof surpluswine.Surplus sugary drinks may also be used.In Japan, it has been proposed to use rice normally made intosakeas an ethanol source.
Ethanol can be made frommineral oilor 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),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 21.46MJof energy. The overallenergy efficiencyof 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.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 grainsand sold as WDG,Wet Distiller's grains, and DDGS,Dried Distiller's Grains with Solubles, respectively.