Ethanol fermentation 1

Ethanol fermentation

In ethanol fermentation, one glucose moleculebreaks down into two pyruvates (1). The energy

from this exothermic reaction is used to bindinorganic phosphates to ADP and convert NAD+

to NADH. The two pyruvates are then brokendown into two acetaldehydes and give off two

CO2 as a waste product (2). The twoacetaldehydes are then converted to two ethanolby using the H- ions from NADH; converting

NADH back into NAD+ (3).

Alcoholic fermentation, also referred to as ethanol fermentation, is abiological process in which sugars such as glucose, fructose, andsucrose are converted into cellular energy and thereby produce ethanoland carbon dioxide as metabolic waste products. Because yeastsperform this conversion in the absence of oxygen, alcoholicfermentation is considered an anaerobic process.

Alcoholic fermentation occurs in the production of alcoholic beveragesand ethanol fuel, and in the rising of bread dough.

Grapes fermenting during the production of wine.

Ethanol fermentation 2

Chemical process of fermentation of glucose

A laboratory vessel being used for thefermentation of straw.

The chemical equations below summarize the fermentation of sucrose(C12H22O11) into ethanol (C2H5OH). Alcoholic fermentation convertsone mole of sucrose into four moles of ethanol and four moles ofcarbon dioxide, producing two moles of ATP in the process.

The overall chemical formula for alcoholic fermentation is:C6H12O6 + Zymase → 2 C2H5OH + 2 CO2

Sucrose is a dimer of glucose and fructose molecules. In the first stepof alcoholic fermentation, the enzyme invertase cleaves the glycosidiclinkage between the two glucose molecules.

C12H22O11 + H2O + invertase → 2 C6H12O6Next, each glucose molecule is broken down into two pyruvatemolecules in a process known as glycolysis. Glycolysis is summarizedby the equation:

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

The chemical formula of pyruvate is CH3COCOO−. Pi stands for theinorganic phosphate.

As shown by the reaction equation, glycolysis causes the reduction oftwo molecules of NAD+ to NADH. Two ADP molecules are alsoconverted to two ATP and two water molecules via substrate-levelphosphorylation.

Glucose depictedin Haworthprojection

Pyruvate Acetaldehyde Ethanol

Ethanol fermentation 3

Effect of oxygenFermentation does not require oxygen. If oxygen is present, some species of yeast (e.g., Kluyveromyces lactis orKluyveromyces lipolytica) will oxidize pyruvate completely to carbon dioxide and water. This process is calledcellular respiration. But these species of yeast will produce ethanol only in an anaerobic environment (not cellularrespiration).However, many yeasts such as the commonly used baker's yeast Saccharomyces cerevisiae, or fission yeastSchizosaccharomyces pombe, prefer fermentation to respiration. These yeasts will produce ethanol even underaerobic conditions, if they are provided with the right kind of nutrition.During batch fermentation, the rate of ethanolproduction per milligram of cell protein is maximal for a brief period early in this process and declines progressivelyas ethanol accumulates in the surrounding broth. Studies demonstrate that the removal of this accumulated ethanoldoes not immediately restore fermentative activity, and they provide evidence that the decline in metabolic rate isdue to physiological changes (including possible ethanol damage) rather than to the presence of ethanol. Severalpotential 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 causallyrelated to the fall in fermentative activity during batch fermentation.

Baking bread

The formation of carbon dioxide — a byproduct of ethanol fermentation — causesbread to rise.

Ethanol fermentation causes bread dough torise. Yeast organisms consume sugars in thedough and produce ethanol and carbondioxide as waste products. The carbondioxide forms bubbles in the dough,expanding it into something of a foam.Nearly all the ethanol evaporates from thedough when the bread is baked.

Alcoholic beverages

All ethanol contained in alcoholic beverages(including ethanol produced by carbonicmaceration) is produced by means offermentation induced by yeast.

• Wine is produced by fermentation of the natural sugars present in grapes; cider and perry are produced by similarfermentation of natural sugar in apples and pears, respectively; and other fruit wines are produced from thefermentation of the sugars in and other kinds of fruit. Brandy and eaux de vie (e.g. slivovitz) are produced bydistillation of these fruit-fermented beverages.

• Mead is produced by fermentation of the natural sugars present in honey.• Beer, whiskey, and vodka are produced by fermentation of grain starches that have been converted to sugar by the

enzyme amylase, which is present in grain kernels that have been malted (i.e. germinated). Other sources of starch(e.g. potatoes and unmalted grain) may be added to the mixture, as the amylase will act on those starches as well.Whiskey and vodka are also distilled; gin and related beverages are produced by the addition of flavoring agentsto a vodka-like feedstock during distillation.

• Rice wines (including sake) are produced by the fermentation of grain starches converted to sugar by the moldAspergillus oryzae. Baijiu, soju, and shōchū are distilled from the product of such fermentation.

Ethanol fermentation 4

• Rum and some other beverages are produced by fermentation and distillation of sugarcane. Rum is usuallyproduced from the sugarcane product molasses, while the related drink cachaça is produced from raw cane juice.

In all cases, fermentation must take place in a vessel that allows carbon dioxide to escape but prevents outside airfrom coming in. This is because exposure to oxygen would prevent the formation of ethanol, while a buildup ofcarbon dioxide creates a risk the vessel will rupture or fail catastrophically, causing injury and property damage.

Feedstocks for fuel productionYeast 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. In temperate regions, corn or sugar beets are used.In the United States, the main feedstock for the production of ethanol is currently corn. Approximately 2.8 gallons ofethanol 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 asfeed for livestock. A bushel of corn produces about 18 pounds of DDGS (320 kilograms of DDGS per metric ton ofmaize). Although most of the fermentation plants have been built in corn-producing regions, sorghum is also animportant feedstock for ethanol production in the Plains states. Pearl millet is showing promise as an ethanolfeedstock for the southeastern U.S. and the potential of duckweed is being studied.[1]

In some parts of Europe, particularly France and Italy, grapes have become a de facto feedstock for fuel ethanol bythe distillation of surplus wine. In Japan, it has been proposed to use rice normally made into sake as an ethanolsource.[2]

Cassava as ethanol feedstockEthanol can be made from mineral oil or from sugars or starches. Starches are cheapest. The starchy crop withhighest 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 wheatflour. Nigeria and Ghana are already establishing cassava-to-ethanol plants. Production of ethanol from cassava iscurrently 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 yieldbetween 25-40 tonnes per hectare (with irrigation and fertilizer),[3] and from a tonne of cassava roots, circa 200 litersof ethanol can be produced (assuming cassava with 22% starch content). A liter of ethanol contains circa 21.46[4] MJof 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 bacteriumZymomonas mobilis.

Ethanol fermentation 5

Byproducts of fermentationEthanol fermentation produces unharvested byproducts such as heat, carbon dioxide, food for livestock, and water.

Microbes used in ethanol fermentation•• Yeast•• Zymomonas mobilis•• schizosaaharomyces

References[1] Tiny Super-Plant Can Clean Up Hog Farms and Be Used For Ethanol Production (http:/ / news. ncsu. edu/ news/ 2009/ 04/

wmschengstompduckweed. php)[2] Japan Plans Its Own Green Fuel (http:/ / www. npr. org/ templates/ story/ story. php?storyId=10184012) by Steve Inskeep. NPR Morning

Edition, May 15, 2007[3] Agro2: Ethanol From Cassava (http:/ / agro2. com/ our-cassava-products/ ethanol/ )[4][4] Pimentel, D. (Ed.) (1980). CRC Handbook of energy utilization in agriculture. (Boca Raton: CRC Press)

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