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Ethanol fermentation, also referred to as alcoholic fermentation, is a biological process in which sugars such as glucose, fructose, and sucrose are converted into cellular energy and thereby produce ethanol and carbon dioxide as metabolic waste products. Because yeasts perform this conversion in the absence of oxygen, ethanol fermentation is classified as anaerobic.
Ethanol fermentation occurs in the production of alcoholic beverages and ethanol fuel, and in the rising of bread dough.
The chemical equation below summarizes the fermentation of glucose, whose chemical formula is C6H12O6. One mole of glucose is converted into two moles of ethanol and two moles of carbon dioxide:
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.[1] Glycolysis is summarized by the chemical equation:
The chemical formula of pyruvate is CH3COCOO−. Pi stands for the inorganic phosphate. 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. For more details, refer to the main article on glycolysis.
The fermentation process does not require oxygen. If oxygen is present, some species of yeast (Kluyveromyces lactis, Kluyveromyces lipolytica) oxidize pyruvate completely to carbon dioxide and water. This process is called respiration. Thus these yeasts produce ethanol only in an anaerobic environment.
However, many yeasts such as the commonly used baker's yeast Saccharomyces cerevisiae, and Schizosaccharomyces pombe, prefer fermentation to respiration. These yeasts will produce ethanol even under aerobic conditions given the right sources of nutrition.
Ethanol fermentation is responsible for the rising of bread dough. 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 into something of a foam. Nearly all the ethanol evaporates from the dough when the bread is baked.
All alcoholic beverages, including those produced by carbonic maceration, are produced by ethanol fermentation by yeast. Wine and brandy are produced by fermentation of the natural sugars present in fruits, especially grapes. Beer and whiskey 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 germinated. Amylase-treated grain or amylase-treated potatoes are fermented for the production of vodka. Rum is produced by fermentation of cane sugar. In all cases, the fermentation must take place in a vessel that allows carbon dioxide to escape, but prevents outside air from coming in, as exposure to oxygen would prevent the formation of ethanol.
Similar yeast fermentation of various carbohydrate products is used to produce much of the ethanol used for fuel.
The dominant ethanol feedstock in warmer regions is sugarcane.[2] In temperate regions, sugar beet is sometimes used instead.[2][3]
In the United States, the main feedstock for the production of ethanol is currently corn.[2] 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).[4] 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.[5]
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.[6] In Japan, it has been proposed to use rice normally made into sake as an ethanol source.[7]
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 25-40 tonnes per hectare (with irrigation and fertilizer),[8] 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[9] 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.
Ethanol fermentation produces unharvested byproducts such as heat, food for livestock, and water.[10]
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