Ethanol fermentation

Ethanol fermentation

Ethanol fermentation is the biological process by which sugars such as glucose, fructose, and sucrose are converted into cellular energy and thereby producing ethanol and carbon dioxide as metabolic waste products. Yeasts carry out ethanol fermentation on sugars in the absence of oxygen. Because the process does not require oxygen, ethanol fermentation is classified as anaerobic. Ethanol fermentation is responsible for the rising of bread dough, the production of ethanol in alcoholic beverages, and for much of the production of ethanol for use as fuel.

The chemical process of fermentation

The chemical equation below summarizes ethanol fermentation, in which one hexose molecule is converted into two ethanol molecules and two carbon dioxide molecules:

C6H12O6 → 2 C2H5OH + 2 CO2

The process begins with a molecule of glucose being broken down by the process of glycolysis into pyruvate:

C6H12O6 → 2 CH3COCOO + 2H+

This reaction is accompanied by the reduction of two molecules of NAD+ to NADH and a net of two ADP molecules converted to two ATP plus the two water molecules.

Pyruvate is then converted to acetaldehyde and carbon dioxide. The acetaldehyde is subsequently reduced to ethanol by the NADH from the previous glycolysis, which is returned to NAD+:

CH3COCOO + H+ → CH3CHO + CO2

CH3CHO + NADH → C2H5OH + NAD+

Yeast will perform the above two reactions only if oxygen is excluded from the environment. Otherwise yeast will oxidize pyruvate completely to carbon dioxide and water.

Uses

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.

The production of all alcoholic beverages, except those produced by carbonic maceration, employs ethanol fermentation by yeast. Wines and brandies are produced by fermentation of the natural sugars present in fruits, especially grapes. Beers, ales, and whiskeys employ fermentation of grain starches that have been converted to sugar by the application of 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. Fermentation of cane sugar is the first step in producing rum. In all cases, the fermentation must take place in a vessel that is arranged to allow carbon dioxide to escape, but that 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.

Feedstocks for fuel production

The dominant ethanol feedstock in warmer regions is sugarcane. In temperate regions, this accessibility has been somewhat replicated by selective breeding of the sugar beet.

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 as DDGS (distillers dried grains with solubles) that can be used to fulfill a portion of the diet of livestock. A bushel of corn produces about 18 pounds of DDGS. . 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.

In some parts of Europe, particularly France and Italy, wine is used as a feedstock due to a massive oversupply termed wine lake. Japan is hoping to use rice wine (sake) as an ethanol source.

Ethanol market forecast

Main players will be Brazil, USA, EU, and tropical developing countries. EU can currently (2007) produce ethanol in large quantities with a mineral-oil based chemical process for US$0.57 per liter . USA produces ethanol for circa US$0.32 per liter, mainly from corn starch. Brazil produces ethanol for circa US$0.27 per liter, from sugarcane. Tropical developing countries do not produce very large amounts of ethanol yet.

Brazil is the largest producer, but it will not be able to meet EU's needs for many years to come, assuming that it will expand ethanol production at maximum possible rate. USA is expected to become self-supplying (to avoid high oil prices), but is not expected to become a major exporter. EU also wants to avoid high oil prices, and is starting to require a minimum ethanol percentage in automobile fuels, so it wants to import ethanol. Ethanol can be made from mineral oil or from sugars or starches, cheapest of which are starches, and 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 food and as cheap admixture to wheat flour; Nigeria and Ghana are already establishing cassava-to-ethanol plants ; Brazil is doing that too (sugarcane and cassava grow on very different types of soil) ; and so are many other countries.

EU expects that combined effect of increasing ethanol production will be able to meet its needs in 2012. Therefore it is expect that in 2012 price of ethanol will drop from maybe US$0.42 to maybe US$0.30 (FOB Africa). 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 future situation remains uncertain. Currently, cassava can yield more than 40 tons per hectare (with irrigation and fertilizer), and from a ton of cassava roots, circa 200 liter of ethanol can be produced (assuming cassava with 22% starch content), and a liter of ethanol contains circa 10.7 MJ of energy. Overall energy efficiency of cassava-root to ethanol conversion is circa 32%.

Cassava plants can grow in poor soils, are drought resistant, and need a minimum temperature of 17 °C. They can use solar radiation up to 300 W/m² (equivalent to lightly clouded tropical sky), and optimum water use is 100 to 150 cm (slightly less than rainfall in rain forest). For compensating for nutrients taken up, Cassava's fertilizer demand is (in kilograms of nutrient per ton cassava): N:21, P:10, K:42, Ca:7, Mg:4 , so if fertilizer prices go up, so does ethanol price.

Starch price (food-quality starch from Thailand) is circa 0.22 US$/kg, and from 1 kg starch, 0.9 liter of ethanol can be produced, so, producer price would be 0.24 US$/liter plus cost of conversion from starch to ethanol. A US$10 million conversion plant can convert circa 80 million liters per year, so total cost of ethanol from cassava currently is near USA's production price. Due to improvements being made in this relatively new industry, producer price would become lower, probably near that of Brazil, and maybe even lower than that.

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

Most of this information can be found on FAO's website.

Microbes used in ethanol fermentation

See also

References

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