Ethanol fermentation

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(1) A glucose molecule is broken down via glycolysis, yielding two pyruvate molecules. The energy released by this exothermic reactions is used to phosphorylate two ADP molecules, yielding two ATP molecules, and to reduce two molecules of NAD+ to NADH. (2) The two pyruvate molecules are broken down, yielding two acetaldehyde molecule and giving off two molecules of carbon dioxide. (3) The two molecules of NADH reduce the two acetaldehyde molecules to two molecules of ethanol; this converts NADH back into NAD+.

Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. It also takes place in some species of fish (including goldfish and carp) where (along with lactic acid fermentation) it provides energy when oxygen is scarce.[1]

Ethanol fermentation is the basis for alcoholic beverages, ethanol fuel and bread dough rising.

Biochemical process of fermentation of sucrose

A laboratory vessel being used for the fermentation of straw
Fermentation of sucrose by yeast

The chemical equations below summarize the fermentation of sucrose (C12H22O11) into ethanol (C2H5OH). Alcoholic fermentation converts one mole of glucose into two moles of ethanol and two moles of carbon dioxide, producing two moles of ATP in the process.

C6H12O6 + 2 ADP + 2 Pi → 2 C2H5OH + 2 CO2 + 2 ATP

Sucrose is a sugar composed of a glucose linked to a fructose. In the first step of alcoholic fermentation, the enzyme invertase cleaves the glycosidic linkage between the glucose and fructose molecules.

Next, each glucose molecule is broken down into two

pyruvate molecules in a process known as glycolysis.[2]
Glycolysis is summarized by the equation:

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

CH3COCOO− is pyruvate, and Pi is inorganic phosphate. Finally, pyruvate is converted to ethanol and CO2 in two steps, regenerating oxidized NAD+ needed for glycolysis:

1. CH3COCOO− + H+ → CH3CHO + CO2

catalyzed by pyruvate decarboxylase

2. CH3CHO + NADH + H+ → C2H5OH + NAD+

This reaction is catalyzed by alcohol dehydrogenase (ADH1 in baker's yeast).[3]

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.

Related processes

Main article:
Fermentation (biochemistry)

Fermentation of sugar to ethanol and CO2 can also be done by Zymomonas mobilis, however the path is slightly different since formation of pyruvate does not happen by glycolysis but instead by the Entner–Doudoroff pathway. Other microorganisms can produce ethanol from sugars by fermentation but often only as a side product. Examples are[4]

Gallery

  • Grapes fermenting during wine production.
    Grapes fermenting during wine production.

Effect of oxygen

Fermentation does not require oxygen. If oxygen is present, some species of yeast (e.g., Kluyveromyces lactis or Kluyveromyces lipolytica) will oxidize pyruvate completely to carbon dioxide and water in a process called cellular respiration, hence these species of yeast will produce ethanol only in an anaerobic environment (not cellular respiration). This phenomenon is known as the Pasteur effect.

However, many yeasts such as the commonly used baker's yeast

aerobic
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.

Bread baking

The formation of carbon dioxide – a byproduct of ethanol fermentation – causes bread to rise.

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]

Alcoholic beverages

Primary fermentation cellar, Budweiser Brewery, Fort Collins, Colorado

Ethanol contained in alcoholic beverages is produced by means of fermentation induced by yeast.

In all cases, fermentation must take place in a vessel that allows carbon dioxide to escape but prevents outside air from coming in. This is to reduce risk of contamination of the brew by unwanted bacteria or mold and because a buildup of carbon dioxide creates a risk the vessel will rupture or fail, possibly causing injury or property damage.[citation needed]

Feedstocks for fuel production

Yeast 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.[8] In temperate regions, corn or sugar beets are used.[8][9]

In the United States, the main feedstock for the production of ethanol is currently corn.

duckweed is being studied.[11]

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.[12] Surplus sugary drinks may also be used.[13] In Japan, it has been proposed to use rice normally made into sake as an ethanol source.[14]

Cassava as ethanol feedstock

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 and 40 tonnes per hectare (with irrigation and fertilizer),

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.

Byproducts of fermentation

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 of biogas, are referred to as Distillers grains and sold as WDG, Wet Distiller's grains, and DDGS, Dried Distiller's Grains with Solubles, respectively.

Microbes used in ethanol fermentation

See also

References

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  4. ISBN 978-0470015902. Archived from the original
    (PDF) on 2014-09-08. Retrieved 2014-09-08.
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  6. PMID 20316063
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  7. ^ "Wie aus altem Brot Alkohol wird". Deutschlandfunk Nova. 2 March 2024. Retrieved 7 March 2024.
  8. ^ a b c James Jacobs, Ag Economist. "Ethanol from Sugar". United States Department of Agriculture. Archived from the original on 2007-09-10. Retrieved 2007-09-04.
  9. ^ "Economic Feasibility of Ethanol Production from Sugar in the United States" (PDF). United States Department of Agriculture. July 2006. Archived from the original (PDF) on 2007-08-15. Retrieved 2007-09-04.
  10. ^ "Ethanol Biorefinery Locations". Renewable Fuels Association. Archived from the original on 30 April 2007. Retrieved 21 May 2007.
  11. ^ "Tiny super-plant can clean up hog farms and be used for ethanol production". projects.ncsu.edu. Archived from the original on July 18, 2020. Retrieved 2018-01-18.
  12. ^ Caroline Wyatt (2006-08-10). "Draining France's 'wine lake'". BBC News. Retrieved 2007-05-21.
  13. ^ Capone, John (21 November 2017). "That unsold bottle of Merlot is probably winding up in your gas tank". Quartz. Retrieved 21 November 2017.
  14. ^ Japan Plans Its Own Green Fuel by Steve Inskeep. NPR Morning Edition, May 15, 2007
  15. ^ "Agro2: Ethanol From Cassava". Archived from the original on 2016-05-19. Retrieved 2010-08-25.
  16. ^ Pimentel, D. (Ed.) (1980). CRC Handbook of energy utilization in agriculture. (Boca Raton: CRC Press)
  17. ISBN 978-0-7414-0646-0.[page needed
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  18. doi:10.1016/0308-8146(86)90009-9
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  19. doi:10.1002/j.2050-0416.1973.tb03541.x
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  20. PMID 4954284
    . (Article in French)
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