Ethanol fermentation

• Read in another language
• Watch this page
• Edit

In ethanol fermentation, (1) one glucose molecule breaks down into two pyruvates. The energy from this exothermic reaction is used to bind the inorganic phosphates to ADP and convert NAD+ to NADH. (2) The two pyruvates are then broken down into two acetaldehydes and give off two CO2 as a by-product. (3) The two acetaldehydes are then converted to two ethanol by using the H- ions from NADH, converting NADH back intoNAD+.                    Ethanol fermentation, also calledalcoholic fermentation, is a biological process which converts sugars such asglucose, fructose, and sucrose intocellular energy, producing ethanol andcarbon dioxide as by-products. Becauseyeasts perform this conversion in the absence of oxygen, alcoholicfermentation is considered an anaerobicprocess. It also takes place in some species of fish (including goldfish andcarp) where (along with lactic acid fermentation) it provides energy when oxygen is scarce.^[1]Ethanol fermentation has many uses, including the production of alcoholic beverages, the production of ethanol fuel, and bread cookingA laboratory vessel being used for the fermentation ofstrawFermentation of sucrose by yeastThe chemical equations below summarize the fermentation of sucrose (C₁₂H₂₂O₁₁) into ethanol (C₂H₅OH). Alcoholic fermentation converts onemole of glucose into two moles of ethanol and two moles of carbon dioxide, producing two moles of ATP in the process.The overall chemical formula for alcoholic fermentation is:      C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂Sucrose is a dimer of glucose and fructose molecules. In the first step of alcoholic fermentation, the enzymeinvertase cleaves the glycosidic linkagebetween the glucose and fructose molecules.C₁₂H₂₂O₁₁ + H₂O + invertase → 2 C₆H₁₂O₆Next, 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 P_i + 2 NAD⁺ → 2 CH₃COCOO^- + 2 ATP + 2 NADH + 2 H₂O + 2 H⁺CH₃COCOO⁻ is pyruvate, and P_i 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 ADPmolecules are also converted to two ATP and two water molecules viasubstrate-level phosphorylation.

Related processesEdit

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]

• Heterolactic acid fermentation in which Leuconostoc bacterias produce Lactate + Ethanol + CO₂
• Mixed acid fermentation whereEscherichia produce ethanol mixed with lactate, acetate, succinate, formate, CO₂, and H₂
• 2,3-butanediol fermentation byEnterobacter producing ethanol, butanediol, lactate, formate, CO₂, and H₂

Primary fermentation cellar, Budweiser Brewery, Fort Collins, ColoradoAll ethanol contained in alcoholic beverages (including ethanol produced by carbonic maceration) is produced by means of fermentation induced by yeast.^{[citation needed]}Wine is produced by fermentation of the natural sugars present in grapes;cider and perry are produced by similar fermentation of natural sugar in apples and pears, respectively; and other fruit wines are produced from the fermentation of the sugars in any 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 agents to a vodka-like feedstock during distillation.

• Rice wines (including sake) are produced by the fermentation of grain starches converted to sugar by the mold Aspergillus oryzae. Baijiu, soju, and shōchū are distilled from the product of such fermentation.
• Rum and some other beverages are produced by fermentation and distillation of sugarcane. Rum is usually produced from the sugarcane product molasses.

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.

^[