ALCOHOLIC FERMENTATION

Ethanol fermentation, also referred to as alcoholic fermentation, is a biological process in which sugars such as glucos, 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 process of fermentation of glucose

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:

C₁₂H₂₂O₁₁ +H₂O + invertase →2 C₆H₁₂O₆

C₆H₁₂O₆ + Zymase → 2C₂H₅OH + 2CO₂

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. Glycolysis is summarized by the chemical equation:

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

The chemical formula of pyruvate is CH₃COCOO⁻. P_i 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.

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  Glucose depicted in Haworth projection
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  Pyruvate
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  Acetaldehyde
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  Ethanol

 

Effect of oxygen

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.