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.
The overall chemical formula for alcoholic fermentation is:
- C6H12O6 → 2 C2H5OH + 2 CO2
Sucrose is a dimer of glucose and fructose molecules. In the first step of alcoholic fermentation, the enzyme invertase cleaves the glycosidic linkage between the glucose and fructose molecules.
- C12H22O11 + H2O + invertase → 2 C6H12O6
Next, each glucose molecule is broken down into two pyruvate molecules in a process known as glycolysis. 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).
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.
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
- Heterolactic acid fermentation in which Leuconostoc bacterias produce Lactate + Ethanol + CO2
- Mixed acid fermentation where Escherichia produce ethanol mixed with lactate, acetate, succinate, formate, CO2, and H2
- 2,3-butanediol fermentation by Enterobacter producing ethanol, butanediol, lactate, formate, CO2, and H2