Fermentation is a general term for anaerobic metabolism of glucose . Some anaerobic bacteria, such as lactobacilli, produce lactate, while others have alternative mechanisms for anaerobic oxidation of NADH formed during glycolysis. During fermentation in yeast, the pathway of glycolysis is identical with that in the RBC, except that pyruvate is converted into ethanol. The pyruvate is first decarboxylated by pyruvate decarboxylase to acetaldehyde, releasing CO2. The NADH produced in the GAPDH reaction is then re-oxidized by alcohol dehydrogenase, regenerating NAD+ and producing ethanol (Fig. 11.6). Ethanol is a toxic compound, and yeast die when the ethanol concentration in their medium reaches about 12%, which is the approximate concentration of alcohol in natural wines.
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