The conversion of certain organic compounds by enzymes secreted by microorganisms. These oxidation-reduction (redox) reactions take place in the absence of air or free oxygen (anaerobiosis). The type of fermentation depends on the nature of the reaction product — alcohol or ethanol, acetic acid, lactic acid, malic acid, propionic acid or butyric acid. Very little energy is released by fermentation. It is useful in biofuels, and the process for converting plant carbohydrates into ethanol is based on alcoholic fermentation.