However, in these later cases in addition to ethanol, .other undesirable products are also produced so that often they are not used for alcohol production. Fermentation is often most active under anaerobic conditions, when carbohydrate is converted into ethanol via pyruvate and acetaldehyde. Under aerobic conditions on the other hand very little alochol is produced, .most of the carbohydrate getting oxidized to CO2 + H2O.

Furthermore, ethanol is inhibitory for yeast growth at high concentration, although yeast strains differ in their tolerance to ethanol's inhibitory action.
Commercial production of ethanol involves three steps:
(i) preparation of substrate,
(ii) fermentation and
(iii) distillation.