The Syrup is first diluted to reach a 13 to 22% fermentable sugars mash, and thereafter sent to the fermentation unit. Theoretically, the maximum conversion efficiency of glucose to ethanol is 51 percent on a weight basis. However, some glucose is used by the yeast for the production of cell mass and for metabolic products other than ethanol. In practice, between 40 and 48 percent of glucose is converted to ethanol. With 45% fermentation efficiency, 1.000 kilograms of fermentable sugar produce about 570 liters of pure ethanol.
Ethanol is separated from the mash by distillation, a physical process in which the water and ethanol components of the solution are separated by differences in boiling point/ vapor pressure.
This ethanol is then fed to a rectifying column, the product of which is the isotropic mixture. Ethanol and water form an azeotrope, or constant boiling solution, of about 95 percent alcohol and five percent water. The five percent water cannot be separated by conventional distillation. The production of pure, water-free (anhydrous) ethanol requires a dehydration step following distillation. The method employs molecular sieves that selectively absorb water on the basis of the difference in molecular size between water and ethanol.