Today, a vintner creating the next great cabernet, a moonshiner making whiskey, and an engineer processing biomass employ brewer’s yeast, the fungus Saccharomyces cerevisiaea, to convert glucose or fructose in plant juices into ethanol. This reaction proceeds until ethanol concentrations reach between 14.0% and 15.5% by volume, the upper limit of tolerance for the yeast. After fermentation, the vintner, moonshiner, or engineer distills the liquid to remove water and produce nearly pure ethanol.
Distillation separates chemicals by their differences in boiling temperatures. First, the fermentation solution (mash) is heated in a still to above 78.4°C (the boiling point of ethanol) but below 100°C (the boiling point of water). Ethanol vapors rise to the top of the still, where they cool and condense into a liquid again. Finally, the distillate collects into a holding tank. Stills designed for biofuels produce a distillate that is up to 95.6% ethanol and as low as 4.4% water. By contrast, stills designed for alcoholic beverages do not seek such purity; their distillates contain lower concentrations of ethanol and higher concentrations of natural flavors.
The last step of bioethanol production is dehydration. The most common method is to pass the distillate through a molecular sieve that absorbs the remaining water. A molecular sieve is a material, such as an aluminosilicate clay, with pores large enough to trap water molecules but too small to restrict ethanol molecules from passing through. Another drying method is via the addition of an organic solvent, such as benzene or cyclohexane, that forms a chemical association with the water and ethanol. This association has a boiling point of 64.9°C and can be removed by a second distillation. This method leaves traces of benzene or cyclohexane, rendering the product undrinkable (in other terms, denatures the ethanol) and not subject to liquor taxes. The end product of these dehydration methods is 99% or even purer ethanol, ready for combustion.
The processes of sugar extraction from the plant biomass, fermentation of the sugars, and distillation and dehydration of the ethanol have an overall efficiency of about 87% in terms of the energy content of the ethanol produced over that of the sugar initially present in the sugarcane. 
 Macedo, I. C., M. R. L. V. Leal, and J. E. A. R. da Silva (2004) Assessment of Greenhouse Gas Emissions in the Production and Use of Fuel Ethanol in Brazil, Government of the State of São Paulo, http://www.unica.com.br/i_pages/files/pdf_ingles.pdf.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents