Fermentation of sugars obtained from hydrolysis of starch or cellulosic biomass is an alternative to using petrochemicals as the feedstock in production of ethanol. One of the many commercial processes to do this uses an enzyme to hydrolyze starch in corn to maltose (a disaccharide consisting of two glucose units) and oligomers consisting of several glucose units. A yeast culture then converts the maltose to ethyl alcohol and carbon dioxide:

As the yeast grows, 0.0794 kg of yeast is produced for every kg ethyl alcohol formed, and 0.291 kg water is produced for every kg of yeast formed. For use as a fuel, the product from such a process must be around 99.5 wt% ethyl alcohol. Corn fed to the process is 72.0 wt% starch on a moisture-free basis and contains 15.5 wt% moisture. It is estimated that 101.2 bushels of corn can be harvested from an acre of corn that each bushel is equivalent to 25.4 lbm of corn, and that 6.7 kg of ethanol can be obtained from a bushel of corn. What acreage of farmland is required to produce 100,000 kg of ethanol product? What factors (economic and environmental) must be considered in comparing production of ethanol by this route with other routes involving petrochemical feedstocks?

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C12H2011 + H2O (+ yeast) → 4C2H5OH + 4CO2 (+ yeast + H2O)