Ethanol Fermentation and Feedstock

The ethanol fuel manufacturing process is a combination of biochemical and physical processes based on traditional unit operations. Ethanol is produced by fermentation of sugars with yeast. The fermentation crude product is con­centrated to fuel-grade ethanol via distillation. The organisms of primary interest to industrial fermentation of ethanol include Saccharomyces cerevisiae,

S. uvarum, Schizosaccharomyces pombe, and Kluyveromyces spp., among which Saccharomyces cerevisiae is most commonly utilized.

Feedstock for ethanol fermentation are either sugar or starch-containing crops. These "biomass fuel crops" (tubers and grains) typically include sugarbeets, sugarcane, potatoes, corn, wheat, barley, Jerusalem artichokes, and sweet sorghum. Sugar crops such as sugarcane, sugarbeets, or sweet sor­ghum are extracted to produce a sugar-containing solution or syrup that can be directly fermented by yeast. Starch feedstock, however, must go through an additional step that involves starch-to-sugar conversion, as is the case for grain ethanol. Needless to say, sugar crops are simpler to convert to ethanol than starch crops. Therefore, the ethanol production cost, excluding the feed­stock cost, is substantially lower for sugar crops than for starch crops.