Traditionally, the primary substrates for the industrial production of acetone-butanol were corn and molasses. The use of molasses as a fermentation substrate offers some advantages as compared to corn, such as easy handling and utilization by saccharolytic Clostridium species. Cane molasses was used in the commercial production of butanol in South Africa until about 1980 (Ezeji et al. 2004a). Solventogenic Clostridium species express the genes for potent amylolytic enzymes constitutively and do not need amylases added to substrate to metabolize starchy products. For this reason, other starchy substrates such as millet, wheat, rice, cassava, tapioca, and potatoes have been used successfully for growth and ABE produc­tion by solventogenic Clostridium species. The United States has the capacity to produce 13 billion gallons of biofuel per year from corn alone, and any further increase in biofuel pro­duction will most likely come from utilization of feedstocks other than corn grain because of limitations in supply (Gray et al. 2006). Production of butanol from low-cost lignocellu — losic biomass that does not compete with food crops may be the important aspect to meeting the DOE target (to blend 7.5 billion gallons of renewable fuels into gasoline by 2012), and for biobutanol production to become economically viable as well as sustainable (Ezeji and Blaschek 2008b).

Substrate cost has long been recognized as having the most influence on butanol price and has been identified as a major factor affecting economic viability of butanol production by fermentation (Qureshi and Blaschek 2000). To produce butanol at a competitive price, the use of more economic agricultural residues such as DDGS, corn fiber, corn stover, wheat straw, rice straw, and wood has been evaluated by many investigators including the labora­tories of the authors. Butanol-producing cultures are able to use a wide variety of carbohy­drates such as starch, cellobiose, sucrose, glucose, fructose, mannose, dextrin, galactose, xylose, and arabinose. The use of these carbohydrates by butanol-producing cultures illus­trates the potential to ferment sugars derived from hydrolysates of agricultural residues to butanol (Qureshi and Ezeji 2008).