Corn fiber is a coproduct of the corn wet-milling industry. It is a mixture of corn kernel hulls and residual starch not extracted during the wet-milling process. Corn fiber is composed of approximately 40% hemicellulose, 12% cellulose, 25% starch, 10% protein, 3% oil, and 10% other substances such as ash and lignin (Singh et al., 2003). Corn fiber represents a renewable resource that is available in significant quantities from the corn dry — and wet-milling industries. Approxi­mately 6.3 x 106 dry tons of corn fiber is produced annually in the United States. Typically 4.5 lb of corn fiber is obtained from a bushel (56 lb) of corn, which can be converted to about 3.0 lb of fermentable sugars. The major fermentable sugars from hydrolysis of lignocellulosic biomass, such as softwood, hardwood and grasses, rice and wheat straw, sugarcane bagasse, corn stover and corn fiber, are D-glucose and D-xylose (except that softwood also contains substantial amounts of mannose) (Sedlak and Ho, 2004). Industrial Saccharomyces yeast strains used for fermenting sugars to ethanol lack the ability to utilize xylose, one of the major end products of hemicellulose hydrolysis. This is a major obstacle for the utilization of corn fiber or other forms of lignocellulosic-based biomass.

Economically, it is important that both xylose and glucose present in corn fiber be fermented to butanol in order for this renewable biomass to be used as feedstock for butanol production. Solventogenic clostridia have an added advan­tage over many other cultures as they can utilize both hexose and pentose sugars (Singh and Mishra, 1995) released from lignocellulosic biomass upon hydrolysis to produce butanol. Fond and Engasser (1986), during their evaluation of the fermentation of lignocellulosic hydrolysates to butanol by C. acetobutylicum ATCC 824, demonstrated that the culture utilized both xylose and glucose, although xylose was utilized more slowly than glucose and also supported lower butanol production. However, C. beijerinckii BA101 has been shown to utilize xylose and can effectively coferment xylose and glucose to produce butanol (Ebener et al., 2003). Parekh et al. (1988) produced acetone-butanol from hydroly­sates of pine, aspen, and corn stover using C. acetobutylicum P262. Similarly Marchal et al. (1984) used wheat straw hydrolysate and C. acetobutylicum, while Soni et al. (1982) used bagasse and rice straw hydrolysates and C. saccharoper — butylacetonicum to convert these agricultural wastes into butanol.

An important limitation of corn fiber utilization comes from the pretreatment and hydrolysis of corn fiber to glucose and xylose. Saccharification of corn fiber can readily be achieved by treatment with dilute H2SO4. However, this acid — catalyzed reaction leads to the degradation of glucose to hydroxy methyl furfural (HMF) and xylose to furfural at the temperatures of hydrolysis, resulting in inhibition of fermentation by these degradation products. Other degradation prod­ucts include syringaldehyde, acetic, ferulic, and glucuronic acids. The formation of these degradation products lowers the yield of fermentable sugars obtained from the corn fiber and the degradation products are inhibitory to yeast and bacterial fermentations. C. beijerinckii BA101 is able to completely utilize enzyme-hydrolyzed corn fiber to produce acetone-butanol, but performed poorly in the bioconversion of acid-hydrolyzed corn fiber to acetone-butanol due to the presence of inhibitory compounds generated during hydrolysis (Ebener et al., 2003). Therefore, the development of strains that can tolerate the inhibitory compounds generated during acid pretreatment and hydrolysis of corn fiber remains a priority.