Corn steep water (CSW) is a by-product of the corn wet-milling industry and contains large amounts of substances derived from the fermentative conversion of carbohydrates, proteins, and lipids during corn steeping. Currently, CSW is evaporated to 50% solids and marketed primarily as an economical livestock feed supplement in the cattle industry. CSW is a rich complement of important nutri­ents such as nitrogen, amino acids, vitamins, and minerals and was proposed to be a good substitute for yeast extract (Hull et al., 1996). This finding is important as it impacts the economics of butanol production from corn.

An economic analysis performed by Qureshi and Blaschek (2000a), demon­strated that the fermentation substrate was one of the most important factors that influenced the price of butanol. Development of a cost-effective biomass-to- butanol process can only be commercially viable if cheaper commercial substrate such as liquefied corn starch and CSW can be used in combination with toxic product removal by gas stripping (Ezeji et al., 2005). It is interesting to note that C. beijerinckii BA101 when grown on liquefied corn starch-CSW medium pro­duced levels of acetone-butanol equal to or higher than the levels produced when grown on glucose-based yeast extract medium (Ezeji et al., 2005). The fermen­tation time for liquefied corn starch — and saccharified liquefied corn starch-CSW media were 120 and 78 h, respectively, while the fermentation time for glucose — based yeast extract medium was 68 h. The presence of sodium metabisulfite (Na2S2O5; a preservative) in the liquefied starch and CSW was found to result in inhibition of C. beijerinckii BA101 and also may have affected the secretion of amylolytic enzymes by the culture, which is necessary for efficient hydrolysis and utilization of starch and oligosaccharides. However, it appears that the use of CSW has a great potential for the bioconversion of corn to acetone-butanol. The presence of Na2S2O5 in the CSW may be a major problem in a long-term fermentation by C. beijerinckii BA101. During a long-term fermentation using CSW and C. beijerinckii BA101, removal of Na2S2O5 from CSW by oxidation is recommended (Ezeji et al., 2005).