About 77.1 million metric tons of wheat straw is produced in the United States annually, and this quantity has the potential to produce 54 million metric tons of fermentable sugars (http:// www. biofuelscenter. org). Wheat straw, which is composed of 67% of cellulose and hemicel — lulose (Table 3.1), can serve as a cheap substrate for butanol production by solventogenic Clostridium species. Qureshi et al. (2007, 2008b, c) have pioneered research on potential use of wheat straw as substrate for ABE production. During one investigation, five different processes were evaluated for ABE from wheat straw by Clostridium beijerinckii P260 (Qureshi et al. 2008b). The five processes were fermentation of pretreated wheat straw (Process I), separate hydrolysis and fermentation of wheat straw to ABE without removing sediments (Process II), simultaneous hydrolysis and fermentation of wheat straw without agitation (Process III), simultaneous hydrolysis and fermentation with additional sugar sup­plementation (Process IV), and simultaneous hydrolysis and fermentation with butanol recov­ery by gas stripping (Process V). In these studies, process IV and V achieved maximum ABE concentrations of 17.92 g/L and 22.42 g/L, resulting in ABE productivities of 0.19 and 0.31 g/L/h, respectively. In the control experiment (glucose), an ABE productivity of 0.30 g/L/h was achieved (Qureshi et al. 2008b).

To improve C. beijerinckii 260 tolerance to toxic compounds generated during pretreat­ment and hydrolysis of wheat straw biomass and enhance fermentability of acid pretreated wheat straw, Qureshi et al. (2008d) used alkaline peroxide and electrodialysis treatment methods to detoxify acid pretreated wheat straw hydrolysates prior to ABE fermentation. A maximum ABE concentration of 22.17g/L with reactor productivity of 0.55 g/L/h was pro­duced, compared to ABE concentration of 21.37 g/L with reactor productivity of 0.30 g/L/h produced by the control (glucose). To improve ABE productivity further, Qureshi et al. (2008c) used C. beijerinckii P260 to produce butanol from wheat straw hydrolysates in a fed-batch reactor with simultaneous ABE recovery by gas stripping. A total of 192.0g ABE was produced in a 2.5 L bioreactor (1 L reaction volume) at the end of fermentation, resulting in an ABE productivity of 0.77g/L/h and ABE yield of 0.44. Simultaneous hydrolysis and fermentation of wheat straw to butanol and/or fed-batch fermentation of wheat straw hydro­lysates and simultaneous butanol recovery by gas stripping are, therefore, attractive options with great potential of replacing glucose for butanol production.

Conclusion

With continuous world population growth and energy demand, current production of liquid biofuel (ethanol) from food crops such as corn and sugar cane is unsustainable. The use of agricultural residues to produce liquid biofuels—(n particular, the production of butanol— holds great interest as a means for generating sustainable transportation fuel and feedstock chemicals. The possibility of using agricultural residues such as corn fiber, corn stover, wheat straw, as well as energy crops such as switchgrass, Miscanthus, and so on for butanol production and incorporating gas stripping in an in-line product recovery process has gener­ated considerable interest. Simultaneous butanol fermentation and recovery has dramatically improved production of butanol from wheat straw. Considerable progress has been made in the development of biomass pretreatment and hydrolysis technologies. Progress has also been made on the improvement of some technologies that were previously developed so that use is more environmentally compatible and generating little or no lignocellulosic deg­radation compounds that are toxic to fermenting microorganisms. Availability of efficient biomass pretreatment technology will help in utilizing bio-based conversion of agricultural residues into value-added products to become attractive. By employing in-line recovery systems during butanol fermentation, substrate inhibition and butanol toxicity to the culture are drastically reduced, with ABE productivity dramatically increased. Given that butanol is an excellent potential fuel and the United States is rich in lignocellulosic biomass, butanol production from agricultural residues is crucial in future energy production endeavors.

Acknowledgm ents

This work was supported by funding from Northeast Sungrant (Cornell University) Award/

Contract # GRT00012344, National Research Initiative of the USDA Cooperative State

Research, Education and Extension Service, grant number 2006-35504-17419, and Seed grant

from Ohio Agricultural Research and Development Center (OARDC), Wooster.

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