Production of isobutanol by fermentation is looking attractive owing to two main reasons: (1) the higher tolerance of producing microorganisms to isobutanol, usually 36.9—51.9 g/l as compared to n-butanol (called butanol in the later sections of this chapter), which is 20—30 g/l in selected hyper-producing strains and (2) having a lower boiling point (108 °C vs 118 °C) than butanol, which comparatively may be economical to recover. The above titer values for the isobutanol are without simultaneous product recovery (Baez et al., 2011). However, in this report, it was mentioned that in situ recovery by gas stripping improves isobutanol production. To the authors’ knowledge, this is the only report where isobutanol fermentation and recovery were performed simultaneously.

Fermentative production of isobutanol or butanol can be economically achieved in two ways: (1) by developing the high-tolerant or high-producer strain, which also offers some benefits during the recovery process, and (2) using energy-efficient process engi­neering techniques to simultaneously remove the toxic product. Interestingly, the first approach has been reported for isobutanol fermentation (Baez et al., 2011) with great success. For butanol producing strains, numerous attempts have been made to improve perfor­mance; however, success has been limited, with maximum titer stagnating around 21 g/l (total acetone butanol ethanol (ABE) 32.6 g/l) (Chen and Blaschek,

1999) . Nevertheless, butanol has drawn significant amount of attention from the process engineering point of view. One of the main focuses has been the develop­ment of integrated process technologies where fermen­tation and simultaneous product recovery have been integrated. The reader is directed to a couple of following reports, where much higher production of ABE than in a batch system has been achieved. Employing such inte­grated systems (gas stripping and perstraction), cumula­tive ABE production from 461 g/l to 698 g/l (Ezeji et al., 2013; Jeon and Lee, 1989) has been achieved compared to 21 g/l butanol or 51.9 g/l isobutanol. It should also be noted that several simultaneous product recovery systems such as adsorption, liquid—liquid extraction, pervaporation, ionic liquid extraction, and reverse osmosis have been investigated (Qureshi et al., 2013). Also, other advances have been made for butanol pro­duction from agricultural residues such as wheat straw (Qureshi et al., 2007; Qureshi et al., 2008a), barley straw (Qureshi et al., 2010a), corn stover (Parekh et al., 1988; Qureshi et al., 2010b), switchgrass (Qureshi et al., 2010b), and distillers dry grains and soluble (Ezeji and Blaschek, 2008).

CONCLUSION AND
FUTURE PERSPECTIVE

Given the higher microbial tolerance of isobutanol and its greater volatility in comparison to butanol, it is likely that simultaneous product recovery using gas stripping, perstraction, and/or pervaporation would achieve even higher production levels than reported for butanol, thus benefiting economics of isobutanol production process. To make this biofuel even more attractive, recent advances in fermentation of lignocellu — losic biomass such as separate hydrolysis of ligno — cellulosic biomass, combined with fermentation, and product recovery separate hydrolysis, fermentation, and recovery (SHFR), and simultaneous saccharification, fermentation, and recovery (SSFR) should be applied (Qureshi et al., 2013). In a process where wheat straw was used to produce butanol by SSFR, 192 g/l ABE was produced from 430 g/l of lignocellulosic sugars (Qureshi et al., 2008b). At this stage, engineering producing micro­organisms by applying similar method already reported by Higashide et al. (2011) to utilize pentose sugars such as arabinose and xylose as substrates for the production of isobutanol is looking promising.

Owing to the fact that butanol producing cultures have the potential to be tolerant to isobutanol, it is reasonable to express isobutanol-producing genes in sol — ventogenic Clostridium species. Such an undertaking would have two advantages: (1) ability of the developed strain to utilize pentose sugars that accounts for about 30—40% of carbohydrates present in lignocellulosic biomass and (2) the developed strain may produce higher titers of isobutanol than yeast or E. coli.