Fermentation processes are exothermic as its products contain less energy than substrates. Theoretically, mass and energy yield of ABE fermentation is 37 and 94%, respectively calculated on the basis of energy of combustion and products ratio in the fermentation. During the study, it was suggested that yield of ABE fermentation might not be possible to meet its 100%, whereas product yield less than 25% can cause the economical unfeasibility even with any process development [50]. In the account of above fact, strain improvement may be a necessary step to enhance the theoretical yield. In this direction, many endeavors have been made to engineer the strain or transfer the gene in a heterogeneous host organism. But, to time none of genetically engineered strain produced higher yield than native organism [4]. However, the most valuable strain, C. beijerinckii BA101, has been developed through chemical mutagenesis from native organism, C. beijerinckii N-CIMB 8052 [80, 81]. C. beijerinckii BA101 can generate 19-20 g l-1 solvent, which is much higher than the native and other organisms [25, 40]. Through recent endeavors in process development for butanol production using C. beijerinckii BA101, improved solvent concentration (20-30 g l-1), solvent yield (0.30-0.50 g g-1), and reactor productivity (0.30-1.74 g l-1) have been achieved [64]. A high productivity of

15.8 g l-1 h-1 has also been achieved in immobilized reactor. Due to the high concentration of solvents, this organism leads ABE fermentation to be economical. An economic evaluation of ABE fermentation from corn using C. beijerinckii BA101 reported butanol cost of US$0.25 lb-1. The improvement in yield from 0.42 to 0.45 g g-1 resulted in lesser butanol cost of US$0.20 lb-1 [49].

Apart from the yield, other vital factor is feedstock in economics of ABE fermentation, it almost contributes to 60% of the total production cost of butanol [82, 83]. Utilization of none of starch and sugar-containing crops can make this fermentation economically feasible in the present scenario. Moreover the
continuous use of these food materials can cause the food shortage. On the basis of recent studies, cheaper agriculture biomass (lignocellulosic materials) and indus­trial wastes were found suitable for sustainable production of butanol. Still, efforts are being made for scaling-up the process for economical industrial production using lignocellulosic biomass.