Abstract
In lieu of rising crude oil prices, exhaustion of petroleum feed stocks and environmental challenges, only renewable fuels have the potential to match the energy requirements of the future. Among the various renewable fuels, butanol has recently gained a lot of attention because of its advantages over other biofuels. Its microbial production by clostridia through ABE fermentation is being explored for improved yield and cost effectiveness. Using lignocellulosic wastes successfully for butanol production through ABE fermentation is a major breakthrough to deal with the future energy crisis. Genetic engineering of microbes to increase the carbon and redox balance, cell recycling, media optimization, mathematical modelling and tolerance improvement strategies are being attempted to overcome the hurdles of high production cost, by products formation leading to low yield and product toxicity. Along with genetic engineering major research is cantered on heterologous host engineering for improved butanol production and tolerance. This review highlights the recent advances in improving yield and tolerance to butanol in both Clostridial and heterologous hosts from genetic engineering and fermentation methodology aspects.
 Int. J. Appl. Sci. Biotechnol. Vol 7(2): 130-152
Highlights
Increasing crude oil prices and awareness about the finite life span of fossil fuels have resulted in increased demand of renewable fuels that can be derived from sustainable resources
ABE fermentation based on corn cob residues (CCR) treated with cellulose (48 FPU/g at pH 4.8) followed by Lime treatment resulted in production of 16.8g/L ABE with 8.2g/L butanol (Zhang et al, 2012)
Acid/Alkali Pre-Treatment and Enzymatic Hydrolysis Barley straw pre-treated with 1% H2SO4 (v/w) followed by enzymatic hydrolysis based butanol fermentation by Clostridium beijerinckii P260 produced 7.09 g/L ABE while barley straw hydrolysate (BSH) treated with lime prior to fermentation led to 26.64g/L of ABE and 18.01g/L butanol (i) (Qureshi et al, 2010)
Summary
Increasing crude oil prices and awareness about the finite life span of fossil fuels have resulted in increased demand of renewable fuels that can be derived from sustainable resources. D) Formation of byproducts as acetone and ethanol, leading to costly downstream processing making the process economically less preferable All these limitations have led to renewed interest of the researchers in improving the yield of butanol by cost cutting of the fermentation process (either by improving the efficiency of fermentation process, manipulations in the native Clostridium sp., exploration of renewable and economical substrate and engineering a new potential microbial host for butanol production). Though underestimated or misinterpreted as “ generation biofuel” butanol has been produced since decades both as a by-product along with acetone as well as major fermentation product and is being used as very important industrial solvent Today it is coming out as more potential fuel and solvent over the existing ones (Schwarz et al, 2006). Removal of these inhibitors by evaporation, lime treatment, XAD resin treatment and charcoal adsorption etc. have been successfully employed (Lutke-Eversloh et al, 2011; Bharathiraja et al, 2017; Silva et al, 2013). (Table 1)
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