Recent Developments in Pretreatment and Enzymatic Hydrolysis for Cellulosic Bioethanol Production

Second-generation biofuels like bioethanol from lignocelluloses instead of sugar or starch have demonstrated environmental and economic benefits. Bioethanol is conventionally produced from sugar through a two-step saccharification and fermentation, primarily by yeasts (like Saccharomyces cerevisiae) and bacteria (like Zymomonas mobilis, Escherichia coli and Klebsiella oxytoca) either separately or simultaneously. Pretreatment of the substrates reduces the size, breaks down the fibres, swells and softens the biomass, reduces the crystallinity, solubilises hemicellulose and/or removes lignin or has a combined effect of any of these, with a major objective to enhance the available surface area for enzymatic hydrolysis. Pretreatments could be physical (microwave and steam explosion, ultrasound, mechanical and extrusion), chemical (acids, alkalis, ozone, organosolvs and ionic liquids) and biological (cellulases, hemicellulases and other accessory enzyme-based measures). Physicochemical pretreatments and combined physical and chemical measures involve ammonia fibre explosion, ammonia recycle percolation, wet oxidation, CO2 explosion, etc. A combination of physicochemical followed by biological pretreatment (hydrolysis by cellulases, hemicellulases and accessory enzymes) is reportedly the most effective strategy for high sugar yields that would further facilitate the net bioethanol conversion potential. This write-up is an effort to put in perspective the technological breakthroughs and recent advancements in bioethanol production for energy self-sufficiency from sustainable/renewable sources.