Consolidated Bioprocessing of Lignocellulosic Feedstocks for Ethanol Fuel Production

Ethanol fuel can be produced renewably from numerous plant and waste materials, but harnessing the energy of lignocellulosic feedstocks has been particularly challenging in the development of this alternative fuel as a substitute for petroleum-based fuels. Consolidated bioprocessing has the potential to make the conversion of biomass to fuel an economical process by combining enzyme production, polysaccharide hydrolysis, and sugar fermentation into a single unit operation. This consolidation of steps takes advantage of the synergistic nature of enzyme systems but requires the use of one or a few organisms capable of producing highly efficient cellulolytic enzymes and fermenting most of the resulting sugars to ethanol with minimal byproduct formation while tolerating high levels of ethanol. In this review, conventional ethanol production, consolidated bioprocessing, and simultaneous saccharification and fermentation are described and compared. Several wild-type and genetically engineered microorganisms, including strains of Clostridium thermocellum, Saccharomyces cerevisiae, Klebsiella oxytoca, Escherichia coli, Flammulina velutipes, and Zymomonas mobilis, among others, are highlighted for their potential in consolidated bioprocessing. This review examines the favorable and undesirable qualities of these microorganisms and their enzyme systems, process engineering considerations for particular organisms, characteristics of cellulosomes, enzyme engineering strategies, progress in commercial development, and the impact of these topics on current and future research.