Direct microbial conversion
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Process development is reviewed for ethanol production from cellulosic biomass via direct microbial conversion (DMC). Experimental data addressing cellulase production and ethanol tolerance are also presented for the candidate DMC organismsClostridium thermocellum andClostridium thermosaccharolyticum. Two potential paths are identified for obtaining organisms for use in DMC. Path 1 involves modification of excellent ethanol producers, so that they also become good cellulase producers; Path 2 involves modification of excellent cellulase producers, so that they also become good ethanol producers. Cellulase production, ethanol tolerance, and ethanol selectivity are considered for both Path 1 and Path 2 organisms. It is concluded thatin situ cellulase production has the potential to allow cost reductions relative to state-of-the-art process designs on the order of 50¢/gal. Based on the data available, the value of cellulase production bythermocellum corresponds to 90% of this amount. However, each process path has a strategic obstacle to be overcome: high-level cellulase expression and secretion for Path 1, and high ethanol selectivity for Path 2. Ethanol tolerance is not seen as a primary factor in choosing between DMC and other ethanol process alternatives.