Optimization of direct bioconversion of cellulose into biofuels: medium improvement, scale-up and use of alternative nutrients
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Despite the long-term economic and environmental benefits of cellulosic biofuel production, low rates of cellulose utilization and products syntheses are major techno-economical barriers to the commercialization. Optimized medium composition and low-cost nutrient source could greatly enhance the feasibility of large-scale biofuels synthesis by direct cellulose fermentation using a consolidated bioprocessing (CBP) approach. This study developed an improved growth medium for Clostridium thermocellum, an excellent cadidate for CBP that utilizes cellulose to produce ethanol, hydrogen, and other value-added biochemicals. An experimental design to determine the importance of nutrient components and concentrations on H2 and ethanol production from cellulose by C. thermocellum initially considered seven growth nutrients. Three most significant components - α-cellulose, yeast extract, and magnesium chloride were investigated in detail for their influence on rates and yields of H2 and ethanol production during cellulose fermentation by C. thermocellum. To explore individual and interactive effects of these nutrients on ethanol and hydrogen (H2) production, a central composite face-centered design and the response surface methodology was applied to predict optimum nutrient compositions for H2 and ethanol production. Experimental verification of predicted optima produced about 3-fold and 4-fold more H2 and ethanol respectively compared with the reference medium. These small-scale results were successfully verified in large-volume (7L), atmospheric cultures. Irrespective of culture conditions, relative improvement in rates and productivities of H2 and ethanol in optimized medium compared with reference medium were consistent with small-volume cultures. Various ethanol distillery co-products were tested for their potentials to replace expensive medium ingredients. Medium prepared with these co-products show excellent ability to suppport cell-growth and production of ethanol and H2 at concentrations equivalent to those generated from the reagent grade medium. Utilization of these low-cost nutrient sources to replace expensive reagent ingredients may potentially contribute to the viability of both grain-based ethanol and cellulosic biofuels. With medium optimization, scale-up and use of low-cost nutrient sources, this study represents one of the very few systematic research approaches to improve direct bioconversion of cellulosic biomass into biofuels.