Biorefining Mixed Sugars Using High Densities of Growth-arrested Corynebacteria

Research and development of renewable energies has recently regained prominence given anticipated shortages of fossil-fuel-based energies and parallel rising prices of fossil-derived fuels and chemicals. This changing economic landscape combines with environmental degradation to constitute powerful forces of change [36]. Particularly, the quantities of ethanol produced via biotechnological processes to displace petroleum as a transportation fuel are expected to dramatically increase in the coming years [8]. At present, the bulk of the industrial ethanol production is ensured via the fermentation of easily accessible sugars, for example derived from corn starch, sugar cane or sugar beet. The yeast Saccharomyces cerevisiae is the primary organism to ensure these fermentations [3]. However, the cost analysis of the existing production processes clearly demonstrates that their economic performances are tightly linked to the costs of fermentation raw materials [27], and thus indirectly to the amount –and limitation– of arable land that can be redirected for the production of these materials. Furthermore, the flattening experience-curve observed by industrial manufacturers suggests that radical innovation is needed to make bio-ethanol a competitive commodity compared to fossil fuels [35]. As a result, it becomes increasingly clear that the global scale implementation of ethanol as a primary transportation fuel will only become a reality once sugars can be extracted from abundant and easily accessible lignocellulosic biomass in a cost-effective manner [9, 26, 35]. However, current technological limitations of yeast-based processes include their inability to use the pentose xylose, a major component of typical lignocellulosic fractions, and their sensitivities to the presence of various growth inhibitors that are generated at the pre-treatment step of the saccharification process of lignocellulosic materials [16].