MANIPULATING THE PRODUCTION OF SOLVENTS VERSUS 1,3-PROPANEDIOL IN C. PASTEURIANUM FERMENTATIONS USING REDOX BALANCE AND PH STRATEGIES

摘要

The current study was performed at the 5L scale in support of the development of a commercial process to convert crude glycerol generated in biodiesel production to the products 1,3-propanediol and the solvents butanol and ethanol by fermentation with Clostridium pasteurianum. We have clearly shown that by controlling the fermentation pH in the range of 4.7 to 5.9, the CO_2 production rate can be manipulated such that the flux of carbon through the pathways producing 1,3-propanediol, butanol and ethanol can be increased or decreased. This has been shown to be a direct response to alternative mechanisms for balancing cellular redox potential. The biochemical pathways leading to CO_2 and energy production in C. pasteurianum convert nicotinamide adenine dinucleotide (NAD~+) to the reduced form (NADH_2), thus transferring electrons. Pathways that can regenerate NAD~+ for continued cellular energy production are limited and include solvent production (butanol and ethanol) or 1,3-propanediol. An increase in fermentation pH resulted in an increase in the specific cell growth rate and thus the CO_2 production rate. Consequently the pathway leading to 1,3-propanediol was preferred, as shown by increased 1,3-propanediol production, presumably due to its simplicity which accommodated the higher rate of NAD~+ regeneration. The effect on cellular electron transfer was also confirmed by the redox potential signal during these fermentations. The application of a 'staged' pH program was used to optimize cell growth,1,3-propanediol and solvent production in batch studies. The knowledge gained in batch studies was the basis for designing a continuous fermentation process utilizing both pH and redox strategies applied to crude glycerol conversion to the desired products, while minimizing crude glycerol pre-treatment requirements.