Process and metabolic engineering for enhanced propionic and acetic acid fermentations

The disposal of the byproducts, such as cheese whey, corn steep liquor and corn fiber is an urgent issue facing dairy industry and corn processor. In this study, a novel recycle batch, immobilized cell bioreactor was developed to use these low-grade byproducts for the production of propionic and acetic acids. Compared to the conventional fermentations, product yield and productivity from this immobilized cell bioreactor increased 69% and 209% in propionic acid fermentation with corn steep liquor as the substrate. Also, 65 g/L of propionic acid, which was the highest concentration ever reported in the propionic acid fermentation, and 75 g/L of acetic acid, were obtained from the immobilized cell bioreactor. The specific growth rate test indicated that cells adapted in the bioreactor had developed a better tolerance to the inhibiting products. It was also observed that immobilized cells gave a long-term stable performance.; The optimal product formation in propionic acid fermentation was evaluated based on a modified dicarboxylic acid pathway. The stoichiometric analysis results showed that Hexose Monophosphate Oxidation Pathway (HMP) could participate in glucose glycolysis in propionic acid fermentation. With an increasing percentage of HMP participation in glucose glycolysis, propionic acid yield increased from 0.31 g/g to 0.61 g/g and acetic acid yield decreased from 0.22 g/g to 0 g/g. These results indicated that acetate formation in propionic acid fermentation could be eliminated by reducing biomass formation and directing more glucose to HMP.; A metabolic engineering approach was taken to study the effect of acetate kinase (Ack), one of the key enzymes involved in acetic acid formation, on fermentation pattern. An ack deleted mutant was selected by disrupting ack, encoding acetate kinase in P. acidipropionici. The fermentation results with the ack deleted mutant demonstrated that acetate kinasse had a profound impact on cell growth and a minor effect on fermentation profile. Several possible pathways responsible for acetate formation in the ack deleted mutant were discussed.