Glucose overflow metabolism and mixed-acid fermentation in aerobic large-scale fed-batch processes with Escherichia coli

Industrial 20-m(3)-scale and laboratory-scale aerobic fed-batch processes w ith Eschelichia coli were compared. In the large-scale process the observed overall biomass yield was reduced by 12% at a cell density of 33 g/l and f ormate accumulated to 50 mg/l during the later constant-feeding stage of th e process. Though the dissolved oxygen signal did not show any oxygen limit ation, it is proposed that the lowered yield and the formate accumulation a re caused by mixed-acid fermentation in local zones where a high glucose co ncentration induced oxygen limitation. The hypothesis was further investiga ted in a scale-down reactor with a controlled oxygen-limitation compartment . In this scale-down reactor similar results were obtained: i.e. an observe d yield lowered by 12% and formate accumulation to 238 mg/l. The dynamics o f glucose uptake and mixed-acid product formation (acetate, formate, D-lact ate, succinate and ethanol) were investigated within the 54 s of passage ti me through the oxygen-limited compartment. Of these, all except succinate a nd ethanol were formed; however, the products were re-assimilated in the ox ygen-sufficient reactor compartment. Formate was less readily assimilated, which accounts for its accumulation. The total volume of the induced-oxygen -limited zones was estimated to be 10% of the whole liquid volume in the la rge bioreactor. It is also suggested that repeated excretion and re-assimil ation of mixed-acid products contribute to the reduced yield during scale-u p and that formate analysis is useful for detecting local oxygen deficiency in large-scale E. coli processes.