Minimally-disruptive method to estimate the volumetric oxygen mass transfer coefficient (kLa) for recombinant Escherichia coli fermentations

In the biopharmaceutical industry, accurate prediction of the oxygen uptake rate (OUR) is critical to understanding cell health, where OUR is related to the oxygen transfer rate (OTR) and the culture dissolved oxygen (DO). Key to accurate OTR assessment is an accurate volumetric oxygen mass transfer coefficient (kLa) estimate, where kLa represents the oxygen driving force from gas to liquid phase. Common approaches to estimate kLa have significant limitations, such as disruptive to the culture by stopping the oxygen supply or the use of only cell-free buffered solution. Yet, it is well-known that cell secretions and additions (i.e., base and antifoam) can dramatically can affect kLa, and accumulate during the culture. This study describes a novel, minimally-disruptive method to estimate kLa by halving the gas flow rate periodically throughout the fermentation. This approach was used to estimate kLa at multiple times for two recombinant Escherichia coli strains cultured in ambr250 modular vessels. In one case, the duration of the halved gas flow was varied. In the second case, the effects of only cell secretions was examined. As the oxygen supply was only halved, the risk of culture loss was significantly lower compared to the dynamic gas-out method.