Increased total air pressure versus oxygen limitation for enhanced oxygen transfer and product formation in a Pichia pastoris recombinant protein process

Two strategies to increase the productivity of secreted Thai Rosewood β-glucosidase in Pichia pastoris processes were evaluated. Both methods were based on increasing the oxygen transfer rate (OTR) in the process by simple means. Increasing the driving force for the diffusion from the air bubbles to the medium by elevating the air pressure, from 1.2 to 1.9 bar increased the oxygen uptake rate (OUR) by 59% while increasing the driving force by accepting oxygen limitation increased the OUR by 35%. The OTR increased less than in proportion to the increased solubility in the high-pressure process, which indicates that air bubble compression reduces the volumetric oxygen transfer coefficient (KLa). Even though the methanol consumption increased almost in proportion to the OTR in both processes the biomass production did not increase as much. This is explained as a higher maintenance demand for methanol in the oxygen limited (0.027 g g−1 g−1) and high-pressure processes (0.035 g g−1 g−1), compared to 0.022 g g−1 g−1 in the methanol limited reference process. However, in spite of the low effect of increasing OTR on the biomass production the total β-glucosidase yield increased almost in proportion to the increased methanol consumption and reached highest value in the high-pressure process, while the β-glucosidase purity was highest in the oxygen-limited process due to release of less contaminating proteins.