It is demonstrated that bioreactor construction is of practical importance to some recombinant protein production processes. Here we focus on a case where the oxygen mass transfer capability of the bioreactor depicts a limitation to productivity that must be overcome by technical measures. Choosing different impellers can significantly enhance the mass transfer capability of the bioreactor. This is shown through the concrete example of cultivation of Pichia pastoris on methanol. In order to operate the bioreactor at biomass concentrations larger than 150 g kg (-1) and specific growth rates larger than 0.03 h(-1), it is usually necessary to supplement the aeration with molecular oxygen. When, however, as is shown here, the standard Rushton turbines in the bioreactor are replaced by a set of lower power number impellers with improved air handling capabilities it is possible to operate the reactor at even more demanding growth and biomass conditions without additional oxygen while achieving even higher productivities. Here we took a combination of two hollow blades and a single hydrofoil impeller in order to enhance the mass transfer rates. Its advantage is shown at the 10-litre laboratory-scale with a Pichia: pastoris culture that produces the recombinant protein dipeptidylpeptidase IV. The enzyme serves as a target for the development of specific inhibitors to control blood glucose homeostasis. Optimization of the fermentation process and higher enzyme yields allow inhibitor evaluation and co-crystallization studies.