The biomass productivity of some fermentation processes is limited by the oxygen availability in the medium. Consequently, it is necessary to know the microorganism’s oxygen demand as well as the volumetric oxygen transfer rate of the fermenter. In this study, the oxygen transfer performance of two lab fermenters was evaluated from the volumetric oxygen transfer coefficient (KLa). Here the aeration system of these two fermenters was investigated by operating with two different impellers, i.e., the Rhuston turbine and the pitched-blade impeller, according to the stirring speed, the air flow rate, the broth-culture rheological properties, and the presence of antifoam agents in the medium. The experimental results show that the trends of KLa against stirring speed, obtained by using the two impellers, are different. Furthermore, the results highlight the strong lowering of KLa with increasing the viscosity of the medium. The formation of foam by using a typical rich medium, composed by peptone and yeast extract, reduces the transfer of oxygen. In these conditions the addition of antifoam into the medium is fundamental to improve the oxygen transfer and consequently the fermentation duration in the case of high agitation speed.