The results presented here are not obvious. It is not only the local energy dissipation density the impeller makes within the culture what determines kLa. Additional to the dispersing shear the flow generated by the impellers in order to distribute the bubbles acrossthe entire culture is of importance. This is a possible reason for the fact that different impellers at the same power input often lead tothe same mass transfer rate although their shear is lower but theflow higher or vice versa.