Measurements in model media (Sieblist et al., 2011b) showedthat the oxygen mass transfer in an aerated stirred tank reactormay depend on the number of impellers employed. At the samepower input, they found larger kLa values in an aerated stirredtank agitated with two impellers than in the same reactor equippedwith three impellers. This fact, not predicted by the standard cor-relations, has to be investigated in cultivation experiments. Forthat purpose fermentations were performed with both impellerarrangements.