By means of improved feedback control kLa measurements become possible at a precision and reproducibility
that now allow a closer look at the influences of power input and aeration rate on the
oxygen mass transfer. These measurements are performed online during running fermentations without
a notable impact on the biochemical conversion processes. A closer inspection ofthe mass transfer during
cultivations showed that at least the number of impellers influences mass transfer and mixing: On the
laboratory scale, two hollow blade impellers clearly showed a larger kLa than the usually employed three
impeller versions when operated at the same agitation power and aeration rate. Hollow blade impellers
are preferable under most operational conditions because of their perfect gas handling capacity. Mixing
time studies showed that these two impeller systems are also preferable with respect to mixing. Furthermore
the widths of the baffle bars depict a significant influence on the kLa. All this clearly supports the
fact that it is not only the integral power density that finally determines kLa.