Out-of-phase operating conditions, a hitherto unknown phenomenon in shaking bioreactors.

One of the important parameters in characterising fermentations of aerobic microorganisms is the specific power consumption. A new method has been introduced which enables the accurate determination of the power consumption in shaking bioreactors. It is based on torque measurements in the drive and the appropriate compensation of the friction losses. Measurements of the power consumption revealed the phenomenon of the liquid being 'out-of-phase' for the first time for shaking bioreactors. This occurs at certain operating conditions and is characterised by an increasing amount of liquid not following the rotating movement of the shaker table, thus reducing the specific power consumption, mixing and the gas/liquid mass transfer. With respect to this, different hydrodynamic cases have to be distinguished. All these cases have in common, however, that the probability of 'out-of-phase' conditions increases with lower shaking diameters, lower filling volumes, larger number and sizes of baffles and higher viscosity. For unbaffled flasks with a nominal volume </=1l the 'out-of-phase' phenomenon is described in the form of a newly defined non-dimensional Phase number (Ph). To avoid the (unidentified) development of a screening project in unfavourable directions or even its complete failure, researchers must be aware of the 'out-of-phase' phenomenon. The experimental protocols have to be carefully selected so that the occurrence of such unwanted hydrodynamic conditions is not possible under all experimental circumstances.