The liquid-liquid extraction for product separation from fermenter broths, which still contain the cells, is very advantageous. This whole broth extraction has hydrodynamic problems, if the broth is very viscous. In a gravity extractor the drop sedimentation and the drop separation is disturbed, if the solvent is dispersed into the broth. If the broth is dispersed, an unusable entrainment of the solvent occurs, when the broth drops coalesce. For the quantification of these effects some experimental methods were developed. The sedimentation behaviour of single drops can be calculated with the aid of single drop investigations and with a falling sphere apparatus. The sedimentation velocity and the coalescence behaviour of drop swarms can be detected with a special solvent drop column. The entrainment that occurs at the coalescence of broth drops can be measured with a special broth drop column. With the informations received, it is possible to decide, whether the specific fermenter broth can be processed by a gravity extractor and to choose a suitable extractor design. The methods give the information to find out whether the solvent or the broth has to be the dispersed phase.