The economic competitiveness of bio-based production processes is often hindered by the high costs associated with downstream processing, compared to fossil-based methods. Liquid-liquid extraction is a widely used technique for aqueous fermentation systems and offers significant cost-saving potential, especially if extraction could be performed directly from the fermentation broth without prior cell separation. However, this is often hindered by the formation of "crud" — a deposit or emulsion at the interface between two partially settled phases. This study investigates the liquid-liquid phase separation of Corynebacterium glutamicum DM 1933 fermentation broths using five different solvents. We systematically examined the impact of cell surface properties, modified through nutrient concentration, on crud formation. In addition, the variation in salt concentration and pH after fermentation was analysed. Our findings show that the present nutrient concentration influences the cell surface properties and, consequently, crud formation. A more hydrophilic cell surface was present at a lower phosphate concentration, whereas a more hydrophobic cell surface was measured for a lower nitrogen and iron concentration. With a more hydrophobic cell surface, the fermentation broth showed a large crud phase, while a decrease in crud formation could be seen for fermentation broths with a more hydrophilic cell surface. Furthermore, the crud formation is influenced by the pH, cell and salt concentration and strongly by the used solvent.
Eberz et al. (Wed,) studied this question.