Emulsification challenges in the microbial production of advanced biofuels

The extracellular microbial production of isoprenoids and alkanes by engineered microorganisms is emerging as an alternative for conventional diesel and jet fuel. However, the production costs are currently too high to make these biofuels an economically feasible alternative. One of the factors contributing to the high production costs is the difficult downstream processing of the formed dispersion, consisting of immiscible biofuel droplets dispersed in the fermentation mixture. The density difference between the two phases can be used for separation, but the oil droplets must be large enough to be affected by gravitational forces. Furthermore, the droplets have to coalesce to form a continuous oil phase.

Components present in the fermentation mixture can stabilize the droplets and cause emulsion formation, for instance by preventing coalescence or decreasing droplet mobility. These components can be grouped into three classes: biosurfactants, bioemulsifiers, and surface active particles. Examples such components are glycolipids or lipoproteins released by cells, cells that have a partial hydrophobic surface, or substrate components that increase the viscosity of the fermentation mixture. Because the stabilizing mechanisms vary per class of stabilizing component, the required process technology solutions to overcome the difficulties in product recovery vary with the stabilizing component. Identifying the main mechanisms and causes for emulsion formation during fermentative processes results in a better understanding of the processes on the micro scale, which provides insight in the downstream processing challenges in the large scale process can be addressed.