Microbial advanced biofuels production: overcoming emulsification challenges for large-scale operation

• Microbial production and secretion of isoprenoids and alkanes is emerging as an alternative for diesel and jet fuel replacements. However, for production to be feasible at a large scale, the process technology must be competitive.
• Under fermentation conditions, a multiphase mixture is formed and the product droplets can be stabilized by (i) surfactants and emulsifiers, (ii) Pickering stabilization, and/or (iii) fermentation medium properties. These mechanisms are also encountered in other multiphase fermentations where an organic phase is present as a substrate, a product, and/or an auxiliary phase.
• Separation of the product droplets as a continuous oil phase may require the droplet size to be increased, the creaming rate to be enhanced, and/or emulsions to be destabilized. Several process technology alternatives addressing these aspects exist and have been widely applied on a large scale in fields such as petroleum engineering and wastewater treatment, but have not been fully exploited in bioprocessing to date.

Isoprenoids and alkanes produced and secreted by microorganisms are emerging as an alternative biofuel for diesel and jet fuel replacements. In a similar way as for other bioprocesses comprising an organic liquid phase, the presence of microorganisms, medium composition, and process conditions may result in emulsion formation during fermentation, hindering product recovery. At the same time, a low-cost production process overcoming this challenge is required to make these advanced biofuels a feasible alternative. We review the main mechanisms and causes of emulsion formation during fermentation, because a better understanding on the microscale can give insights into how to improve large-scale processes and the process technology options that can address these challenges.