Efficient in-situ separation design for long-term sophorolipids fermentation with high productivity

Until now, the production of microbial biosurfactants has been limited to the use of first-generation (food) substrates. Moreover, the results on sophorolipids yield and productivity reported by state-of-the-art fed-batch processes for sophorolipids production remain unsatisfactory and lead to a significantly high cost per kg of the product. With an aim to address these problems, this study demonstrates a long-term and high-efficiency sophorolipids production method using a semi-continuous integrated production-separation system which utilizes food waste as a substrate. An average volumetric productivity of 2.43 g L⁻¹ h ⁻¹ and the overall sophorolipids yield on the substrate (i.e. the combination of glucose and oleic acid as carbon source) of 0.73 g g⁻¹ was achieved within 240 h by fed-batch and separation fermentation. Moreover, the potential of sustaining high production efficiency during long-term fermentation times (480 h) was investigated. No reduction in process efficiency was observed, i.e. average volumetric productivity and an overall sophorolipids yield of 2.39 g L⁻¹ h ⁻¹ and 0.73 g g⁻¹ were obtained, respectively. Notably, a very high biomass concentration of 117.2 g L⁻¹ was observed in fed-batch fermentation and separation, which is the highest cell density ever reported for Starmerella bombicola. The separation efficiency was also calculated to evaluate the in-situ separation performance of the developed process quantitatively. In 480-h fermentation, the average separation efficiency of 74.3% and overall separation efficiency of 93.0% was achieved after six separation cycles, suggesting that both separation design and process control were successful in laboratory-scale fermentation. These findings demonstrate the potential of the developed sophorolipids fermentation-separation system for application at industrial scale.