Sophorolipids (SLs) are regarded as one of the most promising biosurfactants. However, high production costs are the main obstacle to extended SLs application. Semi-continuous fermentation, which is based on in-situ separation, is a promising technology for achieving high SLs productivity. In this study, the sedimentation mechanism of SLs was analyzed. The formation of a hydrophobic mixture of SLs and rapeseed oil was a key factor in sedimentation. And the hydrophobicity and density of the mixture determined SLs sedimentation rate. On this basis, ultrasonic enhanced sedimentation technology (UEST) was introduced, by which the sedimentation rates were increased by 46.9% to 485.4% with different ratio of rapeseed oil to SLs. UEST-assisted real-time in-situ separation and semi-continuous fermentation were performed. SLs productivity and yield were 2.15 g/L/h and 0.58 g/g, respectively, simultaneously the loss ratio of cells, glucose, and rapeseed oil were significantly reduced. This study provides the new horizon for optimization of the SLs fermentation process.

Sophorolipids (SLs) are regarded as one of the most promising biosurfactants. They have a low toxicity and are easily degradable without polluting the environment. However, high production costs are the main obstacle to extended SLs application. Semi-continuous fermentation is a promising technology for achieving high SLs productivity, which is based on in-situ separation. In this study, the sedimentation mechanism of SLs was analyzed. The formation of a hydrophobic mixture of SLs and oil was a key factor in sedimentation. The hydrophobicity and density of the mixture determined SLs sedimentation rate. According to the mechanism, ultrasonic enhanced sedimentation technology (UEST) was introduced, by which the sedimentation rates were increased by 46.9% to 485.4% with different Oil/SLs ratios. UEST-assisted real-time rational in-situ separation and semi-continuous fermentation were performed. We observed that SLs productivity and yield were 2.15 g/L and 0.58 g/g, whereas the loss ratio of cells, glucose, and oil was reduced by 68.2%, 16.2%, and 65.5%, respectively. In-situ SLs separation efficiency and rate were increased by 34.5% and 26.4%, respectively. This study provides the foundation and new horizon for the optimization of the SLs fermentation process.

Propanol have been widely used as a precursor for erythromycin synthesis in industrial production. However, the knowledge on the exact metabolic fate of propanol was still unclear. In the present study, the metabolic fate of propanol in industrial erythromycin-producing strain S. erythraea E3 was explored via 13C labeling experiments. An unexpected pathway in which propanol was channeled into tricarboxylic acid cycle was uncovered, resulting in uneconomic catabolism of propanol. By deleting the sucC gene, which encodes succinyl-CoA synthetase that catalyse a reaction in the unexpected propanol utilization pathway, a novel strain E3-ΔsucC was constructed. The strain E3-ΔsucC showed a significant enhancement in erythromycin production in the chemically defined medium compared to E3 (786.61 vs 392.94 mg/L). Isotopic dilution mass spectrometry metabolomics and isotopically nonstationary 13C metabolic flux analysis were employed to characterize the metabolic differences between S. erythraea E3 and E3-ΔsucC. The results showed that compared with the starting strain E3, the fluxes of pentose phosphate pathway in E3-△sucC increased by almost 200%. The most significant difference located in the tricarboxylic acid cycle was also found. The flux of the metabolic reaction catalyzed by succinyl-CoA synthetase in E3-ΔsucC was almost zero, while the glyoxylate bypass flux significantly increased. These new insights into the precursor utilization of antibiotic biosynthesis by rational metabolic engineering in S. erythraea provide the new vision in increasing industrial production of secondary metabolites.