4. Discussion
SLs has been regarded as a green and sustainable biosurfactants, but their high production costs limit large-scale application and development. [21-23]Coupling fermentation on the basis of in-situ product separation is one of the most effective ways to reduce production costs and improve productivity (Table 2). Wang designed a simple in-situ separation device coupled with the bioreactor and adopted high cell density fermentation to achieve high-efficient production of SLs. [24] After running for 480 h, the productivity and yield of SLs were 2.39 g/L/h and 0.73 g/g, respectively, with an average separation efficiency of 74.3%. Zhang developed a new bioreactor with dual ventilation pipes containing dual sieve plates (DVDSB) for semi-continuous fermentation of SLs.[15] By regulating the air pressure and different sources of oil (transgenic or non-transgenic oil), the gravity sedimentation of SLs at the bottom of bioreactor could be accomplished with SLs titer, productivity, and yield of 477 g/L, 1.59 g/L/h and 0.60 g/g, respectively. Otherwise, Dolman suggested that the glucose concentration could adjust the density of fermentation broth thereby controlling the ups and downs of SLs. [25] The whole fermentation process lasted 1023 h to produce 623 g/L SLs, and during the SLsin-situ separation process, the recovery rate of SLs was as high as 86.0%, which was 9 times concentration of previously reported. Apart from SLs separation by gravity sedimentation, Liu realized the SLs separation in the upper layer of the broth through the principle of froth flotation. Subsequently, part of biomass and glucose could be recovered by washing the settled SLs, and the final titer and productivity of SLs were 342 g/L and 1.55 g/L/h for 221 h, respectively.[20] Though these strategies have achieved relatively satisfactory results in the in-situ separation of SLs, the mechanism of SLs separation either by gravity sedimentation or by froth flotation was still not been clearly elucidated, which led to a blind separation process and low separation efficiency. Oil or glucose concentration has been pointed out to possess a significant impact on the settlement of SLs. [15] However, these researches focused more on the final results and failed to further analyze the causes of the sedimentation, resulting in big differences in different studies.
Herein, the sedimentation mechanism of SLs was discussed and it was found that the O/S ratio was the key factor affecting the morphology and subsequent settling of SLs. In terms of sedimentation rate, dependent on the particle size of mixture and density difference. To improve the efficiency of SLs separation and reduce the loss of substances during in-situ separation process, low glucose concentration and appropriate O/S ratio should be adopted. On the other hand, the UEST can accelerate the particle collision, make the particle become larger, eliminate the air bubbles, and further reduces the settling resistance.[26] Palme and Hincapié Gómez applied ultrasonic enhanced deposition technology in the separation and recovery of microalgae and yeast cells, respectively. [16, 18]In this work, UEST was introduced to enhance the sedimentation of SLs, simultaneously greatly reduce the losses of rapeseed oil, glucose, and biomass duringin-situ separation process. Moreover, semi-continuous fermentation by in-situ separation of SLs with UEST was conducted. Compared to the batch fermentation (168 h), the fermentation cycle was more than 2-fold longer (378 h), and the average productivity and yield from semi-continuous fermentation were increased by 26.5% and 23.4% respectively. This could be contributed to that the semi-continuous fermentation could separate out SLs and toxic substances in time, and allow the cells being in the optimal production environment, which also proved the feasibility of semi-continuous fermentation as a model for the efficient production of SLs. In comparison with other semi-continuous fermentations of SLs, both the productivity and yield in this work were at high levels, especially for the index of YSLs/DCW representing the SLs production yield to the DCW, its value reached 36.1 g/gDCW, which was 86.1% higher than the highest value in the literatures. Furthermore, the specific SLs productivity (QP/DCW) also exhibited a 41.2% higher than the highest value ever reported. It could be expected to achieve a more ideal production efficiency in combination with high cell density strategy, laying a foundation for further industrial application.