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.