1. Introduction
Sophorolipids
(SLs), which are regarded as one of the most promising biosurfactants,
have been widely used in cosmetic, food, pharmaceutical fields due to
their low toxicity and high biodegradability. [1,
2]. Many microorganisms can synthesize SLs, includingCandida ,Pichia , andRhodotorula .[3-5]Candida bombicola is the most widely adopted specie due to its
high efficiency of SLs production. [6]SLs
are the mixtures of many structural derivatives consisting of two parts,
a hydrophilic sophorose and a hydrophobic hydroxyl fatty acid with
general 16-18 carbon atoms. [7] The differences of
SLs structures are dependent on the degree of acetylation, the length of
carbon chain, the unsaturation of fatty acids, and internal
esterification, which further determined the properties of SLs.[8-10] Therefore, SLs fermentation process is a
complex multiphase system of gas, liquid, and solid, not only containing
SLs with different structural properties (solid crystallization, oily,
etc.), but also possessing bubbles, lipids, glucose, cells, and other
substances in the fermentation broth. This poses a great challenge to
the effective regulation and optimization of the fermentation process.
Biosurfactants
have
the
advantages of green production and environmental friendliness, compared
with chemical surfactants. However, the high production cost of SLs is
still the main factor that hinders their development and application.[11-12]Therefore, it is necessary to develop adequate process control
strategies to improve production
efficiency.
To date, great progress has been made on SLs optimization in batch
fermentations. However, the raise of SLs concentration during
fermentation process leads to a significant increase in viscosity, which
further results in the difficulty in mixing and mass transfer, and
finally causing a sharp decline in fermentation efficiency.[13] Semi-continuous fermentation basing onin-situ separation of products can segregate part of SLs from the
fermentation broth, thereby improving the rheological characteristics of
broth and maintaining efficient synthesis of SLs. Different types of
devices have been developed to improve the efficiency of in-situseparation. [14] In this regard, gravity
sedimentation is the main method for in-situ separation of SLs.
Zhang found that the density of the fermentation broth could be adjusted
by controlling the glucose concentration, so as to achieve the
sedimentation of SLs. [15] Subsequently,
genetically modified soybean oil was used to achieve the sedimentation
and separation of SLs.
Although
these studies have achieved good results, the mechanism of SLs
sedimentation in a heterogeneous fermentation system has not yet been
clearly elucidated, thus it is difficult to rationally control SLs
sedimentation and separation.
Generally, a large number of cells
is lost during SLs in-situ separation process, so the efficiency
of subsequent SLs synthesis will be significantly affected. Moreover,
SLs have a strong affinity with oil, which also leads to the loss of
oil, thereby reducing the substrate conversion rate. On the other hand,
the enhancement of separation efficiency also contributes to the
efficient production of SLs.
Ultrasonic
enhanced sedimentation technology (UEST) consists of the use of particle
agglomeration by
acoustophoresis,
which causes increased gravitational force from larger effective
particle size to increase the
sedimentation
rate. UEST has been applied in improving microalgae production, sludge
settling and dehydration efficiency. [16, 17]Otherwise, Palme reported that UEST was applied in SLs fermentation forin-situ separation of cells, and the cell recovery was up to
99.0% without affecting the cell activity. However, it should be noted
that only 10% of SLs could be recovered by this strategy.[18]
In this study, the influences of
rapeseed oil concentrations on SLs morphology and sedimentation were
investigated, and then the mechanism of SLs sedimentation was further
discussed. On this basis, UEST was
introduced for in-situ separation of SLs, and finally a rational
semi-continuous fermentation process was developed for highly efficient
SLs production.