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 (Jahan et
al., 2020; Van Bogaert et al., 2007). Many microorganisms can synthesize
SLs, includingCandida(Gaur et al., 2019), Pichia(Thaniyavarn et al., 2008), andRhodotorula (Sen et al.,
2017). Candida bombicola is
the most widely adopted specie due to its high efficiency of SLs
production (Zhou et al., 2019).
SLs
are the mixtures of many structural derivatives consisting of two parts,
a hydrophilic sophorose and a 16-18 carbon hydrophobic hydroxyl fatty
acid with general 16-18 carbon atoms (Fig. 1) (Chen et al., 2020). 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 (David et al., 2003; Hu and Ju, 2001; Jimnez‐Pealver
et al., 2020). 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
(Ashby et al., 2013; Ma et al., 2020).
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 leads to an increase in viscosity, which further results in
the difficulty in mixing and mass transfer, and finally causing a sharp
decline in fermentation efficiency (Jia et al., 2017). Semi-continuous
fermentation basing on in-situ separation of products can
segregate part of SLs from the fermentation broth, thereby improving the
rheological characteristics and maintaining efficient synthesis of SLs.
Different types of devices have been developed to improve the efficiency
of in-situ separation (Wang et al., 2019). In this regard,
gravity sedimentation is the main method for in-situ separation
of SLs. Zhang et al. (2018) found that the density of the fermentation
broth could be adjusted by controlling the glucose concentration, so as
to achieve the sedimentation of SLs. 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.
In general, 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 improvement 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 (Hincapié Gómez et al., 2015;
Maddikeri et al. 2015). Otherwise, Palme et al. (2010) reported that
UEST was introduced in SLs fermentation for in-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.
In this study, the influences of
oil concentrations on SLs morphology and sedimentation were
investigated, and then the mechanism of SLs sedimentation was further
analyzed. 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.