Sophorolipids Characteristics, Production, and Applications - Biosurfactants: Research Trends and Applications - page 113

1993, 1997; van Bogaert, et al., 2007). Several strategies can be applied in order to
develop the cost-effective method. As a concern, to maintain the stationary phase
of the microbes for better production of the sophorolipid, the process itself should
be optimized like using appropriate medium, culture conditions, product recovery
techniques, etc. The cost-effective substrate replacement and their optimization
are a secondary approach to have an industrial-scale production. The conventional
engineering is not enough to achieve the goal; thus, the overproducing mutant and
recombinant strains should also be created to obtain maximum productivity. The
approaches mentioned earlier should take place together to resolve the issues of
the commercialization and making them as a market-compatible product. The
yeast cells can be maintained in stationary phase up to 10 days while produc-
ing sophorolipid (van Bogaert et al., 2011). Sophorolipid production media were
designed and optimized for the carbon and nitrogen sources, and it was found that
around 100 g/L glucose was used as the carbon source with variable concentra-
tions of yeast extract or corn steep liquor as the nitrogen source. Supplementation
of lipophilic or hydrophobic carbon as carbon source, along with additional glu-
cose feedings, can help to increase the yield either in batch or fed-batch (Shin
et al., 2010).
One of the other most important factors is temperature optimization to achieve
the good production of sophorolipids. The optimum growth of C. bombicola ATCC
22214 was reported at 28.8°C, while the production of sophorolipid was favored at
21°C (Gobbert et al., 1984). But most of the fermentation was carried out at 25°C
or 30°C, as the higher temperature favors high biomass production and relatively
lower glucose consumption (Casas and Garcia-Ochoa, 1999). The pH of medium
also plays a crucial role in the production. During the exponential growth phase, pH
drops till 3.5 or below. But it should ideally be kept at 3.5 for optimal sophorolipid
production (Gobbert et al., 1984). Oxygen concentration is one of the important
constraints during the entire fermentation process. The culture broth should be sup-
plied with an adequate amount of oxygen because the yeast cells are very sensitive
to oxygen and good aeration conditions are important for sophorolipid produc-
tion as the cytochrome P450 monooxygenase uses molecular oxygen (Guilmanov
et al., 2002; van Bogaert et al., 2007). The optimal aeration for high sophorolipid
yield expressed in terms of oxygen transfer rate lies between 50 and 80 mM O 2 /(Lh)
(Shin et al., 2010).
In order to achieve a high amount of sophorolipid, the supplementation of hydro-
phobic carbon substrate is an important concern. In the last few decades, a number of
substrates have been screened to design the production media such as alkanes, oils,
fatty acids, and their corresponding esters. In most cases, relatively pure substrates
are used. But one can go further and exploit waste streams such as biodiesel by-
product streams, soybean dark oil, waste frying oil, corn oil, and molasses (Solaiman
et al., 2004, 2007; Kim et al., 2005; Pekin et al., 2005; Fleurackers, 2006). An over-
view of various experiments in relation to different substrates or feeding strategies is
given in Table 4.2. It has been reported previously that the vegetable oils are rich in
oleic acid and promote sophorolipid production. Consequently, rapeseed oil is one of
the substrates of choice (Davila et al., 1994; Daniel et al., 1998a,b; Kim et al., 2009;
Shin et al., 2010).