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increased PUFA production (Čertík et al. 2013). Moisture content regulation is
also another important aspect of SSF. Low moisture can result in substrate swelling along with decreased levels of nutrient solubility, while high moisture causes
stickiness in substrate particles, thereby resulting in decreased levels of gaseous
exchange and porosity. The preferred moisture content of substrate in SSF is
60–65 and 70–75% for ω-3 and ω-6 fatty acid synthesis, respectively. A pH of not
more than 8 and less than 5, i.e. pH in the range of 6–7, is usually preferred for
PUFA production (Asadi et al. 2015). It is also possible to control the direction of
higher fatty acid synthesis by supplementing the substrate with oils. The addition
of exogenous oils such as linseed oil and sunflower oil can direct lipid synthesis
in favour of omega-3 (EPA) fatty acid synthesis, while sesame seeds promote
omega-6 fatty acid synthetic pathway. Both linseed and sunflower oils, direct precursors of omega-3 fatty acid pathway, contain alpha-linoleic acid. Sesame seeds
are known to contain inhibitors of delta-5 desaturase enzyme. Nutritionists and
dieticians often recommended ω-6/ω-3 ratio of PUFA to be 5:1 or less (Patel et al.
2020). Therefore, this strategy can also be applied to balance ω-6/ω-3 ratio of
food products as in Mortierella alpina which normally produces arachidonic acid
and can be also made to produce EPA by addition of linseed oil (Asadi et al. 2015;
Sláviková and Čertik 2005).
7.5 Processing Methods for Extraction of PUFA-Rich Oil
The PUFA-rich oil produced by fungal fermentation often undergoes downstream
processing steps of lipid recovery and refining for application in food and other
nutraceutical products. Two methods of oil recovery are currently employed for
lipid recovery: dry methods that use solvents for extraction of lipids from dried cell
mass and wet methods where the biomass is directly treated with enzymes without
the use of solvents. Dry methods mainly include initial physical separation of biomass from culture media employing techniques such as centrifugation, flocculant
addition and sedimentation depending upon the viscosity and composition of the
culture media. The separated biomass is then dewatered by techniques such as spray
drying, heating or freeze drying followed by the addition of an organic solvent. The
mixture is then de-solventized to obtain a crude lipid extract and the solvent is generally recycled. To improve the efficiency of lipid extraction, other methods of fungal cell wall disruption such as acid treatment, microwave-assisted solvent extraction
and physical disruption of fungal cell wall are also frequently employed in dry
methods. A general outline of the above process is given in Fig. 7.3. The lipids produced by these methods are not pure and more prone to oxidation. Therefore, further refining of these oils is carried out by techniques such as degumming, bleaching
and alkali refining (Khot et al. 2020; Ji et al. 2015; Ji and Ledesma-Amaro 2020;
Cheng et al. 2019).
R. Gupta and S. Gaur
increased PUFA production (Čertík et al. 2013). Moisture content regulation is
also another important aspect of SSF. Low moisture can result in substrate swelling along with decreased levels of nutrient solubility, while high moisture causes
stickiness in substrate particles, thereby resulting in decreased levels of gaseous
exchange and porosity. The preferred moisture content of substrate in SSF is
60–65 and 70–75% for ω-3 and ω-6 fatty acid synthesis, respectively. A pH of not
more than 8 and less than 5, i.e. pH in the range of 6–7, is usually preferred for
PUFA production (Asadi et al. 2015). It is also possible to control the direction of
higher fatty acid synthesis by supplementing the substrate with oils. The addition
of exogenous oils such as linseed oil and sunflower oil can direct lipid synthesis
in favour of omega-3 (EPA) fatty acid synthesis, while sesame seeds promote
omega-6 fatty acid synthetic pathway. Both linseed and sunflower oils, direct precursors of omega-3 fatty acid pathway, contain alpha-linoleic acid. Sesame seeds
are known to contain inhibitors of delta-5 desaturase enzyme. Nutritionists and
dieticians often recommended ω-6/ω-3 ratio of PUFA to be 5:1 or less (Patel et al.
2020). Therefore, this strategy can also be applied to balance ω-6/ω-3 ratio of
food products as in Mortierella alpina which normally produces arachidonic acid
and can be also made to produce EPA by addition of linseed oil (Asadi et al. 2015;
Sláviková and Čertik 2005).
7.5 Processing Methods for Extraction of PUFA-Rich Oil
The PUFA-rich oil produced by fungal fermentation often undergoes downstream
processing steps of lipid recovery and refining for application in food and other
nutraceutical products. Two methods of oil recovery are currently employed for
lipid recovery: dry methods that use solvents for extraction of lipids from dried cell
mass and wet methods where the biomass is directly treated with enzymes without
the use of solvents. Dry methods mainly include initial physical separation of biomass from culture media employing techniques such as centrifugation, flocculant
addition and sedimentation depending upon the viscosity and composition of the
culture media. The separated biomass is then dewatered by techniques such as spray
drying, heating or freeze drying followed by the addition of an organic solvent. The
mixture is then de-solventized to obtain a crude lipid extract and the solvent is generally recycled. To improve the efficiency of lipid extraction, other methods of fungal cell wall disruption such as acid treatment, microwave-assisted solvent extraction
and physical disruption of fungal cell wall are also frequently employed in dry
methods. A general outline of the above process is given in Fig. 7.3. The lipids produced by these methods are not pure and more prone to oxidation. Therefore, further refining of these oils is carried out by techniques such as degumming, bleaching
and alkali refining (Khot et al. 2020; Ji et al. 2015; Ji and Ledesma-Amaro 2020;
Cheng et al. 2019).
R. Gupta and S. Gaur