Enrichment of n-3 PUFA contents on triglycerides of fish oil by lipase-catalyzed trans-esterification under supercritical conditions

The immobilized 1,3-regiospecific lipase, Mucor miehei, was employed to catalyze the trans-esterification reaction to enrich n-3 polyunsaturated fatty acid (n-3 PUFA) content of triglycerides (TG) in Menhaden oil under supercritical carbon dioxide (SCCO2). A supercritical reacting system with a syringe pump to pressurize CO2 to supercritical condition and a high-pressure liquid pump to transfer the substrate into the reactor is applied to carry out experiments. Through the saponification of Menhaden oil followed by urea inclusion, 80.1 wt% of n-3 PUFA can be concentrated with containing 29.4 wt% EPA and 41.8 wt% DHA. The profile of conversion shows a constant increase at beginning 5 h, but after which the conversion levels off. Pressure up to 103.4 bars has a significant positive effect on the conversion of n-3 PUFA onto TG. Further increasing pressure, the conversion rate decreases due to the transformation of the spatial structure of lipase and leads to deactivate. The enzyme exhibits a good performance and stability in the region of 323 K. Ethanol is used as a co-solvent and its maximal addition is 10 wt% of the total substrates. The optimal substrate molar ratio of TG to n-3 PUFA is about 1/4 by considering the TG inhibition. The conversion in SCCO2 appears 40 wt% higher than in n-hexane at ambient pressure after 5 h. The ping-pong bi-bi mechanism was proposed for the kinetic model of the reaction. The final rate equation can be simplified and expressed by a modified Michaelis–Menten equation. Based on the evaluated kinetic constants, there was 7.3% deviation of the total n-3 PUFA content between the experimental and the predicted data.