Production of structured triglycerides rich in n-3 polyunsaturated fatty acids by the acidolysis of cod liver oil and caprylic acid in a packed-bed reactor: equilibrium and kinetics

Abstract

Structured triglycerides (ST) enriched in n-3 polyunsaturated fatty acids (PUFAs) (eicosapentaenoic acid, EPA, and docosahexaenoic acid, DHA) in position 2 of the triglyceride backbone were synthesised by acidolysis of cod liver oil (CLO) and caprylic acid (CA) catalysed by the 1,3-specific immobilised lipase Lipozyme IM. The reaction was carried out in three ways: (1) in a batch reactor (where the influence of temperature on the incorporation of CA into the CLO triglyceride was studied); (2) in an immobilised lipase packed-bed reactor (PBR) by recirculating the reaction mixture from the exit of the bed to the substrate reservoir (product recirculation) to determine the equilibrium composition; and (3) in a PBR without recirculation. A “lag” period of duration inversely proportional to the initial water amount of the lipase, was observed when new lipase was used. Apparently, during this “lag” period the hydro-enzymatic layer that surrounds the lipase surface reaches its water equilibrium content. A reaction scheme, where only the fatty acid in the positions 1 and 3 of the glycerol backbone were exchanged by CA, was proposed. The exchange equilibrium constants between CA and the native fatty acids of CLO were determined. The n-3 PUFAs (EPA and DHA) were the most resistant native fatty acids to exchange with exchange equilibrium constants of 1.32 and 0.28, respectively. Also, average reaction rates and kinetic constants of exchange of CA and native fatty acid of CLO were calculated. Low kinetic constants were observed for EPA, DHA and palmitic acid. For acidolysis reaction in the continuous mode PBR, the lipase amount/(flow rate × substrate concentration) ratio ( m L / q[TG] 0) could be considered as the intensive variable of the process for use in scale up of the PBR. A simple equation was proposed for the prediction of the fatty acid composition of the ST at the exit of the PBR as a function of the intensive variable m L / q[TG] 0. At equilibrium, the ST produced had the following composition: CA 57%, EPA 5.1%, DHA 10.0% and palmitic acid 6.3% (only considering the major fatty acids). In addition, the proportion of EPA and DHA that esterified the position 2 of the ST was 13.5%, which represented 44% of the total fatty acids in the position 2 of the resultant ST.