A rapid method to determine the oxidation kinetics of n-3 fatty acids in fish oil

The resistance of unsaturated fatty acids against oxidation was determined by monitoring gas chromatographically changes in three n-3 fatty acids (octadecatetraenoic (C18:4n-3), eicosapentaenoic (C20:5n-3) and docosahexaenoic acid (C22:6n-3)) in mackerel oil that had been exposed to temperatures of 80, 90 and 100 degrees C to accelerate oxidation. Fatty acids contained in the oils were converted into methylesters by reaction with trimethylsulfoniumhydroxide taking place immediately in the injection parr of the gas chromatograph. Results were expressed as weights fractions of C18:4n-3, C20:5n-3 and C22:6n-3 by comparison to an internal standard (methyl tricosanoate ester, C23:0). The repeatability values (r) obtained for the GC method used in this study were 2.90 mg/g oil, 1.49 mg/g oil and 2.31 mg/g oil for C18:4n-3, C20:5n-3 and C22:6n-3, respectively. A brief thermal pretreatment of the oil-trimethylsulfoniumhydroxide solution, or using a trimethylsulfoniumhydroxide/oil fraction above the stoichometric relation did not influence the results, i.e. the concentration of n-3 fatty acids obtained The oxidation reactions kinetics were modelled in terms of the hydroperoxide formation mechanisms from unsaturated fatty acids.