Use of focused open vessel microwave-assisted extraction as prelude for the determination of the fatty acid profile of fish - a comparison with results obtained after liquid-liquid extraction according to Bligh and Dyer

A method based on microwave-assisted extraction (MAE) was developed to extr act lipids from fish for the determination of the fatty acid composition. M icrowave-assisted extraction was performed with an open-vessel extraction a pparatus similar to the system of Soxhlet. The solvent was an equivolume mi xture of ethyl acetate and cyclohexane. The solvent forms a ternary azeotro pe with water, and water is separated after re-condensation in a water trap . With this technique, extraction can be performed without pre-drying of fi sh tissues. After lipid extraction, the esterification was performed with t rimethylsulfonium hydroxide (TMSH) and the fatty acid methyl esters (FAMEs) were determined by GC/FID. The results were compared with those obtained a fter liquid-liquid extraction according to Bligh and Dyer. Fillets of macke rel (Scomber scombrus) and livers of cod (Gadus morhua) were used as the sa mple material. The water content (n=7) of the cod livers was 36.2+/-1.6% an d that of mackerel fillets was 74.5+/-0.5%. The lipid contents (n=5) using the MAE method were 5.6+/-0.4% and 62.6+/-3.1% in mackerel and cod liver, r espectively. Relative levels of 58 fatty acids (two co-eluted) were determi ned with GC/FID on two capillary columns with different polarity. These wer e 10 saturated, 24 unsaturated, and 24 unidentified peaks which are most li kely unsaturated or branched-chain fatty acids. After MAE the composition o f the 57 peaks was virtually the same as that obtained with the Bligh and D yer method. The advantages are, however, that MAE is faster, requires less solvent, and avoids the use of chlorinated solvents and so is more environm entally friendly. Our results confirm that MAE is a well-suited alternative to the Bligh and Dyer method for the extraction of lipids followed by dete rmination of the fatty acid profile.