A quantitative lipid extraction method for residue analysis of fish involving nonhalogenated solvents

Numerous intercalibration exercises have indicated that the in Sweden frequently used, so called Jensen extraction method for total lipids and lipophilic pollutants gave satisfactory yields when applied to fatty aquatic organisms. However, a comparison with the classical Bligh and Dyer method and the forerunner, the Folch methods, revealed that in the case of very lean fish (fat content below 1%, e.g., cod), the lipid yields were about 25% too low for the Jensen method; consequently, residue levels quoted on a lipid weight basis were correspondingly too high. To rectify the unacceptably low fat recovery from lean marine organisms, the Jensen extraction method has been modified to give recoveries not significantly different from the Folch and Bligh and Dyer methods. In the modified version, acetone is replaced by 2-propanol and part of the hexane is replaced by diethyl ether. Comparison between the modified Jensen method and the Folch method for cod muscle gave the same recovery of total lipids but slightly lower than that obtained with the Bligh-Dyer method. A possible explanation for this small difference is discussed. It is anticipated that the reported increased yield for cod is due to the superior solubility of phospholipids in 2-propanol as compared to acetone. The possible use of correction factors for previously reported contaminant residual levels of lean and medium fat fish calculated on lipid basis in the future is suggested.