Extraction of Oil from High-Moisture Tuna Livers by Subcritical Dimethyl Ether: A Comparison with Different Extraction Methods

Tuna livers, which are currently discarded in the fish industry, are a valuable source of marine oil rich in n-3 polyunsaturated fatty acids (PUFAs), vitamin A, and vitamin D. In this study, subcritical dimethyl ether extraction (SDEE) is applied in the extraction of oil from high-moisture tuna livers. Furthermore, other different extraction methods including wet reduction (WR), enzymatic extraction (EE), and supercritical carbon dioxide fluid extraction (SFE-CO2) are used for comparisons, to assess the effect of different extraction methods on yield, oil quality, flavor (volatile compounds and sensory assessing), vitamin contents, and fatty acids composition. Compared with traditional methods, the high-quality liver oil can be obtained with excellent yields by SDEE and SFE-CO2, because they can prevent the oxidation of lipids, and, reduce the damage of PUFAs and vitamins effectively. Furthermore, only minor differences in volatile compounds and vitamins contents are observed between SDEE-oil and SFE-CO2-oil, which is attributed to the differences in solubility to the solvents used. The oil with higher contents of vitamin A and alpha-vitamin E can be obtained by SDEE. The main limitations of SFE-CO2 are energy-consuming due to high pressure used and the necessary pre-treatment of freeze-drying. However, these drawbacks could be overcome in SDEE because the pressure employed in SDEE is quite low (0.8 MPa) and the freeze-drying is unnecessary. Therefore, SDEE would be a simple, effective, and promising technique to obtain the high quality oil rich in n-3 PUFAs and vitamins in the fish oil industry. Practical Applications: Subcritical dimethyl ether extraction (SDEE) is a rising technology in oil extraction. It can extract oil from the high-moisture materials without freeze-drying under relatively low pressure. As is known, freeze-drying is a necessary pre-processing step in ordinary sub- and super-critical fluid extraction. The high quality of oil can be obtained by SDEE, similar with the quality of the oil obtained by supercritical fluid extraction, with dramatically declined operational costs. Subcritical dimethyl ether extraction (SDEE) can extract oil from the high moisture tuna livers without freeze-drying under relatively low pressure. Furthermore, the quality of SDEE-oil is similar to the quality of oil obtained by supercritical carbon dioxide extraction, but substantially better than the oil obtained by wet reduction and enzymatic extraction.