EXTRACTION OF ETHYL AND METHYL-ESTERS OF POLYUNSATURATED FATTY-ACIDS WITH AQUEOUS SILVER-NITRATE SOLUTIONS

Experiments on the extraction of ethyl and methyl esters of polyunsaturated fatty acids (PUFA) such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from various organic solvents into aqueous silver nitrate solutions were performed. The esters of PUFAs with high degree of unsaturation such as DHA and EPA could be selectively extracted into aqueous solutions. On the basis of the slope analysis, the extracted complex of PUFA ester (PUFA-E) was deduced to be (PUFA-E.nAg)n+ where n is the number of the carbon-carbon double bonds in PUFA. The distribution ratio D increased drastically with a decrease in temperature and also by addition of water-soluble alcohols such as methanol, ethanol, and n-propanol to aqueous solutions. The value of D was found to be highly dependent on the organic solvent used as the diluent of PUFA-E, and the D value of EPA ethyl ester (EPA-Et) was correlated by the activity coefficient of EPA-Et in the organic phase estimated by the UNIFAC equation.