Comparative Study of the Oxidative Stabilities of Palm Oil and Olive Oil.

Abstract

Palm oil and olive oil samples were subjected to methylene blue sensitized photoxidation in 10% methanol in dichloromethane in order to compare their oxidative stabilities using duration for oxidation to be observed and extent of product formation. Progress of the reaction was monitored by thin layer chromatography (TLC) which indicated that products were formed in the palm oil after 13 hours of irradiation and 10 hours for the olive oil. Chemical analysis of the reaction mixtures with triphenylphosphine (TPP) and potassium iodide/ starch indicator/thiosulphate solution indicate that peroxy compounds are formed during the sensitization. TLC results indicate that the TPP-reduction products showed lower R_f values than their precursors suggesting that alcohols are obtained from hydroperoxides. TLC also shows a dark polar spot indicating the formation of triphenylphosphine oxide. Column chromatographic separation of reaction mixtures on silica gel isolated reaction products for both oils. The proton NMR spectra of the unoxidized oils samples and their reaction products show that the major components of the oils are triacylglycerols. The spectra of the reaction products showed reductions in intensities in the peaks due to the protons of olefinic (5.2-5.4ppm), allylic (1.9-2.0 ppm) and bis-allylic (2.2-2.3 ppm) groups, indicating that oxidation preferentially affected these double bonds systems in the structures of the triacylglycerols.  Additional peaks were observed at 5.7-5.8 ppm and 7.0-7.3 ppm in the spectra of the palm oil and at 5.6-5.7 ppm and 6.8-8.0 ppm in the spectra of olive oil. The reduction in peaks and heights of new peaks observed were more pronounced in the spectra of the olive oil indicating its higher abundance in unsaturated fatty acids and a corresponding lower oxidative stability.

Keywords: Palm oil, Olive oil, Methylene blue, Photoxidation, Dye-sensitizer, Hydroperoxide, Singlet Oxygen.