n Journal of Emerging Trends in Engineering and Applied Sciences - Methylene blue-sensitized photoxidation of olive oil

Volume 4, Issue 3
  • ISSN : 2141-7016
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Extra virgin olive oil was photoxidized in the presence of methylene blue as sensitizer to investigate quality changes as indicated by hydroperoxide formation. Progress of the reaction was monitored by thin-layer chromatography. Initial oxidation was observed after ten hours of irradiation. The peroxy content of the oxidation reaction mixture was analyzed by reaction with triphenylphosphine, potassium iodide/starch indicator and iron (II) sulphate/ammonium thiocyanate reagent. Column chromatographic separation of the photoxidized reaction mixture and the triphenyl-phosphine (TPP)-reduced reaction mixture on silica gel afforded products. The oxidation product isolated from the oxidation reaction mixture also reacted with the test reagents. Fourier Transform infra-red (FT-IR) spectrum of the initial oil and the isolated reaction products confirm that the major constituents of the oil are triacylglycerols (TAGs). The absorption band at 3005 cm-1due to the cis-olefin CH stretch vibrations observed in the unoxidized oil shows that the oil is rich in monounsaturated fatty acids. The major changes in FT-IR of the isolated products compared to that of the initial oil show that the cis-double bonds were mostly affected in the irradiation process. The absorption band responsible for this structural unit was not observed in the spectra of the reaction products, rather a new band at 968.64 cm-1 due to isolated transolefins is observed in the spectra of the reaction products. Also, TAG-hydroperoxides were produced as shown by both FT-IR (absorption bands at 3401 and 811.23 cm-1) and chemical analyses. The absorption band, 811.23 cm-1 due to O-O stretch of the peroxy linkage was no longer observed in the spectrum of the TPP-reduced product, indicating its removal in the reduction reaction. The results indicate that exposure to light in the presence of a sensitizer accelerates photoxidation of vegetable oils.

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