PHOTOSENSITIZED OXIDATION OF OLEIC-ACID, METHYL OLEATE, AND OLIVE OIL USING VISIBLE-LIGHT

Oleic acid, methyl oleate and olive oil were photosensitized in the presence of rose bengal to produce the monohydroperoxide intermediates. The optimum factors affecting the hydroperoxide formation rate were discussed from the viewpoint of the temperature dependence, solvent effect, added amount of sensitizer and light intensity. The peroxide value (POV) increased linearly with irradiation time in the range 10-40 degrees C. Both auto-oxidation and photo-oxidation caused an increase in the POV at 50 degrees C. For a solvent effect, the peroxide formation gave a maximum in methanol (dielectric constant epsilon=33) for oleic acid, in ethanol (epsilon=24) for methyl oleate, and in n-hexane+ethanol (50/50, epsilon=11) for olive oil. The mutual solubilization of lipid, sensitizer and solvent is one of the important kinetic features. When the sensitizer concentration increases, the reaction rate increases to reach a constant value of 10(-3) molar equivalent for oleic acid and 2x10(-3) molar equivalent for methyl oleate. However, olive oil exhibits the maximum rate at 2x10(-4) molar equivalent of the ratio of rose bengal to acyl group. The peroxide formation rate for olive oil decreases with increasing sensitizer concentration because of poorly mutual solubilization. The peroxide formation of three kinds of lipid is excellent under a light intensity of 1200 mu W cm(-2).