Lipid Oxidation in Water-in-Olive Oil Emulsions Initiated by a Lipophilic Radical Source

The lipophilic 2,2′-azobis(2,4-dimethylvaleronitrile) (AMVN) was used to study thoroughly the oxidation reaction in a model water-in-olive oil emulsion system. This radical species decomposes thermally generating a constant flux of radicals in the oil phase. The dissociation constant kd in olive oil at 40 °C for AMVN was calculated as 2.5 × 10-4 min-1 and the rate of initiation of the oxidation reaction, Ri was calculated by using vitamin E as antioxidant. The olive oil oxidation in emulsion was monitored by measuring the hydroperoxide concentration by a sensitive fluorimetric method. The DPPP (diphenyl-1-pyrenylphosphine) was used as a probe because it reacts stoichiometrically with hydroperoxides to yield a fluorescent product, the diphenyl- 1-pyrenylphosphine oxide (DPPP-O). Oxidation data together with emulsion droplet size data showed that in the presence of radical initiator and a large interface, the oxidation reaction is accelerated in W/Olive oil emulsion with respect to whole oil. The mediation of the surface area of water droplets is surely involved in this process because the addition of saturated solutions of ascorbic acid (AA) dispersed in the oil brings about the strong reduction of the oxidation rate even in the presence of the highest AMVN quantity.