Quantification of antioxidant capacity in a microemulsion system: synergistic effects of chlorogenic acid with...

We report herein a characterization of an oil-in-water (o/w) microemulsion consisting of 12% methyl linoleate (or styrene in weight %), 29% surfactant Tween-20, 15% n-butanol, and 44% 75 mM potassium phosphate buffer (pH 7.04). The oil phase droplet size, determined by dynamic light scattering, is 19.3 nm with polydispersity at 0.103. When methyl linoleate is replaced with styrene, the droplet size increases to 63 nm but with much narrower polydispersity at 0.047. The droplet size of styrene is confirmed by polymerization of the styrene in the microemulsion. The polystyrene particles isolated have diameters of approximately 70 nm determined by a scanning electronic microscope. Both microemulsions remain stable for two months as expected for a thermodynamically stable system. The methyl linoleate oxidation is induced by AAPH, and the effects of radical scavengers are evaluated in a high throughput fashion using an oxygen sensor coated 96-well microplate Oxygen Biosensor Systems. From the oxygen consumption kinetic curves, the antioxidant capacity can be calculated using Trolox as the standard. The synergistic effect of hydrophilic antioxidants and alpha-tocopherol was measured for representative flavonoids. Chlorogenic acid has the best synergistic effect of 44.8%. The assay, coined as ORAC(E) standing for oxygen radical absorbance capacity in microemulsion, provides a model system in evaluating antioxidant capacity of phenolic compounds in a heterogeneous system relevant to food and cosmetic applications.