High-throughput quantitation of peroxyl radical scavenging capacity in bulk oils

Autoxidation of methyl linoleate (8:2 mixture with decane, 37 degrees C) was induced by 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN, 17.7 mM) and the kinetics of oxygen consumption monitored using a 96-well microplate coated with an oxygen-sensitive fluorescence probe, a ruthenium dye, embedded in a silicone matrix at the bottom of the microplate. The probe does not participate in the reaction; instead, its fluorescence intensity is inversely proportional to the solution oxygen concentration as it changes during oxidation. In the absence of antioxidants, the oxidation rate has a linear relationship with the square root of the initiator concentrations. This is in agreement with theoretical autoxidation kinetics equations. In the presence of tocopherol-type antioxidants, a sharp lag phase appears. The quantitation of the antioxidant capacity is achieved using the area under the curve (AUC) approach. The assay has a 2 h running time, a linearity range from 1.56 to 18.7 microM (Trolox), and a limit of quantitation at 2.7 microM Trolox equivalency. The peroxyl radical scavenging capacities of several cold-pressed and organically grown plant seed oils were quantified along with the tocopherol concentrations of the oils. Tocopherols contribute only a fraction of the peroxyl radical scavenging capacity of the oils, and there is poor correlation between total tocopherol concentrations and radical scavenging capacity, suggesting that the antioxidant capacity of oils is due not only to tocopherols but also to other lipid-soluble antioxidants.