MEASUREMENT OF HYDROPEROXIDES IN EDIBLE OILS USING THE FERROUS OXIDATION IN XYLENOL ORANGE ASSAY

The application of the Fox2 (ferrous oxidation in xylenol orange, version 2) method to the measurement of hydroperoxides in edible oils is described. Sample preparation involves dissolving oil (10 mg) in 1 mL of propan-1-ol and mixing an aliquot (100 mu L) with 900 mu L of the FOX2 reagent in a 1.5 mL microcentrifuge tube. Absorbance of the supernatant is read at 560 nm following incubation for 30 min at room temperature and centrifugation at 12000g for 5 min. Analysis by the FOX2 assay of 21 varieties of freshly opened vegetable oils revealed that edible oils contained 0.96-6.74 mmol kg(-1) of hydroperoxides. The intra-assay coefficient of variation for the FOX2 assay was generally less than 10%. There was a strong negative correlation (Spearman's rank correlation) between initial hydroperoxide levels and total tocopherols in the oil (r(s) = -0:48; p ( 0.025). Oils aged for 6 months at room temperature after opening showed an increase in hydroperoxide content which correlated negatively with total tocopherol content (r(s) = -0.39; p < 0.05). Surprisingly, there was a significant negative correlation between initial thiobarbituric acid reactive material (TBARM) values and hydroperoxide content as well as a significant positive correlation between total tocopherols and initial TBARM values. These results suggest that TBARM values for edible vegetable oils may not accurately predict true peroxidation and confirm that the FOX2 assay is appropriate for the determination of ''peroxide values''.