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

The application of the Fox2 (ferrous oxidation in xylenol orange, vers ion 2) method to the measurement of hydroperoxides in edible oils is d escribed. Sample preparation involves dissolving oil (10 mg) in 1 mL o f propan-1-ol and mixing an aliquot (100 mu L) with 900 mu L of the FO X2 reagent in a 1.5 mL microcentrifuge tube. Absorbance of the superna tant is read at 560 nm following incubation for 30 min at room tempera ture and centrifugation at 12000g for 5 min. Analysis by the FOX2 assa y of 21 varieties of freshly opened vegetable oils revealed that edibl e oils contained 0.96-6.74 mmol kg(-1) of hydroperoxides. The intra-as say coefficient of variation for the FOX2 assay was generally less tha n 10%. There was a strong negative correlation (Spearman's rank correl ation) between initial hydroperoxide levels and total tocopherols in t he oil (r(s) = -0:48; p ( 0.025). Oils aged for 6 months at room tempe rature 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 be tween initial thiobarbituric acid reactive material (TBARM) values and hydroperoxide content as well as a significant positive correlation b etween total tocopherols and initial TBARM values. These results sugge st that TBARM values for edible vegetable oils may not accurately pred ict true peroxidation and confirm that the FOX2 assay is appropriate f or the determination of ''peroxide values''.