Quantitative Analysis of Tocopherol Degradation and Radical Scavenging Activity During Lipid Oxidation in Bulk Soybean and Corn Oils

Accurate shelf-life prediction for fats and oils is essential, yet traditional lipid oxidation models are often time-consuming and unreliable. Since antioxidants deplete as oxidation progresses, tracking their loss alongside oxidation products could improve lag phase predictions. This study investigates a rapid, cost-effective spectrophotometric test to quantify antioxidant depletion in soybean and corn oils for potential use in mathematical modeling. Results showed that α-tocopherol was fully degraded by the end of the oxidation lag phase, while (γ + β)- and δ-tocopherols concentrations remained at > 70% (soybean oil) and 65% (corn oil). DPPH scavenging activity initially declined with tocopherol loss but later increased (up to 79%), likely due to lipid radical interference. Further analysis confirmed DPPH reacts with free radicals, compromising its specificity to only detecting antioxidants. To address this, the ABTS assay was tested, requiring prior antioxidant extraction from oil due to its water-soluble nature. Unlike DPPH, ABTS inhibition dropped to zero once all tocopherols were depleted, confirming its higher specificity. However, this depletion did not align with the oxidation lag phase, as (γ + β)- and δ-tocopherols were not completely depleted at the end of the lag phase. These findings highlight three key insights: (i) (γ + β)- and δ-tocopherols are less effective than α-tocopherol in inhibiting lipid oxidation in commercial oils, persisting even after oxidation begins; (ii) the direct application of DPPH in lipid-containing matrices can yield misleading results, as it reacts with lipid radicals during oxidation; (iii) while ABTS specifically tracks antioxidant depletion, it might be unsuitable for kinetic modeling due to minimal change during the lag phase.