Determination of antioxidant capacity as a tool for mathematical modeling to predict the lipid oxidation lag phase

Rancidity is one of the most serious issues with oil and fat quality, causing deterioration, lowering nutritional value, and increasing food waste. Predicting the shelf-life of fats and oils is typically difficult and time-consuming, as it might take many months before rancidity occurs. Antioxidants are depleted during oxidation, and their depletion is mostly complete by the conclusion of the lag phase. As a result, establishing mathematical models based on the kinetics of antioxidant depletion in relation to the generation of lipid oxidation products may be a useful method for predicting the shelf-life of oils. The aim of this study was to develop a quick and low-cost approach, such as a spectrophotometric test, to quantify the loss of antioxidants naturally found in soybean and corn oil (stored at 55 °C), which could subsequently be utilized for modeling. Rancidity was assessed by measuring lipid hydroperoxides (spectrophotometric assay) and headspace hexanal (GC-FID), while antioxidant loss was assessed by determining tocopherol content by normal-phase HPLC and total antioxidant capacity by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical spectrophotometric assay. The complete degradation of αlpha- and gamma-tocopherols coincided with the end of the oxidative lag phase, while delta-tocopherol was still detectable at 47% (soybean) and 85% (corn). Unexpectedly, DPPH antioxidant capacity was never entirely depleted. Once it reached a minimum value corresponding to the lag phase (15-19%), it then increased to 79%. Subsequent research showed that the DPPH radical reacted not only with antioxidants but also with free radicals. The DPPH-radical products exhibited absorbance spectra similar to unoxidized DPPH explaining the increase in DPPH in the later stages of storage. That result highlights the limited applicability of DPPH to be used as an analytical tool beyond the lag phase, as its specificity for antioxidants falls beyond that point.