Oxidation Mechanisms in Heterogeneous Food Matrices: Comparison of Autoxidation of Methyl Linoleate and Methyl Linolenate in Bulk Oil and Oil-in-Water Emulsions

Autoxidation pathways and stability of methyl linolenate (MLN) and methyl linoleate (ML) were compared in bulk oil and oil-in-water (O/W) emulsions under dark, sealed storage at 37 °C, using peroxide value (PV), GC, HPLC-MS/MS, and nuclear magnetic resonance (NMR) for characterization. Both esters rapidly entered the oxidation propagation stage in both systems, initially forming cis-trans monohydroperoxides (ZEMHP) via hydrogen abstraction, which was more favorable in the emulsion system. Notably, high proportions of secondary hydroperoxides formed concurrently, increasing with oxidation: MLN initially produced dominant bicyclic endoperoxides (BCEHP) and minor monocyclic endoperoxides, summing to 34.1% (C_HPs at 95.6 mmol/kg); ML initially formed dominant trans-trans monohydroperoxides (EEMHPs), summing to 45.1% (C_HPs at 86.7 mmol/kg), and epoxy hydroperoxides at the late oxidation stage. Moreover, MLN exhibited higher apparent oxidation stability than ML in emulsions, which might be due to higher hydrogen abstraction activation energies for cyclic peroxyl radicals (PRs) to form cyclic endoperoxides calculated by DFT calculations.