Mechanism of Water-Enhanced Volatile Aldehyde Release in Oil Fumes from Thermal Oxidation of Oleic Acid: Insights from Synchrotron Radiation Photoionization and Gas Chromatography–Mass Spectrometry

Thermal oxidation of edible oils during high-temperature cooking produces complex fumes containing harmful volatile compounds. However, the role of water, a common co-reactant in practical cooking, remains insufficiently understood. In this study, oleic acid was used as a model compound to investigate thermal oxidation. Online monitoring using synchrotron radiation photoionization mass spectrometry (SR-PIMS) revealed that water significantly increased the emission of volatile acetaldehyde and acrolein, with maximum increases of 164% and 123% at 10% water addition. Complementary offline GC-MS analysis showed enhanced formation of (<i>E</i>)-2-decenal, (<i>E</i>,<i>E</i>)-2,4-decadienal, and (<i>E</i>)-2-undecenal, suggesting these unsaturated aldehydes may be key intermediates. Mechanistically, oleic acid underwent free radical-mediated peroxidation to form (<i>E</i>)-2-decenal, (<i>E</i>)-2-undecenal, and (<i>E</i>,<i>E</i>)-2,4-decadienal. These intermediates subsequently decomposed into acetaldehyde and acrolein via hydration, retro-aldol condensation, and hydroperoxide scission, with water accelerating both processes. Overall, these findings highlight water's critical role in promoting the generation of harmful volatile aldehydes in oil fumes.