Revealing matrix-dependent contributions of molecular oxygen and interfacial water to oxidized triacylglycerol formation during frying of soybean and palm oils.

The respective roles of molecular oxygen and interfacial water in oxidized triacylglycerol (OxTG) formation during frying remain unclear across oils with different triacylglycerol matrices. In this study, purified soybean oil and palm oil were used as frying media for starch-based potato models at 180 °C under Air/H₂O, 18O₂/D₂O, and 16O₂/H₂18O conditions. Frying oxidation was characterized by 1H nuclear magnetic resonance (1H NMR), lipidomics, OxTG isotope-ratio analysis, glycerol-core aldehyde (GCA) profiling, and electron spin resonance (ESR) spin trapping. Soybean oil showed greater deterioration than palm oil, with higher total polar compounds, broader OxTG formation, greater GCA accumulation, and stronger trapped-radical signals. Lipidomics showed that soybean oil generated diverse OxTGs, mainly from polyunsaturated triacylglycerol precursors, whereas palm oil showed a narrower OxTG profile associated with less-unsaturated backbones. Isotope tracing demonstrated that oxygen incorporated into hydroxy- and hydroperoxy-containing OxTGs originated predominantly from molecular oxygen. For OxTG species detected in both oils, the primary O₂-dominant labeling pattern was generally retained, although isotope-enrichment magnitude differed between matrices. The weaker and species-dependent H₂18O responses suggested that interfacial water mainly modulated OxTG accumulation rather than serving as the principal oxygen source, with a more heterogeneous response in soybean oil than in palm oil. These findings provide atom-resolved evidence for matrix-dependent OxTG formation during frying and support oil selection and process control strategies to limit undesirable oxidation products.