Epoxy Stearic Acid in High Oleic Peanut Oil During Thermal Oxidation: From Formation Mechanism to Quality Evaluation

ABSTRACT Epoxy stearic acid (ESA) is an oxidation product formed during the thermal processing of oils, but its formation mechanism and relevance to frying oil quality remain insufficiently understood. This study combined a pure oleic acid thermo‐oxidation model with a controlled frying system to clarify ESA formation and evaluate frying quality deterioration in high oleic peanut oil (HOPO). In oleic acid heated at 180°C for 36 h, content decreased from 98.9% to 40.1%. Hydroperoxide and ESA both showed rise–fall trends, peaking at 4.75 mmol/kg (8–10 h) and 419.8 μg/mL (8–12 h), respectively. 1 H NMR showed faster depletion of olefinic than allylic hydrogens during 0–16 h, indicating enhanced side reactions such as cyclization/addition. Dynamical analysis indicates that the addition reaction governing the formation of ESA is dominant: (i) hydroperoxide homolysis followed by radical cyclization and (ii) direct addition of peroxyalkyl radicals to C=C bonds. Under 180°C frying, the ESA content in both HOPO and peanut oil (PO) increased continuously, rising from 104.7 to 966.6 μg/mL in the former and from 132.9 to 683.3 μg/mL in the latter. Meanwhile, the contents of conjugated dienes and conjugated trienes increased significantly, and total polar compounds gradually accumulated. The time for HOPO to reach the 27% total polar compounds threshold was 49 h, compared to only 36 h for PO. Furthermore, viscosity and color increased more rapidly during late frying. Correlation analysis indicated that ESA correlated well with conventional frying quality indicators. Notably, HOPO has a longer frying life, but it accumulates more ESA. These findings further clarify ESA formation mechanism and support its potential use as a quality indicator.