Heating methods generate different amounts of persistent free radicals from unsaturated fatty acids

Highlights: • Stove heating increases the polarity of the unsaturated fatty acids. • Stove-heating generates EPR signals, and quickly dissipated during cooling. • Microwave-reheating enhances EPR signals which last for over 2 h. • Ring structures are detected in microwave-reheating. • Long-lasting free radicals should be examined in the fried food. -- Abstract: Three unsaturated fatty acids (UFAs), namely linolenic acid, linoleic acid and oleic acid, were selected to investigate the generation of persistent free radicals during heating and reheating by stove or microwave. Stove-heating and -reheating generated significant EPR signals, and quickly dissipated during cooling. When the stove-heated samples were reheated by microwave, the EPR signals were further enhanced and lasted for over 2 h. FTIR characterization showed the breaking of CC and CO bonds and LF-NMR confirmed the increased polarity after stove heating. Microwave reheating following the stove heating generated much more small molecular chemicals according to GC–MS analysis, including some ring structures, which were not detected in stove heating or microwave heating alone. We thus proposed that these ring structures, such as benzene, were involved in the formation and stabilization of free radicals. This work highlighted that the relatively long-lasting free radicals should be carefully examined in the fried food.