Volatile components of deep-fried soybean oil as indicator indices of lipid oxidation and quality degradation

The present study investigated the lipid oxidation degree of soybean oil during regularly discontinuous 40 h-deep-frying process. Electron spin resonance (ESR) spectroscopy technique was applied to identify and quantify the formed radicals, along with evaluation of physicochemical parameters including acid value (AV), peroxide value (PV), p -anisidine value ( p -AnV), polar compounds (PC), fatty acid composition and volatile profile. Results showed the AV, p -AnV, PC and free radical of frying oil samples increased significantly with the increasing frying time. The results of fatty acids showed that unsaturated fatty acid such as C18:1 and C18:2 decreased by 19.98% and 14.58%, respectively, with prolonged frying time, while the content of C16:1, trans C18:1 and C18:2 increased by 20.38%, 425% and 42.86%, respectively, when compared to the fresh oil samples. In contrast, the content of saturated fatty acid had little change. In total, 37 volatile compounds were detected revealing a complex aroma profile of frying soybean oil, composed of 15 aldehydes, 8 alcohols, 4 ketones, 4 acids, 5 alkanes and 1 furan. Principal component analysis and hierarchical clustering analysis indicated that hexanal, heptanal, ( E )-2-hexenal, octanal, ( E )-2-heptenal, nonanal, ( E )-2-octenal, undecanal, ( E,E )-2,4-heptadienal, ( E )-2-decenal, 2-undecenal, ( E,E )-2,4-decadienal, 1-pentanol, 2,2-dimethyl-3-hexanol, ( Z )-2-dodecenol, 1-octen-3-ol, pentanoic acid, octanoic acid, nonanoic acid and 2-pentyl-furan may be potential markers for evaluating lipid oxidation of frying soybean oil.