Trans -Hydroxy, Trans -Epoxy, and Erythro -dihydroxy Fatty Acids Increase during Deep-Frying.

Deep-frying of food is a common cooking technique causing thermal oxidation of fatty acids (FA). Here, we investigated for the first time the formation of hydroxy-, epoxy- and dihydroxy-FA derived from oleic, linoleic (LA), and α-linolenic acid (ALA) during frying. Potato chips were fried in high-oleic sunflower oil for 4 × 5 cycles on 2 days, and the oil was comprehensively analyzed by liquid chromatography-tandem mass spectrometry. During frying, the E , Z -9- and E , Z -13-hydroperoxy-LA and -ALA concentrations decrease while their corresponding hydroxy-FA remain constant. The concentrations of both E , E -9-/13-hydroperoxy-LA and E , E -9-/13-hydroxy-LA increase with the frying cycles, which is also found for the concentration of trans -epoxy-FA. The increase in trans -epoxy-FA is more pronounced than that of the corresponding cis -epoxy-FA, exceeding their concentrations on the second day of frying. This selective change in the cis -/ trans -epoxy-FA ratio is also observed for their hydrolysis products: concentrations of erythro -dihydroxy-FA, derived from trans -epoxy-FA, increase during frying stronger than threo -dihydroxy-FA derived from cis -epoxy-FA. Based on these data, we suggest that the ratio of E , E -/ E ,Z-hydroxy-FA, in combination with the cis -/ trans -epoxy-FA ratio, as well as the threo -/ erythro -dihydroxy-FA ratio are promising new parameters to evaluate the heating of edible oils and to characterize the status of frying oils.