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 alpha-linolenic acid (ALA) during frying. Potato chips were fried in high-oleic sunflower oil for 4 x 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.

Automated summary

New parameters related to the condition of frying oils were discovered by studying the oxidation products of fatty acids during deep-frying.