Formation of cyclic fatty acids during the frying process

Abstract

Cyclic fatty acids formed during frying were at one time thought to be highly toxic. Fears have receded somewhat, but this stimulated considerable research. In our work, the structures of all the cyclic monomers formed from oleic, linoleic, and linolenic acids in heated frying oils were determined. The compounds were first isolated by urea fractionation and reversed-phase high-performance liquid chromatography (HPLC). The mixtures were simplified by silver ion HPLC, and then examined by gas chromatography-mass spectrometry in the form of the picolinyl esters and 4,4-dimethyloxazoline derivatives. In addition, hydrogenation and deuteration aided the characterization. Assignment of double bond configurations was accomplished by gas chromatography linked to Fourier-transform infrared (IR) spectroscopy. Surprisingly, a simpler range of compounds was formed from the trienoic acid than from the diene. Linolenic acid gave four basic cyclic diene structures (two with cyclopentene and two with cyclohexene rings), each of which existed as isomers with different double bond and ring conformations. Some bicyclic fatty acids were formed from linoleate. The mechanism of the reaction is believed to involve intramolecular rearrangements via free radical catalysis with traces of hydroperoxides as the initiators.