Cyclic fatty acids formed during frying were at one time thought to be high
ly toxic. Fears have receded somewhat, but this stimulated considerable res
earch. In our work, the structures of all the cyclic monomers formed from o
leic, 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 spectrome
try in the form of the picolinyl esters and 4,4-dimethyloxazoline derivativ
es. In addition, hydrogenation and deuteration aided the characterization.
Assignment of double bond configurations was accomplished by gas chromatogr
aphy linked to Fourier-transform infrared (IR) spectroscopy. Surprisingly,
a simpler range of compounds was formed from the trienoic acid than from th
e diene. Linolenic acid gave four basic cyclic diene structures (two with c
yclopentene and two with cyclohexene rings), each of which existed as isome
rs with different double bond and ring conformations. Some bicyclic fatty a
cids were formed from linoleate. The mechanism of the reaction is believed
to involve intramolecular rearrangements via free radical catalysis with tr
aces of hydroperoxides as the initiators.