- Food lipid systems are very complex and so is oxidative
degradation that render them unpalatable for human consumption.
The significance of lipid degradation and the key reactions involved
have been thoroughly studied and are well understood today. This
is not true, however, of the secondary products of the autoxidation
reactions. Of the many hydroperoxide decomposition products, the
carbonyls are generally considered the most important. Much work
has been published on the identification of carbonyls in autoxidizing
lipids and model systems, and in practically every case where oxidation
mechanisms were considered, the authors have repeatedly
referred to fatty acid hydroperoxides as immediate precursors. At
the present time, there are a number of carbonyl compounds that are
always observed in oxidized lipid systems that can not be explained
by generally accepted mechanisms. In view of this, it seemed feasible
to look for other possible substrates in lipids that are readily oxidized and that might account for some of the variety of carbonyls
observed. The carbonyl compounds, themselves a product of autoxidation,
were selected for this study.
In a preliminary study, the carbonyl products of carefully
autoxidized methyl linolenate was qualitatively and quantitatively
compared to the theoretically derived carbonyls to determine those
compounds not accountable by currently accepted mechanisms. The
analysis included TBA number, peroxide value, total saturated and
unsaturated carbonyl content and total saturated and unsaturated
volatile carbonyl content. The volatile monocarbonyls were isolated
by reduced pressure steam distillation, separated as 2, 4-dinitrophenylhydrazones
by column chromatography and analyzed.
Forty-seven percent of the oxygen consumed by linolenate
was found in the hydroperoxides and 54 percent was found in the total
carbonyl content. The monocarbonyls identified were ethanal, propanal,
butanal, but-2-enal, pent-2-enal, hex-2-enal, hept-2-enal,
oct-2-enal, hexa-2, 4-dienal, hepta-2, 4-dienal, and nona-2, 4-dienal.
One member from each of the major monocarbonyl classes
encountered in autoxidizing lipids was autoxidized under controlled
conditions. The carbonyls selected were n-nonanal, non-2-enal,
hepta-2, 4-dienal, and oct-l-en-3-one. Samples which had consumed
0.25 and 0.5 moles of oxygen per mole of sample were analyzed for
peroxide, malonaldehyde, and acid production. The degradation products were analyzed, as 2, 4-dinitrophenylhydrazones, using column,
paper, and thin layer chromatography, ultraviolet and infrared
spectroscopy, and melting point determinations.
Non-2-enal and hepta-2, 4-dienal oxidized immediately with
no induction period, whereas, n-nonanal had an induction period of
approximately 12 hours and oct-1-en-3-one did not oxidize when held
at 45°C for 52 hours. The major oxidation product of non-2-enal
was non-2-enoic acid. However, eight carbonyl compounds were
identified as degradation products. These listed in order of decreasing
concentrations were ethanal, n-heptanal, α-ketooctanal, n-octanal,
propanal, glyoxal, α-ketononanal, α-ketoheptanal, and malonaldehyde
(measured by the TBA reaction).
The major oxidation products of hepta-2, 4-dienal were polymers.
The carbonyl compounds produced by autoxidation, listed in
order of decreasing concentrations, were propanal, ethanal, cisbut-
2-en-l, 4-dial, n-butanal, α-ketopentanal, glyoxal, malonaldehyde
(measured by TBA reaction), α-ketohexanal, α-ketoheptanal.
Hepta-2, 4-dienal produced 10 times more malonaldehyde and had a
more pronounced pro-oxidant effect on methyl linoleate than non-2-
enal. n-Nonanal had no pro-oxidant effect at the concentration used
The above results coupled with data in the literature again
clearly illustrated the complexity of oxidative degradation of food lipids. It was shown that many of the carbonyls in oxidized lipids
could originate from the oxidation of the initially formed carbonyl
compounds. This could account for many of the carbonyls which
have been identified in oxidized lipids, but not explained by generally
accepted oxidation mechanisms.