The autoxidative degradation of some secondary products of autoxidizing lipids Public Deposited

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  • 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 (0.1 percent). 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.
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