Edward Selke

Analysis of autoxidized fats by gas chromatography-mass spectrometry: VII. Volatile thermal decomposition products of pure hydroperoxides from autoxidized and photosensitized oxidized methyl oleate, linoleate and linolenate

To clarify the sources of undesirable flavors, pure hydroperoxides from autoxidized and photosensitized oxidized fatty esters were thermally decomposed in the injector port of a gas chromatograph-mass spectrometer system. Major volatile products were identified from the hydroperoxides of methyl oleate, linoleate and linolenate. Although the hydroperoxides from autoxidized esters are isomerically different in position and concentration than those from photosensitized oxidized esters, the same major volatile products were formed but in different relative amounts. Distinguishing volatiles were, however, produced from each type of hydroperoxide. The 9- and 10-hydroperoxides of photosensitized oxidized methyl oleate were thermally isomerized in the injector port into a mixture of 8-, 9-, 10- and 11-hydroperoxides similar to that of autoxidized methyl oleate. Under the same conditions, the hydroperoxides from autoxidized linoleate and linolenate did not undergo significant interconversion with those from the corresponding photosensitized oxidized esters. The compositions of the major volatile decomposition products are explained by the classical scheme involving carboncarbon scission on either side of alkoxy radical intermediates. Secondary reactions of hydroperoxides are also postulated, and the hydroperoxy cyclic peroxides from methyl linoleate (photosensitized oxidized) and methyl linolenate (both autoxidized and photosensitized oxidized) are suggested as important precursors of volatiles.

Photosensitized oxidation of methyl linoleate: Secondary and volatile thermal decomposition products

Studies of photosensitized oxidation of methyl linoleate show that the greater relative concentration of 9- and 13-hydroperoxides than 10- and 12-hydroperoxides is characteristic of singlet oxygenation and not due to either simultaneous autoxidation or type 1 photosensitized oxidation. Cyclization of the internal 10- and 12-hydroperoxides accounts for their lower relative concentrations. Secondary products separated by silicic acid and high pressure liquid chromatography were characterized spectrally (IR, UV,1H-NMR,13C-NMR, GC-MS). Major secondary products included diastereomeric pairs of 13-hydroperoxy-10,12-epidioxy-trans-8-octadecenote (I and III) and 9-hydroperoxy-10,12-epidioxy-trans-13-octadecenoate (II and IV); minor secondary products included hydroperoxy oxy genated and epoxy esters. Thermal decomposition of the hydroperoxy cyclic peroxides produced hexanal and methyl 10-oxo-8-decenoate as major volatiles from I and III and methyl 9-oxo-nonanoate and 2-heptenal from II and IV. Hydroperoxy cyclic peroxides may be important sources of volatile decomposition products of photooxidized fats.

Analysis of autoxidized fats by gas chromatography-mass spectrometry. IX. Homolytic vs. Heterolytic cleavage of primary and secondary oxidation products

To elucidate the genesis of volatile lipid oxidation products, thermal homolytic and acid heterolytic decomposition processes were compared. Secondary oxidation products were decomposed thermally (200 C), and the volatiles formed were identified by capillary gas chromatography-mass spectrometry (GC-MS). Oxidation products also were decomposed in the presence of HCl-methanol, and the resulting dimethyl acetals were identified by GC-MS. The volatile thermal decomposition products were those expected by homolytic β-scission on both sides of the hydroperoxide group. No dialdehydes were identified under our thermal decomposition conditions. In contrast, the acetals formed by acid decomposition were those expected by selective heterolytic scission between the hydroperoxide group and the allylic double bond. Dialdehydes identified from acid decomposition of cyclic peroxides and dihydroperoxides included malonaldehyde and 2,4-hexadienedial.

Photosensitized oxidation of methyl linoleate monohydroperoxides: Hydroperoxy cyclic peroxides, dihydroperoxides, keto esters and volatile thermal decomposition products

Previous studies of lipid secondary oxidation products have been extended to 6-membered hydroperoxy cyclic peroxides from the singlet oxygenation of a mixture of 9- and 13-hydroperoxides from autoxidized methyl linoleate. The oxidation product was fractionated by silicic acid chromatography with diethyl ether/hexane mixtures, and selected fractions were separated by polar phase high performance liquid chromatography. Products characterized by thin layer chromatography, gas liquid chromatography, ultraviolet, infrared, nuclear magnetic resonance and mass spectrometry included: 6-membered cyclic peroxides (13-hydroperoxy-9,12-epidioxy-10- and 9-hydroperoxy-10,13-epidioxy-11-octadecenoates), dihydroperoxides (8,13- and 9,14-dihydroperoxyoctadecadienoates) and keto dienes (9- and 13-oxooctadecadienoates). The 6-membered hydroperoxy cyclic peroxides are apparently formed by 1,4-addition of singlet oxygen to 9- and 13-hydroperoxides withtrans, trans-conjugated diene systems. Thermal decomposition of the 6-membered hydroperoxy cyclic peroxides at 200 C produced methyl 9-oxononanoate and hexanal as the major volatiles. Other volatiles included 2-pentylfuran, pentane, 4-oxo-2-nonenal, methyl furanoctanoate and methyl 9,12-dioxo-10-dodecenoate.

Volatiles from thermal decomposition of isomeric methyl (12S, 13S)-(E)-12,13-epoxy-9-hydroperoxy-10-octadecenoates

Two epimers of methyl (12S,13S)-(E)-12,13-epoxy-9-hydroperoxy-10-octadecenoate were isolated after esterification of a mixture of fatty acids obtained from decomposition of (13S)-(9Z,11E)-13-hydroperoxy-9,11-octadecadienoic acid by an Fe++-cysteine catalyst. These epimeric epoxyhydro-peroxyoctadecenoates were decomposed by heat (210 C) in the injection port of a gas chromatograph, and the cleavage fragments were subsequently separated by gas chromatography (GC) and identified by mass spectrometry (MS). Among the scission products obtained, the most prominent in the GC peak profile were methyl octanoate and methyl 9-oxononanoate. Other peaks were identified as pentane, 1-pentanol, hexanal, 2-heptanone, 2-pentylfuran, methyl heptanoate, 2-octenal, 4,5-epoxy-2-decenal, methyl 8-(2-furyl)-octanoate, 11-oxo-9-undecenoate and methyl 13-oxo-9,11-tridecadienoate. In addition, 3,4-epoxynonanal, methyl 8-oxooctanoate, 3-hydroxy-2-pentyl-2,3-dihydrofuran and methyl 10-oxodecanoate were tentatively identified. Except for the furan compounds, the formation of the fragmentation products could be explained by conventional free-radical scission mechanisms.

Thermal and metal-catalyzed decomposition of methyl linolenate hydroperoxides

Much work has been reported on the volatile oxidative products of fats and their impact on flavor deterioration, cellular damage and the decrease in safety of fatcontaining foods. However, relatively little information is available on the mechanism of hydroperoxide decomposition. Pure methyl linolenate hydroperoxides were decomposed thermally at 150 C and catalytically with ferric chloride-ascorbic acid at room temperature. The volatile decomposition products were collected on porous polymer (Tenax) traps and concentrated by gel permeation chromatography. The total volatile products showed significant differences in composition by capillary gas chromatography-mass spectrometry (GC-MS). Thermal decomposition produced much more methyl octanoate (60.1%) and less 2,4-heptadienal (0.5%) than catalytic decomposition (13.2 and 60.8%, respectively). The volatiles from the ferric chloride-ascorbic acid system also contained unique products tentatively identified by GC-MS as isomers of chloromethyl butene. These results may have important implications in evaluating precursors of flavor deterioration in vegetable oils containing linolenate and in understanding better the biological significance of lipid peroxidation.

Thermal decomposition of methyl oleate hydroperoxides and identification of volatile components by gas chromatography-mass spectrometry

The role of methyl oleate hydroperoxides as precursors of volatile compounds was investigated by thermal decomposition in the injector port of a gas chromatograph attached to a computerized mass spectrometer. The major volatile compounds identified correspond to those formed from triolein heated in air at 192 C.

Analysis of autoxidized fats by gas chromatography-mass spectrometry: VIII. Volatile thermal decomposition products of hydroperoxy cyclic peroxides

Secondary oxidation products are important sources of volatiles because of their susceptibility to further decomposition. Volatiles from the thermal decomposition of hydroperoxy cyclic peroxides have been identified by capillary gas chromatography followed by mass spectrometry (GC-MS). By using a saturated hydroperoxy cyclic peroxide as a synthetic model, the thermal decomposition pathways have been elucidated. Main cleavage occurs between the peroxide ring and the carbon-bearing hydroperoxide group. Volatiles produced were generally similar to those from corresponding monohydroperoxides. New volatiles identified included methyl furan octanoate, methyl ketones, and conjugated diunsaturated aldehyde esters. The general fragmentation observed between the peroxide ring and the hydroperoxide-bearing carbons is sufficiently predictable that it can be used as a tool for the structural characterization of hydroperoxy cyclic peroxides. Hydroperoxy cyclic peroxides from autoxidized and photosensitized oxidized methyl linolenate are suggested as important precursors of volatiles that may affect flavor quality of lipid-containing foods.

Dinitrogen (15N2) fixation and acetylene reduction in free-living strains ofRhizobium

Dinitrogen (15N2) fixation of four free-livingRhizobium strains ranged from 0.8 to 2.3 μmol/mg biomass N. Parallel-grown cultures liberated 4–8 μmol hydrogen and reduced 12–23 μmol acetylene, giving a mean ratio of reduced acetylene-to-fixed15N2 of 12. This ratio contrasts with lower values others have observed for asymbiotic diazotrophs.

Thermal Decomposition of Methyl Linoleate and Methyl Linolenate Hydroperoxides Analyzed by Capillary Gas Chromatography

Much work has been reported on the volatile oxidation products of unsaturated fats because they cause rancidity in foods and cellular damage in the body. Decomposition of fatty ester hydroperoxides creates a wide range of carbonyl compounds, hydrocarbons, furans, and other materials that contribute to flavor deterioration of foods and that are implicated in biological oxidation. Interaction of some of these degradation products with DNA, proteins, and enzymes may be involved in cell-damaging reactions.2–4 Although fatty acid hydroperoxides are the recognized precursors of volatile secondary products, relatively little information is available on the mechanism of their decorposition. New information on the volatile lipid oxidation products is needed to better evaluate precursors of oxidative deterioration in lipid-containing foods and to understand the biological significance of lipid peroxidation.