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. Presented at 72nd Annual Meeting of the American Oil Chemists Society, New Orleans, LA, May 1981.     

Chemical lonization-mass spectrometry of secondary oxidation products from methyl linoleate and linolenate

Chemical ionization-mass spectrometry (CI-MS) with a direct exposure probe was used to analyze a series of hydroperoxy cyclic peroxides and dihydroperoxides obtained from methyl linoleate and linolenate by either autoxidation or photosensitized oxidation. The mass spectra obtained with isobutane and ammonia as reacting gases showed fragmentation patterns similar to those ditions of gas chromatography (GC)-electron impact (EI)-MS. Because the fragmentation patterns obtained under either CI-MS with a direct exposure probe or GC-(EI)-MS conditions are sufficiently predictable, these techniques are powerful analytical tools for the structural characterization of lipid oxidation products. These techniques are also useful to elucidate the cleavage pathways to volatile lipid oxidation products of flavor and biological significance.     

Products of the surface oxidation of methyl linoleate

Methyl linoleate with 5% by weight of methyl palmitate as an internal standard was deposited as a monolayer (20% by weight) on Silica gel H and oxidized at 7,25, and 40°C. Oxidation was followed by iodometric PV practiced directly on aliquots of the silica gel. Lower temperatures gave higher PV maxima but after longer times. Oxidation of methyl linoleate at all three temperatures followed first-order kinetics, and the energy of activation was 15.0 kcal/mol. The products recovered from the chloroform-acetic acid layer of the peroxide determination were analyzed by GC and identified by El- and CI-MS. Calculation based on methyl palmitate as an internal standard showed that the total peak area decreased to about 40% that of the original methyl linoleate when the residual methyl linoleate was reduced to less than 2%. The chief nonscission products (NSP) of linoleate oxidation were epoxy, hydroxy, hydroxy-epoxy, dihydroxy, and trihydroxy methyl esters. The greatest NSP concentrations were obtained about the time of the greatest PV, and the yield at 40°C was greater than those at 7 and 25°C. Scission products (SP) increased rapidly until the greatest PV was reached. After this time, SP declined slightly and plateaued at 40°C, but at 7 and 25°C, SP continued to increase slowly with further oxidation.     

Volatile products from mild oxidation of methyl linoleate. Analysis by combined mass spectrometry-gas chromatography

The volatile products from autoxidation of methyl linoleate have been analyzed by combined mass spectrometry-gas chromatography. The principal components were pentanal, hexanal, amyl formate, methyl octanoate, and substituted dioxolanes. Minor components included esters, alcohols, ketones, aldehydes, hydrocarbons, and acetals. Certain unsaturated carbonyl compounds, previously reported, were not detected. A laboratory of the W. Utiliz. Res. Devel. Div., ARS, USDA.     

Nutritive value of methyl linoleate and its thermal decomposition products

Methyl linoleate was heated for 10 hrs. at 300°C, in the absence of air and fractionated by alembic distillation and urea adduct-formation. Intestinal absorptions of the urea adduct-forming monomeric nonadduct-forming monomeric, and dimeric fractions were determined. It was found that dimers were half as well absorbed as the monomers. When fed to rats, dimers were better accepted and exhibited some toxicity symptoms different from the non-adduct-forming monomers. The dimers caused diarrhea, irritability, and loss of hair during the early period of administration. The nonadduct-forming monomers were lethal and produced an increase in liver weight. Both fractions depressed growth. With the technical assistance of Oscar giacomantone and Perla Mordujovich.     

Reaction of histidine with methyl linoleate: Characterization of the histidine degradation products

Efforts have been made to characterize the products that result from interactions between L-histidine (free base) and peroxidizing methyl linoleate (ML) in a model system consisting of reactants dispersed on a filter paper. Imidazole lactic acid and imidazole acetic acid are identified as breakdown products when histidine is incubated with ML, methyl linoleate hydroperoxide (MLHPO), or n-hexanal over a period of 3 weeks. Two other reaction products are found to give back histidine upon acid hydrolysis. These products are though to be Schiff's base compounds which result from the condensation of the histidyl α-amino group and carbonyl groups of reactive aldehydes formed during ML peroxidation. Most of the detectable reaction products have the imidazole moiety intact indicating the high relative reactivity of the functional groups, especially the amino group, associated with the α-carbon. Such high reactivity provides an explanation for the low concentrations of ninhydrin-positive free amino compounds that are, at best, barely detectable on thin layer chromatography. Presented at the AOCS Meeting, Chicago, September 1976.     

Concurrent oxidation of accumulated hydroperoxides in the autoxidation of methyl linoleate

Summary 1. Kinetic studies showed that concurrent oxidation of preformed hydroperoxides may be expected to take place at all stages of the autoxidation of methyl linoleate. The rate of oxidation relative to the rate of autoxidation of unoxidized ester is determined chiefly by the extent of the accumulation of hydroperoxides. 2. Infrared spectral analysis of hydroperoxides oxidized to various degrees indicated thattrans, trans diene conjugation and isolatedtrans double bonds produced in the autoxidation of methyl linoleate are related to the concurrent oxidation of the accumulated hydroperoxides. 3. The low absorptivity observed for diene conjugation, compared to that which may be expected for the exclusive production ofcis, trans diene conjugated hydroperoxide isomers during the autoxidation of methyl linoleate is attributed to the concurrent oxidation of accumulated hydroperoxides. 4. The effect of antioxidants in giving a well-defined induction period in the oxidation of hydroperoxides isolated from autoxidized methyl linoleate indicated that the oxidation proceeds by a chain reaction. 5. The primary reaction products of the oxidation of hydroperoxides isolated from autoxidized methyl linoleate were found to be polymers formed in a sequence of reaction involving the diene conjugation. 6. Studies on the autoxidation of methylcis-9,trans-11-linoleate showed thatcis, trans isomerization of the conjugated diene took place with the concurrent production of isolatedtrans double bonds and loss of diene conjugation. Hormel Institute publication no. 138. Presented before the American Oil Chemists’ Society, Philadelphia, Pa., Oct. 10–12, 1955. This work was supported by a grant from the Hormel Foundation.     

Effect of oil-droplet size on the oxidation of microencapsulated methyl linoleate

Effects of oil-droplet size, the weight ratio of oil to wall material and the storage temperature on the oxidation of methyl linoleate microencapsulated with maltodextrin by spray-drying were examined. The oxidation of methyl linoleate was more retarded for the microcapsules prepared from the emulsion having smaller oil droplets. The oxidation was more suppressed for the microcapsules having a lower weight ratio of oil to wall material. The fraction of unoxidized methyl linoleate leveled off after 10- to 15-days storage. The level, Y(infinity), depended on the weight ratio. The dependence of Y(infinity) on the weight ratio was analyzed based on the percolation theory, and the three-dimensional model of the theory was suitable to express the dependence. The effect of the storage temperature on the oxidation of microencapsulated methyl linoleate was also examined, and the activation energy was evaluated. The value of the energy suggested that the oxidation itself was a rate-limiting step for the oxidation of methyl linoleate encapsulated with maltodextrin.     

Effect of water-binding agents on the catalyzed oxidation of methyl linoleate

Autoxidation of methyl linoleate was studied in model systems containing added metals and different water-binding agents including cellulose, dextran and glycerol. In all systems, addition of water was increasingly antioxidant up to a critical level of water activity, above which further additions promote oxidation. The specific level of water activity at which oxidation rates are minimal and the water contents at this critical activity depend on the composition of the system. Results of the present study and much of the conflicting data in the literature can be explained as follows. A small amount of water is tightly bound to polysaccharides and does not affect lipid oxidation. Additional amounts of water are antioxidant because of their ability to hydrate metallic catalysts and to form hydrogen bonds with hydroperoxides. At high water contents the water's solvent action mobilizes catalysts, thereby overcoming the antioxidant effects. The present study indicates that water bound to glycerol is still capable of mobilizing catalysts. Depression of water activity by addition of agents such as glycerol has different effects on oxidation than does depression of water activity by other means. Interaction between the effects by system composition and water activity must be considered when either is varied to maximize storage stability of foods     

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.     

Oxidation products of α-tocopherol formed in autoxidizing methyl linoleate

The oxidation products of¹⁴C-α-tocopherol formed by heating with methyl linoleate in an air atmosphere at 60 C or 100 C were investigated. The products included a dimer, trimer and dihydroxy dimer, α-tocopherol quinone and unidentified degradation compounds. The dimer and trimer constituted the major products present after heating for 70 hr at 60 C. After 70 hr at 100 C most of the¹⁴C-α-tocopherol had been converted to degradation products; part of the¹⁴C originallt present in the 5-methyl group was recovered as¹⁴CO₂ and¹⁴CH₃OH.     

Structure of monohydroperoxides formed by chlorophyll photo-sensitized oxidation of methyl linoleate

The structures of the hydroperoxides prepared by photochemical oxidation of methyl linoleate in the presence of chlorophyll were determined by gas liquid chromatography-mass spectrometry (GLC-MS) of the trimethylsilyl (TMS) derivatives of the hydroxyesters. The position of the double bonds were deduced via hydroxylation with OsO₄, followed by GLC-MS analysis of the TMS derivatives. Structures of the original hydroperoxides were elucidated from these data. One of 28 papers presented at the Symposium “Metal-Catalyzed Lipid Oxidation,” ISF-AOCS World Congress, Chicago, September 1970.     

Analysis of fat acid oxidation products by countercurrent distribution methods. V. Low-temperature decomposition of methyl linoleate hydroperoxide

Methylcis, trans diene conjugated linoleate hydroperoxide isolated by counterenrrent distritbution from 4°C, auatoxidation of methyl linoleate was stored in atmospheres of oxygen and of nitrogen at 4°C. in darkenss. Besides manometric changes, infrared and ultraviolet characteristics, peroxide value, diene conjugation, and molecular weights were followed on samples removed at various periods of storage up to 53 days. These same analyses were obtained on fractions obtained by counter-current distributions. Evidence for the reaction that occurs on storage in oxygen may be summarized thus: 1 mole oxygen absorbed by linoleate hydroperoxides destroys 1 molecis, trans diene conjugation, 1/2 mole peroxide group, and 1 mole linoleate hydroperoxide; dimers of varying polarities, scission acids, and isolatedrans bonds are formed. Since to volume changes were observed in the nitrogen storage of methyl linoleate hydroperoxide, changes in chemical and physical characteristics can only be related to time of storage. Storage in nitrogen at 4°C, destroys diene conjugation, peroxides, and linoleate hydroperoxide and produces dimers of varying polaritics, seission neids, and isolatedrans bonds. Destruction of diene conjugation was one-fourth as rapid in a nitrogen atmosphere as in oxygen. While differences in reactions and products were observed between oxygen and nitrogen storage, particularly in rates and in countereurrent distribution patterns, the similarity of products from oxygen and nitrogen storage is remarkable. One methyl linoleate hydroperoxide is formed regardless of storage atmosphere, dimirization and attendant destruction of double bonds and peroxides proceed. This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture.     

Prooxident activities of chlorophylls and their decomposition products on the photooxidation of methyl linoleate

Although chlorophylls in oils affect oxidative stabilities, little has been known about the prooxidant activity of the chlorophylls or their decomposition products. To evaluate the prooxidant activities of chlorophyll derivatives, chlorophyll (CHL) A and B, pheophytin (PHY) A and B and pheophorbide (PHO) A and B were added to methyl linoleate (ML). The sample oils were then subjected to photooxidation at O C and the peroxide value and absorbance at 234 nm were measured. Chlorophylls catalyzed oxidation of methyl linoleate at concentrations greater than 2.2×10⁻⁹mol/g ML, and CHL B showed a stronger prooxidant activity, ca. twice that of CHL A under the same light intensity. PHY and PHO exhibited stronger prooxidant activities than CHL, the prooxidant activity of PHO being the strongest among them. Moreover, photosensitive activity was found in PHY as well as in CHL. These results suggest that particular attention should be paid to the decomposition products of CHL that affect the quality of vegetable oils.     

Determination of degree of oxidation of methyl linoleate and linolenate by weighing method

Autoxidation of methyl linolenate and linoleate was investigated at room temperature in the light and dark during a period of 2 mon using a weighing method accompanied with ¹H NMR measurements. In the light, the increase in weight, or autoxidation process, of the two compounds began immediately after the start of the experiment. In the dark, the weight of methyl linolenate began to increase after a delay of 1 wk, and the weight of methyl linoleate after 40 d. The maximal increases in the weights of the compounds were somewhat greater in the dark than in the light; the weight of methyl linolenate in the dark increased by 22.3% and in the light by 20.8%. The corresponding values for methyl linoleate were 16.9 and 14.4%, respectively. Results of the NMR measurements were compared to the weighing results, and a table was constructed representing the correlation between selected multiplets and corresponding weight increments.     

Effect of browning reaction products of phospholipids on autoxidation of methyl linoleate

Methyl linoleate was allowed to autoxidize in bulk phase at 50 C in the presence of either synthetic phospholipids consisting of saturated fatty acids or egg yolk phospholipids to estimate the effect of base group and fatty acid moiety of phospholipids on oil autoxidation. Dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE) exhibited poor antioxidant activity at 50 C and showed no synergistic effect with α-tocopherol. The addition of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) of egg yolk accelerated the oxidation of methyl linoleate. However, egg yolk PC and PE collected at various stages of heating at 180 C inhibited hydroperoxide formation at the initial stage of oxidation. This effect could be attributed to the browning products formed during heating reaction. Thus, browning color products formed from unsaturated phospholipids at high temperatures may influence oil stability, although the base group of phospholipids did not exert any significant effect.     

Autoxidation of methyl linoleate. Separation and analysis of isomeric mixtures of methyl linoleate hydroperoxides and methyl hydroxylinoleates

The mixture of conjugated diene hydroperoxide isomers obtained from autoxidation of methyl linoleate was separated by high performance liquid chromatography (HPLC). Four major isomers were obtained from adsorption chromatography and identified as the 9 and 13 positional isomers having thetrans-trans andcis-trans configurations. The latter geometrical isomers have thetrans double bond adjacent to the hydroperoxide group. The hydroxy compounds (methyl hydroxylinoleates) obtained from the hydroperoxides by NaBH₄ reduction were similarly separated but with improved resolution. This is the first instance of the complete separation of these compounds and provides a rapid method for their analysis. Unlike adsorption chromatography, reversed-phase chromatography separates the mixtures only according to the geometrical isomerism of the double bonds and not according to the position of the hydroxy or hydroperoxide function.     

Gas chromatographic study on high-temperature thermal degradation products of methyl linoleate hydroperoxides

Chromatographic techniques were used to separate secondary products generated by thermal degradation of methyl linoleate hydroperoxides (MLHP). The MLHP were obtained by oxidation, selected, and concentrated by solid-phase extraction (SPE) and thin-layer chromatography (TLC). The purified MLHP were then thermo-degraded in the gas-chromatographic glass liner and analyzed on-line by gas chromatography-mass spectrometry (GC-MS). The MLHP were also thermodegraded and collected in a short silicic acid-packed column, eluted, separated by TLC, and then analyzed by GC. By considering the elution in TLC, the GC retention times and the GC-MS analyses, it was possible to characterize the mono- and the dioxygenated secondary products, particularly those having a boiling point higher than methyl linoleate. The peaks that corresponded to the mono-oxygenated products (epoxy, hydroxy, and keto) were identified, and, on the basis of their MS spectra, molecular structures were proposed. A specific elution order was suggested for keto derivatives: 9-keto,Δ^10,12- and 13-keto,Δ^9,11-octadecadienoate. The hydroxy derivatives, which show the typical fragmentations of 9-hydroxy,Δ^10,12- and 13-hydroxy,Δ^9,11-octadecadienoate, were also identified. On the other hand, identification of the di-oxygenated compounds was more difficult, and, therefore, it was not possible to indicate each positional isomer; however, their elution order could be epoxy-hydroxy and epoxy-keto derivatives.     

Effect of water activity on secondary products formation in autoxidizing methyl linoleate

The role of water activity on the formation of peroxides and carbonyl compounds during lipid oxidation is important to know because there could be either beneficial or detrimental effects of water activity on lipid oxidation in stored foods. Therefore, methyl linoleate was chosen as a model lipid and was autoxidized to 1% at water activity ranging from 0.02 to 0.79 at 37°C. Oxygen uptake was monitored manometrically. The peroxide and carbonyl contents were determined upon termination of the autoxidation studies. Methyl linoleate autoxidation was characterized by three phases: i) an initial induction period of no oxygen absorption; ii) a slow rate of oxygen absorption, up to 0.15% oxidation; and iii) a relatively faster rate of oxygen absorption beyond 0.15% up to 1% oxidation. Water activity had considerable influence during the first phase. There was no induction period at or below water activity 0.22. The induction period begins at water activity 0.32 and could be extended to a limit with increase in water activity. Once the induction period was passed water activity had no influence on the rate of oxidation. However, during the second and third phases water activity becomes important in the stabilization of peroxides/hydroperoxides and decides the course of secondary reactions that follow peroxide decomposition. Higher water activity values, particularly water activity 0.67, tended to stabilize peroxides. Water activity had considerable influence on the formation of secondary products of autoxidation as evidenced by the variation in the type and quantity of carbonyl compounds at different water activity values.     

Effect of α-tocopherol on the volatile thermal decomposition products of methyl linoleate hydroperoxides

α-Tocopherol and 1,4-cyclohexadiene were tested for their effect on the thermal decomposition of methyl linoleate hydroperoxide isomers. The volatiles generated by thermolysis in the injector port of a gas chromatograph at 180°C were analyzed by capillary gas chromatography. In the presence of either α-tocopherol or 1,4-cyclohexadiene, which are effective donors of hydrogen by radical abstraction, volatile formation decreased in all tests, and significant shifts were observed in the relative distribution of products in certain hydroperoxide samples. When an isomeric mixture of methyl linoleate hydroperoxides (cis, trans andtrans, trans 9- and 13-hydroperoxides) was decomposed by heat, the presence of α-tocopherol and 1,4-cyclohexadiene caused the relative amounts of pentane and methyl octanoate to decrease and hexanal and methyl 9-oxononanoate to increase. A similar effect of α-tocopherol was observed on the distribution of volatiles formed from a mixture of thetrans,trans 9- and 13-hydroperoxides. This effect of α-tocopherol was, however, insignificant with purecis,trans 13-hydroperoxide of methyl linoleate. The decrease in total volatiles with the hydrogen donor compounds, α-tocopherol and 1,4-cyclohexadiene, indicates a suppression of homolytic β-scission of the hydroperoxides, resulting in a change in relative distribution of volatiles. The increase in hexanal and methyl 9-oxononanoate at the expense of pentane and methyl octanoate in the presence of hydrogen donor compounds supports the presence of a heat-catalyzed heterolytic cleavage (also known as Hock cleavage), which seems to mainly affect thetrans,trans isomers of linoleate hydroperoxides.