Identification of the most intense odor compounds formed during autoxidation of methyl linolenate at room temperature

The volatile compounds formed during autoxidation of methyl linolenate (MLe) at 22–24°C were analyzed by a technique which reveals the flavor compounds having the highest flavor units (ratio of the concentration to the odor treshold). After a reaction time of 48 hr, when ca. 20 mol percent of the MLe had been converted into hydroperoxides,trans, cis-2,6-nonadienal followed by 1,cis-5-octadien-3-one,rans,cis-3-5-octadien-2-one andcis-3-hexenal showed the highest flavor units. After 102 hr 1,cis-5-octadien-3-one was by far the most important odor compound, followed bycis-3-hexenal andtrans,cis-2,6-nonadienal. These results were presented at the AOCS Annual Meeting in New Orleans in May 1987.     

The isolation and characterization of the polymers formed during the autoxidation of ethyl linolenate

Summary Pure ethyl linolenate was prepared and autoxidized; and dimeric and trimeric fractions were separated by solvent fractionation. These fractions were subjected to quantitative, functional group-analysis. On a basis of oxygen known to be present but not detected in the functional groups the possibility that polymerization had taken place through an oxygen linkage was considered. The mild nature of the reagent causing depolymerization suggested that this linkage was of the peroxide type rather than the dioxane ring. A report of work done under contract with the U.S. Department of Agriculture and authorized by the Research and Marketing Act of 1946. The contract was supervised by the Northern Regional Research Laboratory of the Bureau of Agricultural and Industrial Chemistry, Peoria, Ill. Presented at the American Oil Chemists' Society Meeting, October 12–14, 1954, Minneapolis, Minn.     

Evidence for hydroperoxide formation in the autoxidation of methyl linolenate

Summary 1. The results of chemical, spectral, and distillation analyses on the isolated oxidized fractions of three samples of methyl linolenate autoxidized at 0°C. to peroxide values from 600 to 760 m.e./kg. indicated that about 60% of the products consisted of cis,transconjugated diene methyl octadecatrienoate-monohy-droperoxide. 2. A product was isolated from the reduced peroxide concentrates which consisted of about 90% cis,trans- conjugated diene methyl monohydroxyoctadecatrienoate. 3. Analytical micromolecular distillation analyses showed that the total oxidized material isolated from samples of autoxidized methyl linolenate contained only about 15% polymeric material under the conditions of autoxidation employed in this study. 4. Linolenate was shown to be similar to linoleate in forming a cis,trans-conjugated monomeric monohydroperoxide as a major initial product of autoxidation at 0°C. This work was supported in part by a contract between the Office of Naval Research, Department of the Navy, and the University of Minnesota. It was presented at the spring meeting of the American Oil Chemists’ Society, New Orleans, May, 1953.     

Hydroperoxides formed by ferrous ion-catalyzed oxidation of methyl linolenate

An emulsion of methyl linolenate was allowed to oxidize with a catalyst of ferrous sulfate and ascorbic acid. Three oxidation products were isolated, and their hydrogenated derivatives were characterized as the isomeric mixture of methyl monohydroxyoctadecanoate (monoOH), methyl 9,16-dihydroxyoctadecanoate (diOH), and the isomeric mixture of methyl trihydroxyoctadecanoate (triOH). The monoOH isomers and diOH apparently were derived from methyl monohydroperoxyoctadecatrienoate (monoHPO) and methyl dihydroperoxyoctadecatrienoate (diHPO), respectively. Two triOH isomers (the 9,10,12- and 13,15,16-isomers) were thought to be derived from the products containing cyclic peroxide-hydroperoxide structure. 9,16-diHPO was produced by the incubation of monoHPO with ferrous sulfate and ascorbic acid. Moreover, the experiment using¹⁸O₂ demonstrated that mono-HPO yielded 9,16-diHPO by reacting with oxygen molecule. 9,10,13- and/or 9,12,13- and 12,13,16- and/or 12,15,16-triOH isomers were also detected in the hydrogenated derivatives of oxidation products from monoHPO.     

Further observations on the dicarbonyl compounds formed via autoxidation of methyl linoleate

a-Dicarbonyls isolated from oxidized methyl linoleate and conclusively identified as DNP-osazones³ were glyoxal, methyl glyoxal,a-keto hexanal,a-keto heptanal, anda-keto octanal. Technical Paper No. 2012, Oregon Agricultural Experiment Station.     

Hydrogenation of methyl linolenate. I. The formation of methyl isolinoleate on the hydrogenation of methyl linolenate

Summary A highly purified sample of methyl linoleate was prepared from linolenic acid obtained by debromination of hexabromostearic acid. The methyl linolenate was hydrogenated to varying degrees of saturation, using palladium on barium sulfate as the catalyst. Ethyl acetate was used as the solvent and all hydrogenations were conducted at room temperature and atmospheric pressure. The hydrogenated samples were analyzed for their fatty acid composition (as methyl esters). The relative reactivities of the various polyunsaturated acids towards hydrogenation were calculated and may be represented by the following whole numbers: oleic (including isooleic) acid, 1; isolinoleic acid, 5; linoleic, 27; linolenic, 27. A procedure was outlined for effecting a concentration of an isolinoleic acid (methyl ester) by low temperature crystallization. The fraction isolated contained 95.8% methyl isolinoleate. Contribution No. 798 from the Department of Chemistry, University of Pittsburgh.     

Hydroperoxide formation during autoxidation of conjugated linoleic acid methyl ester

The aim of this study was to investigate whether hydroperoxides are formed in the autoxidation of conjugated linoleic acid (CLA) methyl ester both in the presence and absence of α‐tocopherol. The existence of hydroperoxide protons was confirmed by D₂O exchange and by chemoselective reduction of the hydroperoxide groups into hydroxyl groups using NaBH₄. These experiments were followed by nuclear magnetic resonance (NMR) spectroscopy. The ¹³C and ¹HNMR spectra of a mixture of 9‐hydroper‐oxy‐10‐trans,12‐cis‐octadecadienoic acid methyl ester (9‐OOH) and 13‐hydroperoxy‐9‐cis, 11‐trans‐octadecadienoic acid methyl ester (13‐OOH), which are formed during the autoxidation of methyl linoleate, were studied in detail to allow the comparison between the two linoleate hydroperoxides and the CLA methyl ester hydroperoxides. The ¹³CNMR spectra of samples enriched with one of the two linoleate hydroperoxide isomers were assigned using 2D NMR techniques, namely Correlated Spectroscopy (COSY), gradient Heteronuclear Multiple Bond Correlation (gHMBC), and gradient Heteronuclear Single Quantum Correlation (gHSQC). The ¹³C and ¹H NMR experiments performed in this study show that hydroperoxides are formed during the autoxidation of CLA methyl ester both in the presence and absence of α‐tocopherol and that the major isomers of CLA methyl ester hydroperoxides have a conjugated monohydroperoxydiene structure similar to that in linoleate hydroperoxides.     

Red pigment-forming substances from autoxidized linolenate: Identification of prostaglandin-like substances

The chemical structures of lipid degradation products capable of reacting with amino acids and forming red pigments were investigated. The red pigment-forming substances (RPS's) derived from autoxidized linolenate in triglyceride of linseed oil were purified successively by gel chromatography on Sephadex LH-20, column chromatography and TLC on Silica gel 60, and HPLC on μ-Porasil. Consequently, three types of RPS's were isolated. IR spectra of RPS's were similar, except for slight differences in the fingerprint region (1300—650 cm⁻¹). These substances included the OH group (3500 cm⁻¹ region), conjugated aldehyde (νC=0 1688 cm⁻¹, νC=C 1635 cm⁻¹) and ketone (νC=0 1740 cm⁻¹) in their molecules. RPS's were analyzed by GLC and GC-MS after derivatization with dimethylhydrazine and/or trimethylsilyl reagents, before and after the reduction with NaBH₄ and/or hydrogenation with PtO₂. The fragmentation patterns indicated the presence of an ethyl group in addition to the functional groups described above, and the molecular formula was estimated to be C₁₀H₁₄O₃. Further elucidation of the structures was obtained by¹³C- and¹H-NMR analyses, and evidence was obtained for the presence of a hydroxypentanone ring, a PG-like structure. The sequence of the protons on the ring carbons was unequivocally deduced from the double resonance experiments. All the data taken together suggested that the RPS's were the stereoisomer of 3-(2-ethyl-5-hydroxy-3-oxo) cyclopentanyl-2-propenal. Presented at the General Meeting of JSSF held in Tokyo University of Fisheries, Tokyo, April 1983.     

Photosensitized oxidation of methyl linolenate. Secondary products

Previous studies of secondary oxidation products by high-pressure liquid chromatography (HPLC) of autoxidized methyl oleate, linoleate and linolenate and photosensitized-oxidized linoleate are extended to photosensitized-oxidized linolenate. Photosensitized-oxidized linolenate was fractionated by silicic acid chromatography with diethyl ether/hexane mixtures. Selected silicic acid chromatographic fractions were separated by polar phase HPLC and characterized by thin layer and gas liquid chromatography and by ultraviolet, infrared, nuclear magnetic resonance and mass spectrometry. Secondary products from the photosensitized oxidation mixtures (containing 8.2 to 29.0% monohydroperoxides) included keto- and epoxy-dienes (0.4–1.6%), hydroperoxy epidioxides (0.8–4.9%), hydroperoxy bicyclic monoenes (0.1–0.3%), dihydroperoxides (1.0–5.6%), and hydroperoxy bisepidioxides (0.7–1.6%). Some of these secondary products are new and unique to photosensitized oxidation. Cyclization of the 10-, 12-, 13- and 15-hydroperoxides of linolenate would account for their lower relative concentration than that found for the 9- and 16-hydroperoxides. Dihydroperoxides may be derived from monohydroperoxides by singlet oxygenation or free radical oxidation. The hydroperoxy bis-epidioxides may be formed by further serial cyclization of the hydroperoxy epidioxides from 10- and 15-monohydroperoxides. Dihydroperoxides, hydroperoxy epidioxides and hydroperoxy bis-epidioxides are suggested as important flavor precursors in oxidized fats. The mention of firm names or trade products does not imply that they are endorsed by the US Department of Agriculture over other firms or similar products not mentioned.     

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.     

Determination of pentane formed during autoxidation of oils contained in solid samples

A convenient and practical technique for the measurement of pentane formed during peroxidation of solid materials containing unsaturated oils was developed. The test has a sensitivity below. 1. parts per million (ppm) and can be used to monitor shelf-life of a variety of products. A special glassware was designed to perform a single extraction-concentration step. The pentane is recovered in 2 ml of hexane and analyzed by gas chromatography. This method correlates well with taste panel scores.     

Thermal polymerization of methyl linolenate,alpha- and beta-eleostearates

Summary The rates of polymerization of alpha and beta eleostearates agree with second order kinetics, as would be expected for a bimolecular Diels-Alder addition. The all-trans, beta isomer reacts faster than thecis, trans, trans alpha isomer, in agreement with knowncis, trans effects on diene activity. The polymerization of normal linoleate follows an apparent first order reaction. It is suggested that conjugation is the slow rate determining monomolecular reaction, as has been proposed for the non-conjugated linoleate isomers. Paper No. 177, Journal Series, Research Laboratories, General Mills Inc. Presented at the 28th fall (Paul Bunyan) meeting of the American Oil Chemists’' Society, Oct. 12, 1954, Minneapolis, Minn.     

Autoxidation of tissue lipids. II. Monocarbonyl compounds formed by the autoxidation of methyl eicosapentaenoate,methyl docosahexaenoate,and cod-liver oil

Fatty acid analysis of autoxidized cod-liver oil with a peroxide value of 192 showed significant degradation of only eicosapentaenoic, docosapentaenoic, and docosahexaenoic acids of the linolenate family. Purified, mildly autoxidized cod-liver oil with a peroxide value of 28, methyl eicosapentaenoate, and methyl docosahexaenoate produced carbonyl patterns in agreement with the accepted mechanism for olefinic autoxidation. In all cases the major products were propanal andn-pent-2-enal as predicted, andn-hex-2-enal andn-hept-2-enal as reported in the literature for linolenate. In addition, the same cod-liver oil, which had been heated to 188C in vacuum for 1 hr to decompose completely the hydroperoxides before carbonyl analysis, showed the presence ofn-hepta-2,4-dienal as predicted.     

Countercurrent distribution of alkali-isomerized methyl linolenate with an argentation system

The isomerized product obtained by heating linolenic acid with potassium hydroxide in ethylene glycol at 165C for 30 min was separated into urea-adduct-forming (AF) and nonurea-adductforming (NAF) materials. Both were converted to methyl esters and fractionated by countercurret distribution (CCD) between hexane and 0.2N silver nitrate in 90% methanol. Some of the CCD fractions were further fractionated by low-temperature crystallization from acetone. The AF material consisted largely of trienoate containing triene conjugation and of trienoate containing diene conjugation. The NAF material was composed of the same two components and cyclic esters.     

Autoxidation of methyl linolenate: Analysis of methyl hydroxylinolenate isomers by high performance liquid chromatography

The mixture of methyl hydroxylinolenates obtained by reduction of the hydroperoxide isomers formed by autoxidation of methyl linolenate was resolved by high performance liquid chromatography into eight major components. These are positional isomers with the hydroxyl group at positions 9, 12, 13, and 16. Two geometrical isomers of each positional isomer are present; these differ in the configuration of the conjugated double bonds (cis-trans andtrans-trans). Autoxidation of methyl linolenate is regioselective and favors the formation of positional isomers 9 and 16.     

Selective hydrogenation with copper catalysts: I. Isolation and identification of isomers formed during hydrogenation of linolenate

Methyl linolenate was hydrogenated with 10% copper chromite catalyst at 150 C and atmospheric hydrogen pressure. The product was separated into monoene, diene and triene fractions by countercurrent distribution. These fractions were further separated into various geometrical isomers. The double bond location in the various fractions was determined by reductive ozonolysis. Double bonds in bothcis andtrans monoene fractions, as well as incis,trans andtrans,trans conjugated dienes, were extensively isomerized. A monoene containing vinylic unsaturation was one of the major products. The nonconjugated dienes were mostly dienes whose double bonds were widely separated. Results are explained on the basis of conjugation of the double bonds in linolenate followed by hydrogen addition. Presented in part at the symposium “Hydrogenation Process,” Division of Industrial Engineering Chemistry, 157th American Chemical Society Meeting, Minneapolis, April 1968. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Quantitative GLC determination of methyl linolenate in the presence of conjugated dienes

Methyl linolenate and cis,trans conjugated isomers of methyl linoleate are not resolved by gas liquid chromatography because they have the same retention time on polar columns used for separation of methyl ester mixtures. A new column of intermediate polarity made from a mixture of OV-17 and OV-225 separated these critical pairs and enabled quantitative determination of methyl linolenate in partially hydrogenated or conjugated esters. Presented at the AOCS meeting, San Francisco, April, 1979.