Autoxidation of fatty acid monolayers adsorbed on silica gel: II. Rates and products

Unsaturated fatty acid monolayers on silica gel have been autoxidized, and the rate of fatty acid disappearance and products obtained from those membrane-like assemblies have been studied Fatty acid monolayers consisting of pure linoleic acid, linolelaidic acid, and oleic acid were autoxidized at 60 C. The rates of autoxidation of linoleic acid and linolelaidic acid monolayers followed by the disappearance of substrates are considerably faster than that in bulk phase, and the rates conform to apparent first order kinetics. Autoxidation of linoleic and linolelaidic acid monolayers, unlike bulk phase, produced only a small amount of diene conjugation, and the major products formed were identified as 9,10-epoxy and 12,13-epoxyoctadecenoic acid in roughly equal quantities. The epoxidation is stereospecific, withcis andtrans olefins givingcis andtrans epoxides, respectively. Oleic acid was autoxidized to only a small extent during 27 hr and produced no detectable amount of epoxide.     

Autoxidation products of 2,4-decadienal

2,4-Decadienal was autoxidized by purging a purified sample with oxygen. An analysis of the autoxidative degradation products was made with tandem gas chromatography-mass spectrometry. Additional information was obtained from the determination of the melting point of the dinitrophenylhydrazone derivatives and IR absorbency data. Pentane, furan, ethanal, hexanal, acrolein, butenal, 2-heptenal, 2-octenal, benzaldehyde, glyoxal,trans-2-buten-1,4-dial, acetic acid, hexanoic acid, 2-octenoic acid, 2,4-decadienoic acid and benzene were identified. Technical Paper No. 3069, Oregon Agricultural Experlment Station.     

Autoxidation of synthetic isomers of triacylglycerol containing eicosapentaenoic acid

Several triacylglycerols (TAG) that contained eicosapentaenoic acid (EPA) were chemically synthesized and stored at 25°C to assess the influence of TAG structure on oxidative stability and formation of oxidation products. Oxidative stability was evaluated by oxygen consumption during storage of the TAG. Autoxidation products of TAG were analyzed by high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). Results showed that a 2:1 (mole/mole) mixture of trieicosapentaenoylglycerol (EEE) and tripalmitoylglycerol (PPP) was most susceptible to autoxidation. The oxidative stability of TAG that contained EPA and palmitic acid was negatively correlated with the moles of EPA in a single TAG molecule. When TAG with one EPA and two other fatty acids were oxidized, chainlength of constituent fatty acids hardly affected the oxidative stability of EPA-containing TAG molecules, except for stearic acid. HPLC and LC-MS analyses showed that monohydroperoxides were major oxidation products regardless of type of TAG. Bis- and tris-hydroperoxides were formed during autoxidation of EEE and dieicos-apentaenoylpalmitoylglycerol. Monohydroperoxy epidioxides were found in all autoxidized TAG. These observations suggested that TAG structure affected the oxidation of TAG with highly unsaturated fatty acids.     

Autoxidation rates of various esters of safflower oil and linoleic acid

Autoxidation rates of five types of safflower oil esters increased in the following order: monoacylglycerol (MG), methyl ester (ME), diacylglycerol (DG), triacylglycerol (TG) and sucrose ester (SE). The differences in autoxidation rate were confirmed by measuring autoxidation of similar esters of linoleic acid. The order of the oxidation rates corresponded to the number of acyl groups per molecule. This relationship was explainable by the idea that intramolecular free radical chain reaction between actyl groups of esters occurs more rapidly than intermolecular chain reaction. The position of acyl groups also affected the oxidation rates of esters in that 1,3-dilinolein autoxidized more rapidly than 1,2-dilinolein. This difference in oxidation rate may result from a closer arrangement of acyl groups in 1,3-dilinolein than those in the 1,2-isomer.     

Autoxidation of polyunsaturated triacylglycerols. IV. Volatile decomposition products from triacylglycerols containing linoleate and linolenate

Trilinoleoylglycerol (LLL), trilinolenoylglycerol (LnLnLn) and four synthetic triacylglycerols were autoxidized and the volatile products were investigated to determine the effect of fatty acid glyceride position on the mechanism of hydroperoxide decomposition. Capillary gas chromatography provided a sensitive method to follow the volatile oxidation products of mixtures of LLL and LnLnLn and of synthetic triacylglycerols containing linoleate and linolenate in different known positions. The relative amount of linoleate oxidation was determined by analyzing for hexanal, 2-heptenal and 2,4-decadienal, and the relative amount of linolenate oxidation by analyzing for 2,4-heptadienal and 2,4,7-decatrienal. The volatiles from pure monohydroperoxides of LLL and LnLnLn were compared with those of the corresponding triacylglycerols by capillary gas chromatography. Significant differences in the distribution of volatile products were observed depending on the triacylglycerols precursor. A 1∶1 mixture of LLL and LnLnLn autoxidized at 40°C showed an equal contribution of linolenate and linoleate volatiles at a peroxide value of 34. The synthetic triacylglycerols LLnL and LLLn (L, linoleic; Ln, linolenic acid) formed initially about the same total volatiles, whereas LnLnL formed more volatiles than LnLLn. The ratio Ln to L volatile products was the same for the diL triacylglycerols, and higher for LnLnL than for LnLLn. This new information should permit us to better understand the influence of triacylglycerol structure on the relative oxidative stability of unsaturated triacylglycerols. Presented in part at the 80th Annual American Oil Chemists' Meeting, Cincinnati, OH, May 3–6, 1989.     

Ion exchange resins and ethyleneimine polymer as antioxidants II: Autoxidation products

The antioxidant effects of ion exchange resins and ethyleneimine polymer on the autoxidation products of methyl linoleate in a heterogeneous reaction system are discussed. Results from analyses of the various autoxidation products from linoleate samples with and without the antioxidants showed that the addi-tion of the antioxidants did not change the original autoxidation mechanism of methyl linoleate. How-ever, the antioxidants did retard the autoxidation in response to their antioxidant activity and, compared with a linoleate control, changed the yields of some autoxidized products such as an increased amount of conjugated diene hydroperoxides in linoleate samples with added ion exchange resins.     

Autoxidation of polyunsaturated triacylglycerols. II. Trilinolenoylglycerol

The hydroperoxides and secondary products formed from trilinolenoylglycerol autoxidized at 40°C were isolated and characterized to clarify the mechanism of oxidative deterioration of polyunsaturated vegetable oils. The products were purified by high performance liquid chromatography (HPLC) and identified either as intact triacylglycerols spectrophotometrically, or after lipolysis (pancreatic lipase) and capillary gas chromatography and gas chromatography-mass spectrometry. The products included 9-, 12-, 13-, and 16-mono-,bis-, tris-hydroperoxy, 9- and 16-hydroperoxy epidioxy, 9- and 16-hydroperoxy bicycloendoperoxy and 9,12-, 13,16-, and 9,16-dihydroperoxy linolenoylglycerols. The mono-hydroperoxides and sydroperoxy epidioxides were the only main products initially formed at peroxide values (PV) below 30.Bis- and tris-hydroperoxides were formed consecutively as minor products from the mono-hydroperoxides at PV's between 31 and 47. Hydroperoxy bicycloendoperoxides and monodihydroperoxides were also formed as minor secondary products at PV's above 75. HPLC analyses show a small preference for the formation of 16-hydroperoxides on the 1(3)-position over the 2-position of trilinolenoylglycerol. However, there was no selectivity for the formation of the 9-, 12- and 13-hydroperoxides and for the hydroperoxy epidioxides between the 1(3)- and the 2-positions in trilinolenoylglycerol. Mono-hydroperoxides and hydroperoxy epidioxides of linolenate triacylglycerols may be important precursors of volatile compounds contributing to oxidative deterioration of vegetable oils. Presented at the 79th Annual American Oil Chemists' Society Meeting, Phoenix, Arizona, May 8‐12, 1988.     

Bildung von Hydroperoxy-epidioxiden bei der Autoxidation von Linolsäurephenylester

Formation of Hydroperoxy-epidioxides during the Autoxidation of Phenyl Linoleate Phenyl linoleate was autoxidized at 37° C for 72 hours. Among the peroxides formed the phenyl 13-hydroperoxy-erythro and threo-10, 12-epidioxy-8(E)-octadecenoates and the phenyl 9-hydroperoxy-erythro and threo-10, 12-epidioxy-13(E)-octadecenoates were detected by co-chromatography (HPLC) with reference substances which had been prepared by photosensitized oxidation of phenyl linoleate. The hydroperoxy-epidioxides were silylated and analysed by mass spectrometry. The hydroperoxy-epidioxides comprised about 5% of the peroxides. The presence of α-tocopherol inhibited the formation of the hydroperoxy-epidioxides.     

Autoxidation of methyl esters of cyclopentenyl fatty acids

The early stages of the autoxidation of methyl hydnocarpate, chaulmoograte and gorlate in air have been examined at 40, 60 and 80 C, and the initial products have been compared by several methods with those derived from methyl oleate and linoleate autoxidized at 60 C. To supplement information about oxygen absorption and peroxide development in relation to time, other information about the early products, and some information about the reduced products, have been obtained by ultraviolet (UV) and infrared (IR) spectrophotometry, and by thin layer chromatography (TLC). The kinetic and other data presented in this study strongly support the conclusion that the methyl esters of cyclopentenyl fatty acids yield initial autoxidation products that, although they are primarily peroxides, differ in some ways (as expected) in the kinetics of their formation and their chemical nature, compared to those of oleate and linoleate. Nevertheless, all the data obtained strongly support the surmise that the peroxides are formed autocatalytically by a chain mechanism, and that secondary products not derived from peroxide decomposition, are formed pari passu in lesser, but increasing amounts with increasing temperature, probably from free radical intermediates. The autoxidation of esters of cyclopentenyl fatty acids has potential importance in several ways, 3 of which are mentioned briefly.     

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.     

Analysis of fat deterioration-comparison of some photometric tests

Eight photometric methods (e.g., measurement of the diene absorption, ferrous isothiocyanate test, thiobarbituric acid value test, anisidine value, Kries test) for analysis of the oxidative deterioration of fats were compared with respect to sensitivity against autoxidized methyl linoleate and methyl linolenate. The ferrous isothiocyanate test was found to be the most sensitive method followed by the measurement of diene absorption. For assessment of the specificity, highly autoxidized methyl linoleate was separated by column chromatography into the following classes of compounds: volatile carbonyl compounds, monohydroperoxides, and polar peroxides-1 and-2. The influence of each class of compounds on the results of the eight tests was determined.     

Autoxidation of polyunsaturated triacylglycerols. I. Trilinoleoylglycerol

The hydroperoxides and secondary products formed from trilinoleoylglycerol autoxidized at 40°C were isolated and characterized to clarify their contribution to oxidative deterioration of vegetable oils. The products were purified by high performance liquid chromatography (HPLC) and identified, as intact triacylglycerols, by ultraviolet, infrared,¹H NMR and¹³C NMR analyses, and after derivatization by lipolysis, gas chromatography, and gas chromatography-mass spectrometry. The main, primary products included mono-,bis- and tris-9-hydroperoxy-trans-10,cit-12-; 9-hydroperoxy-trans-10,trans-12; 13-hydroperoxy-cis-9,trans-11; and 13-hydroperoxy-trans-9,trans-11-linolenoyl glycerols. The structures of the minor secondary products analyzed after derivatization were consistent with known oxidative degradation products of linoleate hydroperoxides. HPLC analyses showed that thebis- and tris-hydroperoxides were formed from the mono-hydroperoxides during autoxidation at peroxide values above 18 and 28 meq/kg. Studies on the further oxidation of the mono-hydroperoxides support a mechanism for the consucutive formation ofbis- and tris-hydroperoxides from the monohydroperoxides. HPLC analyses showed that no preferential oxidation occurred between the 1(3)- and 2-triglyceride positions. Hydroperoxides of linoleate triacylglycerols may be important precursors of volatile compounds contributing to off-flavors of vegetable oils. Presented at the 79th Annual American Oil Chemists' Society Meeting, Phoenix, Arizona, May 8–12, 1988.     

Analysis of autoxidized fats by gas chromatography-mass spectrometry: II. Methyl linoleate

The gas chromatography-mass spectrometry (GC-MS) approach developed in the preceding paper was applied for qualitative and quantitative investigations of autoxidation products of methyl linoleate. A GC-MS computer summation method was standardized with synthetic 9- and 13-hydroxyoctadecanoate. Equal amounts of 9- and 13-hydroperoxides were found in all samples of linoleate autoxidized at different temperatures and peroxide levels. The results are consistent with the classical free radical mechanism of autoxidation involving a pentadiene intermediate having equivalent sites for oxygen attack at carbon-9 and carbon-13. Minor oxygenated products of autoxidation indicated by GC-MS include keto dienes, epoxyhydroxy monoenes di- and tri-hydroxy monoenes. These hydroxy compounds are presumed to be present in the form of hydroperoxides. The quantitative GC-MS method was found suitable for the analysis of autoxidized mixtures of oleate and linoleate. By this method, it is possible to determine the origin of the hydroperoxides formed in mixtures of these fatty esters. Presented at the AOCS Meeting, Chicago, September 1976.     

The oxidized-metallic and grassy flavor components of autoxidized milk fat

OCt-1-en-3-one accounts for the metallic flavor of autoxidized milk fat. In combination with small amounts of aldehydes it gives an oxidized flavor to milk. The grassy flavor component observed in autoxidized milk fat is shown to betrans- cis-2,6-nonadienal. Mechanisms are proposed for the production of oct-l-en-3-one from linoleate and 2,6-nonadienal from linolenate.     

Interference of peroxides with the determination of total carbonyls in autoxidized fats

The contribution of hydroperoxides to the carbonyl content of autoxidized fats measured by a colorimetric 2,4-dinitrophenylhydrazone procedure has been studied. Carbonyls were determined in radiation oxidized methyl myristate, in autoxidized methyl esters of oleie, linoleic and linolenic acids and in autoxidized oils, before and after reduction of hydroperoxides to hydroxyl groups. The results indicate that hydroperoxides decompose to carbonyl compounds during the carbonyl determination and give carbonyl contents that are too high. The extent of the interference depends on the nature of the peroxides and, therefore, on the fatty acid composition of the material and on other factors probably associated with the conditions during autoxidation and subsequent storage. For these reasons it is not possible to apply a correction for peroxide interference based on the determined peroxide value. Carbonyl determinations on autoxidized lipids should be preceded by reduction of the peroxides to non-carbonyl compounds, and care should be taken to prevent losses of low molecular weight carbonyls during this procedure. Presented at the AOCS meeting, Houston, 1965.     

Analysis of polymers from autoxidized marine oils by gel permeation HPLC using a light-scattering detector

A high-performance gel-permeation chromatographic method was developed for the isolation and quantification of polymers from autoxidized fish oils. The chromatographic system consists of a single pump, a 500 Å styrene/divinylbenzene column, and a light-scattering “mass” detector. Dichloromethane was used as the mobile phase, and quantifications were carried out using glycerol as internal standard. The polymer content of commercially available fish oils was found to be around 2%, but the value increased rapidly during oxidation at 35°C in air under light. The oxidized oils were also characterized by peroxide value, thiobarbituric acid value, ultraviolet (UV) measurements, thin-layer chromatography (TLC)/densitometry, and capillary gas chromatography. No correlation between polymer content and standard oxidation assessment parameters for the fish oils analyzed was found. However, a relationship between the polymer increase and the decrease of total fatty acids was observed, indicating that the polymers may be the main oxidation product in autoxidized marine oils.     

The Autoxidation of Hept-3-yne

In the reaction mixtures of the oxidation of hept-3-yne with molecular oxygen as products of the attack on the CC triple bond heptane-3,4-dione, propionic and butyric acids and a very small amount of 2-ethylvaleric acid were found. Hept-2-en-4-one and hept-3-en-5-one were probably present, but could not be identified unambiguously. As in the case of the isomeric octynes the main primary reaction products were the hydroperoxides formed by attack on the C H bonds in α-position to the CC triple bond.     

Color tests for the detection of alpha-dicarbon compounds formed in the autoxidation of fats

Summary Tests have been devised for detectingα-dicarbonyl compounds in autoxidized fats. The tests are based upon the conversion of theα-dicarbonyl compounds to their dioximes, formation of the nickelous, cupric and bis-pyridine-ferrous derivatives of the dioximes, and extraction of these colored metallic dioxime derivatives into organic solvent phases. The presence ofα-dicarbonyl compounds in a number of autoxidized fatty materials has been demonstrated by these tests. Peroxides in autoxidized fatty materials could be destroyed by treatment with ferrous chloride without destruction ofα-dicarbonyl compounds. This investigation was aided by the Jonathan Bowman Fund for Cancer Research.     

Effects of filtration through activated carbons on peroxide,thiobarbituric acid and carbonyl values of autoxidized soybean oil

Effects of filtration bleaching on peroxide value (PV), thiobarbituric acid value (TAV) and carbonyl value (CV) of autoxidized soybean oil were investigated by using twenty-three kinds of activated carbon in order to improve oil quality. From the decreases in PV, TAV and CV and from the physical and chemical properties of activated carbons, it was suggested that hydroperoxides, aldehydes and ketones were adsorbed on the acid sites distributed over the surface or within the pores of the activated carbons while the autoxidized soybean oil flowed through the packed column. The residual tocopherols in autoxidized soybean oil and treated soybean oil were determined during storage. The decrease in oxidative stability of treated soybean oil seemed to be caused by elimination ofα-,β-andγ-tocopherols.δ-Tocopherol was chemically more stable thanα-,β- andγ-tocopherols in autoxidized soybean oil.     

Autoxidation products from cholesterol

Thin-layer chromatography and gas-liquid chromatography were used to separate and identify products from the autoxidation of cholesterol. Both cholesterol-a-epoxide and 1,4-cholestadien-3-one were found. In addition, several other compounds, identified also by previous workers, were shown to be present. Autoxidation in bulk was quite slow at 82C, requiring several weeks for development of detectable quantities of decomposition products. The reaction could be accelerated by irradiation with ultraviolet light or by heating the sterol above its melting point.