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.     

Autoxidation of polyunsaturated triacylglycerols. III. Synthetic triacylglycerols containing linoleate and linolenate

Four triacylglycerols containing linoleate (L) and linolenate (Ln) in specific positions were synthesized to determine the effect of fatty acid position on their relative rates and products of autoxidation. Analyses by reversedphase high performance liquid chromatography (HPLC) showed that autoxidation of L- and Ln-containing triacylglycerols form monohydroperoxides and hydroperoxy epidioxides as the main products. The peroxyl radicals of internal 12- and 13-mono-hydroperoxides of Ln triacylglycerol components cyclized rapidly and their relative triacylglycerol position had no influence on their rates of cyclization. A good linear relation was obtained between total HPLC peak areas (detected at 235 nm) of the main oxidation products and peroxide values. Reversed phase HPLC analyses thus provide a useful method to estimate oxidation of polyunsaturated triacylglycerols. The ratios of Ln to L mono-hydroperoxides were twice the ratios of Ln to L in the triacylglycerol substrates. Ln triacylglycerol components, therefore, oxidized twice as much as the L components. At 40°C, LnLnL oxidized slightly faster than LnLLn with respective induction periods of 45 and 47 hr. LLnL oxidized faster than LLLn with respective induction periods of 56 and 60 hr. Dilinolenoyl-linoleoylglycerols are, therefore, slightly less stable to oxidation when Ln is in the 1,2- than the 1,3-triacylglycerol position. Dilinoleoyl-linolenoylglycerols are less stable when L is in the 1,3- than the 1,2-triacylglycerol position. Presented in part at the 80th Annual American Oil Chemists' Society Meeting, Cincinnati, Ohio, May 3–6, 1989     

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.     

Autoxidation of polyunsaturated fatty esters on Silica

The stability of unsaturated methyl esters of fatty acid adsorbed on silica gel and silicic acid was studied by gas liquid chromatographic estimation of disappearance relative to a saturated internal standard. Variables included silica-ester ratio, agitation, adsorbent particle size, and degree of unsaturation. Under the conditions of the experiment, destruction of substrate unsaturated ester at 80 C was more a function of time than of unsaturation or initial purity.     

Autoxidation of polyunsaturated fatty acids: II. A suggested mechanism for the formation of TBA-reactive materials from prostaglandin-like endoperoxides

The nature and mechanism of formation of the thiobarbituric acid (TBA)-reactive material produced in the autoxidation of polyunsaturated fatty acids (PUFA) or their esters has been studied. On the basis of chemical studies and     

Stereospecific analysis of triacylglycerols rich in long-chain polyunsaturated fatty acids

Six oils of marine, algal, and microbial origin were analyzed for stereospecific distribution of component fatty acids. The general procedure involved preparation ofsn-1,2-(2,3)-diacylglycerols by partial deacylation with ethylmagnesium bromide or pancreatic lipase, separation of X-1,3- andsn-1,2(2,3)-diacylglycerols by borate thin-layer chromatography, resolution of thesn-1,2- andsn-2,3-enantiomers by chiral phase high-performance liquid chromatography following preparation of dinitrophenylurethane derivatives, and determination of the fatty acid composition by gas chromatography. Unexpected complications arose during a stereospecific analysis of triacylglycerols containing over 33% of either 20∶4 or 22∶6 fatty acids. Thesn-1,2(2,3)-diacylglycerols made up of two long-chain polyunsaturated acids migrated with the X-1,3-diacylglycerols and required separate chiral phase resolution. Furthermore, the enzymatic method yieldedsn-1,2(2,3)-diacylglycerols, overrepresenting the polyenoic species due to their relative resistance to lipolysis, but prolonged digestion yielded correct composition for the 2-monoacylglycerols. The final positional distribution of the fatty acids was established by pooling and normalizing the data from subfractions obtained by norman- and chiral-phase separation of diacylglycerols. The molecular species of X-1,3-,sn-1,2- andsn-2,3-diacylglycerol dinitrophenylurethanes were identified by chiral-phase liquid chromatography/mass spectrometry with electrospray ionization, which demonstrated a preferential association of the paired long-chain acids with thesn-1,2- andsn-2,3-diacylglycerol isomers.     

On the Autoxidation of Vitamin D Preparations II: The Autoxidation of Ergocalciferol

Ergocalciferol was used as a model for the autoxidation studies of vitamins D. It was shown that ergocalciferol is sensitive to light, moisture and heat in the addition of oxygen. The autoxidation proceeds through isomerisation to carbonyl compound formation without the development of peroxidic groups.     

Autoxidation of polyunsaturated fatty compounds in mackerel oil: Formation of 2,4,7-decatrienals

The formation of potentially “fishy” off flavor components, especially 2,4,7-decatrienals, in various rancid mackerel oils has been semiquantitatively investigated using preparative thin layer chromatography (TLC) and gas liquid chromatography (GLC) methods. A combination of 2 GLC analyses can be directly employed for free aldehyde analysis. This GLC method is faster and gives a better recovery than the alternative TLC proceeding through the dinitrophenylhydrazone derivatives of the carbonyl compounds. Kinetic relations between decatrienal formation and the degree of autoxidation of polyenoic fatty compounds present in mackerel oil are discussed. The decreases in major polyenoic fatty acids rancid oils, measured by the ratios 1 $$\frac{{18:4\omega 3 + 20:5\omega 3 + 22:6\omega 3}}{{14:0 + 16:0 + 18:0}}$$ or 1 $$\frac{{total polyenoic acid}}{{14:0 + 16:0 + 18:0}}$$ can be related to the formation of 2,4,7-decatrienals and other unsaturated aldehydes.     

Autoxidation of fats. II. Preparation and oxidation of methyl oleate-maleic anhydride adduct

Summary The adduct of maleic anhydride and methyl oleate has been prepared and characterized. The physical and chemical properties of the adduct indicate that it is composed of one mole of each of the reactants and that the double bond of methyl oleate has not been saturated. Oxidative fission of the adduct showed that the adduct consisted of a mixture of four isomers which yielded octanoic, nonanoic, suberic, and azelaic acids on ozonolysis. Isolation of these four acids from the fission products indicates that the addition reaction probably proceeds by a mechanism analogous to that proposed by Farmer, Koch, and Sutton for autoxidation. In view of the similarity between the reactions of maleic anhydride and oxygen with unsaturated compounds, the foregoing observations appear to support the free radical chain mechanism for the autoxidation of fats. One of the laboratories of the Bureau of the Agricultural and Industrial Chemistry Agricultural Research Administration, U. S. Department of Agriculture.     

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 of fatty materials in emulsion. II. Factors affecting the histidine-catalyzed autoxidation of emulsified methyl linoleate

Several factors which affect autoxidation of methyl linoleate in emulsion have been examined. Data are presented which indicate: 1) In the presence of histidine, the ionic (anionic) emulsifiers examined promote autoxidation of emulsified methyl linoleate, but nonionic emulsifiers do not. 2) The concentration of an emulsifier affects the rate of oxygen absorption. 3) Inorganic salts (0.1 M or less) such as sodium chloride, sodium acetate and sodium sulfate affect oxygen absorption of emulsified methyl linoleate prepared with either ionic or nonionic emulsifiers. In histidine-catalyzed autoxidation there is a suppressing effect in the case of the ionic and a promotional effect in the case of the nonionic. In uncatalyzed autoxidation, these salts have a promotional effect in ionic emulsions and none in nonionic emulsions. 4) Sodium phosphate buffers completely suppress autoxidation due to histidine catalysis, but do not suppress the normal uncatalyzed autoxidation of emulsified methyl linoleate. 5) The pro-oxidative effects of histidine and histidine-metal ion complexes on emulsified unsaturated materials is not limited to polyolefins but also includes mono-olefinic compounds. Presented at the AOCS meeting in Toronto, October, 1962. E. Utiliz. Res. and Dev. Div., ARS, USDA.     

Oxidative decomposition of oxirane and methyloxirane polyethers. II [1]. Autoxidation of Dipropylene Glycol Isomers

The oxidative decomposition of two structural isomers of dipropylene glycol, i.e. 2,4-dimethyl-3-oxapentandiol-1,5 ( 1 ) and 4-oxaheptandiol-2,6 ( 2 ) was studied. The maximum rate of oxidation of glycol 1 was ca. 20 times lower than that of glycol 2 . In the products of oxidation 15 different compounds were detected for the former glycol and 20 compounds for the latter. The products of decomposition were the mixtures of water and carboxy acid, acetal, alcohol, aldehyde, ketone, ester, unsaturated ether and hydroperoxy type of compounds of different structure. The mechanism of product formation is discussed.     

Synthesis of regioisomerically pure triacylglycerols containing n‐3 very long‐chain polyunsaturated fatty acids

There is growing scientific evidence that consumption of n‐3 very long‐chain polyunsaturated fatty acids (n‐3 VLC‐PUFA) helps in brain and eye development, and protects against a range of common degenerative diseases. This has provided the impetus to the scientists to develop new and renewable sources for these important fatty acids so that the food industry is able to produce and market products fortified with n‐3 VLC‐PUFA. The bioactive efficacy and stability of food products containing n‐3 VLC‐PUFA may be determined not only by the amount of n‐3 VLC‐PUFA present but also by the positional distribution of these acids within the triacylglycerol (TAG) molecules (regiopurity). Studies of the effects of positional distribution on the functionality of n‐3 VLC‐PUFA containing oils have been hampered by a general lack of pure TAG regioisomers for experimentation. This paper reviews methods that have been used for the synthesis of TAG regioisomers containing n‐3 VLC‐PUFA, with special reference to those in which one n‐3 VLC‐PUFA occurs in combination with two long‐chain saturated acids.     

Triacylglycerols of winter butterfat containing configurational isomers of monoenoic fatty acyl residues. II. Saturated dimonoenoic triacylglycerols

Triacylglycerols of Finnish winter butterfat containing one saturated and two monoenoic fatty acyl residues were studied. With silver ion high-performance liquid chromatography (HPLC), molecules were separated according to the difference in the configuration of one fatty acyl moiety. The distribution of the saturatedcis,trans-dimonoenoic and saturatedcis,cis-dimonoenoic triacylglycerols according to their acyl carbon numbers was compared by means of reversed-phase HPLC and tandem mass spectrometry. Furthermore, two examples of the fatty acid composition of a specified molecular weight species were shown. The fatty acid compositions of corresponding saturatedcis,trans-dimonoenoic and saturatedcis,cis-dimonoenoic triacylglycerols were similar; however, there may be differences in the proportions of different fatty acid combinations or in the distribution of fatty acids between primary and secondary glycerol positions.     

Autoxidation of saturated fatty acids. II. The determination of the site of hydroperoxide groups in autoxidizing methyl palmitate

The monohydroperoxides of methyl palmitate oxidized at 150C were concentrated, purified, and reduced to yield a mixture of isomers of monohydroxy palmitate. No evidence of unsaturation in these molecules could be obtained, and no direct method for the resolution or identification of the individual isomers was found. Nuclear magnetic resonance (NMR) spectroscopy of the reduced esters suggested that the hydroxy groups were not located at either end of the chain of the fatty ester. To establish the location of the initial oxidative attack on saturated esters, the reduced esters of oxidized methyl palmitate were chemically cleaved at the location of the hydroxy group by means of chromium trioxide oxidation and by the Beckmann transformation. The resulting mono- and dibasic fatty acids were methylated and analyzed by gas chromatography. This analysis indicated that the oxidation of methyl palmitate did not occur selectively at one location along the ester. Although all carbon atoms can apparently be oxidized, preferential oxidation was observed toward the center of the molecule.     

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 phosphatidylcholine liposomes

Autoxidation of pure soybean phosphatidylcholine liposomes at 40 C was found to proceed without an observed induction period, but otherwise, the rates of disappearance of the linoleic acid (70% of total) and linolenic acid (6% of total) followed typical autocatalytic kinetics. Incorporation of 0.05 mol % of tocopherol into the liposomes produced an induction period of about 7 hr under the condition used for the incubation. The produts formed from the autoxidation of pure soybean phosphatidylcholine liposomes were mostly 9- and 13-hydroperoxyoctadecadienoates (isolated as hydroxy esters). The yield of hydroperoxides withcis,trans configuration was about the same as those withtrans,trans configuration throughout incubation period. After extensive autoxidation, a large quantity of trihy droxyoctadecenoate was also produced. When a large quantity of dipalmitoyl phosphatidylcholine was incorporated into soybean phosphatidylcholine liposomes, the rate of autoxidation decreased and was found to conform to apparent first-order kinetics. In this system, the yield oftrans, trans hydroperoxides was much greater than that ofcis,trans isomers at all stages of autoxidation. Late in the autoxidation of the mixed liposomes, both trihydroxyoctadecenoate and hydroxyepoxyoctadecenoate were produced in substantial quantities.     

Autoxidation of semicrystalline polyethylene

Thermal oxidation in linear polyethylene is mainly confined to disordered regions in which scission reactions cause crystallization and eventual deterioration of mechanical properties. Gel formation is negligible at 100°C. As degradation proceeds, comparable changes occur in the intrinsic viscosities of melt and solution-crystallized liner polymers, indicating that chain folds are regularly arranged and are resistant to oxidative scission. Breakdown is much more extensive in branched and crosslinked polymers since crosslinking retards oxidative crystallization and branching increases the volume fraction of substrate ultimately accessible to oxygen.