A kinetic study of the autoxidation of methyl linoleate and linoleic acid emulsions in the presence of sodium chloride

Autoxidation of linoleic acid and methyl linoleate emulsions in aqueous buffer solutions was studied by the rate of oxygen uptake. The oxidation rates of methyl linoleate emulsions increased with an inerease in the pH of the buffer solution. With linoleic acid, oxidation rates rose until the increase reached its peak at pH 5.50 and then decreased gradually to a minimum at pH 8.00. Oxidation rates of methyl linolente and linoleic acid emulsious decreased with increased concentration of NaCl in the system. The effect of variation of pH of the emulsion in the range investigated was similar to that in emulsions without NaCl. There was no evidence that NaCl accelerated the oxidation rates in the system. The observed inhibitory effect of NaCl may result from the decreased solubility of oxygen in the emulsion with the increased concentration of NaCl. Consequently the availability of oxygen would be a limiting factor in oxidation rates. The activation energy for the monomolecular and bimolecular reactions of methyl linoleate and linoleic acid autoxidation was found to be independent of the pH value and sodium chloride concentration of the system. The energy of activations for the monomolecular and bimolecular reactions of methyl linoleate and of linoleic acid are 22,000, 18,200, 19,600, and 16,400 cal./mol., respectively. Spectrophotometric studies of the autoxidized emulsions of linoleic acid and its methyl ester indicate that the magnitude of the absorption at 2325 Å is the same at different pH values. On the contrary, the secondary products showing absorption at 2775 Å are to some extent dependent on the pH value of the emulsion.     

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.     

Role of the bases and phosphoryl bases of phospholipids in the autoxidation of methyl linoleate emulsions

Methyl linoleate, emulsified in borate buffer with sodium lauryl sulfate, was used to study the pro- or antioxidant effect of 0-phosphocholine, 0-phosphoethanolamine, 0-phosphoserine, as well as the corresponding nonphosphoryl compounds. Oxygen uptake was calculated from rate data obtained at 37 C with an oxygen electrode. The results were similar for the corresponding phosphoryl and nonphosphoryl bases. 0-Phosphocholine and choline had little effect at either pH 7.9 or 10.2. 0-Phosphoethanolamine and ethanolamine significantly increased oxygen uptake at pH 7.9, but significantly decreased uptake at pH 10.2. 0-Phosphoserine and serine decreased oxygen uptake at both pH values. The catalytic activities of the bases investigated may be attributed to their functional groups. The phosphoryl and β-hydroxy groups exhibited no catalytic activity in the autoxidation of methyl linoleate emulsions at either pH 7.9 or 10.2. The α-carboxyl amino group of 0-phosphoserine and serine decelerated autoxidation at both pH values. The amino group H₃N⁺ of the primary amine accelerated autoxidation, but the H₂N: group and the reverse effect. Since the quaternary amino group (CH₃)₃N⁺ did not affect autoxidation at either pH 7.9 or 10.2, we concluded that the presence of the N−H bond may be necessary for the prooxidant activity of an amine, and that the presence of a pair of free electrons on the nitrogen of an amine is necessary for its antioxidant activity. Kinetically, the autoxidation of methyl linoleate emulsion without added base was in agreement with Farmer’s proposed mechanism involving a bimolecular dissociation of hydroperoxides. However, methyl linoleate emulsion at pH 7.9 and 37 C in the presence of ethanolamine or 0-phosphoethanolamine was autoxidized by a mechanism involving a combined mono- and bimolecular dissociation of hydroperoxides. Submitted as partial requirement for a Ph.D. degree in Agricultural Chemistry. Presented in part at the AOCS Meeting, San Francisco, April 1969.     

Kinetic investigation into glucose-, fructose-, and sucrose-activated autoxidation of methyl linoleate emulsion

Autoxidation of methyl linoleate emulsions in aqueous phosphate buffer solutions in the presence of glucose, fructose, and sucrose has been studied by the rate of oxygen uptake. Oxidation rates increased with increasing concentration of sugars in the system. At comparable molar ratios of sugar to methyl linoleate the rate of oxidation in the presence of fructose was greater than with glucose which, in turn, was greater than with sucrose. Oxidation rates increased with pH until a maximum rate was reached at pH 8.00. There was less conjugation and more CO₂ with fructose than with glucose at a comparable level of oxygen uptake and pH value. This suggested concurrent oxidation of methyl linoleate and sugars; fructose is the most readily oxidized of those studied. Sugars seemed to be effective only in combination with the resulting methyl linoleate hydroperoxide. The effect of sugars rests in an activation of the decomposition of the linoleate hydroperoxide, and on the acceleration of the autocatalysis. The activation energy values for the autoxidation of methyl linoleate emulsions in the presence of sucrose, glucose, and fructose are 14.9, 10.6, and 10.6 K. Cal./mol. at pH 5.50; 16.0, 10.8, and 10.4 K. Cal./mol. at pH 7.00; and 14.4, 10.2, and 8.8 K. Cal. at pH 8.00, respectively. Addition of ascorbic acid to the system at zero time or after 2 hrs. increased oxygen absorption. It appeared that oxidized methyl linoleate caused co-oxidation of the ascorbic acid. Additions of nordihydroguaiaretic acid, propyl gallate, and hydroquinone to the system were ineffective in stopping oxidation when they were added after oxidation had commenced. They reduced effectively the rate of oxidation when added at zero time. Presented at the 52nd annual meeting, American Oil Chemists' Society, St. Louis, Mo., May 1–3, 1961. American Meat Institute Foundation Journal Paper No. 215.     

Vergleichende Untersuchungen über die Wirkung von Antioxidantien,Aminosäuren und Casein auf die Autoxidation ungesättigter Fettsäuren

Comparative Investigations of the Effect of Antioxidants, Amino Acids and Casein on the Autoxidation of Unsaturated Fatty Acids The antioxidants α-tocopherol and ascorbyl palmitate (AP), the amino acids histidine and methionine as well as casein were added to emulsions of methyl linoleate (ML) in water, and the influence of these components on autoxidation of ML was determined by measuring the rate of oxygen uptake using the Warburg method. The monohydroperoxides of ML being the primary autoxidation products were analyzed by HPLC; hexanal being their main fragmentation product which essentially contributes to rancidity of foods was determined by head space gas chromatography. α-Tocopherol on the one side acts as a radical scavenger by its hydrogen donating effect discernible by a high ratio of cis/trans- to trans/trans-isomers of the ML-hydroperoxides, on the other side it stabilizes these intermediary products thus inhibiting the formation of hexanal. In the presence of Fe(II)-ions AP has a prooxidative effect by causing a rapid decomposition of hydroperoxides, whereas in the initial oxidation phase histidine and methionine have no or only a minor antioxidative effect which increases later on, connected with an inhibitory effect on peroxide decomposition creating hexanal. During ML autoxidation the different components added to the ML emulsion including methionine bound in the casein, which has a strong antioxidative effect, were also oxidized.     

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.     

On the kinetics of the autoxidation of fats. II. Monounsaturated substrates

The autoxidation of methyl oleate and oleic acid shows some differences as compared to the autoxidation of linoleate,e.g., the formation of water at an early stage. Linearization of experimental data on the autoxidation to high oxidation degrees of methyl oleate and other monounsaturated substrates shows that the rate equations previously derived for methyl linoleate in the range of 1–25% oxidation are valid, provided the correct expression for the remaining unreacted substrate is used. With monounsaturated substrates, part of the oxygen is consumed by a secondary oxidation reaction almost from the beginning, and only a certain constant fraction α of the total O₂ consumption is consumed in hydroperoxide formation. The fraction α is different for methyl oleate, oleyl alcohol, oleic acid andcis 9-octadecene, but the rate constant for the hydroperoxide formation is the same for all of them when experimental conditions are the same. The main difference between oleate and linoleate autoxidation is the much faster decomposition of the oleate hydroperoxides relative to their slow formation.     

Studies of the fluorescent products of lipid oxidation in aqueous emulsion with glycine and on the surface of silica gel

The formation of fluorescent products in the reaction of methyl linoleate hydroperoxide with glycine in aqueous emulsions correlated directly with the decrease in diene conjugation and the increase in thiobarbituric acid (TBA) reactive substances. These correlations also were reflected in the course of the oxidation of methyl linoleate in aqueous emulsions with glycine and indicated that glycine reacted with products of peroxide decomposition as opposed to intermediates of autoxidation in hydroperoxide formation. Thin layer chromatography (TLC) and selective solvent extraction demonstrated that the products of the reaction contained many substances with a fluorescent spectrum similar to those of lipofuscin pigments. When methyl esters of polyunsaturated fatty acids or other polyunsaturated lipids underwent oxidation adsorbed on silica gel particles, products with similar fluorescent spectral properties were formed illustrating that fluorescent substances were formed in a variety of reactions associated with the oxidation of unsaturated lipids.     

Phenothiazine derivatives as new antioxidants for the autoxidation of methyl linoleate and their reaction mechanisms

The effect of new antioxidants, such as phenothiazine derivatives, on the autoxidation of methyl linoleate was evaluated by estimating the induction period using the weighing method. Peroxide values (iodometry and IR), refractive indices, mol wts, and UV and IR spectra were measured to investigate the extent of the autoxidation. The induction period evaluated from the weighing method gives almost the same value as that from the peroxide values. It was shown that some new phenothiazine derivatives are remarkably effective antioxidants, and, besides, the mechanism for the autoxidation of methyl linoleate containing phenothiazine derivatives as antioxidants is probably of the same type as that for the substrate alone. This study also investigated the reaction mechanism of phenothiazine derivative antioxidants by determining the electron spin resonance spectra for the antioxidants in the autoxidation of methyl linoleate. Then, the following mechanism was proposed. That is, within the induction period, these inhibitors hold stable nitroxide radicals (>NO*) in the reaction between the antioxidant amino radical (>N*), produced by the reaction of the antioxidant with ROO* or O₂, and the peroxy radical (ROO*). Besides, the more superior the phenothiazine derivative antioxidant, the more inactive the antioxidant makes oxygen and the peroxy radical for the methyl linoleate autoxidation and also for the antioxidant oxidation. Presented at the 12th World Congress of ISF, Milan, 1974.     

Prooxidant effects of inorganic chromium compounds in the autoxidation of methyl linoleate

The prooxidant property of inorganic chromium compounds was determined in methyl linoleate free from natural antioxidants and metals. Prooxidant properties of inorganic chromium compounds appeared in order of sodium chromate > chromium (VI)-oxide > chromium chloride > potassium chromate > chromium (III)-oxide > potassium dichomate. In comparison with the control, additions of chromium compounds induced different amounts of autoxidation products derived from methyl linoleate, such as small amounts of hydroperoxides and conjugated dienes and large amounts of hydroxy groups,α,β,γ,δ-unsaturated carbonyls, isolatedtrans double bonds, polymers, and free radicals. From these analytical data, the catalysis of chromium compounds in the autoxidation of methyl linoleate seemed to be based on their abilities of abstracting a hydrogen from methyl linoleate and decomposing hydroperoxides derived from the autoxidation of methyl linoleate.     

Oxidation of Methyl Linoleate in Oil-in-Water Micro- and Nanoemulsion Systems

Oxidation of methyl linoleate in O/W emulsions having droplets of median diameters ranging from 17 nm to 8.0 μm was carried out at 40°C. The oxidation process was analyzed on the basis of a kinetic equation of the autocatalytic type. The induction period was found to be shorter and the oxidation rate constant lower for emulsions with smaller oil droplets. The stoichiometry between methyl linoleate and oxygen was observed to be independent of both the size of oil droplet and the type of the surfactant and was found to be unity during the early stage of the oxidation. However, more oxgen was consumed in the oxidation of the methyl linoleate in the later half of the oxidation process.     

Synergistic antioxidant effect of nucleic acids and tocopherols

Nucleic acids acted as synergists with tocopherols in inhibiting the oxidation of methyl linoleate. DNA and RNA enhanced the activity of tocopherols to different extents in the order α->γ->δ-tocopherol. Nucleic acids decreased the rates of consumption of tocopherol in the presence of oxi-dizing methyl linoleate. Nucleic acids also decreased the rate of oxidation of tocopherols by PbO₂. The synergistic effect of nucleic acids seemed to be caused by hydrogen bond formation with tocopherols which protected toco-pherols from direct air oxidation.     

The rates of oxidation of unsaturated fatty acids and esters

The rates of autoxidation of oleic acid, ethyl oleate, linoleic acid, 10,12-linoleic acid, ethyl linoleate, trilinolein, pentaerythritol linoleate, dipentaerythritol linoleate, elaidolinolenic acid, linolenic acid, ethyl linolenate, trilinolenin, and methyl arachidonate have been studied by oxygen uptake in a Warburg respirometer and the results are compared with the rates of enzymatic oxidation of lipoxidase substrates. The increase in the number of double bonds in a fatty acid by one increases the rate of oxidation of the fatty acid or its esters by at least a factor or two. Earlier findings that acids oxidize more rapidly than their esters have been confirmed. The initial rates of lipoxidase oxidation of ethyl linoleate, ethyl linolenate, and methyl arachidonate were found to be essentially the same.     

Droplet composition affects the rate of oxidation of emulsified ethyl linoleate

Our objective was to study the influence of droplet composition on the rate of lipid oxidation in emulsions. A series of oil-in-water emulsions stabilized by a nonionic surfactant (Tween 20) was studied. These emulsions had the same total oil concentration (5 wt%) and initial droplet diameter (0.3 μm), but contained droplets with different ratios of ethyl linoleate (substrate) andn-tetradecane (inert diluent). Lipid oxidation was measured as a function of time by three different methods: gas-chromatographic determination of residual substrate; ultraviolet-visible spectrophotometric determination of conjugated dienes; and measurement of aqueous thiobarbituric acid-reactive substances. All three methods showed similar trends for emulsions of similar composition. The progress of lipid oxidation in the emulsions was dependent on the concentration of ethyl linoleate in the emulsion droplets. At low concentrations (1% oil as substrate), oxidation proceeded at a relatively slow and constant rate. At intermediate concentrations (20%), the oxidation rate was rapid initially and then slowed down with time. At high concentrations (100%), the oxidation rate was slow at first, and then increased with time. An explanation of our results is proposed in terms of the distribution of substrate molecules between the droplet interior and interface, and the ingress of aqueous radicals into the emulsion droplets.     

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.     

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.     

Study on the oxidative rate and prooxidant activity of free fatty acids

Oleic, linoleic and linolenic acids were autoxidized more rapidly than their corresponding methyl esters. Addition of stearic acid accelerated the rate of autoxidation of methyl linoleate and the decomposition of methyl linoleate hydroperoxides. Therefore, the higher oxidative rate of FFA’s than their methyl esters could be due to the catalytic effect of the carboxyl groups on the formation of free radicals by the decomposition of hydroperoxides. Addition of stearic acid also accelerated the oxidative rate of soybean oil. This result suggests that particular attention should be paid to the FFA content that affects the oxidative stability of oils.     

A promising environmentally-friendly manganese-based catalyst for alkyd emulsion coatings

A manganese(IV) complex (MnMeTACN) containing 1,4,7-trimethyl-1,4,7-triazacyclononane (MeTACN), in the presence of polyamines, significantly accelerated the oxidation of ethyl linoleate (EL) emulsions, and appeared to be a potential environmentally-friendly alternative for Co-based driers that are currently widely used for alkyd emulsion coatings. The polyamines greatly enhanced the catalytic activity of MnMeTACN in decomposing the formed hydroperoxides during the oxidation of EL.     

The reaction of an autoxidized lipid with proteins

Evidence is presented which indicates that an interaction occurs between proteins and an autoxidizing unsaturated lipid. Using a model system approach, it has been established that two purified proteins (gelatin and insulin) are chemically modified in the presence of an autoxidizing lipid, methyl linoleate. The insulin-methyl linoleate interaction has been studied chromatographically after acid and alkaline hydrolysis, and also by using the Sanger end group analysis method. The data indicate that lipid intermediates react with theε-amino group of lysine, and also with phenylalanine and glycine, the N-terminal amino groups of insulin. Hydrogen fluoride solubility and enzyme hydrolysis determinations indicate that the autoxidation products of methyl linoleate interact with protein to produce new chemical entities through cross-linking.     

Mechanism of lipoxidase reaction. I. Specificity of hydroperoxidation of linoleic acid

Linoleate hydroperoxides from autoxidation of methyl linoleate and from lipoxidase oxidation of linoleic acid are compared. Data indicate an equal amount of methyl 9- and 13-hydroperoxyoctadecadienoate produced by autoxidation of methyl linoleate, and the exclusive formation of 13-hydroperoxyoctadeca-9,11-dienoic acid from the incubation of lipoxidase with linoleic acid. As a result of these findings, a specific mechanism for the reaction of lipoxidase with linoleic acid is postulated. Presented at the AOCS Meeting, Philadelphia, October 1966. This work was conducted under a Postdoctoral Resident Research Associateship established at the Northern Laboratory by ARS, USDA, in association with the National Academy of Sciences-National Research Council. No. Utiliz. Res. Dev. Div., ARS, USDA.