Reactions of fatty materials with oxygen. XVI. Relation of hydroperoxide and chemical peroxide content to total oxygen absorbed in autoxidation of methyl oleate

Summary Methyl oleate has been autoxidized at 100°, 80°, and 60° in the Barcroft-Warburg apparatus. Samples have been analyzed for total peroxide by the iodometric method and for hydroperoxide by the polarographic method. These peroxide values have been compared with each other and with total oxygen absorbed. The relation of chemical peroxide (y_c) to oxygen uptake (x) is expressed by y_c=1.09x^0.936. This equation is equally valid at the three temperatures for the first 150 millimoles (15%) of oxygen absorbed per mole of methyl oleate. Similarly, the hydroperoxide content (y_h) for the first 150 millimoles of oxygen absorbed at 80° and 100° is given by the equation y_h=1.02x^0.936. The ratio of hydroperoxide to chemically determined peroxide was, on the whole, constant throughout the entire range of oxidation (15–300 millimoles of oxygen absorbed per mole), and averaged about 95%. It has been shown unequivocally that the major portion of the peroxides formed in the autoxidation of methyl oleate are hydroperoxides, confirming conclusions of recent investigators. A small but significant amount of non-hydroperoxidic peroxide appears to be formed concurrently. Paper XV is reference 6. Presented at the Spring meeting of the American Oil Chemists' Society, San Antonio, Tex., April 11–14, 1954. A laboratory of the Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture.     

Thermal oxidation of fractionated polypropylene in solution

Thermal oxidation of fractionated polypropylene was carried out in trichlorobenzene under atmospheric oxygen at 125°C with conventional oxygen uptake. The oxidizability of the polymers is discussed on the basis of the oxygen uptake curves and the properties of the polymers. Fractions of atactic polypropylene oxidized easily at the initial stage of the oxidation and showed neither autoxidation phenomena nor the induction period observed in the isotactic polymer. Hydrogenation of the ether-soluble fraction by a Wilkinson catalyst gave a polymer which was, according to infrared spectrometry, free from impurity groups such as hydroperoxide, carbonyl, and unsaturation groups. The hydrogenated fraction was more stable to thermal oxidation than the unhydrogenated fraction and showed an induction period. The results indicate that the initiation process of the oxidation of polypropylene is apparently dependent on the impurities such as hydroperoxide, carbonyl, and unsaturation and that stereoregularity of the polymer affects the kinetic dependence of the oxidation.     

Reactions of fatty materials with oxygen. XVIII. Catalytic hydrogenation of autoxidized methyl oleate and oleic acid. Preparation of monohydroxystearic acids

Summary Autoxidation of methyl oleate and oleic acid beyond the peak peroxide values followed by catalytic hydrogenation gave mixed monohydroxystearic acids in high yield. The complicated autoxidation mixture which contains peroxides, hydroxy, carbonyl, and oxirane compounds was simplified considerably in composition by this procedure. For complete reduction of the double bond, and the carbonyl and oxirane groups, hydrogenation was conducted at about 150° and 150 lbs.. Peroxides were reduced at room temperature. Catalysts used were palladium on carbon and Raney nickel. The selective reduction of peroxides in autoxidation mixtures has been studied by chemical and catalytic means. Peroxides were converted largely to carbonyl compounds rather than to the anticipated hydroxy compounds. Palladium-lead on calcium carbonate is an excellent catalyst for reducing peroxides with hydrogen. tert-Butyl hydroperoxide, 12- ketostearic acid, stearone,cis-9,10-epoxystearic acid and methyl oleate peroxide concentrate were employed as model substances in determining hydrogenation conditions. Paper XVII. is reference 5. Presented at the fall meeting of the American Oil Chemists' Society, Minneapolis, Minn., Oct. 11–13, 1954. A laboratory of the Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture.     

Thiobarbituric acid-reactive malondialdehyde formation during superoxide-dependent,iron-catalyzed lipid peroxidation: Influence of peroxidation conditions

A systematic study of the influence of biological lipid peroxidation conditions on lipid hydroperoxide decomposition to thiobarbituric acid-reactive malondialdehyde is presented. A superoxide-dependent, iron-catalyzed peroxidation system was employed with xanthine oxidase plus hypoxanthine plus ferric iron-adenosine diphosphate complex as free radical generator. Purified cardiac membrane phospholipid (as liposomes) was the peroxidative target, and 15-hydroperoxy-eicosatetraenoic acid was used as a standard lipid hydroperoxide. Exposure of myocardial phospholipid to free radical generator at physiological pH (7.4) and temperature (37°C) was found to support not only phospholipid peroxidation, but also rapid lipid hydroperoxide breakdown and consequent malondialdehyde formation during peroxidation. Under lipid peroxidation conditions, oxidative injury to the phospholipid polyunsaturated fatty acids required superoxide radical and ferric iron-adenosine diphosphate complex, whereas 37°C temperature and trace iron were sufficient for lipid hydroperoxide decomposition to malondialdehyde. Harsh thiobarbituric acid-test conditions following peroxidation were not mandatory for either lipid hydroperoxide breakdown or thiobarbituric acid-reactive malondialdehyde formation. However, hydroperoxide decomposition that had begun in the peroxidation reaction could be completed during a subsequent thiobarbituric acid test in which no lipid autoxidation took place. Iron was more critical than heat in promoting the observed hydroperoxide decomposition to malondialdehyde during the lipid peroxidation reaction at 37°C and pH 7.4. These data demonstrate that the radical generator, at physiological pH and temperature, serves a dual role as both initiator of membrane phospholipid peroxidation and promotor of lipid peroxide breakdown and thiobarbituric acid-reactive malondialdehyde formation. Consequently, peroxidation reaction conditions can directly influence lipid hydroperoxide decomposition, malondialdehyde production and system thiobarbituric acid-reactivity. In vivo, decomposition of lipid peroxides to malondialdehyde during radical-mediated, metal-catalyzed membrane peroxidation may represent an integral component of oxidative tissue injury rather than a mere consequence of hydrolyzing the peroxidized biological sample in a thiobarbituric acid test.     

Degradation of poly(vinyl chloride). II. Kinetic investigation of thermal and radiation-induced polyene formation at low temperatures

The degradation arising from purely thermal treatments as well as with accompanying exposure to γ-radiation has been followed under vacuum by visible and ultraviolet spectroscopy in the temperature range from 80° to 130°C. As corroborated by extensive literature reports, absorbance changes have been considered suitable to provide valuable information on the unsaturation sequences produced. In agreement with the results obtained in the same conditions by following the evolution of HCl,¹ a catalytic action has been ascribed to HCl depending on the efficiency of removal of HCl. Furthermore, both the purely thermal and radiation-induced uncatalyzed degradations yield length distribution functions that are constant with time and have been found to be accounted for by the previously formulated free-radical mechanism, also from a quantitative point of view.     

Bestimmung von Sauerstoff und Stickstoff sowie Erfassung primärer Oxydationsprodukte in Ölen,Fetten und Emulsionen auf physikalisch-chemischem Wege I: Methodik

Determination of Oxygen and Nitrogen as well as the Detection of Primary Oxydation Products in Oils, Fats and Emulsions by Physico-chemical Methods A displacement method for the determination of oxygen and nitrogen in oils, fats and emulsions is reported. The method also serves to calibrate the described oxygen determination procedures by Tödt and by Clark. A procedure has been developed with the help of Clark-cell, with which the oxygen content of oils, fats and emulsions can be determined and continuously registered. A polarographic procedure for the determination of fatty hydroperoxides is also described. It is shown with the help of an autoxidation experiment, that the absorbed oxygen is mostly used up in the formation of hydroperoxides.     

Bunsen coefficient for oxygen in marine oils at various temperatures determined by an exponential dilution method with a polarographic oxygen electrode

A polarographic oxygen electrode has been applied to an exponential dilution method for the determination of the solubility of oxygen in oils. Results are compared with other chemical and physical methods for herring and olive oils and the same oils subjected to partial oxidation. The Bunsen coefficients for oxygen in nine marine oils have been determined by this procedure between 20 and 80 C, with a relative standard deviation of ±7% or less. The densities and viscosities of these oils have been measured for the same temperature range. In general, the Bunsen coefficient for oxygen in marine oils increases with an increase in temperature between 20 and 60 C, but then rapidly decreases between 60 and 80 C to a value lower than that for room temperature. It appears that autoxidation should not be the major cause of this effect, as the measurement rate was relatively rapid. Some tentative correlations between the solubility of oxygen in marine oils and the fatty acid composition, iodine value, density and viscosity are discussed briefly.     

Oxidative stability of polyunsaturated fatty acids: effect of squalene

The propensity of polyunsaturated fatty acids (PUFAs) to undergo oxidation plays an important role in the integrity of biological membrane and lipid containing foods. The ability of squalene (SQ), a naturally occurring dehydrotriterpene present in animal and plant tissues, to protect linoleic, linolenic, arachidonic and docosahexaenoic acids against temperature‐dependent autoxidation and UVA (ultraviolet A, 320‐380 nm) mediated oxidation was assessed. The oxidation of PUFAs was protected in varying degrees, with highest protection observed for linolenic, arachidonic and docosahexaenoic acids. Linoleic acid was less protected. At a molar ratio of 7:1 (PUFA:SQ) the inhibition of the oxidation process was 22% in the presence of linoleic acid and about 50% in presence of the other PUFAs tested. The different protection exerted by SQ against PUFAs with different degrees of unsaturation may be accounted for by the higher stability of octadecadienoic acid hydroperoxide isomers compared with respective PUFA hydroperoxides. Observing mild UVA‐mediated oxidation and the temperature‐dependent autoxidation reactions we found similarities in the oxidation pattern and the protection exerted by SQ. These findings suggest that the reaction of autoxidation is predominant and SQ acts mainly as peroxyl radical scavenger.     

Macro peroxide initiators based on autoxidized unsaturated plant oils: Block/graft copolymer conjugates for nanotechnology and biomedical applications

The autoxidation of unsaturated fatty acids and their esters is one of the most important reactions in food and biological systems. Autoxidation is a type of reaction between air oxygen and unsaturated plant oils. This reaction starts with a hydrogen abstraction on the methylene group adjacent double bond, leaving a radical onto the carbon atom. Molecular oxygen attacks this radical leading to produce peroxide and hydroperoxide derivatives of the oligomerized unsaturated plant oils. Because peroxide groups thermally cleave to produce free radicals, hydroperoxide derivatives of unsaturated plant oil oligomers can be used in the free radical polymerization of vinyl monomers leading to the related block/graft copolymers. The obtained copolymers combined with the biodegradable natural plant oil gain superior properties such as biodegradability and softness. In this review article, the in vitro autoxidation of well-known unsaturated plant oils-soybean oil, linseed oil, and castor oil and some related unsaturated fatty acids-oleic acid, linoleic acid, and ricinoleic acid was carried out under atmospheric conditions with or without exposing white light. Because the autoxidized unsaturated oil/fatty acids contain peroxide/hydroperoxide groups (they are referred to as macro peroxide initiators) that they are used in the free radical polymerization of vinyl monomers to obtain block/graft copolymers. The improved polymerization conditions, characterization, and applications of the obtained products will be discussed.     

Hydrolysis during deodorization of fatty oils. Catalytic action of fatty acids

Summary The hydrolysis of peanut oil at subatmospheric pressures and 180°C. has been studied, using both a static and a dynamic method. The conditions applied to the latter method are identical with those used in the current steam deodorization process. It has been found that, as a result of catalytic action, the rate of hydrolysis is a function of the free fatty acid content and moreover increases in direct proportion with the absolute pressure. From data of the degree of association it has been shown that only the monomeric acid has catalytic action. It is clear therefore that acid catalysis by H⁺ ions must also be involved. It was further established that when the calculation of vaporization efficiency is based upon the reduction in free fatty acid content, hydrolysis may give rise to serious errors.     

A highly sensitive method for the micro-determination of lipid hydroperoxides by potentiometry

A modified method for peroxide value (POV) determination of lipids was developed through the application of potentiometry to conventional POV tests such as the official method of the Japan Oil Chemists’ Society (JOCS). The new method permits a simple and reliable determination of low hydroperoxide levels in the initial stages of lipid autoxidation when only very small amounts of sample are available, even when those levels are measured on less than 10 mg of lipid. Using the present method, hydroperoxide levels as low as 20 nanoequivalents (neq) were determined with reasonable precision. This method is applicable to all lipids tested including oils and fats, free fatty acids, phospholipids, glycolipids and cholesterol esters.     

Oxidation of all‐cis‐7,10,13,16,19‐docosapentaenoic acid ethyl ester. Hydroperoxide distribution and volatile characterization

The isomeric hydroperoxide distribution and the composition of volatiles generated by oxidation of all‐cis‐7,10,13,16,19‐docosapentaenoic acid ethyl ester (DPA Et) were determined. DPA Et was prepared by using seal blubber oils as raw material and purified by urea complexation and reverse‐phase high‐performance liquid chromatography (HPLC). The DPA Et of over 96% purity thus obtained was dissolved in methanol and subsequently divided into two portions. One portion was added with methylene blue and exposed to a tungsten bulb light at 5 °C for photosensitized oxidation. The other portion was added with 2,2'‐azobis (2,4‐dimethylvaleronitrile) as an azo‐radical initiator and kept in the dark at room temperature for autoxidation. Positional isomers of hydroperoxides generated by autoxidation or photosensitized oxidation of DPA Et were separated by normal‐phase HPLC and detected by a fluorescence detection system as well as UV absorption. The peak components were identified by gas chromatography‐mass spectrometry (GC‐MS). Eight isomeric hydroperoxides, including certain amounts of 7‐, 10‐, 11‐, 13‐, 14‐, 16‐, 17‐, and 20‐hydroperoxy docosapentaenoate, were generated by autoxidation of DPA Et. The photosensitized oxidation of DPA Et yielded not only the above eight hydroperoxide isomers but also two additional isomeric hydroperoxides, 8‐ and 19‐hydroperoxy docosapentaenoate, which are characteristic hydroperoxide isomers generated by singlet oxygen‐mediated oxidation. Volatiles formed by autoxidation of DPA Et at 50 °C were collected and analyzed by solid‐phase micro‐extraction and GC/GC‐MS. A number of aldehydes, ketones, alcohols, acids, furans and hydrocarbons were identified. The formation mechanisms of certain volatiles are discussed.     

Anwendung von Antioxydantien zur Verlängerung der Lagerung von Shortenings I: Studium primärer unflüchtiger Autoxydationsprodukte

Application of Antioxidants for Improving the Storage Properties of Shortenings I: Study of Primary Non-Volatile Products of Autoxidation In shortenings stored at 28°C and at 2°–4°C in the presence of various antioxidants (ascorbyl palmitate, butyl hydroxyanisole and propyl gallate) the content of non-volatile autoxidation products, the primary products as well as those formed in the later stages, was determined. The organoleptic properties were determined at the same time. The samples containing ascorbyl palmitate showed the least content of autoxidation products and the best organoleptic properties. For the determination of the products of autoxidation from shortenings the direct UV-spectrophotometric method was as useful as the determination of peroxide number.     

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.     

Influence of synergists: The influence of hydroperoxide decomposers on phenolic inhibitors consumption in oxidized polypropylene

The influence of sulphur‐ and phosphorum‐containing substances (hydroperoxide decomposers) on the kinetics of the consumption of two phenolic antioxidants in polypropylene (PP) was studied. The induction periods of PP autoxidation at 130°C were measured in the presence of inhibiting compositions that consisted of phenolic inhibitors and decomposers of hydroperoxide. Obtained results indicated that the influence of the hydroperoxide decomposer became significant when the concentration of the phenolic antioxidant became close to critical value. It also was shown that the efficiency of the hydroperoxide decomposer significantly depended on the mechanism of the transformation of the phenolic inhibitor; and first of all on the nature of its transformation products. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2239–2243, 2002     

Polarographische Bestimmung von Hydroperoxiden in Ölen

Polarographic Determination of Hydroperoxides in Oils The application of direct current polarography for the determination of hydroperoxides in oils has the following advantages over other methods: 1) specific for the hydroperoxides, 2) independent of the structure of hydroperoxides, 3) even very small amounts (upto 10^?6 mol/1) are detectable and 4) relatively short time is required. In comparison to chemical determination the accuracy of the polarographic determination is ± 3% and in case of very small hydroperoxide content (<10^?5 mol/1), it is ± 10%.     

Photo-sensitized oxidation of unsaturated fatty acid methyl esters. The identification of different pathways

The photo-sensitized oxidation of methyl linolenate and methyl oleate was studied using erythrosine and riboflavin as sensitizers. The complex mixtures of hydroperoxides obtained were analyzed for the proportion of conjugated products and, after reduction to the corresponding mixtures of hydroxystearates, for the distribution of positional isomers. By comparing the mixtures with that obtained from autoxidation, it was shown that the riboflavin reaction involved the “Type 1” mechanism of photosensitized oxidation which proceded via the formation of diene-radicals and yielded the same positional isomers of hydroperoxides as autoxidation. Thus, mixtures of the 8, 9, 10, and 11 positional isomers of allylic hydroperoxides were formed from oleate and the 9, 12, 13, and 16 isomers of conjugated diene-hydroperoxides from linolenate oxidation. The erythrosine reaction, on the other hand, proceded via the “Type 2”. mechanism which involved singlet oxygen as the oxygenating species. The mixtures of isomers resulting from oxidation involving singlet oxygen were different from those obtained by autoxidation. Oleate oxidation gave rise to a mixture of only the 9 and 10 positional isomers while the mixture obtained from oxidation of methyl linolenate contained non-conjugated hydroperoxide isomers (with the hydroperoxide group at positions 10 and 15) as well as the conjugated—9, 12, 13, and 16—isomers.     

A possible role for singlet oxygen in the initiation of fatty acid autoxidation

A mechanism for the initiation of autoxidation in fatty acids is proposed which involves singlet state oxygen, formed through a photosensitization reaction, as the reactive intermediate. Both singlet oxygen generated in a radio-frequency gasdischarge, and photosensitization by natural pigments, were shown to catalyze the oxidation of methyl linoleate. The involvement of singlet oxygen was shown by the identification of nonconjugated hydroperoxides as products common to both photooxidation and singlet O₂ oxidation. Nonconjugated hydroperoxides could not be detected among the free radical autoxidation products. Further proof for the above mechanism was gained by showing that compounds known to react strongly with singlet oxygen, inhibited the photooxidation. With the exception of chlorophyll, all sensitizers could be completely inhibited. Although singlet oxygen formation can account for approximately 80% of the observed chlorophyll photooxidation, at least one other mechanism must be involved. It is postulated that proton abstraction by the photoactivated carbonyl group of chorophyll could account for the remaining 20% of the observed photooxidation. The conclusion is drawn that oxygen, excited to its singlet state by a photosensitization process, plays the important role of forming the original hydroperoxides whose presence is necessary before the normal free radical autoxidation process can begin.     

Kinetic model for autoxidation of β-carotene in organic solutions

Autoxidation of β-carotene was studied experimentally using n-decane as a solvent under various reaction conditions of temperature and dissolved oxygen concentration A novel kinetic model was proposed on the basis of an autocatalytic free-radical chain reaction mechanism. A secondary initiation reaction by decomposition of hydroperoxide and reactions concerned with a β-carotene-derived C-centered radical in propagation and termination processes were taken into consideration in the model. There were four unknown kinetic constants, and the constants were estimated by fitting the model with the experimental data. The fitted results are in good agreement with the experimental data in all stages of the kinetics of autoxidation and over a wide range of oxygen concentrations. The model described not only the appearance of the induction stage but also the effect of the oxygen concentration on the autoxidation rate. In addition, the model predicted the behavior of autoxidation in another solvent at low temperature that had been reported by other researchers.     

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.