Autoxidation of fatty materials in emulsions. I. Pro-oxidant effect of histidine and trace metals on the oxidation of linoleate esters

Aqueous emulsions of methyl or ethyl linoleate (sodium dodecylsulfate as emulsifier) together with such added components as 1-histidine, metal chlorides, buffers, and acid or alkali, were oxidized in the dark with shaking in an oxygen atmosphere. Under optimum conditions (pH 6.5), the linoleate peroxide content, after 2 hr autoxidation at 20C, was increased more than 3-fold by the addition of 1 ppm of ferrous or ferric ions, approximately 20-fold by a 0.01 M concentration of histidine and more than 60-fold by the addition of both histidine and ionic iron. The pro-oxidative effect of other transition metal ions (Cu⁺⁺, Co⁺⁺, Cr⁺⁺⁺, Mn⁺⁺, and Ni⁺⁺) also was investigated. None of these ions had a significant effect alone. Combined with 0.01 M histidine, only Mn⁺⁺ increased peroxidation over that when when histidine alone was added. The pro-oxidative action of histidine was retarded approximately 60% by 0.1 N acetate buffer and completely repressed by 0.05 M phosphate, nonionic emulsifiers, and low and high pH. The threshold concentration of histidine necessary for pro-oxidative action was greater than 0.0001 M. Presented in part at the AOCS meeting, St. Louis, Mo., 1961. Eastern Utilization Research and Development Division, Agricultural Research Service, U.S.D.A.     

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.     

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.     

The kinetics of methyl linoleate emulsion autoxidation in the presence of polyhydroxy compounds

The kinetics of the autoxidation of methyl linoleate emulsions activated by carbohydrates likely to be present in meat, with special reference to the effects of functional groups, number of carbon atoms and configuration have been investigated by the rate of oxygen uptake. On the basis of equimolar concentrations of aldoses in the system, oxidation rate of methyl linoleate increases as the number of carbon atoms in the sugar molecule decreases, reaching a maximum in the presence of glyceraldehyde. Configuration of the aldose has a slight effect on the oxidation rate of methyl linoleate emulsions. At comparable molar ratios of hexose to methyl linoleate, the rate of oxidation was found to be: ketohexose > aldohexose > hexahydroxy alcohol. Replacement of the primary alcohol group in an aldohexose with a methyl group decreases the oxidation rate of methyl linoleate emulsion. An opposite effect is observed when the primary alcohol group is substituted with a carboxyl group, i.e., in the presence of sodium glucuronate. 2-Deoxy-D-glucose and 2-deoxy-D-ribose exhibit a lesser effect on the autoxidation of methyl linoleate emulsion than glucose and ribose, respectively. Oxidation rates in the presence of reducing disaccharides, maltose, lactose and cellobiose, are more rapid than in the presence of the non-reducing disaccharide sucrose. Presented at the AOCS Meeting, Toronto, 1962. American Meat Institute Foundation Journal Paper No. 254.     

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.     

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.     

Argan oil-in-water emulsions: Preparation and stabilization

We prepared stable oil-in-water emulsions of argan oil with two different types of mixtures of nonionic emulsifiers. Three different types of oil (Israeli argan oil, Moroccan argan oil, and soybean oil) were emulsified with mixtures of Span 80 and Tween 80. The optimum HLB value for argon oil was 11.0 (±1.0). The argan oil-in-water emulsions were stable for more than 5 mon at 25°C. Synergistic effects were found in enhancing stability of emulsions prepared with sucrose monostearate. The origin of the oil and the internal content of natural emulsifiers, such as monoglycerides and phospholipids, have a profound influence on its interfacial properties and on the stability of the argan oil-in-water emulsions.     

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.     

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.     

Autoxidation of fatty materials in emulsion. III. Application of GLC and TLC to studies of the histidine-catalyzed autoxidation of methyl oleate

Methyl oleate, emulsified with sodium dodecyl-sulfate, was extensively oxidized, and the amount of oleate which was reacted was determined by GLC using methyl palmitate as an internal standard. The effects of histidine, iron, and histidine plus iron were compare with the uncatalyzed reaction in the presence and absence of ultraviolet light. Results in the absence of ultraviolet light confirmed previous findings that histidine and histidine plus iron are prooxidants at the concentration studied. In the presence of ultraviolet light the rate of oleate oxidation was about 100× faster than that of the nonirradiated reaction, and the effect of the catalysts was almost negligible. The principal products, determined by GLC and TLC, were epoxide and α,β-unsaturated carbonyl. Hydroperoxy, hydroxy, acid and aldehyde compounds were also present. Epoxy stearate, determined by GLC, compared favorably with epoxy stearate as determined by use of the Durbetaki method. Presented at the AOCS Meeting, Chicago, October 1964. E. Utiliz. Res. Dev. Div., ARS, USDA.     

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.     

Effects of ionizing radiations on fats. II. Accumulation of peroxides and other chemical changes

The accumulation of peroxides, carbonyl com-pounds and reducing substances during irradia-tion and post-irradiation storage of pure fatty acid methyl esters has been studied. Irradiation and storage of irradiated methyl myristate under vacuum results in formation of small quantities of these compounds. Irradiation under oxygen gives peroxides and carbonyl com-pounds in yields indicating that every ionization results in the formation of one molecule of each group, and antioxidants have no effect on the formation of these compounds during irradiation. Irradiation of methyl linoleate under vacuum results in destruction of pre-formed hydroperox-ides. During irradiation in oxygen, approximately one-eighth of the peroxides formed arises from the direct reaction of irradiation-induced free radi-cals with oxygen, while the rest is formed through a chain mechanism with an average chain length of 7. Peroxides continue to accumulate in irradiated methyl linoleate stored under oxygen at a rate increasing with initial irradiation dose. Antioxidants have some effect in retarding the formation of peroxides during irradiation of methyl linoleate and during post-irradiation stor-age, but the effect is small compared to their antioxygenic activity toward simple autoxidation. The effect varies with the nature of the antioxi-dant and with irradiation dose. Propyl gallate is much less effective than butylated hydroxy-anisole and appears to be easily destroyed during irradiation. For paper I of this series, see Ind. Eng. Chem.49. 1713 (1957). Presented at the AOCS Meeting in Minneapolis, 1963.     

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.     

The physical and chemical structure of lipids in relation to digestion and absorption

Lipids provide specific functional, nutritional and sensorial properties to foods. The main dietary source of lipids is triacylglycerols. Triacylglycerols can occur in different forms, such as free (oil), droplets (emulsions) or crystallised network (butter). Whatever the form consumed, the triacylglycerols are emulsified in the stomach, as lipolysis takes place at the oil‐water interface. If some emulsifiers are already present at the surface of the lipid droplets, they will affect the lipolysis process. Another crucial parameter is the size of the emulsified droplets, as smaller droplets, providing a larger surface area for lipase binding, will be digested faster. The chain length and degree of saturation of the fatty acids and their position on the triacylglycerol backbone influence their digestion, mode of transport and absorption. Thus the choice of emulsifiers, particle size and fatty acid composition are major factors to take into account when processing food to reduce or increase lipid digestion and absorption.     

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.     

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.     

Emulsifying and self‐emulsified properties of siloxane polymer grafted with easy hydrophile

A series of siloxane polymers (SHE) with varying weight percents of simple nonionic hydrophilic groups have been synthesized and characterized by FTIR and ¹³C‐NMR. Emulsions have been made by self‐emulsification of those polymers in pure water and also by emulsifying silicone oil using those polymers as oil‐in‐water type emulsifiers. The stability of those emulsions have been examined apparently from the phase separation and also from the particle size increase on storage by Transmission Electron Microscopy (TEM) and photo microscopy. The results reviled that SHE 40, having 40 wt % grafted hydrophile, formed the most stable self‐emulsified emulsion, and SHE30 and SHE40 could be used as effective emulsifiers for silicone oil emulsification. The possible use of those emulsions as a defoamer for water‐borne systems has also been investigated by measuring the reduction of foam height of a strongly foamed aqueous solution of sodium lauryl sulfate (SLS). The defoaming ability decreases with the increase in hydrophile wt % in the polymer backbone; however, it increases with the emulsion (defoamer) concentrations. When compared with respect to the total wt % of hydrophobe contents in the emulsion defomer, the self‐emulsified emulsion has shown better defoaming than the silicone oil emulsion, and the results are well in accord with the difference in size of the respective emulsion particles. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2408–2415, 2002     

Effect of water activity on secondary products formation in autoxidizing methyl linoleate

The role of water activity on the formation of peroxides and carbonyl compounds during lipid oxidation is important to know because there could be either beneficial or detrimental effects of water activity on lipid oxidation in stored foods. Therefore, methyl linoleate was chosen as a model lipid and was autoxidized to 1% at water activity ranging from 0.02 to 0.79 at 37°C. Oxygen uptake was monitored manometrically. The peroxide and carbonyl contents were determined upon termination of the autoxidation studies. Methyl linoleate autoxidation was characterized by three phases: i) an initial induction period of no oxygen absorption; ii) a slow rate of oxygen absorption, up to 0.15% oxidation; and iii) a relatively faster rate of oxygen absorption beyond 0.15% up to 1% oxidation. Water activity had considerable influence during the first phase. There was no induction period at or below water activity 0.22. The induction period begins at water activity 0.32 and could be extended to a limit with increase in water activity. Once the induction period was passed water activity had no influence on the rate of oxidation. However, during the second and third phases water activity becomes important in the stabilization of peroxides/hydroperoxides and decides the course of secondary reactions that follow peroxide decomposition. Higher water activity values, particularly water activity 0.67, tended to stabilize peroxides. Water activity had considerable influence on the formation of secondary products of autoxidation as evidenced by the variation in the type and quantity of carbonyl compounds at different water activity values.     

Formation of peroxides in fatty esters. II. Methyl linoleate: application of the polarographic and direct oxygen methods

Summary This study of the prolonged autoxidation of methyllinoleate at 80°C. has included polarographic identification and determination of hydroperoxides, the direct determination of oxygen contents, and catalytic micro-hydrogenation for the determination of unsaturation. The polarographic method has further substantiated the observations of other workers that the principal peroxidic substance formed during the autoxidation of methyl linoleate at 80°C. is a hydroperoxide. The direct oxygen measurements have shown that most of the oxygen absorbed in the initial stages of autoxidation can be accounted for as hydroperoxide. During the latter stages there was a continuous increase of oxygen uptake, half of which can be accounted for as free acid and half as forms other than hydroperoxide, ester, or free acid. By means of the catalytic micro-hydrogenation method it has further been shown that as the autoxidation progresses there is a continuous decrease in unsaturation.     

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.