Autoxidation of methyl linoleate initiated by the ozonide of allylbenzene

Allylbenzene ozonide (ABO), a model for polyunsaturated fatty acid (PUFA) ozonides, initiates the autoxidation of methyl linoleate (18∶2 ME) at 37°C under 760 torr of oxygen. This process is inhibited by d-α-tocopherol (α-T) and 2,6-di-ert-butyl-4-methylphenol (BHT). The autoxidation was followed by the appearance of conjugated diene (CD), as well as by oxygen-uptake. The rates of autoxidation are proportional to the square root of ABO concentration, implying that the usual free radical autoxidation rate law is obeyed. Activation parameters for the thermal decomposition of ABO were determined under N₂ in the presence of radical scavengers and found to be E_a=28.2 ±0.3 kcal mol⁻¹ and log A=13.6±0.2; k_d (37°C) is calculated to be (5.1±0.3)×10⁻⁷ sec⁻¹. Autoxidation data are also reported for ozonides of 18∶2 ME and methyl oleate (18∶1 ME).     

Photo-initiated peroxidation of lipids in micelles by azaaromatics

The monoazaaromatics, pyridine (1), hexyl nicotinate (2), and quinoline (3) and diazaaromatics pyrimidine (4) and purine (5), readily act as photo-initiators for the peroxidation of methyl linoleate in 0.50 M SDS at 37°C giving free radical chain oxidations of linoleate. Quantitative kinetic runs on the order in substrate, RH, and in the rate of chain initiation, R_i, showed that the classical rate law for autoxidation,-d[O₂]/dt=(k _p/(2 k _t ^1/2))[RH]xR _i ^1/2, is applicable to these photo-initiated oxidations. The oxidizability of methyl linoleate under these conditions is 2.92×10⁻² M^−1/2 s^−1/2. These peroxidations were inhibited by chromanol phenolic antioxidants of the vitamin E class, such as lipid-soluble 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC) and water-soluble 2-carboxy-2,5,7,8-tetramethyl-6-hydroxychroman (Trolox) and derived rate constants for inhibition of peroxidation were k _inh (PMHC)=4.35×10⁴ M⁻¹ s⁻¹ and k _inh (Trolox)=2.81×10⁴ M⁻¹ s⁻¹ during inhibited oxidation of methyl linoleate photo-initiated by 4. The products from photo-initiated peroxidation of methyl linoleate by 1 through 5 were determined by reduction and high-performance liquid chromatography analyses to be the 9-and 13-positional hydroperoxides of the four geometrical isomers: cis-9, trans-11 (6), trans-10, cis-12 (7), trans-9, trans-11 (8), and trans-10, trans-12 (9)-octadecadienoates typical of the free radical chain mechanism of lipid peroxidation. Products from dye-sensitized oxidation by Methylene Blue or Rose Bengal of methyl linoleate gave a product distribution of six hydroperoxides typical of oxidation by singlet oxygen. Thermal or photo-initiated peroxidation of methyl linoleate in SDS gave some selectivity of oxidation at the 13-position of the linoleate chain. The ratio of 13-to 9-oxidation varied in the range 1.23 to 1.14 as the cis/trans to trans/trans ratio of geometric isomers varied from 0.44 to 1.25 during photooxidation of increased amounts of linoleate in SDS. This selectivity is attributed to loss of the pseudo symmetry around the pentadienyl system in the lipid chain in the SDS system during the peroxidation.     

Kinetic study of the prooxidant effect of tocopherol. Hydrogen abstraction from lipid hydroperoxides by tocopheroxyls in solution

A kinetic study of the prooxidant effect of vitamin E (tocopherol, TocH) has been carried out. The rates of hydrogen abstraction (k₋₁) from methyl linoleate hydroperoxide (ML-OOH) by α-tocopheroxyl (α-Toc^.) (1) and eight types of alkyl substituted Toc^. radicals, (2–9) in benzene solution have been determined spectrophotometrically. The results show that the rate constants decrease as the total electron-donating capacity of the alkyl substituents on the aromatic ring of Toc^. increases. The k₋₁ value (5.0×10⁻¹M⁻¹s⁻¹) obtained for α-Toc^. (1) was found to be about seven orders of magnitude lower than the k₁ value (3.2×10⁶M⁻¹s⁻¹) for the reaction of α-TocH with peroxyl radical, which is well known as the usual radical-scavenging reaction of α-TocH. The above reaction rates (k₋₁) obtained were compared with those (k₃) of methyl linoleate with Toc^. (1–9) in benzene solution. The rates (k₋₁) were found to be about six times larger than those (k₃) of the corresponding Toc^.. The results suggest that both reactions may relate, to the prooxidant effect of α-TocH at high concentrations in foods and oils. The effect of the phytyl side chain on the reaction rate, of Toc^. in micellar dispersions has also been studied. We have measured the rate constant, k₋₁, for the reaction of phosphatidylcholine hydroperoxide with a Toc^. radical in benzene,tert-butanol and in Triton X-100 micellar dispersions, and compared the observed k₋₁ values with the corresponding values for ML-OOH.     

A kinetic study on autoxidation of α-naphthol-formaldehyde polymer in aqueous alkaline solutions

α-naphthol-formaldehyde polymer having active methylene groups and low average molecular weight of 1,000–2,000 was synthesized and characterized by FT-IR and gel permeation chromatography(GPC). Macroscopic kinetics of autoxidation of the polymer was investigated in an aqueous alkaline solution using UV/Vis spectrophotometry. Autoxidation processes in the presence of oxygen (or air) and hydrogen peroxide were associated with a color change of the solution to blue, which was monitored at 650 nm. The overall reaction rate law, order of the reaction, kinetic rate constants, and a proposed mechanism for the autoxidation reaction were obtained and reported. Infrared spectrophotometry shows that autoxidation takes place via conversion of methylene groups into carbonyls. Furthermore, kinetic data confirms the proposed mechanism for this autoxidation reaction which is of pseudo zero-order with respect to the polymer and first-order to the oxidant. The obtained kinetic rate constants of the autoxidation reaction by air and hydrogen peroxide are 0.0052 ± 7.1 × 10⁻⁴ min⁻¹ and 0.0044 ± 9.7 × 10⁻⁴ min⁻¹ at 25°C, respectively.     

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.     

Autoxidation kinetics for fatty acids and their esters

The autoxidation kinetics for n-3 and n-6 polyunsaturated fatty acids and their esters, which are collectively referred to as polyunsaturated fatty acids (PUFA), were investigated. Changes in the amounts of unreacted n-6 PUFA during the entire period of autoxidation could be expressed by dY/dt−k ₁ Y(1 −Y), wereY was the fraction of unreacted PUFA,t was the time, andk ₁ was the rate constant. For n-3 PUFA, autoxidation had to be separated into two parts. The first half of autoxidation (Y ≥ 0.5) was expressed by the same equation as above, while the latter half (Y<0.5) relates to dY/dt=−k ₂ Y, wherek ₂ was the rate constant. The apparent activation energies and the frequency factors ofk ₁ andk ₂ were evaluated. The apparent activation energies were in a range of 50 to 60 kJ/mol for bothk ₁ andk ₂. The frequency factor became large as the number of double bonds of PUFA increased.     

Kinetic Study of the Prooxidant Effect of α-Tocopherol. Hydrogen Abstraction from Lipids by α-Tocopheroxyl Radical

A kinetic study of the prooxidant effect of α-tocopherol was performed. The rates of allylic hydrogen abstraction from various unsaturated fatty acid esters (ethyl stearate 1, ethyl oleate 2, ethyl linoleate 3, ethyl linolenate 4, and ethyl arachidonate 5) by α-tocopheroxyl radical in toluene were determined, using a double-mixing stopped-flow spectrophotometer. The second-order rate constants (k _p) obtained are <1 × 10⁻² M⁻¹ s⁻¹ for 1, 1.90 × 10⁻² M⁻¹ s⁻¹ for 2, 8.33 × 10⁻² M⁻¹ s⁻¹ for 3, 1.92 × 10⁻¹ M⁻¹ s⁻¹ for 4, and 2.43 × 10⁻¹ M⁻¹ s⁻¹ for 5 at 25.0 °C. Fatty acid esters 3, 4, and 5 contain two, four, and six –CH₂– hydrogen atoms activated by two π-electron systems (–C=C–CH₂–C=C–). On the other hand, fatty acid ester 2 has four –CH₂– hydrogen atoms activated by a single π-electron system (–CH₂–C=C–CH₂–). Thus, the rate constants, k _abstr/H, given on an available hydrogen basis are k _p/4 = 4.75 × 10⁻³ M⁻¹ s⁻¹ for 2, k _p/2 = 4.16 × 10⁻² M⁻¹ s⁻¹ for 3, k _p/4 = 4.79 × 10⁻² M⁻¹ s⁻¹ for 4, and k _p/6 = 4.05 × 10⁻² M⁻¹ s⁻¹ for 5. The k _abstr/H values obtained for 3, 4, and 5 are similar to each other, and are by about one order of magnitude higher than that for 2. From these results, it is suggested that the prooxidant effect of α-tocopherol in edible oils, fats, and low-density lipoproteins may be induced by the above hydrogen abstraction reaction.     

Kinetic study of the reaction between tocopheroxyl radical and unsaturated fatty acid esters in benzene

A kinetic study of the reaction between a tocopheroxyl radical and unsaturated fatty acid esters has been undertaken. The rates of allylic hydrogen abstraction from various unsaturated fatty acid esters (ethyl oleate2, ethyl linoleate3, ethyl linolenate4, and ethyl arachidonate5) by the tocopheroxyl radical (5,7-diisopropyltocopheroxyl6) in benzene have been determined spectrophotometrically. The second-order rate constants, k₃, obtained are 1.04×10⁻⁵ M⁻¹s⁻¹ for2, 1.82×10⁻² M⁻¹s⁻¹ for3, 3.84×10⁻² M⁻¹s⁻¹ for4, and 4.83×10⁻² M⁻¹s⁻¹ for5 at 25.0°C. Thus, the rate constants, k_abstr/H, given on an available hydrogen basis are k₃/4=2.60×10⁻⁶ M⁻¹s⁻¹ for2, k₃/2=9.10×10⁻³ M⁻¹s⁻¹ for3, k₃/4=9.60×10⁻³ M⁻¹s⁻¹ for4, and k₃/6=8.05×10⁻³ M⁻¹s⁻¹ for5. The k_abstr/H values obtained for the polyunsaturated fatty acid esters3,4, and5 containing H-atoms activated by two π-electron systems are similar to each other, and are about three orders of magnitude higher than that for the ethyl oleate2 containing H-atoms activated by a single π-system. From these results, it is suggested that the prooxidant effect of α-tocopherol in edible oils and fats may be induced by the above hydrogen abstraction reaction.     

Kinetics of the thermal decomposition of alkyl hydrogen sulphates

First order rate constants and Arrhenius parameters have been obtained for the thermal decomposition of 1-hexadecyl hydrogen sulphate. From published data on thermal decomposition of lauryl hydrogen sulphate and lauryl ether hydrogen sulphate, first order rate constants and Arrhenius parameters have been obtained. The agreement between the two sets of data for the two alkyl hydrogen sulphates is within the 95% confidence limits, a combined Arrhenius plot giving an activation energy of 12.69 ± 1.97 K cal mol⁻¹ and pre-exponential factor of 10^{(4.68 ± 1.24)} sec⁻¹. For lauryl ether hydrogen sulphate, a 3-point Arrhenius plot gives an activation energy of 9.4 K cal. mol⁻¹ and a pre-exponential factor of 10² sec⁻¹.     

The kinetics and mechanism of metal-catalyzed autoxidation

The autoxidation of organic compounds, RH, occurs by a radical-catalyzed chain reaction to give hydroperoxides, RO₂H, as primary products. The initial rate is -d[O₂]/dt = k_p[RH] {k_i[Cat]/k_t}^1/2, or in the presence of an inhibitor, (In), k_p[RH](k_i[Cat]/k_I[In]), where k_p is the chain propagation rate; k_i[Cat], the rate of radical catalysis; k_t chain termination rate; k_I[In] rate of inhibitor action. As oxidation proceeds the hydroperoxides break down to give further catalytically active radicals and eventually an autoxidation may reach a maximum rate of k _p ² [RH]²/fk_t, independent of the concentration or nature of the catalyst. Photosensitization, by forming singlet oxygen, can catalyze autoxidation by forming peroxides. Compounds of many transition metals, e.g., Co, Mn, Fe, act as secondary catalysts by promoting the rapid formation of radicals from RO₂H molecules by a one-electron transfer reaction RO-OH + M²⁺→RO· + M³⁺ + OH⁻ and the M³⁺ ions are then reconverted to M²⁺ ions giving further radicals. The overall catalytic activity of a metallic ion is controlled by the slower step of the M²⁺⇌M³⁺ + e redox cycle and depends on the electronic structures of the two ions concerned and on the ligand groups attached to them. These effects are discussed in detail since ligand molecules for transition metal ions can be selected so as either to promote or inhibit autoxidation. Special reference is made to biological catalysts, such as the porphyrins, found in food products. Direct activation of oxygen by metallic complexes rarely seems to occur, but direct oxidation of substrates by metallic compounds is possible. This leads to another redox cycle which is utilized in copper-containing enzymes. One of 28 papers presented at the Symposium, “Metal-Catalyzed Lipid Oxidation,” ISF-AOCS World Congress, Chicago, September 1970.     

Radiolytic resistance of DL-α-tocopherol in lipid systems with different degrees of unsaturation

The radiolytic resistance of the DL-α-tocopherol irradiated by low (10⁵rad), medium (10⁶rad) and high (10⁷rad) doses of gamma rays at a molar ratio of 1:1, 1:1 × 10⁻² and 1:1 × 10⁻³ mole in methyl laurate, methyl oleate, methyl linoleate, methyl linolenate and benzene (chosen as solvent media) has been studied. Under the experimental conditions stated, it has been established that, contrary to ordinary autoxidation, the unsaturated lipid systems exert a progressive, protective effect on DL-α-tocopherol as the number of double bonds increases. When the DL-α-tocopherol was in a pure state, for example in benzene and in methyl esters of the fatty acids at a molar ratio 1:1, no effect of ionizing radiation was detected.     

Application of new methods and analytical approaches to research on polyunsaturated fatty acid homeostasis

New methods and analytical approaches are important to challenge and/or validate established beliefs in any field including the metabolism of polyunsaturated fatty acids (PUFA; polyunsaturates). Four methods that have recently been applied toward obtaining a better understanding of the homeostasis of PUFA include the following: whole-body fatty acid balance analysis, magnetic resonance imaging (MRI), ¹³C nuclear magnetic resonance (NMR) spectroscopy, and gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Whole-boby balance studies permit the measurement of both the percentage of oxidation of linoleate and α-linolenate and their conversion to long-chain PUFA. This method has shown that β-oxidation to CO₂ is normally the predominant metabolic fate of linoleate and α-linolenate. Furthermore, models of experimental undernutrition in both humans and animals show that β-oxidation of linoleate and α-linolenate markedly exceeds their intake, despite theoretically sufficient intake of linoleate or α-linolenate. Preliminary results suggest that by using MRI to measure body fat content, indirect whole-body linoleate balance can be done in living humans, ¹³C NMR spectroscopy provided unexpected evidence that linoleate and α-linolenate were metabolized into lipids synthesized de novo, an observation later quantified by tracer mass balance done using GC-C-IRMS. This latter method showed that within 48 h of dosing with ¹³C-α-linolenate, >80% underwent β-oxidation to CO₂ by suckling rats, whereas 8–9% was converted to newly synthesized lipids and <1% to docosahexaenoate. Further application of these recently developed methods in different models should clarify the emerging importance of β-oxidation and carbon recycling in PUFA homeostasis in mammals including humans.     

Antioxidant efficacy of water washed gums as a function of relative humidity in peanut oil

The stability of raw peanut oil to autoxidation at 40 C increased with ambient relative humidity, reaching a maximum stability at 90.5% RH (K_M × 10⁻³ 2.54, 1.83, 0.50 meq O₂/kg/hr at RH of 2, 47.5, 90.5%, respectively). Degumming with water or with phosphoric acid accelerated autoxidation at all test humidities. The trend in the rate of autoxidation was opposite that of the raw oil, increasing with relative humidity. It reached its maximum at 90.5% RH (K_M × 10⁻³ of 3.93, 3.16, 6.65 meq O₂/kg/hr at RH of 2, 47.5, 90.5% respectively for phosphoric acid degummed oil). Adding back the water washed gums removed during degumming to the water degummed peanut oil substantially restored the stability of the oil to autoxidation. These studies indicate that water washed gums retain most of their native state antioxidant activity. Hence, the gums removed during processing of oils could be added back to the oil after final processing to impart increased stability to the oil in some applications. The antioxidant efficacy of water washed gums was as good as that of synthetic antioxidants. These have an advantage over synthetic antioxidants in that they are natural components of most oils and fats.     

The effect of copper,managanese and cobalt acetates on the autoxidation oftrans-9,trans-11-octadecadienoic acid in 90% v/v aqueous acetic acid

The autoxidation oftrans-9,trans-11-octadecadienoic acid in 90% v/v aqueous acetic acid has been studied, at 80 C, with and without copper, manganese and cobalt acetate. The initial 9,11-octadecadienoic acid concentration was 0.05 M in most of the experiments. The metallic acetate concentrations were 10⁻⁵ to 10⁻² M. Copper and manganese acetates retard the autoxidation. With copper acetate, this retardation involves both the induction period and the rapid autocatalytic stage. Manganese acetate prolongs the induction period. A slightly lowered rate is observed at low cobalt acetate concentrations (∼10⁻⁵ M), but higher cobalt acetate concentrations clearly accelerate the autoxidation.     

Quenching mechanism and kinetics of ascorbyl palmitate for the reduction of the photosensitized oxidation of oils

Effects of 0, 500, 1000, and 1500 ppm (wt/vol) ascorbyl palmitate (AP) on the methylene-blue- and the chlorophyll-sensitized photooxidations of linoleic acid or soybean oil, either in methanol or in a solvent mixture (benzene/methanol, 4:1, vol/vol), were studied during storage under 3300 lux fluorescent light for 5 h. Steady-state kinetic approximation was used to determine a quenching mechanism and quenching rate constant of AP in the chlorophyll-sensitized photooxidation of methyl linoleate in a solvent mixture (benzene/methanol, 4:1, vol/vol). Both methylene blue and chlorophyll greatly increased the photooxidation of linoleic acid and soybean oil, as was expected. AP was extremely effective at minimizing both methylene-blue-and chlorophyll-sensitized photooxidations of linoleic acid and soybean oil, and its effectiveness was concentration-dependent. The addition of 500, 1000, and 1500 ppm AP resulted in 69.3, 83.6, and 94.6% inhibition of methyleneblue-sensitized photooxidation of linoleic acid, respectively, after 5-h storage under fluorescent light. AP showed significantly greater antiphotooxidative activity than α-tocopherol for the reduction of methylene blue-sensitized photooxidation of linoleic acid (P < 0.05). The steady-state kinetic studies indicated that AP quenched singlet oxygen only to minimize the chlorophyll-sensitized photooxidation of oils. The calculated total quenching rate of AP was 1.0 × 10⁸ M⁻¹s⁻¹. The present results clearly showed, for the first time, the effective singlet oxygen quenching ability of AP for the reduction of photosensitized oxidation of oils.     

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.     

Water vapor and oxygen permeability of wax films

The water vapor (WVP) and oxygen (O₂P) permeabilities of beeswax (BW), candelilla wax (CnW), carnauba wax (CrW) and microcrystalline wax (MW), formed as freestanding films, were determined. CnW and CrW both had small values for O₂P (0.29 and 0.26 g·m⁻¹·sec⁻¹·Pa⁻¹ × 10⁻¹⁴, respectively), which are less than half the value for high-density polyethylene and about a decade greater than the value for polyethylene terephthalate. O₂P values for BW and MW were about 6−9× greater than those of CnW and CrW. WVP of CnW was 0.18 g·m⁻¹·sec⁻¹·Pa⁻¹ × 10⁻¹², which is about one-half the value for CrW and MW and about one-third the value for BW. The WVP of CnW was somewhat less than that of polypropylene and somewhat greater than that of high-density polyethylene. Differences in permeabilities among the wax films are attributed mainly to differences in chemical composition and crystal type as determined by X-ray diffraction.     

Heated fat-based oil substitutes, oleic and linoleic acid-esterified propoxylated glycerol

Four oils [triolein, trilinolein, oleic acid-esterified propoxylated glycerol (EPG-08 oleate), and linoleic acid-esterified propoxylated glycerol (EPG-08 linoleate)], each without added antioxidants, were heated for 12 h/d at approximately 190°C in a small deep-fat fryer until the polymer concentration exceeded 20%, as determined by high-performance size-exclusion chromatography. Increases in the free fatty acid content, total acid value, food oil sensor value, and p-anisidine value during heating indicated that significant thermal oxidation had occurred in each oil. Capillary supercritical fluid chromatography (SFC) was used to determine the substrate concentration of each oil after each heating interval. The average, apparent first-order reaction rate constant (as determined by SFC) for trilinolein was 0.0348±0.0034 h⁻¹, while the rate for EPG-08 linoleate was 0.0253±0.0032 h⁻¹. The average apparent reaction rate constant for triolein was 0.0256±0.0011 h⁻¹, while the rate for EPG-08 oleate was 0.0252±0.0008 h⁻¹. Triolein contained >20% polymer after 60 h of heating, EPG-08 oleate contained >20% polymer after 36 h of heating, and both trilinolein and EPG-08 linoleate contained >20% polymer after 24 h of heating.     

Effects of quenching mechanisms of carotenoids on the photosensitized oxidation of soybean oil

The effects of 0, 1.0 × 10”⁻⁵, 2.5 × 10⁻⁵, and 5.0 × 10⁻⁵ M β-apo-8'-carotenal, β-carotene, and canthaxanthin on the photooxidation of soybean oil in methylene chloride containing 3.3 × 10⁻⁹ M chlorophyll b were studied by measuring peroxide values and conjugated diene content. β-Apo-8'-carotenal, β-carotene, and canthaxanthin contain 10,11, and 13 conjugated double bonds, respectively. The peroxide values and conjugated diene contents of oils containing the carotenoids were significantly lower (P<0.05) than those of control oil containing no carotenoid. As the number of conjugated double bonds of the carotenoids increased, the peroxide values of soybean oils decreased significantly (P<0.05). The quenching mechanisms and kinetics of the carotenoids in the photosensitized oxidation of soybean oil were studied by measuring peroxide values. The steady-state kinetics study showed that carotenoids quenched singlet oxygen to reduce chlorophyll-sensitized photooxidation of soybean oil. The singlet-oxygen quenching rate constants ofβ- apo-8'-carotenal, β-carotene, and canthaxanthin were 3.06 × 10⁹, 4.60 × 10⁹, and 1.12 × 10¹⁰ M⁻¹sec⁻¹, respectively.     

Relative autoxidative and photolytic stabilities of tocols and tocotrienols

The relative stabilities of selected individual tocols and tocotrienols and of equimolar mixtures of either α- plus γ- or α- plus δ- tocopherols were determined in methyl myristate and methyl linoleate during autoxidation and photolysis. Solutions containing 0.05% of the appropriate tocopherol(s) or tocotrienols were subjected to UV light (254 nm) or to a flow of 4.3 ml/min of oxygen, both at 70 C. Tocopherols (T) and tocotrienols (T−3) were determined by gas chromatography without preliminary separation or purification. Under photolytic conditions, stabilities in increasing order in methyl myristate were γ-T−3<α-T−3<δ-T<α-T <γ-T<5,7-T<β-T and in methyl linoleate were α-T<α-T−3≤γ-T−3≤β-T≤5,7-T <γ-T<δ-T. A solvent effect on the initial rate of photolysis was observed for 5-methyl substituted tocols but not for the tocols with an unsubstituted 5-position or for the tocotrienols. Under autoxidative conditions, stabilities in increasing order in methyl myristate were α-T=α-T−3 <β-T−3<γ-T−3<δ-T−3<γ-T<δ-T=β-T and in methyl linoleate were α-T<α-T−3 <γ-T−3<β-T<γ-T<δ-T. Tocopherols were much more stable during autoxidation in methyl myristate than they were in methyl linoleate. In mixtures, there was no significant protection of α-tocopherol by either γ- or δ-tocopherol under any of the conditions used. However, α-tocopherol was highly effective in protecting γ- and δ-tocopherols in methyl myristate during both photolysis and autoxidation and in methyl linoleate during photolysis. During autoxidation in methyl linoleate, α-tocopherol protection of γ- and δ- tocopherols after 24 hr was slight tough measurable.