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.     

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.     

Cellulose graft copolymers. III. Effects of oxygen on graft copolymerization of ethyl acrylate with γ-irradiated cellulose

The effects of oxygen on graft copolymerization of ethyl acrylate from methanol–water systems with γ-irradiated fibrous cotton cellulose were investigated by electron spin resonance spectroscopy and by the formation of cellulose–poly(ethyl acrylate) copolymer. The concentrations of free radicals in cellulose irradiated dry in an atmosphere of nitrogen at 25°C decreased during postirradiation storage in nitrogen or oxygen. The concentration of free radicals in the irradiated cellulose, moisture regain of the irradiated cellulose, and formation of cellulose–poly(ethyl acrylate) copolymer decreased with increase in temperature and time of postirradiation storage and to a greater extent when stored in oxygen rather than in nitrogen. From the decrease in moisture regain of irradiated cellulose during postirradiation storage, it was concluded that increased intermolecular bonding occurred in irradiated cellulose during storage in both nitrogen and oxygen atmospheres. When irradiated celluloses which had been stored in either oxygen or nitrogen were copolymerized with ethyl acrylate at 60°C, less formation of copolymer was observed than when the copolymerization reactions were conducted at 25°C. It was concluded that there was no evidence for the formation or decomposition of cellulose peroxides during these reactions and that formation of graft copolymer depended primarily on the concentration of free radicals in the irradiated cellulose at the time of copolymerization.     

Melting and chemical behaviors of isothermally crystallized gamma-irradiated syndiotactic polystyrene

Melting and chemical behaviors of isothermally crystallized gamma-irradiated sPS have been investigated using differential scanning calorimetry, FTIR, and X-ray diffraction techniques. Amorphous sPS samples were subjected to gamma radiation in vacuum and in oxygen at different doses from 200 to 1000 kGy. Irradiated samples were heated to 310 °C, cooled to 220-260 °C range, held for 10 min, and re-melted. Three melting endothermic peaks observed for irradiated and non-irradiated samples isothermally crystallized at 220 °C were decomposed into individual Gaussian distributions, and enthalpies of the total melting endotherm and individual peaks were determined. Both α and β crystalline forms coexist in the crystallized irradiated sPS, regardless of the radiation treatment environment. Dose and irradiation environment have a great effect on the melting behaviors and chemical structures of the isothermally crystallized gamma-irradiated sPS. Crystallinity increases with increasing dose of irradiation in both vacuum and oxygen, with level of increase is greater in oxygen. The three melting peak temperatures decrease with increasing dose regardless of irradiation environments. Ketone and aldehyde oxidized products are formed in the isothermally crystallized gamma-irradiated sPS in oxygen through a combination of hydrogen abstraction or chain scission process and hydroxyl free radicals formation.     

Improvement of the photostability of isotactic polypropylene by the incorporation of ethylene

The effect of the incorporation of ethylene on the photostability of isotactic poly(propylene) (iPP) was studied with the aim of improving the photostability. iPP was prepared with a random ethylene sequence (ethylene–propylene random copolymer, rPP), and the photooxidative degradation behavior was compared with that of homogeneous iPP. Both samples were thermally post‐treated under vacuum to ensure the same crystallinity. The degradation behavior was studied by infrared spectroscopy (IR), gel permeation chromatography (GPC), and temperature rising elution fractionation (TREF) measurements. The rates of hydroperoxide and carbonyl formation in the irradiated iPP increased with irradiation time for > 192 h, whereas those in the irradiated rPP are almost constant after 96 h. The change in molecular weight with the irradiation time showed similar behavior, suggesting that the degradation reaction in the irradiated rPP was suppressed after 96 h. The degradation behavior of rPP was thought to be due to the dissociation of the methyl group, which leads to the termination of degradation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1863–1867, 2002     

The antioxidant effect of natural substances on lipids during irradiation of chicken legs

Entire fresh chicken legs were subjected to three pretreatments (packaged in air; packaged under vacuum; or marinated in natural plant extracts and packaged in air) followed by irradiation (0, 3, or 5 kGy). The control and irradiated chicken legs were stored at 4°C and analyzed for FA composition and sensory quality at predetermined intervals. Irradiation dose had a significant (P<-0.01) effect on FA derived from phospholipid but less than on FA derived from a neutral lipid. In general, levels of unsaturated FA decreased as the radiation dose increased; however, for marinated chicken legs irradiated with 5 kGy, levels of linoleic acid (C_18∶2) and arachidonic acid (C_20∶4) derived from the phospholipid fraction were significanlty (P≤0.05) higher than those irradiated in air or under vacuum. The concentration of FA also decreased significantly (P≤0.05) as storage time increased. For chicken legs packaged in air or marinated and then packaged in air, significant (P≤0.01) inverse correlations existed between high-carbon-number PUFA and lower-carbon-number (≤17) saturated FA; this relationship was not apparent in samples irradiated under vacuum. A processing combination of marinating and vacuum packaging might better control lipid oxidation and degradation in irradiated chicken. Panelists found no significant difference (P>0.05) in the flavor and oder intensity of cooked irradiated chicken legs and their nonirradiated equivalents.     

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.     

Electron‐beam curing of hydrogenated acrylonitrile–butadiene rubber

The electron‐beam‐induced crosslinking of hydrogenated acrylonitrile–butadiene rubber (HNBR) was investigated. HNBR sheets were exposed to electron‐beam irradiation in air at a room temperature of 25 ± 2°C over a dose range of 0–20 Mrad. An attempt was made to correlate the structure of the irradiated rubber with the properties. The ratio of chain scission to crosslinking and the gelation dose were determined by a method proposed elsewhere. The gel content and dynamic storage modulus increased with the radiation dose. Fourier transform infrared studies revealed the formation of double bonds and carbonyl and ether groups. These observations were further supported by a thermogravimetric analysis of the carbonaceous residue of irradiated HNBR. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 648–651, 2005     

Electron spin resonance studies on graft copolymerization of gaseous styrene onto preirradiated polypropylene. III. Preirradiation under vacuum followed by exposure to oxygen at −78°C

Grafting of styrene vapor was carried out onto polypropylene γ-irradiated under vacuum followed by exposure to oxygen at ?78°C. A comparison with the results on polypropylene irradiated in an oxygen atmosphere leads to an interesting result. Two samples were irradiated at room temperature under vacuum and in an oxygen atmosphere, respectively, the former being followed by exposure to oxygen. Although irradiation was carried out at room temperature for either sample and nearly the same amounts of the peroxy radicals are trapped, the former showed more grafting than the latter. The origin of this difference was investigated by means of electron spin resonance. Significant differences were observed in ESR spectra as well as in isothermal decays of the trapped peroxy radicals. In the former sample, the trapped peroxy radicals had a higher mobility, and consequently a significant hydrogen abstraction by the peroxy radicals was observed during storage at 40°C. This result supports the conclusion of the previous paper that the radicals effective in the grafting reaction of polypropylene preirradiated in an oxygen atmosphere are the carbon radicals which are produced by hydrogen abstraction by the peroxy radicals.     

Veränderung von fluorpolymeren durch gammastrahlen

Foils of poly(tetrafluorethylene) and of poly(ethylene-co-tetrafluorethylene) were irradiated by gamma rays in the presence of air oxygen. Infrared spectroscopy by MIR technique (multiple internal reflection) shows that in the case of poly(tetrafluorethylene) only chain splitting occurs, oxidational changes could not be observed. Thermomechanical analysis (TMA) reveals that the glass transition temperature is lowered by the irradiation, evidently as a result of an increase in density of the amorphous phase. Contrarily to these results poly(ethylene-co-tetrafluorethylene) undergoes oxidation with formation of carbonyl groups up to an irradiation rate of about 3 · 10⁶ Gy; further irradiation leads to chain splitting. The formation of the carbonyl groups is a reaction of the first order.     

Formation of fluorescent substances from degradation products of methyl linoleate hydroperoxides with amino compound

The degradation products formed from methyl linoleate hydroperoxides by reaction with heme were fractionated by Sephadex LH-20 column chromatography and by reverse-phase high performance liquid chromatography, and the ability of each compound to form fluorescent substances through reaction with amino compound was compared. Maximum formation of fluorescent substances was obtained from monomeric degradation products with amino compound, but low molecular weight aldehydes such as hexanal, 2-hexenal and 2,4-decadienal, formed only a small amount of fluorescent substances. However, the major monomeric degradation products described previously, the hydroxy-, keto- and epoxy-derivatives, do not significantly contribute to the formation of fluorescent substances through reaction with amino compound. It was suggested that formation of fluorescent substances from lipid peroxides with amino compound may originate from a precursor present in monomeric degradation products formed from hydroperoxide of methyl linoleate during lipid peroxidation, and that low molecular weight aliphatic aldehydes are not involved in fluorescent substance formation. Moreover, the majority of TBA-reactive substances in secondary oxidation products prepared from autoxidized methyl linoleate are also unrelated to the formation of fluorescent substances through reaction with amino compound.     

Application of radiation curing in the preparation of polycarbosilane-derived SiC fibers

Polycarbosilane fibers were irradiated by gamma-rays under vacuum and by electron beam in He gas flow or under vacuum at room temperature. Free radicals on Si and C atoms were produced. Most radicals reacted with each other, causing cross-links between polycarbosilane molecules. Some radicals, which did not contribute to cross-linking, were fairly stable under vacuum or in inert gas at room temperature but oxidized on exposure to air. The number of stable radicals under vacuum could be decreased by annealing. The remaining radical concentration was about 1% after annealing at 513 K. By a combination of radiation curing and annealing, SiC fibers with smaller quantities of oxygen were prepared. The mechanical properties of the SiC fibers showed a high tensile strength of 2.5 GPa after heat treatment at 1773 K. On the other hand, polycarbosilane fibers could be cured by radiation oxidation at room temperature, that is, gamma-ray or electron irradiation in oxygen, and the oxygen content could be well controlled by irradiation dose and dose rate. The SiC fibers obtained by the radiation oxidation had an oxygen gradient from the surface to the center which was dependent on the radiation oxidation conditions.     

Photochemical degradation of poly(ethylene terephthalate). II. Effect of wavelength and environment on the decomposition process

Poly(ethylene terephthalate) has been irradiated with UV light of different wavelengths and in various atmospheres. The extent of degradation was monitored by measuring the tensile strength, molecular weight, carboxylic acid endgroups, and fluorescence emission of the polymer. The importance of wavelengths <315 nm in causing deterioration was demonstrated to be mainly due to strong surface absorption which results in surface crazing and ultimate fracture under stress. Irradiation in nitrogen and under vacuum were found to give similar results, but with oxygen present in the system several significant differences were observed. In nonoxidative irradiations, crosslinking and discoloration of the polymer occurred. Under oxidative conditions, chain scissions and fluorescent material build-up resulted, whereas no crosslinking and only slight discoloration was observed. The possibility of a photo-oxidation reaction has thus been suggested, involving hydroperoxide formation, to explain the discrepancy in results obtained for the two types of environments.     

Photochemical oxidation of fatty acid esters with and without chlorophyll

1. It has been confirmed that the principal products formed in the oxidation of methyl oleate by oxygen under a variety of conditions are predominantlytrans hydroperoxides. However no inversion of the double bond occurs in unoxidized oleate. Hence the conversion ofcis totrans double bonds and peroxide formation occur together in the same molecules.
2. The autoxidation of methyl linoleate at low temperature yields predominantlycis,trans conjugated hydroperoxides. Autoxidation at 25°C., oxidation catalyzed by visible light, or ultraviolet light and copper soap catalyzed oxidation at temperatures appreciably above 0°C., lead to the formation primarily oftrans,trans conjugated hydroperoxides. The inversion of the second double bond in this case appears to be independent of the peroxide-forming reactions.
3. The photochlorophyll oxidation of methyl linoleate leads to the formation of some unconjugated hydroperoxides, some of which containtrans double bonds.
4. Under all of the conditions employed in the present investigation, the oxidation of methyl oleate and linoleate led primarily to the formation of monomeric peroxides which retained most of the unsaturation of the parent compound.

     

Interference of peroxides with the determination of total carbonyls in autoxidized fats

The contribution of hydroperoxides to the carbonyl content of autoxidized fats measured by a colorimetric 2,4-dinitrophenylhydrazone procedure has been studied. Carbonyls were determined in radiation oxidized methyl myristate, in autoxidized methyl esters of oleie, linoleic and linolenic acids and in autoxidized oils, before and after reduction of hydroperoxides to hydroxyl groups. The results indicate that hydroperoxides decompose to carbonyl compounds during the carbonyl determination and give carbonyl contents that are too high. The extent of the interference depends on the nature of the peroxides and, therefore, on the fatty acid composition of the material and on other factors probably associated with the conditions during autoxidation and subsequent storage. For these reasons it is not possible to apply a correction for peroxide interference based on the determined peroxide value. Carbonyl determinations on autoxidized lipids should be preceded by reduction of the peroxides to non-carbonyl compounds, and care should be taken to prevent losses of low molecular weight carbonyls during this procedure. Presented at the AOCS meeting, Houston, 1965.     

Influence of electron beam sterilization on polymers when incubated in different media

Polyurethane catheters were irradiated with electron‐beam. We looked at the way the catheter is physically and chemically modified after incubation in different media. An extracting medium (methanol) was compared to other oxidative, enzymatic, and saline aqueous media, mimicking some aspects of biological fluids. Irradiation had a strong impact on the stability of polymer during incubation. For nonirradiated samples incubated in aqueous media, only a small diffusion of additives was observed; no oligomer release was put into evidence, and the polymer was almost not degraded. But the chain branching that occurred because of irradiation created some weaknesses in the polymer chains: irradiated samples were mostly sensitive to chain scission and oligomers released during incubation; this trend increased with the irradiation dose. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Cellulose peroxides derived from carbonylated cellulose and hydrogen peroxide

Cellulose peroxides derived from hydrogen peroxide and cellulose derivative into which a ketone group is introduced by reaction with methyl vinyl ketone were investigated. The amount of peroxide formed on the cellulose substrate increased linearly with increasing carbonyl content of the sample, and sulfuric acid activated the formation of peroxide. The cellulose peroxide was gradually decomposed at 60°C in aqueous medium, and the decomposition was accelerated by addition of ferrous salt or irradiation with light of λ > 300nm. Grafting was initiated by adding methyl methacrylate to the thermal decomposition system under nitrogen. The formation, stability, thermal decomposition, and structure of the cellulose peroxide were discussed in comparison with one derived from aldehyde cellulose and hydrogen peroxide.     

The Radiation Chemistry of CMPO: Part 1. Gamma Radiolysis

Octylphenyl-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) was irradiated using ⁶⁰Co γ-rays, and post-irradiation samples were analyzed for the decrease in CMPO concentration, the appearance of degradation products, and the effects on americium solvent extraction. The –G-value for the radiolytic degradation of CMPO depends on the presence of nitric acid and oxygen during irradiation, with both providing protection from radiolytic degradation. The –G-value does not depend on the γ-ray dose rate or the CMPO concentration. Effects on solvent extraction performance were independent of the change in CMPO concentration, but rather related to the nature of the degradation products. The main products were identified by ESI-MS and were different for irradiation in the presence of nitric acid.     

The effect of orientation on radiation-induced degradation in high density polyethylene

The oxidative degradation of cold drawn highly oriented high density polyethylene is studied by IR spectroscopy. Both gamma and ultraviolet radiation sources are used. It is found that under gamma radiation the oxidative degradation, as determined from carbonyl formation, is reduced very significantly but that the trans-vinylene unsaturation in the polymer increases with draw ratio. Similar results are obtained for samples irradiated in vacuum or when the polymer is stabilized with a radical quencher. Annealing (with free-ends) of the samples restores the rate of carbonyl formation over and above that of the undrawn polymer although restoration after annealing with fixed-ends is not complete. There, is further increase in trans-vinylene development after annealing. In stabilized samples, the effect of annealing on carbonyl and trans-vinylene development appears to depend on draw ratio. Under ultra-violet radiation, the oxidative degradation of drawn and unannealed samples is also reduced with increasing draw ratio. The same effect is observed in stabilized samples. Annealing, once again, restores the rate of oxidative degradation to that of the undrawn polymer. The main unsaturation product during ultraviolet irradiation is the vinyl end group and its development is suppressed with drawing. During ultraviolet irradiation of unannealed drawn samples, cracks, generally perpendicular to the draw direction (intrafibrillar cracks) appear on the sample and with further irradiation they penetrate into the sample. In the case of stabilized and unannealed drawn samples, cracks parallel to the draw direction (interfibrillar cracks) appear first and continued irradiation results in the appearance of intrafibrillar cracks. These observations may have significance in modelling the fibrous structures obtained by drawing semicrystalline polymers.     

The photodegradation of polypropylene. IV. UV stabilizer decomposition

The destruction of various UV stabilizers in polypropylene films during irradiation has been examined both in the presence and absence of air and oxidation products. Although some stabilizers were destroyed in the absence of oxygen, all were more sensitive when irradiated in the presence of air. Several stabilizers were highly effective but were destroyed so rapidly that photoprotection of the polymer must have resulted from stabilizer products. Work with preoxidized polymer and with model hydroperoxides and peroxides indicated that most stabilizers were destroyed by the attack of oxygen-centered radicals, with peroxy radical attack appearing to dominate.