Distribution of monomeric,dimeric and polymeric products of stigmasterol during thermo‐oxidation

When plant sterols are oxidized at moderate temperatures (≤100 °C), products mainly derive from hydroperoxides, but at temperatures close to 200 °C, thermal reactions such as dehydration and condensation become important. Although sterols are often subjected to frying conditions, very little is known of their thermal reactions. In this study, stigmasterol was thermo‐oxidized at 180 °C, and the formation of dimers and polymers and the amounts of monomers were measured by high‐performance size‐exclusion chromatography. The products were further characterized by polarity using solid‐phase extraction fractionation. During heating, the amounts of monomers decreased at a steady rate, and those of dimers and polymers increased. After 3 h of heating, 21% of the material existed in higher‐molecular‐weight products. The amount of polar monomers increased especially during the first hour, demonstrating the formation of oxides and their further reactions, while that of mid‐polar monomers decreased constantly, indicating losses of stigmasterol. Polar dimers contributed to approximately 60% of the dimers, and polar polymers to approximately 78% of the polymers, which suggests that in most higher‐molecular‐weight products at least one of the sterol moieties was oxidized. This study showed that a significant proportion of thermo‐oxidation products are not polar monomeric oxides which are commonly analyzed as oxidation products.     

Esterification affects phytosterol oxidation

The effect of esterification with rapeseed oil fatty acids on the oxidation reactions of sitosterol, campesterol and sitostanol was investigated, as well as the interactions between phytosterol/stanol compounds and the saturated lipid matrix at 100 °C and 180 °C. Free and esterified phytosterols differed in their reactivity in terms of the formation and profile of secondary oxidation products. Phytosteryl esters were more reactive than free phytosterols during prolonged heating at 100 °C. In contrast, free phytosterols were slightly more reactive than phytosteryl esters at 180 °C. The oxidation reactions of phytostanol compounds were low under all conditions studied. Changes in the phytosterol compounds during heating were also studied via the losses in the original phytosterol contents. This study revealed that the formation of secondary oxides did not account for all the phytosterol losses; this indicates the presence of other oxidation products, especially at 180 °C, and during the heating of free sitosterol. Thus, in order to understand the overall deterioration of phytosterol and phytostanol compounds, both the secondary oxide formation and the sterol loss need to be studied. The deterioration of the saturated lipid matrix used in this study was rather low and was mainly associated with the heating temperature and time.     

Oxidation at elevated temperatures: competition between α‐tocopherol and unsaturated triacylglycerols

The competitive oxidation between α‐tocopherol and unsaturated fatty acyls at thermoxidation conditions (180 and 240 °C) was evaluated using purified triacylglycerols from nine fats and oils (refined coconut, palm, tallow, olive, high oleic sunflower, sunflower, corn, soybean, and flaxseed oil). α‐Tocopherol degraded faster in less unsaturated lipids and a linear correlation between the iodine value (x) and the residual tocopherol content (y) was obtained after 2 h of heating at 240 °C (y = 3.72x + 137.5, R² = 0.9463). The formation of polar oxidation products was established and the results were explained by a non‐selective oxidation of unsaturated fatty acyls and α‐tocopherol by highly reactive alkoxyl and hydroxyl radicals generated by decomposition of hydroperoxides.     

Cholesterol oxidation in lard as affected by CLA during heating – A kinetic approach

The objectives of this study were to determine the effect of two CLAs , 9‐cis, 11‐trans CLA and 10‐trans, 12‐cis on cholesterol degradation and cholesterol oxidation products (COPs) formation in lard during heating for varied length of time. No CLAs and approximately 1770.1 µg/mL of cholesterol were detected in lard. Additionally, there was no significant change in the level of cholesterol and COPs in lard during heating at 100°C over a period of 240 min. But, at 150 or 200°C, the degradation of cholesterol was prominent with substantial amount of COPs being formed. Formation profiles of 7‐OOH, 7‐OH, and 7‐keto at 150°C as well as 5,6‐epoxides at both 150 and 200°C were fitted by a first‐order equation, while a pseudo‐second‐order model described the kinetic pathway of 7‐OH and 7‐keto formation at 200°C. The formation of 7‐OOH at 200°C were fitted as multiple first‐order formation and first‐order degradation curves as its level reached a peak at 60 min and declined to zero. Incorporation of 100 µg/mL CLA showed antioxidant activity, whereas a prooxidant activity was observed for CLA at 500 µg/mL. The outcome of this study demonstrated the potential of CLA to be an antioxidant in oil system.     

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.     

Oxygen diffusion mechanisms during high‐temperature oxidation of ZrB2‐SiC

Oxygen diffusion mechanisms during oxidation of ZrB₂‐30 vol% SiC were explored at temperatures of 1500°C and 1650°C using an ¹⁸O tracer technique. Double oxidation experiments in ¹⁶O₂ and ¹⁸O₂ were performed using a modified resistive heating system. A combination of scanning electron microscopy, energy‐dispersive spectroscopy, and time‐of‐flight secondary ion mass spectrometry was used to characterize the borosilicate and ZrO₂ oxidation products. Oxygen exchange with the borosilicate network was observed to occur quickly at the oxygen‐borosilicate surface at both 1500°C and 1650°C, while evidence of oxygen permeation was only observed at 1650°C for short time (<1 min) exposures. At longer times, >5‐9 min, complete oxygen exchange throughout both the borosilicate glass and ZrO₂ was observed at both temperatures preventing identification of the oxygen transport mechanisms, but demonstrating that oxygen transport is rapid in both oxide phases.     

Antioxidative and synergistic effects of bene kernel and hull oils during oxidation of virgin olive oil

Total amounts of conjugated diene hydroperoxides and carbonyl compounds of a virgin olive oil (VOO) and its purified form as affected by 0.1–6% w/w bene kernel (BKO) and hull (BHO) oils were monitored during 16 h heating at 180°C. The VOO was more prone to the production of off‐flavour carbonyl compounds than to the formation of conjugated diene hydroperoxides. The VOOs oxidative stability decreased significantly due to the removal of the indigenous antioxidative compounds. Oxidative stability, especially regarding the secondary oxidation, significantly improved with increasing concentrations of the BKO than with those of the BHO.     

Influence of heating time and oxygen availability on lipid oxidation in meat emulsions

The aim of the present study was to investigate the effect of heating time and oxygen availability on lipid oxidation during chill storage, using different step indicators, in a meat emulsion model. Lipid oxidation was measured by conjugated dienes, hydroperoxides and 2‐thiobarbituric acid reactive substances formation on raw and cooked meat emulsion at 80 °C in different oxygen availability bags, at 0 °C during 35 d. The results obtained showed that heat treatment and oxygen availability affected conjugated dienes, hydroperoxides and 2‐thiobarbituric acid reactive substances formation during chill storage. Besides cooked meat emulsion did not have a similar behaviour to that of raw samples. Thus, is very important to follow primary and secondary products of oxidative deterioration to understand the effect of heat treatment and oxygen availability overall lipid oxidation process in meat emulsion.     

Cholesterol oxidation in heated lard enriched with two levels of cholesterol

Cholesterol oxidation in lard containing two levels of added cholesterol was monitored using capillary gaschromatography. Loss of cholesterol and formation of cholesterol oxidation products (COPs) were measured. Lard samples with 10 times (Test I) and 2 times (Test II) the amount of cholesterol originally found in each batch of lard were heated at 180°C for 10 hr a day for 240 and 160 hr, respectively. Cholesterol steadily decreased throughout the heating period in both tests. Cholesterol loss followed a first-order reaction rate, with a rate constant (k) of −1.18×10⁻³ h⁻¹ for Test I and −9.45×10⁻³ h⁻¹ for Test II. The COPs accumulated during both heating tests. But the amount of COPs formed did not total the amount of cholesterol lost. During heating, thermal degradation of cholesterol likely occurred, and those products were not detected. During cooling, hydroperoxides formed, which further oxidized into the COPs that were detected. The 7-ketocholesterol and 5α,6α-epoxycholesterol were the predominant COPs formed. The isomeric 7α-and 7β-hydroxycholesterols also accumulated in the heating tests. The 3β,5α,6β-cholestantriol was found in very small amounts and the 25-hydroxycholesterol was not detected. Presented in part at the 80th AOCS Annual Meeting, Cincinnati, OH, in May, 1989.     

Evolution of the oxidation process in olive oil triacylglycerol under accelerated storage conditions (40–60°C)

This paper presents an analysis of oxidation in olive oil TAG at different temperatures within a range of 40–60°C in darkness, as measured by the rate of formation of hydroperoxides, their decomposition products, and sensory and chemical flavor deterioration. At 60°C, the oxidation process was relatively fast, and all the indexes of the extent of oxidation behaved in a very similar way. The behaviors of the rate of formation of conjugated dienes (measured as the K ₂₃₂), of oxidized TAG (determined by HPLC), and of carbonyl compounds (measured as anisidine value) were very similar to that of the PV. The induction periods (IP) of the four indexes were less than 36 h. Nevertheless, the IP for K ₂₇₀ and the sum of oxidized TAG dimers and polymers were 5 d. At 50 and 40°C, the rate of formation of secondary oxidation products was lower. Residual linolenic acid or PUFA as determined by GC showed correlation coefficients higher than 0.999 with the Totox index at all of the assayed temperatures. The rancid recognition threshold corresponded to lower values of the oxidation indexes at lower temperature. Moreover, at all assayed temperatures, the rancidity threshold apparently coincided with the IP for the kinetics of 2,4-decadienal formation.     

Some effects of pressure on the oxidation of methyl oleate

The oxidation of methyl oleate under gas pressures up to 800 p.s.i.a. has been investigated. At 120°C. the samples oxidized under pressure built up hydroperoxides rapidly and exploded. There is no necessary connection between hydroperoxide concentration and explosiveness. Oxidations at 100°C are stable, but peroxide formation is drastically reduced. The presence of cobalt acetate increases the instability of the oxidizing oleate, causing it to explode even at 100°C.     

Side-chain autoxidation of stigmasterol and analysis of a mixture of phytosterol oxidation products by chromatographic and spectroscopic methods

Although the structure of phytosterols is closely related to cholesterol, there is a gap in knowledge concerning the formation and occurrence of oxidation products from phytosterols. The main objective of this study was to isolate and characterize some side-chain oxidation products formed after autoxidation of stigmasterol. Another objective was to highlight the difficulties in the analysis of phytosterol and a mixture of their oxidation products by GC. Pure stigmasterol was oxidized at 120°C for 72 h in an air-ventilated oven. Preparative TLC separated the oxidation products, and the products were characterized with GC-MS and NMR. In addition to the common ring-structure oxidation compounds, three semipolar oxidation products—24-ethylcholest-5,22-dien-3β,25-diol, 24-ethylcholest-5,22-dien-3β,24-diol, and 24-ethyl-5,22-choladien-3β-ol-24-one—were characterized for the first time by TLC, GC-MS, and NMR. Moreover, the results of the analysis of a large number of oxidation products from sitosterol, campesterol, and stigmasterol by capillary column GC indicated that further efforts and optimization are required in this area.     

Antioxidant Activities and Phenolic Compositions of Wheat Germ as Affected by the Roasting Process

Wheat germ is a good source for wheat germ oil, and it is a by‐product with highly concentrated nutrients from the wheat flour‐milling industries. In the present study, raw wheat germ was firstly heat‐treated at 180 °C for 20 min in a fluidized bed dryer, and further roasted at 180 °C for different periods of time. Roasting influence on total phenolic content (TPC), antioxidant activities, and phenolic compositions of wheat germ were evaluated. The roasting process significantly increased the TPC and antioxidant activities including free radical scavenging against DPPH and ABTS radicals, FRAP, and ORAC. In particular, the wheat germ roasted at 180 °C for 20 min showed higher antioxidant activity than those roasted at 180 °C for 5 and 10 min. Three major phenolic acids, namely, ferulic, chlorogenic, and caffeic acid, and four main flavonoids, namely, schaftoside and its isomers or adduct of sinapic acid were identified by HPLC. In general, the content of individual phenolic compounds decreased with prolongation of the roasting time except for ferulic acid. The results suggest that the antioxidant activities of wheat germ can be enhanced by roasting, and the enhancement effect might be partially attributed to the formation of Maillard reaction products (MRP).     

Chemical and biological oxidation of Pinus pinaster lignin of the production of vanillin

Oxidation of kraft black liquor with 14·39% solids content, obtained by cooking of Pinus pinaster wood, was studied by three different process; chemical oxidation with molecular oxygen at 120–138°C (oxygen partial pressure ≈ 0·15 MPa); biological (enzymatic) oxidation; and combined oxidation (in the sequence: chemical + enzymatic). Lignin degradation was followed by high pressure liquid chromatography (HPLC). Production of low molecular weight compounds, e.g. vanillin, was monitored by gas chromatography (GC). Chemical oxidation produced about 1.5 g of vanillin per dm³ of liquor (0·904 g per 100 g of solids). Biodegradation products had molecular weights near to that of vanillin and mainly around 100 daltons, but vanillin was not detected. In the combined oxidation process, vanillin was consumed with a concomitant reduction of the HPLC peak.     

Influence of native antioxidants on the formation of fatty acid hydroperoxides in model systems

The effect of alpha‐tocopherol (alpha‐T) and quercetin on the formation of hydroperoxides of linoleic and linolenic acids during autoxidation at 60 ± 1 °C was investigated. Three isomers of hydroperoxides were detected using HPLC. Of isomers of linoleic acid hydroperoxides, 13‐hydroperoxy‐octadecadienoic acid trans‐trans (13‐HPODE t‐t), 9‐HPODE cis‐trans (9‐HPODE c‐t) and 9‐HPODE trans‐trans (9‐HPODE t‐t) were identified, constituting 64, 19 and 17% of the total amount, respectively. For linolenic acid, the components 13‐hydroperoxy‐octadecatrienoic acid trans‐trans (13‐HPOTE t‐t), 9‐HPOTE c‐t and 9‐HPOTE t‐t contributed 7, 33 and 60% to the total, respectively. The different dominant hydroperoxide isomers detected in linoleic and linolenic acids during oxidation are related to their chemical structure and the microenvironment of emulsion droplets. The ratios between specific isomers for both fatty acid hydroperoxides did not change during oxidation with or without antioxidants. Alpha‐T effectively inhibited the oxidation of fatty acids and reduced the formation of hydroperoxides. The total amount of the hydroperoxides decreased along with the increase in the concentration of alpha‐T, 1–40 µM. Quercetin inhibited the oxidation of both fatty acids at similar efficiency only at 40 µM concentration. A synergistic antioxidant effect of quercetin with alpha‐T in a binary system on both fatty acids was observed.     

Effects of β‐carotene and retinal on the formation of methyl linoleate hydroperoxides

In order to clarify the prooxidative role of carotenoids on the oxidation of unsaturated lipids this study examined the effects of β‐carotene and its oxidative breakdown product, retinal, on primary oxidation products of linoleic acid methyl ester. Formation as well as isomer distribution of methyl linoleate hydroperoxides were followed by highperformance liquid chromatography. Oxidation of methyl linoleate without or with added β‐carotene (5, 20, 200 μg/g) or retinal (7, 18, 180, 360 μg/g) was carried out in the dark under air at 40 °C. Both β‐carotene and retinal promoted the formation of hydroperoxides and thus acted as prooxidants in a concentration‐dependent way. Moreover, carotenoids also had an effect on the isomeric distribution of primary oxidation products as high contents of retinal increased the portion (%) of trans,trans‐hydroperoxides. Being thermodynamically more stable isomers than cis,trans‐isomers of hydroperoxides they are known to accumulate during later phases of oxidation or during hydroperoxide decomposition. The results showed that β‐carotene and retinal were not effective hydrogen donors. These findings raise the question that carotenoids and their oxidative breakdown products enhance the decomposition of lipid hydroperoxides and this effect partially explains the prooxidative effect of carotenoids.     

Insight into spent caustic treatment: on wet oxidation of thiosulfate to sulfate

Oxidation of thiosulfate to sulfate is often the rate controlling step during wet air oxidation (WAO) of spent caustic from the refinery and petrochemical industry and exhibits high Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). The kinetics of WAO of thiosulfate was studied in the absence and presence of a heterogeneous copper catalyst. Wet oxidation of thiosulfate to sulfate is a free radical reaction exhibiting an induction period. In non‐catalytic oxidation, almost complete conversion of thiosulfate to sulfate was observed in 12 min at 150 °C and in 8 min at 120 °C in the presence of a heterogeneous copper catalyst at 0.69 MPa oxygen partial pressure. The presence of phenol accelerated thiosulfate oxidation. © 1999 Society of Chemical Industry     

Capabilities of different cooking oils in prevention of cholesterol oxidation during heating

The potential of various cooking oils to prevent cholesterol degradation and/or oxidation, as measured by the production of 7-ketocholesterol during heating at different temperatures, was studied using a cholesterol model system. In the control group (without cooking oil), cholesterol was relatively stable, and 73% of its initial concentration was present after 30 min of heating at 125°C. Less than 30 and 10% of cholesterol remained at 150 and 175°C after 30 min, respectively, and 10% at 200°C after 10 min. In the treatment group, cholesterol mixed with corn, canola, soybean, or olive oil had significantly improved thermal stability. More than 60 and 40% of cholesterol remained at 150 and 175°C after 30 min, respectively. In the control group, 7-ketocholesterol was produced when samples were heated above 150°C, and levels increased consistently during 30 min of heating. At 175 or 200°C, the level of 7-ketocholesterol did not increase further after reaching the highest level after 10 min of heating. 7-Ketocholesterol is not stable above 175°C, and its degradation rate could be much faster than its production at 200°C. 7-Ketocholesterol was not found in samples of cholesterol mixed with corn oil or laboratory-prepared soybean and rice bran oils until the heating temperature was raised to 175°C for 20 min. The levels of 7-ketocholesterol in those treatment groups were greater than that in the control group at 175°C for 30 min. These oils may increase the thermal stability of 7-ketocholesterol and retard its degradation rate.     

Electron spin resonance spectroscopy for evaluation of early oxidative events in semisolid palm oil

Electron spin resonance (ESR) spectroscopy was applied to study early oxidative events in semisolid palm oil in bulk. Oil was stored at mildly accelerated conditions of 50°C for 7 days and the free radical formation was followed with the addition of spin trap N‐tert‐butyl‐α‐phenyl‐nitrone. Dissolution of the oil samples in isooctane prior to ESR measurements stabilized the ESR signal allowing the changes in relative free radical concentrations during oil storage to be monitored. Formation of lipid hydroperoxides as primary lipid oxidation products was found to correlate with the tendency for the formation of free radicals in the oil during the storage and accordingly, ESR spectroscopy may be used to detect the early events in lipid oxidation in palm oil. However, the interference of added rosemary extract (RE) in ESR analyses was seen as an increased ESR signal while the efficiency of RE as antioxidant in palm oil was confirmed by isothermal DSC. Practical applications : ESR spectroscopy may be used to evaluate early events of oxidation in semisolid oils such as palm fat, which is widely used in food industry.     

Fabrication and oxidation behavior of Al4SiC4 powders

Al₄SiC₄ powders with high purity were synthesized by heating the powder mixture of aluminum (Al), silicon (Si), and carbon (C) at 1800°C in argon. The microstructure is characterized as platelike single grain. Both the nonisothermal and isothermal oxidation behavior of Al₄SiC₄ was investigated at 800°C‐1500°C in air by means of thermogravimetry method. It is demonstrated that Al₄SiC₄ powder possesses good oxidation resistance up to 1200°C and is almost completely oxidized at 1400°C. At 800°C‐1100°C, the oxide scales consist of an Al₂O₃ outer layer and a transition layer. Al₄SiC₄ remains the main phase. At 1200°C, some spallation resulting from the increment of Al₂O₃ and the mismatch of thermal expansion coefficient between different product layers can be observed. Above 1300°C, the oxide layer is composed of two part, i.e., large‐scale Al₂O₃ crystals (outer layer) and mullite with less amount of SiO₂ (inner layer). The oxidation behavior changes due to the different oxide products. For the reaction kinetics, a new kind of real physical picture model is adopted and obtains a good agreement with the experimental data. The apparent activation energy is calculated to be 176.9 kJ/mol (800°C‐1100°C) and 267.1 kJ/mol (1300°C‐1400°C).