Autoxidation of methyl linolenate and methyl linoleate: The effect of α-tocopherol

α-Tocopherol substantially affects the distribution of peroxidic compounds formed during the autoxidation of methyl linolenate and methyl linoleate. In the autoxidation of both these unsaturated fatty acid esters the proportion of monohydroperoxides with conjugated double bonds in the trans, trans configuration is reduced until at high concentrations of α-tocopherol (5%) only cis, trans isomers are formed. In the autoxidation of methyl linolenate the proportion of hydroperoxy-epidioxides is reduced and only monohydroperoxides are formed when 5% α-tocopherol is present. The results are discussed in terms of recent findings concerning the mechanism of the autoxidation of unsaturated lipids.     

Contribution of low-molecular-weight antioxidants to the antioxidant capacity of raw and processed lentil seeds

In this study four cultivars of lentil originating from Spain were examined: cv Paula, cv Agueda, cv Almar and cv Alcor. Since consumption of these seeds after heat treatment and as sprouts has been popularised, the impact of cooking (up to 30 min) and germination process (in dark, at 25 degrees C, for up to 4 days) on peroxyl radical-trapping capacity (PRTC) and Trolox-equivalent antioxidant capacity (TEAC) of the processed seeds was addressed. Also, changes in the content of low-molecular-weight antioxidants (LMWA) and soluble proteins in the course of cooking and germination were studied. The analyzed LMWAwere: total phenolics, tocopherols (alpha-T, beta-T, gamma-T, delta-T), reduced glutathione, and L-ascorbic acid. On the basis of the results obtained, the contribution of LMWA and soluble proteins to the PRTC and TEAC of raw, cooked, and germinated lentil seeds was calculated by multiple mean values for the content of investigated compounds and their relative potential with respect to Trolox. The results showed avery high molar percentage contribution of phenolic compounds and low contribution of tocopherols, glutathione, soluble proteins, and ascorbate (only in germinated seeds) to the total TEAC and total PRTC calculated as a sum of data provided for phosphate-buffered and 80% methanolic extracts of raw and processed lentil seeds.     

Tocopherol composition of deodorization distillates and their antioxidative activity

During the last stage of plant oil refining, deodorization distillates containing very important biological substances such as tocopherols, sterols, terpenoids or hydrocarbons are formed as a by-products. This study aimed at evaluating the content and antioxidant capacity of tocopherol concentrates from deodorization distillates obtained after the refining of rapeseed, soybean and sunflower oil. The majority of the matrix substances were eliminated from deodorization distillates by freezing with an acetone solution at -70 degrees C. The tocopherol concentrates obtained in this way contained approximately fivefold more tocopherols than the quantity in condensates after deodorization. Antioxidant activity was investigated by observing the peroxide value at 25 degrees C and using the Oxidograph test. The test medium was lard enriched with the tocopherol concentrates of the three plant oils versus single, synthetic alpha-, gamma- and delta-tocopherols (-T), which served for comparison. In these model systems, all investigated tocopherol concentrates exhibited antioxidant capacity. Their antioxidant effect was significantly lower than that of single delta-T and gamma-T, but significantly higher than alpha-T. The results prove that natural tocopherol concentrates obtained from plant oils are valuable food antioxidants and they also increase the biological and nutritional value of food especially when administered to animal fats or food of animal origin. Tocopherol concentrates can fully replace synthetic antioxidants that have been used thus far.     

Effect of lipid hydroperoxides on pheophytin in the dark

Pheophytin was heated with hydroperoxyoleate in the medium of methyl palmitate, methyl oleate or methyl linoleate to 60°C in inert gas. The rate of pheophytin destruction due mostly to free peroxy radicals generated by hydroperoxide decomposition was proportional to the content of hydroperoxide. Double bonds of fatty acid esters were competitively attacked by free peroxy radicals. In systems containing low levels of oxygen, methyl esters were slowly autoxidized on heating, and pheophytin moderately inhibited their autoxidation. Because of a low concentration of pheophytins the antioxidant activity was more pronounced under conditions of a lower rate of oxidation chain initiation, i.e. in more saturated systems and at a lower content of hydro-peroxides.     

Inhibition of methyl linoleate autoxidation by phenolics and other related compounds under mild oxidative conditions

Methyl linoleate (MeLo) is a commercially available substrate widely used for studying the inhibitory properties of pure compounds and plant extracts against lipid oxidation. In this paper, 13 phenolic (benzoic and cinnamic acids, aldehydes and derivatives, and flavonoids) and other related compounds (butylated hydroxyanisole, butylated hydroxytoluene and tocopherols) as well as nine complex mixtures rich in phenolics (wines) or tocopherols (soybean oil extracts) were assayed for their inhibitory activity against MeLo autoxidation under mild conditions (40 °C, darkness and atmospheric pressure). Samples were also assayed for their free radical‐scavenging capacity against 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH^?). Protocatechuic aldehyde, a compound that has not been previously evaluated by either of the two methods, showed the highest antioxidant activity. Some variations in the antioxidant ranking for some compounds were found between our results and those obtained by other authors using accelerated conditions for MeLo oxidation. Antioxidant activity of wines and soybean oil extracts was related to their richness in phenolics and tocopherols respectively. Correlation between antioxidant capacity measured by the MeLo and DPPH^? methods was found for wines but not for the other samples studied. Therefore the measure of the free radical‐scavenging capacity of a compound is not always a reliable indicator of its lipid oxidation inhibitory ability. Copyright © 2004 Society of Chemical Industry     

Review. Recent advances in lipid oxidation

In a major pathway of the autoxidation of methyl linolenate, peroxyl radicals of the internal hydroperoxides undergo rapid 1,3-tyclisation to form hydroperoxyepidioxides. Because linolenate hydroperoxides are relatively unstable, free radical antioxidants are much less effective in linolenate oils than in linoleate oils. Tocopherols and carotenoids effectively inhibit photosensitised oxidation of vegetable oils. Direct gas chromatographic analyses of malonaldehyde do not correlate with the TBA test. Model fluorescence studies indicate that malonaldehyde may not be so important in crosslinking with DNA. In contrast to oxidised methyl linoleate, oxidised trilinolenin does not form dimers. Although trilinolein oxidises with no preference between the 1(3)-and 2-triglyceride positions, the n-3 double bond of trilinolenin oxidises more in the 1(3)- than in the 2-position. Synthetic triglycerides oxidise in the following decreasing relative rates: LnLnL, LnLLn, LLnL, LLLn (Ln = linolenic and L = linoleic). To estimate the flavour impact of volatile oxidation products their relative threshold values must be considered together with their relative concentration in a given fat.     

Extraction of Lignan Compounds from Roasted Sesame Oil and their Effects on the Autoxidation of Methyl Linoleate

ABSTRACT:  Lignan compounds were extracted from roasted sesame oil and their effects on the autoxidation of methyl linoleate (ML) were studied. Lignan compounds extracted from roasted sesame oil, sesamol, sesamin, and sesamolin, were added to ML, which was then oxidized at 60 ^oC for 18 h in the dark. Alpha-tocopherol was separately added to ML for a reference antioxidant. Degree of ML oxidation was monitored by conjugated dienoic acid (CDA) contents and p -anisidine value (PAV) by AOCS methods, and ML retention by gas chromatography. CDA contents and PAV of samples increased with the oxidation time at 60 ^oC in the dark, and ML decreased. Sesamol-, sesamin-, or sesamolin-added samples showed lower CDA contents, PAV, and ML loss than the samples without lignans during oxidation in the dark, which indicated that lignan compounds lowered the ML autoxidation. The antioxidant activity of sesamol was significantly higher ( P < 0.05) than that of sesamin, sesamolin, or α-tocopherol. Lignan compounds added to ML were degraded during the autoxidation of ML, and the degradation rate was higher in sesamol- than in sesamin-, or sesamolin-added ML, but was lower than in tocopherol-added samples. As the lignan compounds concentration in ML increased, the degradation rate of lignans decreased, and the inhibition of the ML autoxidation by lignan compounds increased. The results strongly suggested that the autoxidative stability of ML could be improved by the addition of sesamol, sesamin, or sesamolin extracted from the roasted sesame oil.     

Mechanisms and Factors for Edible Oil Oxidation

ABSTRACT: Edible oil is oxidized during processing and storage via autoxidation and photosensitized oxidation, in which triplet oxygen (³O₂) and singlet oxygen (¹O₂) react with the oil, respectively. Autoxidation of oils requires radical forms of acylglycerols, whereas photosensitized oxidation does not require lipid radicals since ¹O₂ reacts directly with double bonds. Lipid hydroperoxides formed by ³O₂ are conjugated dienes, whereas ¹O₂ produces both conjugated and nonconjugated dienes. The hydroperoxides are decomposed to produce off‐flavor compounds and the oil quality decreases. Autoxidation of oil is accelerated by the presence of free fatty acids, mono‐ and diacylglycerols, metals such as iron, and thermally oxidized compounds. Chlorophylls and phenolic compounds decrease the autoxidation of oil in the dark, and carotenoids, tocopherols, and phospholipids demonstrate both antioxidant and prooxidant activity depending on the oil system. In photosensitized oxidation chlorophyll acts as a photosensitizer for the formation of ¹O₂; however, carotenoids and tocopherols decrease the oxidation through ¹O₂ quenching. Temperature, light, oxygen concentration, oil processing, and fatty acid composition also affect the oxidative stability of edible oil.     

Isolation and some properties of plastochromanol-8

Conditions suitable for preparative isolation, quantitative separation and determination of plastochromanol-8 (PC-8), and their mild oxidation products were elaborated. Mild oxidation of PC-8 alone and with gammatocopherol (gamma-T) equimolare mixture by benzoquinone gives four and two new products, respectively. These products give Emmerie-Engel reaction. Four of gamma-T, three of PC-8 and two mixed mild oxidation products were found in linseed (Lea number = 49). It was also found that PC-8 derived from plant oils was a natural antioxidant better inhibiting autoxidation than alpha-tocopherol in the model system in which lard was the substrate.     

4-Oxo-2-nonenal as a pro-oxidant during the autoxidation of methyl linoleate in bulk phase

The effect of aldehydic lipid peroxidation products on the autoxidation of methyl linoleate (MeL) was studied at 60 °C in bulk phase. Addition of 4-oxo-2-nonenal (ONE) to MeL accelerated the formation of MeL hydroperoxides. Other tested aldehydes showed no effect on the rate of MeL autoxidation. The pro-oxidative effect of ONE disappeared when pre-existing peroxides and transition metals were removed from MeL. Further addition of ferric ion to the peroxide- and metal-free MeL recovered its pro-oxidative effect. ONE reduced ferric ion to ferrous ion effectively. Furthermore, it had a chelating ability with ferrous ion. The results suggest that the pro-oxidative effect of ONE is due to its ability to reduce transition metals and to chelate the reduced form of metal ions. Thus, ONE could accelerate the metal-dependent lipid peroxidation.     

Oxidative Degradation of β-Cyclodextrin Induced by Lipid Peroxidation

The oxidative degradation of β-cyclodextrin (β-CD) induced by autoxidation of linoleate has been investigated in the solid system composed of β-CD and linoleate. β-CD was oxidized with a propagative oxidation of linoleate to induce the cleavage of its glucosidic linkage and this degradation proceeded proportionally with the moisture content in the solid system. The oxidative cleavage of β-CD gave several kinds of oligosaccharides which were composed of D -erythrose, D -arabinose, D -erythropentosulose, D -xylopentdial-dose, D -glucose and deoxyunsaturated hexose as their reducing terminals. These degradation of β-CD seemed to be initiated by certain radical species formed from the peroxidation of linoleate.     

Temperature Dependence of Autoxidation of Perilla Oil and Tocopherol Degradation

Abstract: Temperature dependence of the autoxidation of perilla oil and tocopherol degradation was studied with corn oil as a reference. The oils were oxidized in the dark at 20, 40, 60, and 80 °C. Oil oxidation was determined by peroxide and conjugated dienoic acid values. Tocopherols in the oils were quantified by HPLC. The oxidation of both oils increased with oxidation time and temperature. Induction periods for oil autoxidation decreased with temperature, and were longer in corn oil than in perilla oil, indicating higher sensitivity of perilla oil to oxidation. However, time lag for tocopherol degradation was longer in perilla oil, indicating higher stability of tocopherols in perilla oil than in corn oil. Activation energies for oil autoxidation and tocopherol degradation were higher in perilla oil (23.9 to 24.2, 9.8 kcal/mol, respectively) than in corn oil (12.5 to 15.8, 8.8 kcal/mol, respectively) indicating higher temperature-dependence in perilla oil. Higher stability of tocopherols in perilla oil was highly related with polyphenols. The study suggests that more careful temperature control is required to decrease the autoxidation of perilla oil than that of corn oil, and polyphenols contributed to the oxidative stability of perilla oil by protecting tocopherols from degradation, especially at the early stage of oil autoxidation.     

Antioxidant properties of lucerne extracts

Aqueous extracts of fresh and dehydrated lucerne were found to exhibit heat-stable antioxidant activity toward the autoxidation of linoleic acid, the lipoxidase-catalysed oxidation of linoleic acid and the lipoxidase-induced carotene oxidation. EDTA increased the antioxidant activity of lucerne extract in all three systems. Alone, EDTA inhibited linoleate autoxidation, but not lipoxidase-catalysed linoleate oxidation, and was only partly inhibitory toward lipoxidase-induced carotene oxidation. The presence of ferulic acid in the acid hydrolysate of lucerne extract could be demonstrated by both paper and gas chromatography. This acid, and the related coumaric and sinapic acids, were shown to inhibit lipoxidase activity. It is suggested that a ferulic acid derivative may play a rôle in the antioxidant effects observed with aqueous lucerne extracts.     

Comparison of antioxidants in primary and secondary oxidations

The relative efficacies of a number of antioxidants including mono- and bis-phenols were compared in gel emulsion systems containing linoleate and β-carotene and subjected to autoxidation and to oxidation by lipoxidase and haemoglobin. They were also tested in liquid emulsions against oxidation of linoleate and vitamin A by lipoxidase. Information on the specificity of antioxidants towards particular systems was obtained.     

Antioxidant composition and activity of barley (Hordeum vulgare) and malt extracts and of isolated phenolic compounds

Phenolics have been identified and quantified in nine varieties of barley and their corresponding malts as flavan-3-ols, flavonols, phenolic acids and apolar esters. Flavan-3-ols are monomers, (+)-catechin and (−)-epicatechin, and polymers constituted mainly by units of (+)-catechin and (+)-gallocatechin. The most abundant compounds were the dimers procyanidin B3 and prodelphinidin B3. The main trimeric procyanidin was procyanidin C2. After malting, the phenolic content decreased for all varieties. Catechin monomers were the most affected. Beside polyphenols, barley and malt extracts contained other antioxidants: carotenoids (lutein and zeaxanthin) and tocopherols (α, δ and γ). The antioxidant activity was measured using three methods: capacity to react with DPPH^. (ARP), inhibition of lipoxygenase activity (LoxI) and inhibition of cooxidation of β-carotene in a linoleate model system (AOP). The inhibition of cooxidation of β-carotene in a linoleate model system did not allow varieties to be discriminated. They all have high antioxidative properties. Using this assay, tocopherols were the best antioxidants. The ARP (antiradical power) was correlated positively with the amount of total flavan-3-ols (r = 0.89) and increased with the degree of polymerisation. The LoxI assay allowed discrimination of the nine varieties of barley and their corresponding malts but was not correlated with any compound, although flavan-3-ols were good inhibitors of lipoxygenase activity. © 1999 Society of Chemical Industry     

Impact of surfactant type, pH and antioxidants on the oxidation of methyl linoleate in micellar solutions

The oxidation of methyl linoleate micelles has been studied, aiming at elucidating the effect of various variables, including surfactant type, pH and antioxidants. The progress of the methyl linoleate oxidation was evaluated by measurement of conjugated dienes hydroperoxides (CDHP) and malondialdehyde (MDA). It was shown that the oxidative stability of methyl linoleate micelles was influenced by surfactant type, with oxidative rate being greater in SDS micelles than in Tween 20 micelles. Besides, the methyl linoleate micelles at pH 6.8 had greater rates of lipid oxidation than their pH 3.0 counterparts. Moreover, the incorporation of VE and VC in the methyl linoleate micelles successfully slowed the formation of hydroperoxides and their subsequent decomposition product MDA. However, the antioxidant activities of VE and VC were related to their concentrations.     

The high performance liquid chromatographic determination of total malondialdehyde in vegetable oil with dansyl hydrazine

A simple high-performance liquid chromatographic (h.p.l.c.) method has been developed for determining total malondialdehyde (MDA), including free MDA and MDA released from its precursor in vegetable oils. Free MDA was reacted with dansyl hydrazine in an acidic medium, and the product, 1-dansyl-pyrazole(DP), was determined by h.p.l.c. The reaction releasing MDA from its precursor requires Fe³⁺ in 5% hydrochloric acid medium. Recoveries of free MDA and tetramethoxypropane (used as an MDA precursor) in vegetable oils were satisfactory. During autoxidation, a remarkable difference between the DP values of methyl oleate, methyl linoleate and some vegetable oils, and that of methyl linolenate was observed. Only methyl linolenate released MDA during autoxidation. For autoxidised methyl linolenate the DP value correlated with the thiobarbituric acid(TBA) value, although the DP value was only 30% of the TBA value.     

Temperature dependence of the autoxidation and antioxidants of soybean, sunflower, and olive oil

Effects of temperature on the autoxidation and antioxidants changes of soybean, sunflower, and olive oils were studied. The oils were oxidized in the dark at 25, 40, 60, and 80 °C. The oil oxidation was determined by peroxide (POV) and p-anisidine values (PAV). Polyphenols and tocopherols in the oils were also monitored. The oxidation of oils increased with the oxidation time and temperature. Induction period decreased with the oxidation temperature; 87 and 3.6 days at 25 and 60 °C, respectively, for sunflower oil. The activation energies for the autoxidation of soybean, sunflower, and olive oils were 17.6, 19.0, and 12.5 kcal/mol, respectively. Olive oil contained polyphenols at 180.8 ppm, and tocopherols were present at 687, 290, and 104 ppm in soybean, sunflower, and olive oils, respectively. Antioxidants were degraded during the oil autoxidation and the degradation rates increased with the oxidation temperature of oils; for tocopherols, 2.1 × 10⁻³ and 8.9 × 10⁻²%/day at 25 and 60 °C, respectively, in soybean oil.     

Annurcoic acid: A new antioxidant ursane triterpene from fruits of cv. Annurca apple

‘Annurca’ is a variety of apple produced in the south of Italy. The phytochemical study of the ethereal extract of the reddened fruits led to the isolation of a new ursane triterpen, as well as five known compounds, which were identified by spectroscopic techniques. The new compound was identified as the acid, 1α,19α-dihydroxyursan-28-oic, and named annurcoic acid. Antioxidant activities of all the isolated compounds were assessed by measuring their ability to scavenge 2,2′-diphenyl-1-picrylhydrazyl (DPPH) radicals and to inhibit the autoxidation of methyl linoleate (MeLo) in vitro.     

Antioxidation reaction mechanism studies of phenolic lignans,identification of antioxidation products of secoisolariciresinol from lipid oxidation

The chain-breaking antioxidation reaction mechanism of a phenolic lignan in a lipid oxidation system was investigated. The 2,2′-azobis(isobutyronitrile) (AIBN)-induced radical oxidation reaction in a large amount of ethyl linoleate in the presence of secoisolariciresinol, one of the potent antioxidative lignans widely distributed in edible plants, produced two types of peroxides as radical termination products, as well as a cyclic derivative. The isolation and structure determination of the peroxides revealed that they have a peroxide linkage with ethyl linoleate or isobutyronitrile at the 1-position of the benzene ring of secoisolariciresinol. The cyclic derivative was also identified as lariciresinol by spectroscopic analysis. Based on the chemical structures of these products, a reaction pathway for the antioxidation reaction of secoisolariciresinol in oxidising lipid media was proposed.