Sensitive and accurate quantitation of monoepoxy fatty acids in thermoxidized oils by gas-liquid chromatography

Lipid oxidation in emulsions: New insights from the past two decades

Lipid oxidation constitutes the main source of degradation of lipid-rich foods, including food emulsions. The complexity of the reactions at play combined with the increased demand from consumers for less processed and more natural foods result in additional challenges in controlling this phenomenon. This review provides an overview of the insights acquired over the past two decades on the understanding of lipid oxidation in oil-in-water (O/W) emulsions. After introducing the general structure of O/W emulsions and the classical mechanisms of lipid oxidation, the contribution of less studied oxidation products and the spatiotemporal resolution of these reactions will be discussed. We then highlight the impact of emulsion formulation on the mechanisms, taking into consideration the new trends in terms of emulsifiers as well as their own sensitivity to oxidation. Finally, novel antioxidant strategies that have emerged to meet the recent consumer's demand will be detailed. In an era defined by the pursuit of healthier, more natural, and sustainable food choices, a comprehensive understanding of lipid oxidation in emulsions is not only an academic quest, but also a crucial step towards meeting the evolving expectations of consumers and ensuring the quality and stability of lipid-rich food products.

Glycerol-bound oxidized fatty acids: formation and occurrence in peanuts

For peanuts, roasted at 170 °C, the formation of selected glycerol-bound oxidized fatty acids (GOFAs), namely 9-oxononanoic acid (9-ONA), azelaic acid (AZA) and octanoic acid, was observed by GC-MS (EI). The content of octanoic acid as well as AZA increased with continuous roasting time (from 59 mg/kg peanut oil to 101 mg/kg peanut oil and from not detectable to 8 mg/kg peanut oil, respectively), whereas the content of 9-ONA initially decreased from 25 mg/kg peanut oil to 8 mg/kg peanut oil (20 min) and increased again up to 37 mg/kg peanut oil following roasting for 40 min. Due to its aldehyde function, 9-ONA could contribute to amino acid side chain modifications as a result of lipation, which could directly influence the functional properties of peanut proteins. Both 9-ONA and octanoic acid are potential markers of thermal processes. Furthermore, in model experiments using methyl linoleate and methyl oleate, up to 18 oxidized fatty acids could be identified as methyl esters, 9-ONA as well as octanoic acid as major components and a faster formation of GOFAs under roasting conditions (170 °C, 20 min). In addition, 9-ONA contributes to the formation of AZA and octanoic acid in both free and bound form as a result of oxidative subsequent reactions in presence of iron (III).

Oxidation and Polymerization of Triacylglycerols: In-Depth Investigations towards the Impact of Heating Profiles

The stability of refined, bleached, and deodorized palm olein (RBDPO) was studied under controlled heating conditions. RBDPO was heated continuously for 24 h at 160, 170, and 180 °C, with oil sampled at four hour intervals. Thermo-oxidative alterations were measured through various parameters, such as monomeric oxidized triacylglycerols (oxTAG), total polar compounds (TPC), polymerized triacylglycerols (PTG), oxidative stability, and fatty acid composition. After 24 h of heating, the TPC and triacylglycerol oligomers showed a linear increase with heating time at all heating temperatures. At the end of the heating study, more epoxy acids were formed than keto and hydroxy acids. Moreover, caprylic acid, which was not present in fresh oil, was formed in significant amounts. The increase in oxTAG was strongly correlated with the increase in the p-anisidine value and total oxidation value. The decreases in diacylglycerol and free fatty acids were strongly correlated with an increase in PTG.

Effect of Accelerated High Temperature on Oxidation and Polymerization of Biodiesel from Vegetable Oils

Oxidation of biodiesel (BD) obtained from the decomposition of biomass can damage the fuel injection and engine parts during its use as a fuel. The excess heating of vegetable oils can also cause polymerization of the biodiesel. The extent of BD oxidation depends on its fatty acid composition. In this study, an accelerated oxidation test of BDs at 95 °C was investigated according to ASTM D 2274 by applying a long-term storage test for 16 weeks. The density, viscosity, and total acid number (TAN) of BDs increased because of the accelerated oxidation. Furthermore, the contents of unsaturated fatty acid methyl esters (FAMEs), C18:2 ME, and C18:3 ME in BDs decreased due to the accelerated oxidation. The 1H-nuclear magnetic resonance spectrum of BDs that were obtained from the accelerated high temperature oxidation at 180 °C for 72 h differed from that of fresh BDs. The mass spectrum obtained from the analysis of the model FAME, linoleic acid (C18:2) methyl ester, which was oxidized at high temperature, indicated the formation of dimers and epoxy dimers of linoleic acid (C18:2) methyl ester by a Diels-Alder reaction.

Simultaneous quantification of epoxy and hydroxy fatty acids as oxidation products of triacylglycerols in edible oils

Epoxy and hydroxy fatty acids are important intermediates during lipid oxidation; quantification of both structures may help evaluate the extent of competition among various lipid oxidation pathways. This article describes a method to simultaneously determine saturated- and unsaturated- epoxy and hydroxy fatty acids derived from oxidation of vegetable oils. The experimental procedures employed transesterification with sodium methoxide, separation of epoxy and hydroxy fatty acid methyl esters (FAME) using solid-phase extraction (SPE), and trimethylsilyl (TMS) derivatization of hydroxy groups. GC-MS was used to identify the epoxy and hydroxy FAME in two different SPE fractions, while GC-flame ionization detection (GC-FID) was used to determine their quantities. Epoxy-octadecanoate/octadecenoate and hydroxy-octadecanoate/octadecenoate/octadecadienoate were determined as lipid oxidation products generated from oxidation of sunflower and canola oils. An isomer of methyl 13-hydroxyoctadeca-9,11-dienoate (13-HODE) TMS ether co-eluted with methyl 15-hydroxyoctadeca-9,12-dienoate TMS ether, which was only present in canola oil; thus, GC-MS-selected ion monitoring (GC-MS-SIM) was used to determine the concentration of 13-HODE. The proposed method has been successfully applied to monitor epoxy and hydroxy fatty acids in sunflower oil and canola oil oxidized at 40 °C.

Techniques for the Analysis of Minor Lipid Oxidation Products Derived from Triacylglycerols: Epoxides, Alcohols, and Ketones

Lipid oxidation can lead to flavor and safety issues in fat-containing foods. In order to measure the extent of lipid oxidation, hydroperoxides and their scission products are normally targeted for analytical purposes. In recent years, the formation of rarely monitored oxygenated products, including epoxides, alcohols, and ketones, has also raised concerns. These products are thought to form from alternative pathways that compete with chain scissions, and should not be neglected. In this review, a number of instrumental techniques and approaches to determine epoxides, alcohols, and ketones are discussed, with a focus on their selectivity and sensitivity in applications to food lipids and oils. Special attention is given to methods employing gas chromatography (GC), high-performance liquid chromatography (HPLC), and nuclear magnetic resonance (NMR). For characterization purposes, GC-mass spectrometry (GC-MS) provides valuable information regarding the structures of individual oxygenated fatty acids, typically as methyl esters, isolated from oxygenated triacylglycerols (TAGs), while the use of liquid chromatography-MS (LC-MS) techniques allows analysis of intact oxygenated TAGs and offers information about the position of the oxygenated acyl chain on the glycerol backbone. For quantitative purposes, traditional chromatography methods have exhibited excellent sensitivity, while spectroscopic methods, including NMR, are superior to chromatography for their rapid analytical cycles. Future studies should focus on the development of a routine quantitative method that is both selective and sensitive.

Identification of Oxidation Compounds of 1-Stearoyl-2-linoleoyl-sn-glycero-3-phosphoethanolamine during Thermal Oxidation

Heat-induced oxidative modification of phosphatidylethanolamine molecular species as potential functional food components was investigated. 1-Stearoyl-2-linoleoyl-sn-glycero-3-phosphoethanolamine (SLPE) was chosen as a model. The optimal temperature for hydroperoxide formation was determined by mass spectrometry. The maximal level of formation of this compound was obtained at 125 °C. The structures of nonvolatile organic compounds (non-VOCs) were identified using liquid chromatography-electrospray ionization mass spectrometry combined with an acid treatment. Kinetics of formation of non-VOCs was monitored over time. Results showed that the level of the SLPE precursor rapidly decreased during thermal oxidation and oxygenated products, such as hydroxyl, oxo, or epoxy groups, were formed. The VOCs formed from oxidized SLPE were determined by headspace solid-phase microextraction followed by gas chromatography-mass spectrometry analysis. The result showed that a saturated methyl ketone (2-heptanone) was the most predominant VOC of SLPE. Kinetics indicated that the formation of VOCs was related not only to the decomposition of hydroperoxides but also to the further decomposition of non-VOCs.

Comparison of the volatiles formed by oxidation of phosphatidylcholine to triglyceride in model systems

The oxidative stability of oleoyl and linoleoyl residues esterified in the form of triglyceride (TAG) and phosphatidylcholine (PC) during thermal treatment was investigated. Headspace solid-phase microextraction (HS-SPME) followed by gas chromatography-mass spectrometry (GC-MS) analysis was used to determine the volatile compounds from oxidized PL and TAG molecular species. The results showed that aldehydes were the major volatile oxidized compounds (VOCs) of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC), and 1,3-distearoyl-2-linoleoyl-glycerol (SLS), while ketones, especially saturated methyl ketones, were the major VOCs of 1,3-distearoyl-2-oleoyl-glycerol (SOS). The monitoring of the oxidative degradation using liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) showed that either monounsaturated or diunsaturated fatty acyl groups were less oxidized when in the form of PCs than when in the form of TAGs. This finding demonstrated that the choline group in the form of PCs could increase the stability of fatty acyl groups to oxidation in comparison to TAGs.

Development and validation of a gas chromatography-flame ionization detection method for the determination of epoxy fatty acids in food matrices

A reliable and suitable method for the determination of epoxy fatty acids in various food matrices based on the Bligh and Dyer lipid extraction procedure was developed and validated. The method involves the use of a methylated epoxy fatty acid as internal standard (IS), extraction of the analytes from the matrices followed by room temperature methylation, a three-step solid phase extraction (SPE) separation of the fatty acid methyl esters (FAMEs), and detection with gas chromatography-flame ionization detection (GC-FID). The method was validated in four different food matrices chosen as model systems, namely, vegetable oils, unprocessed pork, fried potato crisps, and infant formula. The extraction technique allows the method to be applied for routine analysis of a large amount of samples. Intraday repeatability ranged from 1 to 19%, and interday reproducibility ranged from 2 to 9%. The limit of quantification (LOQ) ranged from 3.32 to 20.47 μg g(-1) of sample with recoveries ranging from 94 to 115%. The results verify the accuracy and reproducibility of the analytical technique and its ability to provide reliable quantification of epoxy fatty acids. Finally, levels of epoxy fatty acids in several food products on the Belgian market were screened and are presented.

Aging effects on macadamia nut oil studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

High-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry is successfully used in the detailed molecular analysis of aged macadamia nut oils. The results are consistent with peroxide values, the current industry measure for rancidity, and provide detailed molecular information on the oxidative and hydrolytic degeneration of such oils. Mass analysis of macadamia oil samples stored for extended periods at 6 °C revealed that oils obtained by the cold press method are more susceptible to aging than those obtained using modified Soxhlet or accelerated solvent extraction methods.

Oxidative stability at high temperatures of oleyol and linoleoyl residues in the forms of phosphatidylcholines and triacylglycerols

An investigation was carried out into the stability of fatty acyl groups to heat-induced oxidative changes as affected by their chemical environment. The behavior of oleic and linoleic acyl groups when esterified in triacylglycerols (TAGs) and phosphatidylcholines (PCs) was evaluated. The monitoring of the oxidative degradation using liquid chromatography-mass spectrometry (LC-MS) showed that fatty acyl groups are less likely to be oxidized when in the form of PCs than when in the form of TAGs. In addition, oxidation products from PCs were more stable than those from TAGs. This finding strengthens the idea that the choline group in PCs increases the stability of fatty acyl groups to oxidation in comparison to TAGs.

Quantitative determination of epoxy acids, keto acids and hydroxy acids formed in fats and oils at frying temperatures

A method based on derivatization to fatty acid methyl esters and GC is proposed for the quantitative analysis of hydroxy acids, keto acids and epoxy acids in fats and oils. Isolation of the analytes by solid-phase extraction is proposed to prevent analytical interferences caused by non-altered fatty acids naturally occurring in oils. In addition, hydrogenation is required before the GC analysis to improve repeatability. The analytical method was applied to thermoxidized samples of high linoleic sunflower oil, high oleic sunflower oil and high palmitic sunflower oil. Results showed total levels of these compounds in the order of mg/g of oil in samples with contents of polar compounds ranging from 6.7 to 25.7%. The compounds analyzed constituted major fractions of the oxidized fatty acids.

A laser desorption ionization time-of-flight mass spectrometry investigation into triacylglycerols oxidation during thermal stressing of edible oils

Laser desorption ionization time-of-flight mass spectrometry (LDI-TOF MS) was used to characterize olive and sunflower oils before and after thermally assisted oxidation in order to develop a rapid fingerprinting method for oil that contains unchanged and oxidized components. No matrix was used to assist laser desorption, and simplified mass spectra were obtained in the mass range of interest (m/z 500-1000), where triacyl- and diacylglycerol ions were observed. Sample preparation was reduced to dissolving oil in chloroform saturated with NaCl. Sodiated triacylglycerols (TAGs), their epoxy/hydroxy and hydroperoxy derivatives, as well as TAGs with shortened chain fatty acids (beta-scission products) were clearly observed in the spectra. LDI-TOF MS rapidly provides semiquantitative information about the oxidation level of edible oil, and thus represents a very useful quality control tool.

Direct monitoring of lipid oxidation in edible oils by Fourier transform Raman spectroscopy

Fourier transform Raman spectroscopy has been used to investigate the chemical changes taking place during lipid oxidation in several edible oils. Oxidative degradation of six vegetable oils was accelerated by heating at 160 degrees C. Formation of aldehydes was detected, and saturated as well as alpha,beta-unsaturated aldehydes could be identified with the help of pure component spectra. The formation of conjugated double bond systems and the isomerisation of cis to trans double bonds was observed in the C=C stretching region and found to follow a distinct pattern for the different oils. It was possible to associate these differences to the fatty acid composition. The time-dependent intensity changes in certain Raman bands were compared to conventional parameters used to determine the extent of oxidation in oils, such as anisidine value and K(270), and showed good correlation.

Structural analysis of hydroperoxy- and epoxy-triacylglycerols by liquid chromatography mass spectrometry

Oxidation of triacylglycerols (TAGs) containing oleic acid leads to the formation of several products. This study characterizes hydroperoxy- and epoxy-TAGs including their regio-isomers. For this purpose, epoxy- and hydroperoxy-TAGs, formed by oxidation of 1,2-dipalmitoyl-3-oleoyl-glycerol (PPO) and 1,3-dipalmitoyl-2-oleoyl-glycerol (POP) under air and 18O2, were analysed by reverse phase liquid chromatography-electrospray ionisation-mass spectrometry (LC-ESI-MS) using a triple quadrupole mass analyser, in positive ion mode. Post-column infusion of ammonium formiate was used to obtain intense molecular ion adducts. Pure 1,2-dipalmitoyl-3-epoxystearoyl-glycerol (PPEs) and 1,3-dipalmitoyl-2-epoxystearoyl-glycerol (PEsP), synthesized by epoxidation of the corresponding monounsaturated TAGs, were used to confirm MS/MS identification. The use of 18O2 oxidation experiments permitted unambiguous identification of MS/MS fragmentation pathways of both hydroperoxide and epoxy-TAGs. Fragmentation of hydroperoxy-TAGs are very distinct from their epoxy-TAGs homologues and consist of simultaneous losses of hydrogen peroxide (34 a.m.u.) and water (18 a.m.u.).

Formation and evolution of monoepoxy fatty acids in thermoxidized olive and sunflower oils and quantitation in used frying oils from restaurants and fried-food outlets

The formation and evolution of monoepoxy fatty acids, arising from oleic and linoleic acids, were investigated in olive oil and conventional sunflower oil, representatives of monounsaturated and polyunsaturated oils, respectively, during thermoxidation at 180 degrees C for 5, 10, and 15 h. Six monoepoxy fatty acids, cis-9,10- and trans-9,10-epoxystearate, arising from oleic acid, and cis-9,10-, trans-9,10-, cis-12,13-, and trans-12,13-epoxyoleate, arising from linoleic acid, were analyzed by gas chromatography after oil derivatization to fatty acid methyl esters. Considerable amounts, ranging from 4.29 to 14.24 mg/g of oil in olive oil and from 5.10 to 9.44 mg/g of oil in sunflower oil, were found after the heating periods assayed. Results showed that the monoepoxides quantitated constituted a major group among the oxidized fatty acid monomers formed at high temperature. For similar levels of degradation, higher contents of the monoepoxides were found in olive oil than in sunflower oil. Ten used frying oils from restaurants and fried-food outlets in Spain were analyzed to determine the contents of the monoepoxides in real frying oil samples. Levels ranged from 3.37 to 14.42 mg/g of oil. Results show that, for similar degradation levels, the monoepoxides were more abundant in the monounsaturated oils than in the polyunsaturated oils.

Simultaneous determination of fatty, dicarboxylic and amino acids based on derivatization with isobutyl chloroformate followed by gas chromatography—positive ion chemical ionization mass spectrometry

Gas chromatography-mass spectrometry (GC-MS) with positive ion chemical ionization (PICI) using isobutane as reagent gas was applied for analysis of isobutoxycarbonyl/isobutyl derivatives of 13 fatty, 6 dicarboxylic and 13 amino acids in a single run. For all investigated compounds (except several amino acids) the quasimolecular ions [MH](+) were registered. Asparagine underwent fragmentation via decarboxylation followed by elimination of OC(4)H(9) ([M-117](+)), whereas serine and tyrosine produced the cluster ions [M+C(4)H(9)OCO](+). Estimated detection limits were 6-250 pg in the total ion current (TIC) mode and 3-10 times lower using the selected-ion monitoring (SIM) mode.

Oxidized fats in foods

PURPOSE OF REVIEW

Lipid oxidation is the cause of important deteriorative changes in chemical, sensory and nutritional food properties. In particular, the question of whether oxidized fats in the diet may be detrimental to health is nowadays of the upmost concern, but finding an answer is not easy and requires careful consideration of different aspects of lipid oxidation.

RECENT FINDINGS

In this review, the most recent works on the formation, nature and evaluation of oxidized dietary lipids are addressed; important issues such as the difficulties encountered in estimating their intake and the relationships between oxidants and antioxidants in the diet are discussed, and the latest studies on health implications of oxidized lipids are summarized.

SUMMARY

The current literature reflects various important points. At present, there is no information on the intake of oxidized fats, which is essential to know if the amount of oxidized lipids in normal diets is sufficient to cause the physiological effects claimed. Recently, relevant advances in analytical methodologies for quantitation of specific oxidation compounds have been reported, although their application to improve the analytical definition of the oxidized substrate used in nutritional studies is still a goal to be reached. Alternatively, one of the most promising current tendencies in this field is the study of the molecular targets by which dietary oxidized lipids can influence health. Overall, more selected research based on coordinated multidisciplinary studies is needed to define the role of dietary oxidized fats in health.