Carmen Dobarganes

Oxidative stability of virgin olive oil

Virgin olive oil has a high resistance to oxidative deterioration due to both a triacylglycerol composition low in polyunsaturated fatty acids and a group of phenolic antioxidants composed mainly of polyphenols and tocopherols. Polyphenols are of greater importance to virgin olive oil stability as compared with other refined oils which are eliminated or drastically reduced during the refining process. This paper covers the main aspects related to the oxidative stability of virgin olive oil during storage as well as at the high temperatures of the main processes of food preparation, i.e., frying and baking. Differences between oxidation pathways at low and high temperature are explained and the general methods for the measurement of stability are commented on. The compounds contributing to the oxidative stability of virgin olive oils are defined with special emphasis on the antioxidative activity of phenolic compounds. Finally, the variables and parameters influencing the composition of virgin olive oils before, during and after extraction are discussed.

Interactions between fat and food during deep-frying

In deep-fat frying the food is completely surrounded by the frying fat or oil and different events occur within a few minutes: dehydration of food surface, absorption of fat, formation of flavour compounds, development of surface colour, etc. Due to the drastic conditions applied during deep-frying, the frying fat also undergoes degradation. Although much work has been done on modifications of used frying fats and oils under different conditions, changes in the fried substrate have been much less studied. Particularly, there is minimal information on some physical and chemical aspects of the interactions between frying fats and fried foods. In this paper the main changes in the frying fat due to the nature of the food fried in it as well as modifications in the food as a consequence of the fat or oil used as heat transfer medium are reviewed. Fat absorption and lipid exchanges are the main physical changes involved. Chemical reactions include interaction between food constituents and oxidised lipids as well as hydrolysis of frying fats due to food moisture.

Oxidized fats in foods

Purpose of reviewLipid 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 findingsIn 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. SummaryThe 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.

Loss of tocopherols and formation of degradation compounds in triacylglycerol model systems heated at high temperature

Triolein, trilinolein and a mixture of both (1:1) were heated at 180 °C for 2, 4, 6, 8 and 10 h in the absence of tocopherols or in the presence of α-tocopherol (500 mg kg−1), δ-tocopherol (500 mg kg−1) or a mixture of α-, β-, γ- and δ-tocopherol (200–250 mg kg−1 each). Losses of tocopherols as well as increases in polymeric triacylglycerols were followed. Total polar compounds were also evaluated after 10 h heating. Results demonstrated that the antipolymerisation effect of tocopherols at high temperature depended on the degree of unsaturation affecting to a greater extent the less unsaturated substrate, triolein. The maximum effect for the three substrates was found when the tocopherol mixture was added. Interestingly, α-tocopherol losses were very rapid and independent of the unsaturation of the triacylglycerol system under the conditions used, although degradation of the substrate was significantly higher as the degree of unsaturation increased for any period of heating. © 1999 Society of Chemical Industry

Modeling of alpha-tocopherol loss and oxidation products formed during thermoxidation in triolein and tripalmitin mixtures.

The degradation of α-tocopherol and the formation of α-tocopherol and triacylglycerol oxidation products at high temperatures (150–250°C) over a heating period (0–4h) for a model system ranging between triolein and tripalmitin were modeled by use of an experimental design. The oxidation products of α-tocopherol formed under these conditions were α-tocopherolquinone (1.4–7.7%) and epoxy-α-tocopherolquinones (4.3–34.8%). The results indicate a very high susceptibility of α-tocopherol to capture peroxyl radicals upon oxidation, leading to the formation of polar tocopherol oxidation products. Both α-tocopherolquinone and epoxy-α-tocopherolquinones were not stable upon prolonged heating and were further degraded to other unknown oxidation products. The kinetics of α-tocopherol oxidation were significantly influenced by the triolein/tripalmitin ratio. By increasing the level of triacylglycerol unsaturation the rate of α-tocopherol recovery after heating increased significantly from 2.2 to 44.2% whereas in the meantime triacylglycerol polymerization increased from 0 to 3.7%.

Characterisation of aldehydic acids in used and unused frying oils

Abstract Aldehydic acids, expected to be present in auto-oxidised lipids as a result of decomposition of hydroperoxides by β-scission reactions, were analysed in unused and in frying oils used for two days. The oils studied were partially hydrogenated rapeseed/palm oil mixture, conventional sunflower oil and high oleic acid sunflower oil. The method involved saponification of lipids, elimination of the unsaponifiable matter, transmethylation and further fractionation of the methyl esters into polar and non-polar fractions by silica column chromatography. Both fractions were analysed by high-performance size-exclusion chromatography to determine the levels of non-polar dimers, polar dimers and oxidised monomers. The polar fraction was also analysed by gas chromatography and gas chromatography–mass spectrometry. Using this approach, the aldehydic acid methyl esters: methyl 8-oxooctanoate, methyl 9-oxononoate, methyl 10-oxo-8-decenoate, methyl 11-oxo-9-undecenoate and methyl 12-oxo-9-dodecenoate were characterised in used as well as unused frying oils.

Selection of methylation procedures for quantitation of short-chain glycerol-bound compounds formed during thermoxidation

Five methylation procedures, including base- and acid-catalyzed methods, were tested in thermoxidized methyl linoleate and trilinolein, in order to quantitate major oxidation short-chain glycerol-bound compounds by gas chromatography. Results indicated that transmethylations using KOH in methanol or CH3ONa-CH3OH in tert.-butylmethyl ether were the most appropriate methods, given the excellent reproducibility and practically complete recovery obtained for the compounds of interest, mainly short-chain fatty acids and aldehydic acids. Also, formation of acids from aldehydes during thermoxidation as well as modifications of aldehydic functions under acidic conditions, such as conversion to acetals, were checked using dodecanal as model aldehyde.

Short-Chain Fatty Acid Formation during Thermoxidation and Frying

Formation and evolution of the short-chain fatty acids originated by oxidation and remaining bound to the parent triglyceride, specifically heptanoate and octanoate, were studied during thermoxidation of palm olein, conventional sunflower oil and high-oleic sunflower oil at 180°C, as well as in real used frying oils of unknown history collected by Food Inspection Services. Fatty acids were quantified by gas-liquid chromatography following transesterification of samples and using methyl nonanoate and methyl heptadecanoate as internal standards. Alteration level was determined through analyses of polar compounds and polar fatty acids. Results showed high correlation coefficients between short-chain fatty acids and polar compounds or polar fatty acids, thus suggesting that quantification of short-chain fatty acids is a good indication of the total alteration level in used frying oils.

A detailed identification study on high-temperature degradation products of oleic and linoleic acid methyl esters by GC-MS and GC-FTIR.

GC-MS and GC-FTIR were complementarily applied to identify oxidation compounds formed under frying conditions in methyl oleate and linoleate heated at 180°C. The study was focused on the compounds that originated through hydroperoxide scission that remain attached to the glyceridic backbone in fats and oils and form part of non-volatile molecules. Twenty-one short-chain esterified compounds, consisting of 8 aldehydes, 3 methyl ketones, 4 primary alcohols, 5 alkanes and 1 furan, were identified. In addition, twenty non-esterified volatile compounds, consisting of alcohols, aldehydes and acids, were also identified as major non-esterified components. Furanoid compounds of 18 carbon atoms formed by a different route were also identified in this study. Overall, the composition of the small fraction originated from hydroperoxide scission provides a clear idea of the complexity of the new compounds formed during thermoxidation and frying.

Effect of temperature and addition of α-tocopherol on the oxidation of trilinolein model systems

The effects of temperature and addition of α-tocopherol were evaluated in trilinolein model systems through quantification of oxidized TAG monomers, dimers, and polymers following oxidation at different temperatures. Samples of trilinolein without and with 250 and 500 mg/kg α-tocopherol added were stored at 25, 60, and 100°C. Quantification of oxidized monomers, dimers, and polymers by a combination of adsorption and exclusion chromatography provided a useful measurement for studying the evolution of oxidation. Results showed that the amounts of primary oxidation compounds (trilinolein oxidized monomers) that accumulated during the induction period decreased as the temperature increased, indicating that the slope of the initial linear stage of oxidation depended on temperature. The end of the induction period was marked by a sharp increase in the levels of total oxidation compounds, the initiation of polymerization, and the loss of α-tocopherol. Addition of α-tocopherol did not prevent, but rather delayed, formation of trilinolein oxidized monomers and the initiation of polymerization.