Analysis of vegetable oil volatiles by multiple headspace extraction

Quantitative determination of the volatiles produced from oxidized vegetable oils is an important indicator of oil quality. Five vegetable oils, low-erucic acid rapeseed, corn, soybean, sunflower and high oleic sunflower, were stored at 60°C for four and eight days to yield oils with several levels of oxidation. Peroxide values of the fresh oils ranged from 0.6 to 1.8 while those of the oxidized oils were from 1.6 to 42. Volatile analysis by the multiple headspace extraction (MHE) technique, which includes a pressure and time controlled injection onto the gas chromatography (GC) column (a chemically bonded capillary column), was compared with that obtained by static headspace gas chromatography (SHS-GC). Several volatile compounds indicative of the oxidation of polyunsaturated fatty acids from the vegetable oils were identified and measured by MHE; pure compounds of twelve major volatiles also were measured by MHE, and peak area was determined. Multiple extractions of the oil headspace provided a more reproducible measure of volatile compounds than was obtained by SHS-GC. Concentration of all volatiles increased with increased oxidation as measured by peroxide value of the oil. Presented at the Annual American Oil Chemists' Society Meeting, May 8–12, 1988, Phoenix, AZ.     

Improving the oxidative stability of polyunsaturated vegetable oils by blending with high-oleic sunflower oil

Mixing different proportions of high-oleic sunflower oil (HOSO) with polyunsaturated vegetable oils provides a simple method to prepare more stable edible oils with a wide range of desired fatty acid composition. Oxidative stability of soybean, canola and corn oils, blended with different proportions of HOSO to lower the respective levels of linolenate and linoleate, was evaluated at 60°C. Oxidation was determined by two methods: peroxide value and volatiles (hexanal and propanal) by static headspace capillary gas chromatography. Determination of hexanal and propanal in mixtures of vegetable oils provided a sensitive index of linoleate and linolenate oxidation, respectively. Our evaluations demonstrated that all-cis oil compositions of improved oxidative stability can be formulated by blening soybean, canola and corn oils with different proportions of HOSO. On the basis of peroxide values, a partially hydrogenated soybean oil containing 4.5% linolenate was more stable than the mixture of soybean oil and HOSO containing 4.5% linolenate. However, on the basis of volatile analysis, mixtures of soybean and HOSO containing 2.0 and 4.5% linolenate were equivalent or better in oxidative stability than the hydrogenated soybean oil. Mixtures of canola oil and HOSO containing 1 and 2% linolenate had the same or better oxidative stability than did the hydrogenated canola oil containing 1% linolenate. These studies suggest that we can obviate catalytic hydrogenation of linolenate-containing vegetable oils by blending with HOSO. Presented at the AOCS/JOCS joint meeting, Anaheim, CA, April 25–29, 1993.     

Accretion of Dietary Docosahexaenoic Acid in Mouse Tissues Did Not Differ between Its Purified Phospholipid and Triacylglycerol Forms

Recent studies suggest that dietary krill oil leads to higher omega-3 polyunsaturated fatty acids (n-3 PUFA) tissue accretion compared to fish oil because the former is rich in n-3 PUFA esterified as phospholipids (PL), while n-3 PUFA in fish oil are primarily esterified as triacylglycerols (TAG). Tissue accretion of the same dietary concentrations of PL- and TAG-docosahexaenoic acid (22:6n-3) (DHA) has not been compared and was the focus of this study. Mice (n = 12/group) were fed either a control diet or one of six DHA (1%, 2%, or 4%) as PL-DHA or TAG-DHA diets for 4 weeks. Compared with the control, DHA concentration in liver, adipose tissue (AT), heart, and eye, but not brain, were significantly higher in mice consuming either PL- or TAG-DHA, but there was no difference in DHA concentration in all tissues between the PL- or TAG-DHA forms. Consumption of PL- and TAG-DHA at all concentrations significantly elevated eicosapentaenoic acid (20:5n-3) (EPA) in all tissues when compared with the control group, while docoshexapentaenoic acid (22:5n-6) (DPA) was significantly higher in all tissues except for the eye and heart. Both DHA forms lowered total omega-6 polyunsaturated fatty acids (n-6 PUFA) in all tissues and total monounsaturated fatty acids (MUFA) in the liver and AT; total saturated fatty acid (SFA) were lowered in the liver but elevated in the AT. An increase in the DHA dose, independent of DHA forms, significantly lowered n-6 PUFA and significantly elevated n-3 PUFA concentration in all tissues. Our results do not support the claim that the PL form of n-3 PUFA leads to higher n-3 PUFA tissue accretion than their TAG form.     

AOCS collaborative study on sensory and volatile compound analyses of vegetable oils

An AOCS collaborative study was conducted to determine the effectiveness of sensory analysis and gas chromatographic analyses of volatile compounds in measuring vegetable oils for levels of oxidation that ranged from none to high. Sixteen laboratories from industry, government, and academia in Canada and the United States participated in the study to evaluate the flavor quality and oxidative stability of aged soybean, corn, sunflower, and canola (low-erucic acid rapeseed) oils. Analytical methods included sensory analyses with both flavor intensity and flavor quality scales and gas-chromatographic volatiles by direct injection, static headspace, and dynamic headspace (purge and trap) techniques. Sensory and volatile compound data were used to rank each of the oils at four levels of oxidation—none, low, moderate, and high. For soybean, canola, and sunflower oils, 85–90% of laboratories correctly ranked the oils by either analysis. For corn oil, only 60% of the laboratories ranked the samples according to the correct levels of oxidation by either analysis. Variance component estimates for flavor scores showed that the variation between sensory panelists within laboratories was lowest for the unaged oils. As storage time increased, the variance also increased, indicating that differences among panelists were greater for more highly oxidized oils. Between-laboratory variance of sensory panel scores was significantly lower than within-laboratory variance.     

The Role of Water in Protection Against Thermal Deterioration of Liquid Margarine

The thermal stability of liquid margarine and vegetable oils was investigated by measuring the oxidative stability index (OSI) at temperatures ranging from 90 to 180 °C, whereas total polar compounds (TPC) and tocopherols (vitamin E) were measured during heating at 180 °C in frying trays. Results showed that the OSI of liquid margarine was in the same range as the OSI of vegetable oils at lower temperatures, but at 160 and 180 °C, liquid margarine had significantly higher thermal stability, close to that observed for hard margarine and butter. The increased stability was confirmed by lower levels of TPC and a smaller relative reduction in vitamin E content during heating. Variations between different vegetable oils could partly be explained by differences in degree of saturation and level of vitamin E, with high oleic sunflower oil being the most stable oil at all temperatures. The water in liquid margarine vaporized within 1.5 min at 160 °C, and it is hypothesized that volatile pro‐oxidants are removed with the water, inducing a delay in deterioration. The results indicate a role for water in preventing lipid oxidation and decomposition in fat emulsion products at 160–180 °C, suggesting that liquid margarine, low in saturated fat, may be the healthier and preferable alternative for pan‐frying compared to other liquid vegetable oils.     

Fate of dietary sterols in hydrogenated oils and fats

Samples of table margarines, so-called polyunsaturated table margarines, hydrogenated vegetable oils, and so-called polyunsaturated hydrogenated vegetable oils were shown by infrared spectroscopy to contain hydrogenated components. Examination of the sterols from these oils by argentation thin layer chromatography and gas liquid chromatography did not reveal campestanol, stigmastanol, or Δ²²-stigmastenol, the expected hydrogenation products of the natural sterols. The sterol compositions of the above samples, animal fats, and blends of hydrogenated vegetable oils and animal fats were determined. The compound 24-methyl cholest-7-en-3β-ol was identified tentatively in sunflower and safflower oils.     

Linolenic Acid as the Main Source of Acrolein Formed During Heating of Vegetable Oils

Acrolein, which is an irritating and off-flavor compound formed during heating of vegetable oils, was estimated by the gas–liquid chromatography (GLC). Several vegetable oils such as high-oleic sunflower, perilla, rapeseed, rice bran, and soybean oils were heated at 180 °C for 480 min and then the concentration of acrolein in the head space gas was determined by GLC. The formation of acrolein was greatest in perilla oil among the tested oils, while it was much lower in rice bran oil and high-oleic sunflower oil. There was a good correlation between the level of acrolein and linolenate (18:3n-3) in the vegetable oils. To investigate the formation of acrolein from linolenate, methyl oleate, methyl linoleate, and methyl linolenate were also heated at 180 °C, and the amounts of acrolein formed from them were determined by GLC. The level of acrolein was the greatest in methyl linolenate. Acrolein was also formed from methyl linoleate, but not from methyl oleate. Acrolein in vegetable oils may be formed from polyunsaturated fatty acids, especially linolenic acid but not from glycerol backbone in triacylglycerols.     

Static headspace gas chromatographic analyses to determine oxidation of fish muscle lipids during thermal processing

Oxidation in fish during thermal processing was studied by determining volatile production with a static headspace gas chromatographic system. Different processing temperatures and periods were evaluated to simulate conditions of fish industrial treatments. The major volatiles formed included acetaldehyde, propanal, heptane, 2-ethylfuran, pentanal, and hexanal. Changes in volatile composition were studied for different processing times and temperatures. The method for volatile analyses to determine oxidation in fish muscle was tested by correlation with peroxide value, conjugated diene, and thiobarbituric acid indices. Significant single and multivariate regressions were found between the time of thermal treatment and volatiles produced, showing that the amount of 2-ethylfuran was the best predictor of oxidative stability in fish.     

The development and application of novel vegetable oils tailor‐made for specific human dietary needs

Conventional edible oils, such as sunflower, safflower, soya bean, rapeseed (canola) oils, were modified to obtain high‐oleic, low‐linoleic or even low‐linolenic oils. The aim was to develop salad, cooking and frying oils, that are very stable against lipid peroxidation. They are also suitable for margarine blends, as additives to cheeses and sausages, or even as feed components. Oils containing higher amounts of medium‐chain length or long‐chain polyunsaturated fish oil fatty acids are suitable as special dietetic oils or as nutraceuticals. High‐stearic oils are designed as trans‐fatty acid‐free substitutes for hydrogenated oils. New tailor‐made (designer) oils are thus a new series of vegetable oils suitable for edible purposes, where conventional oils are not suitable.     

A rapid method for determining the oxidation of n-3 fatty acids

The stability of unsaturated fatty acids to oxidation was monitored by following gas chromatographic (GC) analyses of headspace volatiles in comparison to changes in polyunsaturated fatty acids (PUFA) and increases in malonaldehydevia the 2-thiobarbituric (TBA) assay. Pure standards of linoleic acid (Lo) and n-3 fatty acids [eicosapentaenoic (EPA) and docosahexaenoic acid (DHA)] were added to headspace vials, equilibrated in air for 10 min, followed by heating at 80°C in teflon-capped vials for different time intervals. Headspace analysis showed increases in acetaldehyde, propenal, and propanal, corresponding to the oxidation of n-3 fatty acids, whereas hexanal production corresponded to losses of linoleic acid. The analysis of propanal by GC-headspace after only five minutes of heating appeared to be the most effective method of monitoring the oxidation of n-3 fatty acids, as indicated by correlations between TBA values and loss of PUFA. The oxidation of Lo, EPA and DHA appeared to be a function of the number of double bonds. Correlations between PUFA depletion, TBA values and volatile formation indicate that under the prescribed conditions of this experiment, GC-headspace analysis of propanal and pentane/hexanal is an excellent method for following the oxidation of selected n-3 fatty acids and linoleic acid.     

不同植物油脂在近临界水中水解反应动力学的比较

系统地测定了压力10 MPa、温度170℃～240℃范围内橄榄油、花生油、大豆油、红花油等植物油脂在近临界水中无催化水解反应动力学数据。实验结果表明,近临界水中油脂水解反应是一个典型的自催化反应,采用二级自催化反应动力学模型对动力学数据进行了拟合,得到了橄榄油、花生油、大豆油、红花油等四种植物油脂的水解反应活化能分别为41.8 kJ/mol、37.3 kJ/mol、37.7 kJ/mol、31.2 kJ/mol。油脂水解活化能与其碘价密切相关,随着油脂碘价的增加,水解活化能逐渐降低。     

Effects of thermally oxidized triglycerides on the oxidative stability of soybean oil

Soybean oil purified by silicic acid column chromatography did not contain peroxides, free fatty acids, phospholipids or oxidized polar compounds. The purified soybean oil was thermally oxidized at 180°C for 96 hr in the presence of air. The thermally oxidized compounds (31.3%) were separated from the purified soybean oil by gradient elution silicic acid chromatography. Thermally oxidized compounds contained hydroxyl groups, carbonyl groups andtrans double bonds according to the infrared spectrum. Thermally oxidized compounds were added to soybean oil and purified soybean oil at 0, 0.5, 1.0, 1.5 and 2.0% to study the effects of these compounds on the oxidative stability of oil. The oxidative stabilities of oils were determined by gas chromatographic analysis of volatile compound formation and molecular oxygen disappearance in the headspace of oil bottles. The thermally oxidized compounds showed prooxidant effects on the oxidative stabilities of both refined, bleached and deodorized soybean oil and purified soybean oil. Duncan’s Multiple Range Test showed that thermally oxidized compounds had a significant effect on the volatile compound formatiion and oxygen disappearance in the headspace of oil at α=0.05.     

Volatile compounds of oxidized pork phospholipids

Volatile compounds from oxidized pork muscle phospholipids (PL) were analyzed by a purge-and-trap method. Total volatile compounds were highly correlated with thiobarbituric acid-reactive substances, mainly as a consequence of alkanals. Major compounds of the 32 identified substances were alkanals (6023 ng nonane equivalents/mg PL), followed by 2-alkenals (514 ng nonane eq/mg PL) and 2,4-alkadienals (368 ng nonane eq/mg PL). Hexanal (4850 ng nonane eq/mg PL) was the major compound from the oxidation of n-6 fatty acids (mainly linoleic and arachidonic acid). Volatile compounds from the oxidation of n-3 fatty acids were only minor and included 2,4-heptadienal (45 ng nonane eq/mg PL) and 2-pentenal and 2-hexenal (49 ng nonane eq/mg PL). Finally, nonanal, a degradation compound from oleic acid, was present at a low level (200 ng nonane eq/mg PL) and remained constant during oxidation, which confirmed that monounsaturated fatty acids were stable toward metal-catalyzed oxidation. With the exception of ester compounds, identified volatiles were qualitatively similar to those obtained in simpler systems, such as fatty acids or vegetable oils. Quantitatively, the volatile compound composition reflected the fatty acid composition of PL.     

Gas chromatographic separation of the fatty acid methyl esters prepared from dairy products,partially hydrogenated oils,and refined vegetable oils applying a negative temperature gradient

To date no single gas chromatographic method can simultaneously measure all fatty acids (FA), including trans-FA (TFA), that are contained in dairy products, partially hydrogenated oils (PHO), and refined vegetable oils. Using 100% poly(biscyanopropyl siloxane) capillary columns, ruminant and dairy fats are preferentially analyzed by applying temperature programs that separate short chain FA, but not trans-18:3 from 20:1. Refined vegetable oils and PHO are preferentially analyzed by applying isothermal elutions that provide quantification of all 18 carbon TFA including trans-18:3 FA, but not of all short chain FA. In this short communication, we propose a temperature program method capable of simultaneously measuring short chain FA and all 18 carbon TFA including trans-18:3 by applying a negative temperature gradient after the elution of trans-18:1. A simplified version of the method is also described for equipment not able to perform negative temperature gradients.     

Validation of the rancimat test for the assessment of the relative stability of fish oils

The induction periods for the peroxidation of various fish oils at 55–90°C were studied by the Rancimat test. The natural logarithms of the induction periods varied linearly with respect to temperature, with a mean coefficient of −7.5×10⁻²°C⁻¹, which was significantly different from that reported for vegetable oils. The activation energy for the formation of volatile acids had a mean value of 38.9 kJ/mol and was independent of the fish oil source. Peroxide formation under Rancimat test conditions followed first-order kinetics. The same kinetics were followed under Schaal Oven test conditions (forced-air oven, 60°C). On the basis of the results obtained, the Rancimat test appears to be useful in determining the relative stabilities of fish oils without the change in peroxide decomposition kinetics that may occur at elevated temperatures.     

Applications of chromatographic techniques to evaluate enzymatic hydrolysis of oxidized and polymeric triglycerides by pancreatic lipase in vitro

With the aim of studying the susceptibility to enzymatic hydrolysis of oxidized and polymeric triglycerides (TG) that are formed during frying, various chromatographic techniques were applied in combination, i.e., adsorption chromatography, high-performance size-exclusion chromatography (HPSEC), and thin-layer chromatography-flame ionization detection (TLC-FID). Polar fractions, isolated by adsorption chromatography from thermoxidized trilinolein as model system, and real used frying fats and oils, were analyzed by HPSEC before and after incubation with pancreatic lipase in vitro. Also, the influence of degradation level of used frying oils on hydrolysis of intact TG was investigated with the additional aid of TLC-FID. Results showed the high hydrolysis rate of oxidized TG monomers in contrast to the significant discrimination of pancreatic lipase against TG dimers and, particularly, TG polymers. On the other hand, hydrolysis of intact TG can be affected by the presence of dimers and polymers in abused frying oils.     

Tropical Vegetable Oils Do Not Alter Growth Performance in African Catfish through a High n-6 Polyunsaturated Fatty Acids Biosynthesis Capacity

The main objective of this study was to determine the best vegetable oils (VO) for nutrition of African catfish by assessing the effects of a complete replacement of fish oil (FO) by different VO sources on its growth performance, fatty acid composition, and elongase-desaturase gene expression levels. Fish (16.2 g of initial body weight) were fed with five experimental isonitrogenous, isolipidic, and isoenergetic diets in which FO was totally replaced by cottonseed oil (CO), palm oil (PO), desert date oil (DO), or Shea butter (SB). Complete replacement of FO with VO did not affect growth performance except for low values in fish fed SB diet. Muscle n-3 LC-polyunsaturated fatty acids (PUFA) were significantly reduced in fish fed VO-based diets when compared with FO fed fish. However, the muscle arachidinic acid (ARA) levels in phospholipid class were 1.4 to 1.6 times higher in fish fed CO and DO diets than FO fed fish despite the lower ARA suppliers from these VO-based diets, suggesting endogenous LC-PUFA biosynthesis from PUFA precursors in fish fed these VO. The fads2 and elovl5 gene expression levels in liver of fish fed DO were also higher compared to FO controls. Therefore, all the results support the hypothesis that African catfish has higher biosynthesis capacity to convert vegetable n-6 PUFA precursors like linoleic acid (LNA, 18:2n-6) into n-6 LC-PUFA of the ARA type, compared to the conversion of vegetable α-linolenic acid (ALA, 18:3n-3) into n-3 LC-PUFA of the eicosapentanoic acid (EPA) or docosahexanoic acid (DHA) type. The results also indicate that DO can be recommended as the best alternative to FO replacement in African catfish nutrition.     

Formation of Acrolein in the Autoxidation of Triacylglycerols with Different Fatty Acid Compositions

Fish, echium, linseed, and soybean oil triacyglycerols (TAGs) were oxidized at 50 or 60 °C to determine the effect of the polyunsaturated fatty acid composition on the volatile product formation. The analysis of the oxygen consumption and total volatile formation demonstrated that the soybean oil TAG had the highest oxidative stability followed by linseed, echium, and fish oil TAGs. Our results were in agreement with the expected average number of bis‐allylic positions of each TAG. Higher quantities of acrolein (2‐propenal) and propanal were detected using the static headspace gas chromatography method at the early stages of oxidation of echium and fish oil TAGs; however, a considerable amount of propanal and only a small amount of acrolein were found in the oxidized linseed oil TAG. The peak area ratios of acrolein to propanal were 0.115, 0.569, and 2.554 after the 8‐h oxidation of linseed, echium, and fish oil TAG, respectively, suggesting the preferential formation of acrolein, especially during the fish oil TAG oxidation. The acrolein quickly increased during the first stage of oxidation, but thereafter it either did not change or slightly decreased during the fish oil oxidation. Because fish oil induces flavor deterioration from the very early stage of the oxidation, the acrolein formation observed in the present study may be important for fish oil deterioration.     

Dietary polyunsaturated fat in relation to mammary carcinogenesis in rats

High fat diets promote the development of mammary tumors induced in rats by 7,12-dimethylbenz(a)anthracene (DMBA), and polyunsaturated fats are more effective than saturated fats. This difference is related to the linoleic acid content of polyunsaturated vegetable oils, but the amount of linolealte required for maximum tumor promotion appears to be higher than indicated by earlier experiments. Comparison of the effects of a polyunsaturated vegetable oil (corn oil) containing linoleate with a fish oilo (menhaden oil) containing polyunsaturated fatty acids derived from linolenic acid showed that higher dietary mammary tumors, while corresponding levels of menhaden oil had an inhibitory effect. This is further evidence that promotion of mammary tumorigenesis by polyunsaturated vegetable oils may be mediated by prostaglandins or other biologically active eicosanoids derived from n−6 fatty acids.     

Development of steryl ester analysis for the detection of admixtures of vegetable oils

The steryl ester content and composition of 28 samples from 10 vegetable oil types have been determined by isolation of the steryl esters by high-performance liquid chromatography and analysis by gas chromatography. The oils can be classified into oils with a high content (>4000 mg/kg) of steryl esters (corn and rapeseed); oils with a medium content (1400–2400 mg/kg) of steryl esters (sunflower oil and high-oleic sunflower oil); and oils with a low content (<1200 mg/kg) of steryl esters (safflower, soybean, cottonseed, groundnut, olive, and palm oils). The composition of the steryl ester fraction varies to a greater extent for different oil types than for different varieties of the same oilseed. The developed method is promising for authentication of some oils, and is particularly suitable for detecting admixtures of low levels of corn or rapeseed oils.