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.     

Products formed by photosensitized oxidation of unsaturated fatty acid esters

Photosensitized oxidation of unsaturated fatty acid methyl ester was carried out using methylene blue as a sensitizer. Oxidation products, monohydro-peroxides, were identified as trimethylsilyl derivatives. Methyl oleate gave the 9- and 10-isomers; methyl linoleate, the 9-, 10-, 12-, and 13-isomers; and methyl linolenate, the 9-, 10-, 12-, 13-, 15-, and 16-isomers, respectively. The double bond to which the hydroperoxide group attached was shifted to the adjacent position in each isomer. Thus, both conjugated and nonconjugated isomers were present in methyl linoleate monohydroperoxides and methyl linolenate monohydroperoxides. By the inhibition experiment, it was ascertained that the above reaction proceeded via singlet oxygen. The relative rates of methyl oleate, methyl linoleate, and methyl linolenate were 1.0∶1.7∶2.3, respectively. These results obtained from the methyl esters were applied to the photosensitized oxidation of triglycerides purified from vegetable oils, and the reaction mechanism on triglycerides was proposed.     

Analysis of oleate,linoleate and linolenate hydroperoxides in oxidized ester mixtures

The hydroperoxides in oxidized mixtures of methyl oleate, linoleate and linolenate were analyzed by reducing the hydroperoxides to the corresponding hydroxyesters and separating the hydroxyesters from the unoxidized esters by thin layer chromatography (TLC). The hydroxyesters from linolenate were separated from the other hydroxyesters by TLC on silver ion plates. The hydroxyesters were converted to TMS-hydroxy derivatives. The TMS-hydroxyoleate and TMS-hydroxylinoleate were separated by gas chromatography (GC), and all the TMS-derivatives were quantified by GC. The relative rates of oxidation of methyl oleate, linoleate and linolenate in mixtures were ca. 1∶10.3∶21.6. The hydroperoxides formed in the oxidation of soybean and olive oils were similar before and after randomization and similar to corresponding methyl ester mixtures. Journal Paper No. J-9657 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project 2143.     

ADSORPTION BEHAVIOR OF EPOXIDIZED FATTY ESTERS VIA BOUNDARY LUBRICATION COEFFICIENT OF FRICTION MEASUREMENTS

The frictional behaviors of a variety of fatty alkenyl esters and their corresponding fatty epoxide esters (epoxy methyl oleate (EMO) and methyl oleate (MO), epoxy methyl linoleate (EMLO) and methyl linoleate (MLO), epoxy methyl linolenate (EMLEN) and methyl linolenate (MLEN)), epoxidized soybean oil (ESBO), and a commercial epoxidized 2-ethylhexyl transesterified soybean oil (VF) as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. Langmuir critical additive concentrations were determined, which provide the following order of negative adsorption energies: ESBO > VF > EMO ≥ EMLO > EMLEN and MLEN ≥ MLO > MO. Thus, for the similar epoxidized materials the greater degree of epoxidation results in less negative calculated total adsorption energies; this trend is reversed for the olefinic parent systems. This ordering agrees with that obtained via a more complex unconstrained cooperative interaction adsorption model. Fits of the steady-state coefficient of friction (COF) versus concentration data indicate an inverse relation of the obtained interaction parameters (α) with the primary adsorption energies (E). These results demonstrate the complexity of the adsorption mechanism that occurs.     

Effect of α- and γ-tocopherols on thermal polymerization of purified high-oleic sunflower triacylglycerols

The antipolymerization effects of α- and γ-tocopherols were compared in model systems composed of purified high-oleic sunflower triacylglycerols at 180°C. γ-Tocopherol was much more effective as an antipolymerization inhibitor than α-tocopherol, partly due to lower oxidizability/disappearance. Purified triacylglycerols of sunflower, rapeseed, and high-oleic sunflower oils were less stable than their nonpurified forms containing tocopherols. Results confirmed that tocopherols per se can act as antipolymerization agents in high-oleic oils at frying temperatures. No synergism was observed when α- and γ-tocopherols were present together although larger amounts of residuals were left for both tocols. Results suggested that high-oleic/high-γ-tocopherol oils (such as high-oleic canola and high-oleic soybean oils) may provide better frying oils than high-oleic/high-α-tocopherol oils (such as high-oleic sunflower oil).     

Using SPME-GC/MS to Evaluate Acrolein Production in Cassava and Pork Sausage Fried in Different Vegetable Oils

This work presents the quantification of acrolein in cassava and pork sausage fried (temperature of 170 °C) in five different vegetables oils: canola, palm, sunflower, soybean and corn using a method of solid-phase microextraction (SPME) combined with gas chromatography and mass spectrometry. The results showed that the highest concentration of acrolein was found in samples fried in sunflower oil and canola oil. The concentration of acrolein in pork sausage (3.7 and 2.0 ng/g/g) was lower than in cassava (10.2 and 3.8 ng) when fried in sunflower and soybean oils, respectively. In contrast, when the denser oils (canola and palm) were used for frying, the concentration of acrolein in pork sausage (6.3 and 3.8 ng/g) was higher than in cassava (3.7 and 2.8 ng/g). Using corn oil, the concentrations of acrolein in both cassava and sausage were similar (approximately 5 ng/g). The viscosity of the oil, the fatty acid composition, especially the level of saturated and unsaturated fatty acids from the food, and oil uptake are factors that influence the acrolein concentration found in fried food.     

Free radical addition of hydrogen sulfide to conjugated and nonconjugated methyl esters and to vegetable oils

The rate of addition of hydrogen sulfide to high purity methyl oleate, methyl linoleate, methyl linolenate, methyl 9,11-trans,trans-octade-cadienoate and methyl β-eleostearate was investigated at 25 C with UV irradiation. A similar study was carried out with soybean, linseed and tung oils in the absence and presence of 2,2′-azo-bis(isobutyronitrile) with UV photolysis. Initially the reaction of hydrogen sulfide with methyl esters appears to follow pseudo-zero-order kinetics although as the reaction proceeds the kinetics of the polyunsaturated ester reactions become more complex. For nonconjugated systems the overall rate is determined by the initiation step, whereas the overall rate of addition to conjugated systems is a function of the stability of the resonance-stabilized addition radical in the chain transfer step. For methyl esters the following order of reactivity appears to hold: Methyl oleate ≅ methyl linoleate ≅ methyl linolenate >> methyl 9,11-trans,trans-octadecadienoate > methyl β-eleostearate. Using 2,2′-azo-bis(isobutyronitrile) with UV photolysis markedly increases the rate of addition of hydrogen sulfide to nonconjugated vegetable oils. Presented at the AOCS Meeting, New York, October 1968. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Isolation of pure natural linoleic and linolenic acids as their methyl esters by adsorption fractionation on silicic acid

Summary An effective procedure is described for fractionating methyl esters of oils rich in linoleic and linolenic acids by adsorption on silicic acid columns. Pure methyl linoleate from methyl esters of tobacco seed oil and pure methyl linolenate from methyl esters of linseed and perilla oils were isolated by this procedure. These compounds were characterized by the usual physical and chemical constants and by spectrophotometric examination. These natural acid esters differed significantly from corresponding debromination acid esters in the intensity of ultraviolet absorption at their maxima under the conditions of the alkali-isomerization spectrophotometric method of analysis. Presented at the 22nd Fall Meeting of the American Oil Chemists' Society, November 15–17, 1948, in New York City. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Dept. of Agriculture.     

Analysis of autoxidized fats by gas chromatography-mass spectrometry: VII. Volatile thermal decomposition products of pure hydroperoxides from autoxidized and photosensitized oxidized methyl oleate,linoleate and linolenate

To clarify the sources of undesirable flavors, pure hydroperoxides from autoxidized and photosensitized oxidized fatty esters were thermally decomposed in the injector port of a gas chromatograph-mass spectrometer system. Major volatile products were identified from the hydroperoxides of methyl oleate, linoleate and linolenate. Although the hydroperoxides from autoxidized esters are isomerically different in position and concentration than those from photosensitized oxidized esters, the same major volatile products were formed but in different relative amounts. Distinguishing volatiles were, however, produced from each type of hydroperoxide. The 9- and 10-hydroperoxides of photosensitized oxidized methyl oleate were thermally isomerized in the injector port into a mixture of 8-, 9-, 10- and 11-hydroperoxides similar to that of autoxidized methyl oleate. Under the same conditions, the hydroperoxides from autoxidized linoleate and linolenate did not undergo significant interconversion with those from the corresponding photosensitized oxidized esters. The compositions of the major volatile decomposition products are explained by the classical scheme involving carboncarbon scission on either side of alkoxy radical intermediates. Secondary reactions of hydroperoxides are also postulated, and the hydroperoxy cyclic peroxides from methyl linoleate (photosensitized oxidized) and methyl linolenate (both autoxidized and photosensitized oxidized) are suggested as important precursors of volatiles.     

<Emphasis Type="Italic">cis</Emphasis>-, <Emphasis Type="Italic">trans</Emphasis>- and Saturated Fatty Acids in Selected Hydrogenated and Refined Vegetable Oils in the Indian Market

The fatty acid composition of 27 samples of commercial hydrogenated vegetable oils and 23 samples of refined oils such as sunflower oil, rice bran oil, soybean oil and RBD palmolein marketed in India were analyzed. Total cis, trans unsaturated fatty acids (TFA) and saturated fatty acids (SFA) were determined. Out of the 27 hydrogenated fats, 11 % had TFA about 1 % where as 11 % had more than 5 % TFA with an average value of about 13.1 %. The 18:1 trans isomers, elaidic acid was the major trans contributor found to have an average value of about 10.8 % among the fats. The unsaturated fatty acids like cis-oleic acid, linoleic acid and α-linolenic acid were in the range of 21.8–40.2, 1.9–12.2, 0.0–0.7 % respectively. Out of the samples, eight fats had fatty acid profiles of low TFA (less than 10 %) and high polyunsaturated fatty acids (PUFA) such as linoleic and α-linolenic acid. They had a maximum TFA content of 7.3 % and PUFA of 11.7 %. Among the samples of refined oils, rice bran oil (5.8 %) and sunflower oil (4.4 %) had the maximum TFA content. RBD palmolein and rice bran oils had maximum saturated fatty acids content of 45.1 and 24.4 % respectively. RBD palmolein had a high monounsaturated fatty acids (MUFA) content of about 43.4 %, sunflower oil had a high linoleic acid content of about 56.1 % and soybean oil had a high α-linolenic acid content of about 5.3 %.     

Absorption in rats of rapeseed, soybean, and sunflower oils before and following moderate heating

Rapeseed, soybean, and sunflower oil were heated for 15 min in a 5-mm oil layer in a pan at 180°C. The fatty acid composition was almost unaffected by heating, while the polymer content rose slightly and the tocopherol content decreased, except in soybean oil. The absorption of oils before and after heating was investigated in lymph-cannulated rats. Oils were administered as emulsions through a gastrostomy tube and lymph was collected during the next 24 h. The highest accumulated lymphatic transport of total fatty acids was observed after administration of rapeseed oil, and the lowest after heated sunflower oil. The accumulated transport was similar for all unheated oils. The transport of fatty acids was significantly lower in rats receiving heated oil compared to those receiving the corresponding unheated oil. Small increases in polymers may have contributed to the decreased lymphatic transport of oil following heating, although this probably does not fully explain the effect. The absorption of sunflower oil was more affected by heating than the absorption of soybean or rapeseed oil. Furthermore, the largest decrease in total activity of tocopherols following heating was observed in sunflower oil. Overall, these results demonstrate that the absorption of vegetable oils is affected by moderate heating.     

Thermoxidative stability of triacylglycerols from mutant sunflower seeds

Thermoxidative stability was evaluated in triaclyglycerols (TAG) from the oils of the mutant sunflower lines CAS-3, CAS-4, and CAS-8 (with a high percentage of stearic acid), CAS-5 (with a high percentage of palmitic acid), all from standard highlinoleic genetic backgrounds, and the mutant sunflower line CAS-12 (with a high percentage of palmitic acid), from a high-oleic genetic background. These oils contained unusually high contents of TAG molecular species with one or two saturated fatty acids at the sn-1,3 positions. Purified total TAG devoid of tocopherols were subjected to controlled thermoxidative treatment at 180°C. Polymerized TAG were determined at 2-h intervals for 10 h. After this time, total polar compounds, oxidized TAG monomers, TAG dimers, and TAG oligomers were determined. TAG from highly saturated sunflower oils with levels of linoleic acid similar to those found in conventional sunflower oils (40–50%) showed enhanced thermal stability. In these TAG, the amount of polar compounds formed during the thermoxidative treatment was similar to that formed in the high oleic acid line. Excellent results were obtained for the TAG of the CAS-12 oil, which had the highest thermal stability, producing half the amount of polar compounds as the conventional line and less than two-thirds that of the high-oleic line.     

Linalyl oleate as a frying oil autoxidation inhibitor

Linalyl oleate (LO), an interesterification product of linalyl acetate (LA) and methyl oleate catalyzed with sodium methoxide, was studied to determine its effectiveness in retarding oxidative changes in soybean oil heated continuously at 180±5°C for 32 h. The identity of LO was established by GC-MS and NMR. LO was tested at levels of 0.05 and 0.1% and compared with the more commonly used synthetic autoxidation inhibitor methyl silicone (MS) at levels of 5 and 10 ppm. FA changes and conjugated dienoic acid formation were monitored. First-order kinetic equations were used to model the decreases in linoleate (18∶2)/palmitate and linolenate (18∶3)/palmitate ratios. Plots of the data show an inflection point at ∼11 h. Oils with either level of MS and LO had lower reaction rate constants before the inflection points, and lower conjugated diene values and higher 18∶2 and 18∶3 percentages at the end of the 32-h heating period than did oil without additives and with LA. LO could replace methyl silicone in soybean oil during deep-fat frying but at levels about 100 times greater. [We propose to use the term “autoxidation inhibitor” for substances that inhibit autoxidation when added to fats and oils at low concentrations and whose mechanism of action may be unknown. Some may wish to call such substances “antioxidants” but others wish to reserve this term for substances that end free radical chains by hydrogen radical donation. Some refer to methyl silicone as a “polymerization inhibitor”, but this term suggests more about its mechanism of action than seems warranted.]     

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.     

Fractionation of oligomeric triacylglycerides and the relation to rejection limits for used frying oils

Precipitates enriched in oligomeric triacylglycerides were separated from thermally oxidized olive residue oil, conventional and high-oleic sunflower oils, and soybean oil by solvent fractionation in methanol/acetone at 4–5°C for 16 h. Different fractionation conditions were evaluated in an effort to isolate the oligomeric triacylglycerides (OTG). OTG, formed in frying oils upon heating at low concentations, were not detectable with conventional methods to determine polymeric compounds. The best conditions found from the different assays were the following: (i) weight of oil sample-to-solvent volume ratio of 1∶20; and (ii) solvent system methanol/acetone 10∶90 (vol/vol) for monounsaturated oils and 15∶85 (vol/vol) for polyunsaturated oils. Precipitates, enriched in oligomers, were formed when heated oils and used frying oils contained more than 27% polar compounds, a value which is widely accepted as the upper limit for use of frying oils.     

Oxidative Stability of Conventional and High-Oleic Vegetable Oils with Added Antioxidants

The oxidative stability of conventional and high-oleic varieties of commercial vegetable oils, with and without added antioxidants, was evaluated using the oil stability index (OSI). Oil varieties studied were soybean (SOY), partially-hydrogenated soybean (PHSOY), corn (CORN), sunflower (SUN), canola (CAN), high-oleic canola (HOCAN), very high-oleic canola (VHOCAN), oleic safflower (SAF) and high-oleic sunflower (HOSUN). One or more commercial antioxidants were added to the four most stable oils at supplier-recommended levels: rosemary extract (RM; 1,000 ppm), ascorbyl palmitate (AP; 1,000 ppm), tert-butylhydroquinone (TBHQ; 200 ppm), and mixed tocopherols (TOC; 200 ppm). OSI in hours (h) at 110 °C of the conventional oils were 5.2, 7.6, 8.4, 9.8, 10.9 and 14.3 h for SUN, SOY, CAN, CORN, PHSOY and SAF, respectively. OSI of high-oleic variants were 12.9, 16.5 and 18.5 h for HOCAN, HOSUN and VHOCAN, respectively. Maximum OSI values for the four most stable oils when treated with antioxidants, were 40.9, 48.5, 48.8 and 55.7 h for HOCAN, VHOCAN, SAF and HOSUN, respectively. Addition of TBHQ, alone and in combination with other antioxidants, resulted in the greatest increase in oxidative stability of SAF and other high-oleic oils evaluated. AP had a positive synergistic effect when used with TBHQ, while RM decreased TBHQ effectiveness.     

Transesterification of vegetable oil to biodiesel fuel using acid catalysts in the presence of dimethyl ether

The immiscibility of methanol and vegetable oil leads to a mass-transfer resistance in the transesterification of vegetable oil. To overcome this problem, dimethyl ether (DME) was used as an environmentally friendly cosolvent to produce a homogeneous solution. Methylesterifications of corn oil in both the presence and the absence of DME were performed using p-toluenesulfonic acid (PTSA), benzenesulfonic acid and sulfuric acid. PTSA showed highest catalytic activity. The yield of FAME reached 97.1% when 4 wt% of PTSA based on the oil weight was used at 80 °C with a reaction time of 2 h in the presence of DME. The obtained biodiesel was composed of methyl palmitate (9.1 wt%), methyl oleate (33.9 wt%), methyl linoleate (53.5 wt%), methyl linolenate (3.0 wt%) and methyl arachidate (0.5 wt%), and it was similar to the biodiesel compositions from corn oil as reported. The effects of concentrations of FFA and water on FAME yields were also investigated. All results suggested that the reaction rate was greatly improved by the addition of DME to the reaction system.     

Antioxidant Activity of Phytochemicals from Distillers Dried Grain Oil

A distillate was obtained by molecular distillation of oil extracted from distillers dried grains (DDG). The distillers dried grain oil distillate (DDGD) contained phytosterols, steryl ferulates, tocopherols, tocotrienols, and carotenoids. DDGD was tested for its impact on the oxidative stability index (OSI) at 110 °C of soybean, sunflower, and high-oleic sunflower oils, as well as the same oils that were stripped of their natural tocopherols and phytosterols. In addition, the impact of added DDGD on the stability of stripped sunflower oil during an accelerated storage study conducted at 60 °C was also determined. DDGD (0.5–1% w/w) had little impact on the OSI of soybean, sunflower, and high-oleic sunflower oil, but at levels of 0.1–1% it significantly increased the OSI for stripped soybean, sunflower, and high-oleic sunflower oil in a dose-dependent manner. DDGD also delayed peroxide value, conjugated diene, and hexanal formation during accelerated storage of stripped sunflower oil. The antioxidant activity is probably due to the combination of tocopherols, tocotrienols, and steryl ferulates.     

Collaborative Study for the Analysis of Glycidyl Fatty Acid Esters in Edible Oils using LC–MS

An interlaboratory study was conducted to evaluate a method for determining glycidyl fatty acid esters (GE) in edible oils. Samples were dissolved in tert-butyl methyl ether/ethyl acetate and subjected to two solid-phase extraction (SPE) steps. The first SPE step utilized methanol elution from a C18 cartridge, and the second SPE step utilized n-hexane/ethyl acetate elution from a silica cartridge. The final extract was analyzed using liquid chromatography with a single quadrupole mass spectrometer in selected ion monitoring (SIM) mode. Quantification was performed using external standardization. Eighteen samples (9 oils × 2 blind duplicates) were assayed for glycidyl palmitate, glycidyl stearate, glycidyl oleate, glycidyl linoleate and glycidyl linolenate by 17 collaborating laboratories from seven countries. Sample matrices included palm, olive, corn, soybean and rapeseed oils. Repeatability (RSDr) ranged from 6.85 to 19.88 % and reproducibility (RSDR) ranged from 16.58 to 35.52 % for samples containing greater than 0.5 mg/kg of individual GE. HORRAT_R values ranged from 0.62 to 14.70 for determination of total GE. The method provides acceptable results for quantification of GE in edible oils.