Effect of fatty acids and tocopherols on the oxidative stability of vegetable oils

The oxidative stability of vegetable oils is determined by their fatty acid composition and antioxidants, mainly tocopherols but also other non‐saponifiable constituents. The effect of fatty acids on stability depends mainly on their degree of unsaturation and, to a lesser degree, on the position of the unsaturated functions within the triacylglycerol molecule. Vegetable oils contain tocopherols and tocotrienols, especially α‐ and γ‐tocopherols, as their main antioxidants. The antioxidant behavior of tocopherols represents a complex phenomenon as they are efficient antioxidants at low concentrations but they gradually lose efficacy as their concentrations in the vegetable oils increase. The “loss of efficacy” of tocopherols, sometimes referred to as a “pro‐oxidant effect”, is witnessed by an increase in the rate of oxidation during the induction period, despite elongation of this phase. The phenomenon is much obvious for α‐tocopherol, but is also evident for other tocopherols. In agreement with nature's wisdom, the tocopherol levels in vegetable oils seem to be close to the optimal levels needed for the stabilization of these oils. The presence of other antioxidants in the oils, e.g. carotenoids, phenolic compounds, and Maillard reaction products, may synergize with tocopherols and minimize this loss of efficacy.     

Effect of tocopherols on the frying stability of regular and modified canola oils

A study was conducted to compare the relationship between frying stability and levels and degradation rates of tocopherols in regular and three modified canola oils. Oils were heated at 175 ± 2°C for a total of 72 h, with french fries fried intermittently. Frying stability was compared based on the rates of formation of free fatty acids (FFA) and total polar compounds (TPC). Significant differences (P<0.05) were identified between oils using analysis of covariance and t-tests for multiple comparisons. No significant differences were observed in the rates of FFA formation among the canola oils during frying. Nevertheless, regular canola (RCO) and high-oleic, low-linolenic acid canola (HOLLCO) oils produced less FFA compared to higholeic LLCO and HOCO both had significantly (P<0.05) faster rates of TPC formation compared to HOLLCO or RCO. HOLLCO with the highest level of tocopherols (893 mg/kg) exhibited a slow rate of degradation which accounted for a halflife of 48–60 h of frying. RCO, with a lower level of tocopherols (565 mg/kg), however, had the slowest degradation rate with a half-liofe of >72 h. In contrast, HOCO and LLCO with 601 and 468 mg/kg tocopherols, respectively, both exhibited a half-life for tocopherols of 3–6 h of frying. An inverse relatioship was observed between TPC formation and the reduction of tocopherol. Thus, the greater frying stability of RCO and HOLLCO appears to be affected far more by the rate of tocopherol degradation than by any changes in fatty acid composition.     

Flavor and oxidative stability of some linolenate-containing oils

Because crambe, mustard seed, and rapeseed oils, like soybean oil, contain linolenate ester, they have been examined and compared with soybean oil for flavor stability after accelerated storage and after exposure to fluorescent light. Tests showed that the Cruciferae oils did have similar flavor characteristics and that the addition of citric acid did improve their stability. When exposed to light, the citric acid-treated Cruciferae oils differed from soybean oil; they developed a rubbery flavor, whereas soybean oil developed a grassy flavor. Oxidative stability determined by the active oxygen method confirmed results of oven storage tests. This work supports the belief that if linolenic acid is present in an edible oil, it is a precursor to typical off-flavor development. Presented at the AOCS meeting, Chicago, 1964. A laboratory of the No. Utiliz, Res. and Dev. Div., ARS, USDA.     

Determination of tocopherols in vegetable oils

A rapid (ten minute) and selective method for measuring individual tocopherols found in vegetable oils has been developed using High Performance Liquid Chromatography (HPLC) with ultraviolet absorbance detection. The samples are analyzed directly following dissolution in the mobile phase. α -and γ-tocopherols are quantitated based upon their peak areas relative to standard calibration curves. The measurement of β - and δ-tocopherols in the samples is also based upon the calibration data for the α - and δ-tocopherol standards since the individual β- and δ- standards were unavailable. The data obtained are compared with the total tocopherol content as found by a standard colorimetric procedure. The results indicate the HPLC method to be more reliable in the measurement of samples with high α -tocopherol levels. Soybean, safflower, sunflower, cottonseed, corn, peanut and olive oils have been examined using this method.     

Frying quality and oxidative stability of high-oleic corn oils

To determine the frying stability of corn oils that are genetically modified to contain 65% oleic acid, high-oleic corn oil was evaluated in room odor tests and by total polar compound analysis. Flavor characteristics of french-fried potatoes, prepared in the oil, were also evaluated by trained analytical sensory panelists. In comparison to normal corn oil, hydrogenated corn oil and high-oleic (80 and 90%) sunflower oils, high-oleic corn oil had significantly (P<0.05) lower total polar compound levels after 20 h of oil heating and frying at 190°C than the other oils. Fried-food flavor intensity was significantly higher in the normal corn oil during the early portion of the frying schedule than in any of the high-oleic or hydrogenated oils; however, after 17.5 h of frying, the potatoes fried in normal corn oil had the lowest intensity of fried-food flavor. Corn oil also had the highest intensities of off-odors, including acrid and burnt, in room odor tests. High-oleic corn oil also was evaluated as a salad oil for flavor characteristics and oxidative stability. Results showed that dry-milled high-oleic corn oil had good initial flavor quality and was significantly (P<0.05) more stable than dry-milled normal corn oil after oven storage tests at 60°C, as evaluated by flavor scores and peroxide values. Although the high-oleic corn oil had significantly (P<0.05) better flavor and oxidative stability than corn oil after aging at 60°C, even more pronounced effects were found in high-temperature frying tests, suggesting the advantages of high-oleic corn oil compared to normal or hydrogenated corn oils.     

Frying quality and oxidative stability of two unconventional oils

The behavior of crude Sclerocarya birrea kernel oil (SCO) and Sorghum bug (Agonoscelis pubescens) oil (SBO) during deep-frying of par-fried potatoes was studied with regard to chemical, physical, and sensory parameters, such as content of FFA, tocopherols, polar compounds, oligomer TG, volatile compounds, oxidative stability, and total oxidation (TOTOX) value. Palm olein was used for comparison. Whereas potatoes fried in SCO that had been used for 24 h of deep-frying at 175°C were still suitable for human consumption, potatoes prepared in SBO that had been used for 6 to 12 h were not, considering the sensory evaluation. In looking at the chemical and physical parameters, SBO exceeded the limits, after no later than 18 h of use, for the amount of polar compounds, oligomer TG, and FFA recommended by the German Society of Fat Sciences (DGF) as criteria for the rejection of used frying oils. In contrast to SBO, SCO oil did not exceed the limits for the content of polar compounds and oligomer TG during the frying experiment. Only the amount of FFA was exceeded; this was because the amount of FFA at the beginning of the experiment was higher than for refined oils. The results showed that both oils were suitable for deep-frying of potatoes, but remarkable differences in the time during which both oils produced palatable products were found.     

Flavor and oxidative stability of continuously hydrogenated soybean oils

Soybean oil was partially hydrogenated in a continuous system with copper and nickel catalysts. The hydrogenated products were evaluated for flavor and oxidative stability. Processing conditions were varied to produce oils of linolenate contents between 0.4 and 2.7%, as follows: oil flow, 0.6–2.2 liters/hr; reaction temperature, 180–220 C; hydrogen pressure, 100–525 psig, and catalyst concentration, 0.5–1% copper catalyst or 0.1% nickel catalyst.Trans unsaturation varied from 8 to 20% with copper catalyst and from 15.0 to 27% with nickel catalyst. Linolenate selectivity was 9 with copper catalyst and 2 with nickel catalyst. Flavor evaluation of finished oils containing 0.01% citric acid (CA), appraised initially and after accelerated storage at 60 C, showed no significant difference between hydrogenated oils and nonhydrogenated oil. However, peroxide values and oxidative stability showed that hydrogenated oils were more stable than the unhydrogenated oil. CA+TBHQ (tertiary butylhydroquinone) significantly improved the oxidative stability of test oils over oils with CA only, but flavor scores showed no improvement. Dimethylpolysiloxane (MS) had no effect on either flavor or oxidative stability of the oils.     

Comparison of oxidative stability of high- and normal-oleic peanut oils

A peanut breeding line with high-oleic acid and an isogenic sister line with normal fatty acid composition were obtained. Oil was extracted with dichloromethane and processed in the laboratory by alkali neutralization and bleaching. Fatty acid compositions were determined by gas chromatography and application of theoretical response factors. Oils were extracted and processed in duplicate. The oxidative stability of the oils was measured by the Schall oven test (80°C), active oxygen method (AOM) (112°C) and by comparison of oxidation rates on thin-layer chromatography-flame ionization detector (TLC-FID) rods (100°C). Fatty acid analysis indicated that the high-oleic line had 75.6 and 4.7% oleic and linoleic acids, respectively, compared to 56.1 and 24.2% for the normal line. The induction times for the Schall test were 682 and 47 h for high- and normal-oleic oils (P<0.01). The AOM induction times were 69 and 7.3 h for high and normal oils, respectively (P<0.01). The times to reach 50% loss in triglyceride area on TLC-FID were 847 and 247 min for high-oleic compared to normal-oleic oils (P<0.01). The results indicate that high-oleic peanut oil has much greater autoxidation stability as compared to normal oil.     

Fatty acid and tocopherol contents and oxidative stability of walnut oils

Walnuts (Juglans regia L.) were collected during the 1997 harvest from 13 different cultivars of trees grown in a replicated trial in an experimental orchard at Lincoln University. Two U.S. commercial cultivars (Tehama and Vina), three European commercial cultivars (Esterhazy, G139, G120), and eight New Zealand selections (Rex, Dublin’s Glory, Meyric, Stanley, Mckinster, 150, 151, 153) were evaluated. Total lipids were analyzed for fatty acids by capillary gas chromatography, tocopherols by high-performance liquid chromatography, and oxidation stability by Rancimat. The total oil content of the nuts ranged from 64.2 to 68.9% while the stability of the oil ranged from 3.9 to 7.8 h. The oleic acid content of the oils ranged from 12.7 to 20.4% of the total fatty acids, while 18:2 content ranged from 57.0 to 62.5% and the 18:3 contents ranged from 10.7 to 16.2%. Reduced stability of the oil as measured by the Rancimat method appears to be correlated to higher levels of 18:2 in the extracted oil. The total tocopherol contents of these nuts ranged from 268.5 to 436.0 μg/g oil. γ-Tocopherol dominated the profile while α-tocopherol was only 6% of the total content. Peroxide values of the fresh oil were measured spectrophotometrically to give an indication of the overall stability. The levels of total tocopherols when combined with the level of unsaturation in the oil in a multiple regression analysis had a significant relationship (R ²=45.2%, P<0.001) with the peroxide value in the oil. Presented as a poster at the 89th AOCS Annual Meeting, Chicago, Illinois, May 10–13, 1998.     

Organoleptic and oxidative stability of blends of soybean and peanut oils

A table oil or a salad and cooking oil must serve both as an oil for salad dressings and for cooking potatoes in a deep-fat fryer. Blends of peanut and unhydrogenated soybean oil that have been treated with a metal inactivating agent such as citric acid were scored fairly high by a research taste panel after aging for 4 or 8 days at 60 C. Heating the samples to frying temperature resulted in significantly higher room odor scores for peanut oil than for the blends. Blends of hydrogenated or hydrogenated-winterized soybean oil with peanut oil were generally scored about equal to peanut oil in room odor tests. Potatoes fried in these oils were generally given comparable and not significantly different scores. Presented at AOCS Meeting, Houston, May 1971. Northern Marketing and Nutrition Research Division, ARS, USDA.     

Oxidative stability of fat substitutes and vegetable oils by the oxidative stability index method

Oxidative Stability Index (OSI) of carbohydrate fatty acid polyesters, fat substitutes and vegetable oils were measured with the Omnion Oxidative Stability Instrument according to the new AOCS Standard Method Cd 12 B-92 (The Official Methods and Recommended Practices of the American Oil Chemists' Society, edited by D. Firestone, AOCS, Champaign, 1991). The stability of crude and refined, bleached and deodorized (RBD) vegetable oils (soybean, hydrogenated soybean and peanut) were determined at 110°C. In addition, OSI times for sucrose polyesters of soybean oil, butterfat, oleate:stearate and methyl glucoside polyester of soybean oil were determined in the absence and in the presence of 0.02 wt% antioxidants, [Tenox TBHQ (tertiary butylhydroquinone, Tenox GT-2 (from Eastman Chemical Products (Kingsport, TN); and vitamin E (from BASF, Wyandotte, MI)], and the results were compared with those of vegetable oils. Crude oils were most stable (20.4–25.9 h), followed by RBD oils (9.3–10.4 h) for soybean and peanut oils, respectively, and fat substitutes (3.8–6.8 h). Overall, Tenox TBHQ was the best antioxidant for improving the oxidative stability of both vegetable oils and fat substitutes. The sucrose polyester made with oleic and stearic acid was more stable than fat substitutes containing more polyunsaturated fatty acids, such as those from soybean oil, or from short-chain fatty acids, such as from butterfat. Antioxidants enhanced the stability of RBD oils (222% increase) and synthetic fat substitutes (421–424% increase) more than that of crude oils (33% increase). The shapes of the induction curves, not the actual OSI times for fat substitutes and vegetable oils, were similar and sharply defined.     

Seed roasting improves the oxidative stability of canola (B. napus) and mustard (B. juncea) seed oils

Animal fats and partially hydrogenated vegetable oils (PHVO) have preferentially been used for deep‐frying of food because of their relatively high oxidative stability compared to natural vegetable oils. However, animal fats and PHVO are abundant sources of saturated fatty acids and trans fatty acids, respectively, both of which are detrimental to human health. Canola (Brassica napus) is the primary oilseed crop currently grown in Australia. Canola quality Indian mustard (Brassica juncea) is also being developed for cultivation in hot and low‐rainfall areas of the country where canola does not perform well. A major impediment to using these oils for deep‐frying is their relatively high susceptibility to oxidation, and so any processing interventions that would improve the oxidative stability would increase their prospects of use in commercial deep‐frying. The oxidative stability of both B. napus and B. juncea crude oils can be improved dramatically by roasting the seeds (165 °C, 5 min) prior to oil extraction. Roasting did not alter the fatty acid composition or the tocopherol content of the oils. The enhanced oxidative stability of the oil, solvent‐extracted from roasted seeds, is probably due to 2,6‐dimethoxy‐4‐vinylphenol produced by thermal decarboxylation of the sinapic acid naturally occurring in the canola seed.     

Effect of vacuum frying on the oxidative stability of oils

The purpose of this study was to evaluate frying oil quality with different assessment methods during vacuum frying of carrot slices. In six consecutive days, palm oil, lard, and soybean oil were fried under vacuum at 105°C for 20 min each hour in an 8-h shift. Peroxide value, acid value, carbonyl value, total polar components, dielectric constant (Food Oil Sensor reading), viscosity, and fatty acid composition were used to evaluate the quality of these oils. Results showed that palm oil and lard possess greater thermal stability than soybean oil. The decrease in C_18:2/C_16:0 ratio was greater for soybean oil than the other two oils. Of the assessment methods used, peroxide value, carbonyl value, total polar components, and dielectric constant all showed good correlation with frying time and between each other. Viscosity was suitable to assess vacuum-fried lard and soybean oil, but not palm oil. The measurement of dielectric constant, on the other hand, appeared to be unsuitable to assess vacuum-fried soybean oil.     

Evaluation of oxidative stability of canola oils by headspace analysis

Lipid oxidation is a major factor affecting flavor quality and shelf life of vegetable oils. Oxidative stability is therefore an important criterion by which oils are judged for usefulness in various food applications. In this study a method based on headspace analysis was developed to evaluate relative oxidative stability of canola oils. The method does not require the use of chemicals, involves minimal sample preparation, and can be performed on a relatively small sample size in comparison with traditional wet chemical methods. Canola oils freshly extracted in the laboratory from different seed samples were subjected to accelerated oxidation and analyzed for PV by standard methods and headspace volatiles by solid phase microextraction/GC-MS. Forward stepwise regression analysis of the data revealed a relationship between PV and headspace concentration of the volatile lipid oxidation products hexanal and trans,trans-2,4-heptadienal. The PV calculated using this formula correlated (R ²=0.73) with those measured by conventional methods. Presented in part at the 96th Annual Meeting of the AOCS, 1–4 May 2005, Salt Lake City, UT.     

An evaluation of the oxidative and flavor stability of stored soybean oils

Results of 4-year storage tests are reported for crude and refined soybean oils held in 50-gal drums under conditions simulating field tank operations. Once-refined oils stored in filled drums without breathers showed lower peroxide values and lower dimer contents than oil stored in full drums with breathers. Refined oils in half-filled drums exhibited higher storage temperatures and, consequently, higher peroxide values and dimer contents than any other storage condition. Nondegummed and degummed crude oils held in drum storage had lower peroxide values and lower dimer contents than refined oils stored under similar conditions. Relationships are significant not only between storage peroxide values and dimer contents, but also each of these with flavor scores. Evidently, stored crude or stored refined soybean oils with peroxide values under 60 could be deodorized to produce salad-grade oils with initial flavor quality equal to that of oils processed from stocks having considerably lower initial peroxide values. The relative rate of peroxide increase for field tank storage can be estimated from linear regression analysis on data from stor-age of soybean oil in drums. Once-refined soybean oil held under large field tank storage conditions would not be expected to reach peroxide levels of 60 until after 3-4 years, even in warm areas. ARS, USDA. No. Utiliz. Res. Dev. Div.. ARS, USDA.