Frying stability of soybean and canola oils with modified fatty acid compositions

Pilot plant-processed samples of soybean and canola (lowerucic acid rapeseed) oil with fatty acid compositions modified by mutation breeding and/or hydrogenation were evaluated for frying stability. Linolenic acid contents were 6.2% for standard soybean oil, 3.7% for low-linolenic soybean oil and 0.4% for the hydrogenated low-linolenic soybean oil. The linolenic acid contents were 10.1% for standard canola oil, 1.7% for canola modified by breeding and 0.8% and 0.6% for oils modified by breeding and hydrogenation. All modified oils had significantly (P<0.05) less room odor intensity after initial heating tests at 190°C than the standard oils, as judged by a sensory panel. Panelists also judged standard oils to have significantly higher intensities for fishy, burnt, rubbery, smoky and acrid odors than the modified oils. Free fatty acids, polar compounds and foam heights during frying were significantly (P<0.05) less in the low-linolenic soy and canola oils than the corresponding unmodified oils after 5 h of frying. The flavor quality of french-fried potatoes was significantly (P<0.05) better for potatoes fried in modified oils than those fried in standard oils. The potatoes fried in standard canola oil were described by the sensory panel as fishy.     

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.     

Effect of fatty acid composition of oils on flavor and stability of fried foods

Effects of fatty acid composition of frying oils on intensities of fried-food flavor and off-flavors in potato chips and french-fried potatoes were determined. Commercially processed cottonseed oil (CSO) and high-oleic sunflower oil (HOSUN) were blended to produce oils with 12 to 55% linoleic acid and 16 to 78% oleic acid. Analytical sensory panels evaluated french-fried potatoes and pilot plant-processed potato chips. Initially, both foods prepared in CSO (16% oleic/55% linoleic acid) had the highest intensities of fried-food flavor; however, this positive flavor decreased with decreasing levels of linoleic acid. 2,4-Decadienal in potato chips also decreased with decreasing linoleic acid in the oils. Frying oil stability, measured by total polar compounds (TPC), and oxidative stability of potato chips, measured by volatile compounds, showed that HOSUN (78% oleic acid) produced the lowest levels of TPC and the lowest levels of hexanal and pentanal, indicating greater frying oil stability and oxidative stability of the food. However, fresh potato chips fried in HOSUN had the lowest intensities of fried-food flavor and lowest overall flavor quality. Fried-food flavor intensity was the best indicator of overall flavor quality in fresh potato chips. Volatile compounds, TPC, and oxidative stability index directly varied with increasing oleic acid, and were therefore not directly indicative of flavor quality. No oil analysis predicted flavor stability of aged potato chips. Compositions of 16 to 42% oleic acid and 37 to 55% linoleic acid produced fresh fried-food with moderate fried food flavor intensity, good overall flavor quality, and low to moderate TPC levels (chips only). However, in aged food or food fried in deteriorated oil, compositions of 42 to 63% oleic and 23 to 37% linoleic provided the best flavor stability.     

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.     

Long storage stability of biodiesel made from rapeseed and used frying oil

The degree of physical and chemical deterioration of biodiesel produced from rapeseed and used frying oil was studied under different storage conditions. These produced drastic effects when the fuel was exposed to daylight and air. However, there were no significant differences between undistilled biodiesel made from fresh rapeseed oil and used frying oil. The viscosity and neutralization numbers rose during storage owing to the formation of dimers and polymers and to hydrolytic cleavage of methyl esters into fatty acids. However, even for samples studied under different storage conditions for over 150 d the specified limits for viscosity and neutralization numbers had not been reached. In European biodiesel specifications there will be a mandatory limit for oxidative stability, because it may be a crucial parameter for injection pump performance. The value for the induction period of the distilled product was very low. The induction period values for the undistilled samples decreased very rapidly during storage, especially with exposure to light and air.     

High-performance size-exclusion chromatographic studies on a high-oleic acid sunflower oil during potato frying

The behavior of a high-oleic acid sunflower oil used for 75 repeated deep-fat fryings of potatoes, with a fast turnover of fresh oil during frying, was evaluated by measuring the total polar content isolated by column chromatography. The total polar content increased in the oil from 3.6 ± 0.1 (mean ± SD) mg/100 mg unused oil to 7.6 ± 0.4 mg/100 mg oil after being used in 20 repeated fryings, followed by a tendency to reach a near-steady state throughout the successive fryings. Further, the polar fraction was examined by high-performance size-exclusion chromatography. Triacylglyceride dimers increased continuously from 0.18 ± 0.01 mg/100 mg unused oil to 2.42 ± 0.12 mg/100 mg oil at the 40th frying with no further significant changes. The amount of triacylglyceride polymers increased from 0.03 ± 0.00 mg/100 mg unused oil to 0.70 ± 0.01 mg/100 mg oil at the 60th frying, but did not increase further with continued frying. Oxidized triacylglycerides also significantly increased from 1.13 ± 0.06 mg/100 mg oil to 3.58 ± 0.09 mg/100 mg oil at the 50th frying to reach a near-steady state in successive fryings. Diacylglycerides and free fatty acids levels, related to hydrolytic alteration, did not change from the starting oil after continued fryings. Data from this study indicated that repeated fryings of potatoes in high-oleic sunflower oil with a frequent turnover of fresh oil throughout the frying slightly increased the level of polar material in the fryer oil during the first fryings, followed by minor changes and a tendency to reach a near-steady state in successive fryings.     

Low-linolenic acid soybean oil—Alternatives to frying oils

Oil was hexane-extracted from soybeans that had been modified by hybridization breeding for low-linolenic acid (18∶3) content. Extracted crude oils were processed to finished edible oils by laboratory simulations of commercial oil processing procedures. Oils from three germplasm lines N83-375 (5.5% 18∶3), N89-2009 (2.9% 18∶3) and N85-2176 (1.9% 18∶3) were compared to commercial unhydrogenated soybean salad oil with 6.2% 18∶3 and two hydrogenated soybean frying oils, HSBOI (4.1% 18∶3) and HSBOII (<0.2% 18∶3). Low-18∶3 oils produced by hybridization showed significantly lower room odor intensity scores than the commercial soybean salad oil and the commercial frying oils. The N85-2176 oil with an 18∶3 content below 2.0% showed no fishy odor after 10 h at 190°C and lower burnt and acrid odors after 20 h of use when compared to the commercial oils. Flavor quality of potatoes fried with the N85-2176 oil at 190°C after 10 and 20 h was good, and significantly better at both time periods than that of potatoes fried in the unhydrogenated oil or in the hydrogenated oils. Flavor quality scores of potatoes fried in the N89-2009 oil (2.9% 18∶3) after 10 and 20 h was good and equal to that of potatoes fried in the HSBOI oil (4.1% 18∶3). Fishy flavors, perceived with potatoes fried in the low-18∶3 oils, were significantly lower than those reported for potatoes fried in the unhydrogenated control oil, and the potatoes lacked the hydrogenated flavors of potatoes fried in hydrogenated oils. These results indicate that oils with lowered linolenic acid content produced by hybridization breeding of soybeans are potential alternatives to hydrogenated frying oils.     

Impact of frying on key fatty acid ratios of canola oil

The study was carried out to investigate the changes in saturated (SFA), monoene (MUFA), trans (TFA), and polyunsaturated (PUFA) fatty acids and the key fatty acid ratios (SFA/UFA, cis PUFA/SFA, C18:2/C16:0 and C18:3/C16:0) during potato chips frying in canola oil using single bounce attenuated total reflectance FTIR (SB‐ATR‐FTIR) spectroscopy. The data obtained from GC‐FID were used as reference. The calibration of main fat groups and their key fatty acid ratios were developed by partial least square (PLS) regression coefficients using 4000 to 650 cm^?1 spectral range. FTIR PLS regression for the predicted SFA, MUFA, TFA, and PUFA were found 0.999, 0.998, 0.998, and 0.999, respectively, whereas for SFA/UFA, cis PUFA/SFA, C18:2/C16:0 and C18:3/C16:0 the regression coefficients were 0.991, 0.997, 0.996, and 0.994, respectively. We conclude that FTIR‐PLS could be used for rapid and accurate assessment of changes in the main fat groups and their key fatty acid ratios ratio during the frying process. Practical applications: FTIR‐ATR method is very simple, rapid, and environmentally friendly. No sample preparation is required and one drop of oil is enough for FTIR analysis. The proposed method could be applied for quick determination of key fatty acid ratios in the food processing industry.     

Urea complexation for the rapid, ecologically responsible fractionation of fatty acids from seed oil

Urea complex formation is a classic method for fractionating fatty acids from seed and other oils. The method’s simplicity, ease of scaling, and ecological friendliness suggest its reevaluation in regard to modern fractionation challenges. In keeping with this, a simple, quick, inexpensive, robust, and environmentally friendly procedure was developed for reducing the saturated free fatty acid (FFA) content of saponified low-erucic acid rapeseed oil (LEAR). The process involves formation of a homogeneous 65°C solution of FFA and urea in 95% ethanol (5% water), followed by cooling of the resultant urea complex slurry to room temperature. The urea complex and liquid phases are separated by gravity filtration, and the urea isolated in each phase is removed by extraction with 60°C water. Saturated LEAR oil FFA preferentially formed urea complexes easily separated from the noncomplexed, mostly unsaturated FFA, the main product of interest. The effects of single- vs. double-stage fractionations and several other variables (component mass or volume ratios, temperature, ethanol solvent to water ratio) were preliminarily evaluated. Results demonstrated the robustness, reproducibility, and simplicity of the method.     

Effect of after-cooking darkening inhibitors on stability of frying oil and quality of french fries

The effect of two after-cooking darkening inhibitors, sodium acid pyrophosphate (SAPP) and calcium acetate (CaAc), and their combined effect on frying oil stability and quality of french fries produced were evaluated over a period of 72 h. Samples of frying oil and par-fried french fries were taken at 3-h intervals through each experiment and analyzed for selected chemical and physical parameters. As the frying time increased, all the oil samples contained increased amounts of deterioration products. The color index and free fatty acid (FFA) values were highly correlated with frying time. There were no significant effects on oil properties in terms of FFA until 9 h of frying among the 4 pretreatments. However, from 12 to 72 h of frying, oil used to fry potatoes treated with SAPP contained less FFA than oils exposed to CaAc alone or in combination with SAPP. The fat content of par-fried french fries was approximately 0.11 g/g dry matter and remained relatively constant during extended frying. After-cooking darkening of par-fried french fries and final color of the fries were affected by the pretreatment, but not by the frying time.     

Flavor stability and quality of high-oleate and regular soybean oil blends during frying

The percentages of oleate (18∶1), linoleate (18∶2), and linolenate (18∶3) in blended soybean oils (SBO) were evaluated for their impact on flavor stability and quality in fried foods. Six SBO treatments, including a control (conventional SBO, 21.5% 18∶1) and a high-18∶1 SBO (HO, 79% 18∶1), were tested. In addition, these two oils were mixed in different ratios to make three blended oils containing 36.9, 50.7, and 64.7% 18∶1, abbreviated as 37%OA, 51%OA, and 65%OA, respectively. Also, a low-18∶3 (LL) SBO containing 1.4% 18∶3 and 25.3% 18∶1 was tested. Bread cubes (8.19 cm³) were fried in each of 18 oils (6 treatments ×3 replicates). The fresh and stored bread cubes fried in 79%OA were second to the cubes fried in LL in overall flavor quality, were the weakest in intensity of stale, grassy, fishy, cardboard, and burnt flavors by sensory evaluation, and contained the least amounts of hexanal, hexanal, t-2-heptenal, t,t-2,4-nonadienal, and t,t-2,4-decadienal in volatile analysis. Other treatments were intermediate in these sensory and instrumental evaluations, as related to their 18∶1, 18∶2, and 18∶3 concentrations. In general, the results suggested that the overall flavor stability and eating quality of foods fried in the six oil treatments from the best to the poorest would be: LL≥79%OA, 65%OA, 51%OA, 37%OA, and control.     

Protecting oil during frying: A comparative study

The effect of carbon dioxide blanketing (CDB) and vacuum frying (VF) on the frying performance of regular canola oil was evaluated. For 7 h daily and for 7 days French fries were fried in regular canola oil at 185 ± 5 °C without and with CDB and in a vacuum fryer. The extent of changes in the oil was assessed by analysis of total polar compounds (TPC), anisidine value (AV), color component formation and changes in composition of fatty acids and tocopherols. Frying under CDB reduced the amount of TPC by 54%, while 76% reduction was observed during VF compared to standard frying conditions (SFC). Similarly, lower oxidative degradation was observed when measured by AV. At the end of the frying period, the reduction in unsaturated fatty acid content was 3.8, 1.9 and 12.7% when frying under CDB, vacuum and SFC, respectively. The rate of tocopherol degradation was three and twelve times slower in VF when compared to CDB and SFC, respectively.     

Frying stability of high-oleate and regular soybean oil blends

The objective of this work was to study the frying stability of soybean oil (SBO) with reduced linoleate (18∶2) and linolenate (18∶3) and elevated oleate (18∶1) contents. High-oleate SBO [HO SBO, 79% oleic acid (OA)] and a control (conventional SBO, 21.5% OA) were tested as is, as well as blended in different ratios to make three blended oils containing 36.9, 50.7, and 64.7% OA, abbreviated as 37%OA, 51%OA, and 65%OA, respectively. In addition, a low-linolenate (LL) SBO containing 1.4% 18∶3 and 25.3% 18∶1 was tested. Bread cubes (8.19 cm³) were fried in each of 18 oils (6 treatments×3 replicates). We hypothesized that stability indicators would be indirectly related to the total 18∶2 plus 18∶3 percentages and/or the calculated oxidizability. In general, the results were fairly predictable based on total 18∶2 and 18∶3 concentrations. The overall frying stability of the six oil treatments, from the best to the poorest, was: 79%OA, 65%OA, 51%OA, LL≥37%OA, and the control, with respective total compositions for 18∶2 plus 18∶3 of 10.3, 23.6, 36.3, 59.6, 48.9, and 62.8%. The greatly reduced concentration of 18∶3 in the LL SBO made it more stable than the 37%OA, even though the combined composition of 18∶2 and 18∶3 of LL was greater than that of the 37%OA. Blending conventional SBO with HO SBO had a profound effect on the oxidative stability index and color of the blended oils, but the values were not linearly predictable by the percentage of control in the blended oil. Other stability indices, including calculated oxidizability, calculated iodine value, conjugated dienoic acid value, and viscosity, changed in linear response to an increased proportion of the control in the blends.     

Effect of oil replenishment during deep-fat frying of frozen foods in sunflower oil and high-oleic acid sunflower oil

Frying stability of sunflower oil (SO) with 23% oleic acid and 61% linoleic acid, and of high-oleic acid sunflower oil (HOSO) with 74% oleic acid and 13% linoleic acid was studied during 20 discontinuous deep-fat fryings of various frozen foods, with or without frequent replenishment of the used oil with fresh oil. Alterations of both oils were measured by column, gas-liquid and high-performance size-exclusion chromatography. Total polar content and compounds, related to thermoxidative changes, and diacylglycerides, related to hydrolytic changes, increased in all oils during frying but reached higher levels in SO than in HOSO. Nevertheless, the increased levels of diacylglycerides observed may result from the frozen potatoes prefried in palm oil. Oleic acid in HOSO and linoleic acid in SO significantly decreased, but the fatty acid modifications that occurred during the repeated fryings were not only related to thermoxidative alteration but also to interactions between the bath oil and the fat in the fried products. Data from this study also indicated that HOSO performed more satisfactorily than SO in repeated fryings of frozen foods. Moreover, frequent addition of fresh oil throughout the deep-frying process minimized thermoxidative and hydrolytic changes in the frying oils and extended the frying life of the oils.     

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.     

A new spectrophotometric method for the rapid assessment of deep frying oil quality

A new and quick spectrophotometric method was developed to assess deep-frying oil quality. The scanned spectrophotometric curves of the frying oil samples from 350 and 650 nm wavelength changed systematically with the duration of deep frying. The absorbances of the frying oil samples, especially those measured at 490 nm, increased significantly during frying and were significantly correlated to frying time (r ≥0.95, P<0.001). There was a strong correlation between the absorbances of a set of oil samples taken from 0 to 80 h of deep frying and total polar compound contents in the same set of oil samples analyzed using the American Oil Chemists' Society official method (r=0.974, P<0.001). The equation for conversion of the absorbances to total polar compound contents is y=−2.7865x ² +23.782x+1.0309. The absorbances of 10 different types of frying oils with samples taken from 0 to 80 h of deep frying in duplicate were also strongly correlated to total polar compounds in the same oil samples (r=0.953, P<0.001, n=220). The results show that this method is fast, simple, convenient, and reliable.     

Oxidative stability of healthful frying oil medium and uptake of inherent nutraceuticals during deep frying

Four healthful frying oil mediums have been formulated using sunflower (FOB-I), groundnut (FOB-II), mustard (FOB-III), and palm olein (FOB-IV) oils as base oils, and fortified with rice bran and crude sesame oils separately in the ratio of 60∶20∶20 (by vol). Oxidative stabilities have been ascertained by deep-frying potato bajji (potato slices sandwiched with Bengal gram flour) continuously for 60 min for three cycles with a gap of 7 d each. The product had moisture between 12.8 and 16.0% and absorbed fat between 32.5 and 38.1%, making the oil media vulnerable to oxidation. The p-anisidine values for leftover FOB-I and FOB-IV ranged from 10.8 to 24.4 and from 1.5 to 10.7, respectively, indicating that the former was a less and the latter a more stable combination. Hydroperoxide and conjugated dienes were assessed by UV spectrometry at λ_max 230 nm. The O.D. was maximal (1.4) for FOB-I samples for both leftover and absorbed oils for third-cycle experiments. That there was no absorbance for the FOB-III and-IV samples indicated their absence. Estimation of oryzanol and sesamol in oil left over after deep frying and in the oil absorbed by the products indicated that distribution was equal and there was no loss of these active factors during deep frying. The study indicated that sunflower oil blend was the least stable and the palm olein blend was most stable.     

Effects of expeller-pressed/physically refined soybean oil on frying oil stability and flavor of french-fried potatoes

To determine effects of expeller pressing/physical refining of soybean oil (SBO) on frying, studies were conducted with expeller-pressed, physically refined, bleached, deodorized SBO (EPSBO); hexane-extracted, refined, bleached, deodorized SBO+TBHQ; and hydrogenated SBO (HSBO). Oils contained citric acid and dimethylpolysiloxane and were used for 35 h of frying french-fried potatoes. Polar compound levels in EPSBO were similar to SBO+TBHQ or HSBO. Flavor quality of potatoes was evaluated by trained, experienced, analytical sensory panelists. In early frying stages, potatoes fried in EPSBO had significantly lower intensities of fishiness than potatoes fried in SBO+TBHQ. Potatoes fried in HSBO were described as “hydrogenated”. Because of differences in flavor intensities and types, potatoes prepared in EPSBO had significantly better quality scores than those fried in SBO+TBHQ or HSBO during the first 15 h of frying. During later stages (25 and 35 h), potatoes fried in EPSBO had significantly better quality scores than potatoes fried in HSBO. Variations in minor oil constituents may partly explain these differences. EPSBO had less total tocopherols and phytosterols than did SBO at 0-time. During frying, TBHQ in SBO and Maillard reaction products in EPSBO probably inhibited tocopherol loss and therefore improved quality.     

Chemical and physical analyses and sensory evaluation of six deep-frying oils

The performance of three high-oleic canola oils with different levels of linolenic acid [low-linolenic canola (LLC), medium-linolenic canola (MLC), and high-linolenic canola (HLC)], a medium-high-oleic sunflower oil, a commercial palm olein and a commercial, partially hydrogenated canola oil, was monitored by chemical and physical analyses and sensory evaluation during two 80-h deep-frying trials with potato chips. Linolenic acid content was a critical factor in the deep-frying performance of the high-oleic canola oils and was inversely related to both the sensory ranking of the food fried in the oils and the oxidative stability of the oils (as measured by color index, free fatty acid content, and total polar compounds). LLC and sunflower oil were ranked the best of the six oils in sensory evaluation, although LLC performed significantly better than sunflower oil in color index, free fatty acid content, and total polar compounds. MLC was as good as palm olein in sensory evaluation, but was better than palm olein in oxidative stability. Partially hydrogenated canola oil received the lowest scores in sensory evaluation. High-oleic canola oil (Monola) with 2.5% linolenic acid was found to be very well suited for deep frying.