Changes in dielectric constant as a measure of frying oil deterioration

A soybean oil, a hydrogenated vegetable frying shortening and an animal-vegetable shortening were heated at 190 C for 8 hours each day for 4 days with and without the frying of potatoes. Samples were taken periodically and analyzed for various changes normally used to measure frying oil deterioration. The changes in the dielectric constant were determined with a patented instrument called the Food Oil Sensor. This instrument is standardized with 3. sample of the fresh oil, and it then measures the change in the electric capacitance of the heated oil samples. The dielectric constant of all three shortenings increased linearly with heating time. The greatest change occurred in the soybean oil sample and the smallest change in the hydrogenated vegetable shortening. For each shortening the increase was somewhat greater during frying than during heating without frying. Statistically significant correlations were obtained between instrument readings and increase in the total polar materials, the color, the peroxide values, the diene content, and the free fatty acids and the decrease in the iodine values.     

Ziziphi spinosae Semen Oil Enhance the Oxidative Stability of Soybean Oil under Frying Conditions

The objective of this study is to improve the oxidative stability of soybean oil by using Ziziphi spinosae semen oil (ZSSO). In the present study, the oxidative stability, fatty acid composition, tocopherol, and phenolic changes of soybean oil without additives and soybean oil mixed with 5% ZSSO are evaluated during frying at 180 ℃ for 18 h. Tert-butyl hydroquinone (TBHQ) and vitamin E (VE) as common antioxidants are incorporated into soybean oil for comparison. According to the results of oxidative stability assays of conjugated diene value, thiobarbituric acid value, acid values, and total polar compounds, the incorporation of ZSSO significantly restrain the lipid oxidation of soybean oil. After 18 h of frying, the soybean oil samples with ZSSO has more polyunsaturated fatty acids, tocopherols, and DPPH radical scavenging capacity, and less trans fatty acids, compared with TBHQ and VE. In addition, ZSSO-containing soybean oil maintains a high content of phenols during the frying period, which is correlated with the increase in oxidative stability. Therefore, replacing part of soybean oil with ZSSO can effectively reinforce the performance of soybean oil under frying conditions.     

Deterioration of diacylglycerol‐ and triacylglycerol‐rich oils during frying of potatoes

The stabilities of a commercial diacylglycerol‐rich oil (DAG) and a salad oil (TAG) that had been prepared from a mixture of rapeseed and soybean oils were compared while frying potatoes at 180 °C for 3 h. The representative chemical and physical characteristics of the oils were assessed before and after frying, together with the amount of volatile aldehydes in the exhaust of frying. Among the deterioration indications, the carbonyl value, polymer content, and residual polyunsaturated fatty acid content were similar and not significantly different between the TAG and DAG. On the other hand, the characteristics relating to free fatty acids, i.e. the acid value and emission of chemiluminescence at 100 °C, were greater and the smoke and flash points were lower in the DAG than in the TAG. An irritating odor was generated from the DAG after 1 h of frying and got stronger as frying continued. These results suggested that DAG more easily forms free fatty acids under frying conditions than TAG.     

Verhalten und Beurteilung von Fritierfetten in der Praxis

Behaviour and Assay of Frying Fats in the Practice Studies on coconut oil palm oil, peanut oil, soybean oil and hardened peanut and fish oils revealed that the oils rich in polyenoic acids as well as animal fats are unsuitable for prolonged use in frying. The content of oxidized fatty acids, saponification colour number, flavour, acid value, smoke point and appearance served as criteria for assay. The limiting values determining whether the frying fat is edible, and the significance of each of these characteristic properties used for such evaluations are discussed. Finally, a new rapid method is introduced for the direct determination of the degree of oxidative deterioration in frying plants. This methods can be practiced without a prior knowledge of chemistry.     

Oxidative stability of natural and randomized high-palmitic-and high-stearic-acid oils from genetically modified soybean varieties

The oxidative stability of soybean oil triacylglycerols (TAG) obtained from genetically modified soybeans was determined before and after chemical randomization. Soybean oil oxidative studies were carried out under static oxygen headspace at 60°C in the dark and oxidative deterioration was monitored by peroxide value, monometric and oligomeric oxidation products, and volatile compounds. Randomization of the soybean oil TAG improved the oxidative stability compared to the natural soybean oil TAG. Oxidative stability was improved by three factors. Factor one was the genetic modification of the fatty acid composition in which polyunsaturated acids (such as linolenic and linoleic acids) were decreased and in which monounsaturated fatty acids (such as oleic) and saturated acids (palmitic and stearic) were increased. Factor two was the TAG compositional modification with a decrease in linolenic and linoleic-containing TAG and an increase in TAG with stearic and palmitic acids in combination with oleic acid. Factor three was the TAG structure modification accomplished by an increase in saturated fatty acids and a decrease in linoleic and linolenic acids at the glycerol moiety carbon 2. Presented at the AOCS Annual Meeting & Expo, Chicago, IL, May 10–13, 1998.     

The effect of surfactants on the interfacial tension of frying fat

Interfacial tension (IFT) between fat and water was measured as a means of evaluating fats used for frying. The Du Noüy ring method was used with a tensiometer to obtain IFT. The IFT of donut frying fats, which were diluted 1:1 with unused fat, decreased with increasing frying time from 21 mN/m in unused fat to 5 mN/m in fat at the point of discard. To determine which individual chemical components in donut frying fat are responsible for the decreased IFT, various surfactants were added. The addition of surfactants to the water phase (phosphatidylcholine, sodium oleate, and sodium chloride) or oil phase (monoacylglycerols, diacylglycerols, and fatty acids) had varying effects on the IFT of soybean oil against water at 25 ± 0.1°C. The IFT decreased with increasing concentration of monoacylglycerols, sodium oleate, phosphatidylcholine, and sodium chloride. Sodium oleate was the most effective agent in lowering IFT, reducing it almost to zero at a concentration of 0.1%. At a concentration of 0.2%, monoacylglycerols lowered IFT of fresh soybean oil by 14% (monolinolein), 22% (monoolein), and 26% (monostearin). Egg lecithin lowered the IFT by 42%. These surfactants changed the IFT of fresh soybean oil by their adsorption at the oil/water interface. Although sodium chloride is not a surfactantper se, it lowered IFT by salting out surfactants from the aqueous phase onto the interface. The addition of diacylglycerols or fatty acids had virtually no effect on IFT. Presented at the 84th AOCS Annual Meeting & Expo in Anaheim, California, May 1992.     

Performance of Regular and Modified Canola and Soybean Oils in Rotational Frying

Canola and soybean oils both regular and with modified fatty acid compositions by genetic modifications and hydrogenation were compared for frying performance. The frying was conducted at 185 ± 5 °C for up to 12 days where French fries, battered chicken and fish sticks were fried in succession. Modified canola oils, with reduced levels of linolenic acid, accumulated significantly lower amounts of polar components compared to the other tested oils. Canola oils generally displayed lower amounts of oligomers in their polar fraction. Higher rates of free fatty acids formation were observed for the hydrogenated oils compared to the other oils, with canola frying shortening showing the highest amount at the end of the frying period. The half-life of tocopherols for both regular and modified soybean oils was 1–2 days compared to 6 days observed for high-oleic low-linolenic canola oil. The highest anisidine values were observed for soybean oil with the maximum reached on the 10th day of frying. Canola and soybean frying shortenings exhibited a faster rate of color formation at any of the frying times. The high-oleic low-linolenic canola oil exhibited the greatest frying stability as assessed by polar components, oligomers and non-volatile carbonyl components formation. Moreover, food fried in the high-oleic low-linolenic canola oil obtained the best scores in the sensory acceptance assessment.     

Comparative nutritional quality of palmstearin-liquid oil blends and hydrogenated fat (vanaspati)

An attempt was made to use high-melting lowdigestible fat palmstearin as a vanaspati substitute by blending it with polyunsaturated fatty acid-rich liquid oils. This blending produced fat products of zero-trans fatty acid content and melting points below the human body temperature, so that they can be digested easily. The new blended products were fed to male albino rats (Charles Foster strain); the coefficients of digestibilities were 94.2% for palmstearin and rapeseed oil blend, 95.1% for palmstearin and sunflower oil blend, and 96.2% for palmstearin and soybean oil blend, which were somewhat better than the digestibility coefficient of conventional vanaspati (93.6%). Feeding experiments for three months showed comparable results in terms of serum lipid profiles. The blended products significantly increased the total cholesterol level but not the free cholesterol level in serum and liver of rats when compared with those of the conventional vanaspati group of rats.     

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.     

Frying performance of palm oil liquid fractions

Palm oil liquid fractions were used as frying media in household and industrial fryers and were compared to standard edible oils and fats, such as soybean, groundnut, sunflower, rapeseed and tallow. The analytical evaluation covered free fatty acids, viscosity, smoke and flame points, oxidized fatty acids, nonelution material (NEM), UV differential spectra, polymers and foam index. These values measure the extent of the oil degradation, i.e., oxidation, hydrolysis, splitting and polymerization. Moreover, they were combined with other analytical procedures (fatty acid composition, keeping qualities such as the time necessary for an oxygen-absorbing sample to reach a -0.5 psi pressure in a closed system) in order to have a large analytical control during the frying processes. The data collected show the suitability of edible oils and fats for frying purposes and indicate that palm oil liquid fractions perform satisfactorily as frying media. They have low degradation and produce fried foods with acceptable keeping qualities. Presented at the ISF/AOCS World Congress, New York, April 1980.     

Electrochemical hydrogenation of edible oils in a solid polymer electrolyte reactor. Sensory and compositional characteristics of low Trans soybean oils

Soybean oils were hydrogenated either electrochemically with Pd at 50 or 60°C to iodine values (IV) of 104 and 90 or commercially with Ni to iodine values of 94 and 68. To determine the composition and sensory characteristics, oils were evaluated for triacylglycerol (TAG) structure, stereospecific analysis, fatty acids, solid fat index, and odor attributes in room odor tests. Trans fatty acid contents were 17 and 43.5% for the commercially hydrogenated oils and 9.8% for both electrochemically hydrogenated products. Compositional analysis of the oils showed higher levels of stearic and linoleic acids in the electrochemically hydrogenated oils and higher oleic acid levels in the chemically hydrogenated products. TAG analysis confirmed these findings. Monoenes were the predominant species in the commercial oils, whereas dienes and saturates were predominant components of the electrochemically processed samples. Free fatty acid values and peroxide values were low in electrochemically hydrogenated oils, indicating no problems from hydrolysis or oxidation during hydrogenation. The solid fat index profile of a 15∶85 blend of electrochemically hydrogenated soybean oil (IV=90) with a liquid soybean oil was equivalent to that of a commercial stick margarine. In room odor evaluations of oils heated at frying temperature (190°C), chemically hydrogenated soybean oils showed strong intensities of an undesirable characteristic hydrogenation aroma (waxy, sweet, flowery, fruity, and/or crayon-like odors). However, the electrochemically hydrogenated samples showed only weak intensities of this odor, indicating that the hydrogenation aroma/flavor would be much less detectable in foods fried in the electrochemically hydrogenated soybean oils than in chemically hydrogenated soybean oils. Electrochemical hydrogenation produced deodorized oils with lower levels of trans fatty acids, compositions suitable for margarines, and lower intensity levels of off-odors, including hydrogenation aroma, when heated to 190°C than did commercially hydrogenated oil.     

Effect of Different Frying Methods on the Total <Emphasis Type="Italic">trans</Emphasis> Fatty Acid Content and Oxidative Stability of Oils

The main objective of this study was to determine the effect of different frying oils and frying methods on the formation of trans fatty acids and the oxidative stability of oils. Sunflower, canola and commercial frying oils, the most commonly used oils for frying potatoes in the fast food industry, were used as the frying medium. The value for total polar compounds was highest when commercial frying oil was used in the microwave oven (22.5 ± 1.1). The peroxide value, as an indicator of oil oxidation, was lowest for microwave oven frying (2.53 ± 0.03). The K₂₃₂ and K₂₇₀ values were 0.41 ± 0.04 and 0.18 ± 0.02, respectively, for commercial frying oil in the microwave oven. The lowest free fatty acid content was recorded for the commercial frying oil used in the deep‐fat fryer at 190 °C. The highest iodine value was measured for sunflower oil used in the deep‐fat fryer (148.14 ± 0.07), indicating a greater degree of unsaturation. The lowest trans fatty acid value was recorded for sunflower oil in the microwave oven (0.17 ± 0.05), with a higher overall amount of total trans fatty acids observed for oils after frying in the electrical deep‐fat fryer compared to the microwave. Sunflower oil was favourable for both frying methods in terms of the trans fatty acid content.     

Heterogeneous esterification of oil with high amount of free fatty acids

Frying oils have become the newest raw material for the transesterification reaction for the production of biodiesel. However, these compounds usually come with a certain amount of free fatty acids. These impurities can be transformed into esters and the production of biodiesel could be increased.The use of basic resins to perform the esterification reaction into biodiesel is studied in this work. The effect of the most relevant variables of the process such as reaction temperature, molar ratio between alcohol and oil, amount of catalyst and amount of free fatty acids fed with the oil have been analyzed. For this purpose, an ideal frying oil using oleic acid and soybean oil was made. The alcohol used was ethanol.The esterification of free fatty acid using this heterogeneous catalyst appears as a great alternative to purify frying oil; in this case, the final conversion achieved was around 80%.     

Veränderungen der Fette unter Fritierbedingungen und deren analytische Erfassung Beurteilung des Gebrauchswertes und Analytik von Fritierfetten I

Alterations in Fats Under Frying Conditions and Their Analytical Detection: Evaluation of the Quality of Frying Fats and Their Analysis The quality of a frying fat at a certain stage of use depends on its composition, its freshness, and the frying conditions. The frying stability, i. e. the length of time a fat can be heated in the fryer till the content of petroleum ether insoluble oxidized fatty acids reaches a level of 1%, is proportional to the degree of saturation of the fat, and inversely proportional to its specific surface in the fryer and its reversion period. Frying stability can be increased considerably by excluding atmospheric oxygen. Thus, under suitable frying conditions even highly unsaturated fats, such as peanut oil, sunflower oil, and soybean oil, can be used without an increase in the content of oxidized fatty acids above a permitted level. In the choice of fats one should prefer those having lower UV-absorbance and higher SWIFT stability. In order to define the degree of oxidation of used frying fats, it is recommended to determine the content of oxidized fatty acids according to DGF-Standard Method C-III 3 (68). Moreover, acid value, smoke point and UV-absorbance provide valuable information regarding the degree of oxidative changes. A rather loose relationship exists between the saponification colour number and the content of oxidized fatty acids.     

Analysis of thermally abused soybean oils for cyclic monomers

Cyclic monomers derived from the intramolecular condensation of the C₁₈ polyunsaturated fatty acids have been reported to elicit toxic responses when fed to laboratory animals at low dietary levels. This study was undertaken to quantitate the cyclic monomers formed by thermal oxidation induced during deep fat frying to assess the potential toxicity of commonly used vegetable oils. Two separate experiments were designed to study the effects of unsaturation and both intermittent and continuous heating on cyclic monomer formation. Both lightly hydrogenated soybean oil (iodine value [IV]=107) and refined, bleached and deodorized soybean oil were studied. The heated oil sustained substantial chemical and physical alterations, as indicated by IV decreases from 10–15 units, increases in free fatty acids of 5–10-fold and in noneluted material of 18–21%. Selected samples were completely hydrogenated and analyzed for cyclic monomers by gas chromatography. Under chromatographic conditions sufficiently sensitive to detect a cyclic monomer standard at less than 0.5% by weight, no cyclic monomers were detected in any of the heated oil samples. However, after concentration by low temperature crystallization of the hydrogenated samples to remove a major portion of the saturated components interfering in cyclic monomer resolution, about 0.3–0.6% cyclic acids, as well as 0.4–0.9% polar materials, were detected in the heated soybean oils. Components appearing in the gas chromatogram with the same retention time as those in a cyclic monomer standard were further identified by gas chromatography-mass spectrometry as disubstituted cyclic C-18 acids.     

Studies on Quality of Coconut Oil Blends after Frying Potato Chips

The quality (chemical and sensory) of oil blends prepared by blending equal proportions of coconut oil with sesame oil (blend 1), coconut olein with sesame oil (blend 2) and coconut olein with palmolein (blend 3) was evaluated after deep-fat frying of potato chips. After frying, the free fatty acid content did not change, however, the anisidine value increased. Blend 2 had the highest anisidine value (44.0). A marginal decrease in the iodine value and an increase in the diene values were observed in blends 1 and 2. The β-carotene content in blend 3 and tocols in all the three blends were found to decrease after frying. Sensory odor profiles of oil blends after frying showed a decrease in the characteristic coconut oil aroma. The earthy and seedy aroma associated with sesame oil was found to decrease on frying. The sensory profile of potato chips showed a slight bitter taste in the samples fried in blends 1 and 2. However, the intensity of bitterness decreased and the earthy note increased on storage. Blend 3 had the highest overall quality.     

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.     

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.     

Frying Performance of No-<Emphasis Type="Italic">trans</Emphasis>, Low-Linolenic Acid Soybean Oils

Two extruded-expelled physically refined soybean oils with reduced contents of linolenic acid, ultra-low- linolenic acid (ULL, 1.5%) and low-linolenic acid (LL, 2.6%), and a extruded-expelled physically refined control oil (control, 5.3% linolenic acid) were evaluated by frying French fries in a commercial-like setting for 6 h day⁻¹ during 23 days. The oils became darker, increased in yellow color at the beginning, and became redder and less green throughout the process. Free fatty acids levels were not different among the oils until day 14, after which, ULL was different from the control for the remainder of frying. The conjugated dienoic acid values were greatest in the control. Generally, ULL and LL oils had lower percentages of polar compounds than did the control, providing a frying life 2 days longer than the control and ~30% increase in frying time. A trained sensory panel evaluated the French fries on days 2, 5, and 6. Buttery and potato flavors decreased, and rancid and painty flavors increased over frying time for all products. Rancid flavor was highest in the fries from the control oil. Overall, the ULL and LL oils performed better than did the control oil and ULL tended to perform better than the LL.     

Effects of sugar,salt and water on soybean oil quality during deep-frying

The effects of flour dough components (water, sugar and salt) on soybean oil deterioration during deep-fat frying have been investigated. Flour dough sheets made from flour and water were used as the carrier of salt and sugar. Several analyses, including acid value, carbonyl value,p-anisidine value, color, dielectric constant, Fritest, total polar compounds and polymer content, were used to evaluate deterioration of oil quality during deep-fat frying. The relationship between frying time and analytical data was analyzed by Duncan's multiple range test. Oil quality after frying simple flour dough sheets without additives was inferior to that after frying dough sheets with added sugar or salt. The sugar and salt in the flour dough sheets may play a role as water-binding substances during frying. Among the effects of water, salt and sugar, the rate of oil deterioration was found to be highest for water, followed by salt and sugar. The combined addition of salt and sugar had no enhancing effect on the oil deterioration during deep-fat frying.