Determination of the classes of free monocarbonyl compounds in oxidizing fats and oils

Simultaneous equations were derived to distinguish and account for the major free monocarbonyl classes in oxidizing fats and oils. The procedure of Pool and Klose was modified to accommodate the equations. The procedure involves conversion of the free monocarbonyls to DNP-hydrazones by passage of a benzene solution of the sample over an alumina-DNP-hydrazine reaction column. The alkanals, alk-2-enals and alk-2,4-dienals were measured by absorbance readings at 430, 460, and 480 mμ of a benzene alcoholic-KOH hydrazone solution. The average error of the procedure in the analysis of total carbonyls in authentic mixtures was 2.4%. It was found that the qualitative composition of the DNP-hydrazones obtained from milk fat by the reaction column were identical to the volatile carbonyl fraction. Moreover, there was less than 5% difference in the volatile monocarbonyl content of milk fat and the quantity measured by the modified Pool and Klose procedure. The data suggest that the procedure affords a means of measuring volatile or free monocarbonyls of oxidizing lipids. Since the volatile monocarbonyls are directly related to flavor deterioration of oxidizing lipids, it would appear that the free monocarbonyls, as measured by the modified Pool and Klose procedure, should exhibit a corresponding relationship. Technical Paper No. 1544, Oregon Agricultural Experiment Station. This investigation was supported in part by PHS research grant No. EF182, Environmental Health, National Institutes of Health.     

A sensitive method for the determination of carbonyl compounds

2,4,6-Trichlorophenylhydrazine was tested as a reagent for carbonyl compounds. As little as 0.1 of the 2,4,6-trichlorophenylhydrazones (2,4,6-TCPH) could be measured with an electron capture detector, so this reagent should be useful in measuring the carbonyl compounds in oxidized fats at levels near their flavor thresholds. Mixtures of 2,4,6-TCPHs were separated by thin layer chromatography. Alkan-2-one-2,4,6-TCPHs were separated from aldehyde-2,4,6-TCPHs on alumina plates. The alkanal, alk-2-enal and alk-2,4-dienal-2,4,6-TCPHs were separated from each other either on silica gel plates or silica gel-silver ion plates. The derivatives within each carbonyl class were separated by chain length on chromatography media impregnated with phenoxyethanol. The 2,4,6-TCPHs eluted from thin layer plates were determined with an electron capture detector after gas chromatography on a 30 cm column of freeze-dried detergent base coated with a silicone oil. Journal Paper No. J-6842 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, project No. 1856.     

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.     

Carbonyls in oxidizing fat. V. The composition of neutral volatile monocarbonyl compounds from autoxidized oleate,linoleate, linolenate esters,and fats

The steam volatile monocarbonyl compounds in mildly autoxidized esters of oleic, linoleic, linolenic acids, and animal and vegetable fats were quantitatively estimated. The major aldehydes in oleate and linoleate were those that might be expected from the scission of reported monomeric hydroperoxide isomers. The predominance of hept-2,4-dienal and propanal in linolenate suggested that the major monomeric hydroperoxides were 12-and 16-hydroperoxy conjugated dienoic isomers. The number of minor aldehydes increased with degree of unsaturation of the fatty acid. The amounts of monocarbonyl compounds in the fats examined generally agreed with their average fatty acid composition. Appreciable amounts of heptanal in lamb and beef fat and heptanal and decanal in butterfat, under the conditions of oxidation, could not have come from the three unsaturated acids. Heating at 165°C. in all samples increased the proportions of the most unsaturated major aldehydes.     

A modified method for the estimation of total carbonyl compounds in heated and frying oils using 2-propanol as a solvent

A modified method without using benzene for quantitative determination of total carbonyl compounds in heated and frying fats and oils has been developed. The analysis is done by the reaction of 2,4-dinitrophenylhydrazine (2,4-DNP) with aldehydes and ketones in 2-propanol. The optimal wavelength to determine the total carbonyl compounds is 420 nm where the 2,4-dinitrophenylhydrazone (2,4-DNPH) derivatives from saturated and unsaturated aldehydes and ketones had the same molar absorption coefficient. The modified method for the estimation of total carbonyl compounds in heated and frying oils using 2-propanol instead of benzene as a solvent had a good correlation with the conventional method.     

Carbonyls in oxidizing fat. VII. A comparison of methods of isolating monocarbonyl compounds

Comparison of quantitative methods for the isolation of aldehydes from lard oxidized at 23C showed an extreme sensitivity of precursors to isolation conditions. Differences existed in the quantity and class compositions of the monocarbonyl compounds isolated. Acid conditions caused considerable breakdown of alkanal and alk-2-enal precursors. Steam distillation (100C) released alkanals, alk-2-enals, and alk-2,4-dienals. The mild Girard T method apparently produced a small amt of alk-2-enals from precursors, but except in the initial stages isolated the lowest amounts of aldehydes. Complete evaluation of the method for isolation of free aldehydes was not possible because alkanals were released from precursors during the vacuum distillation procedure (50C). The Schwartz (dilute phosphoric acid), Girard T, and vacuum distillation methods isolated similar amounts of alk-2,4-dienals. Curious changes in relationships, possibly indicative of oxidation stages, were observed as autoxidation progressed.     

Chemical reactions involved in the deep-fat frying of foods1

Deep-fat frying is one of the most commonly used procedures for the preparation and manufacture of foods in the world. During deep-fat frying, oxidative and thermal decompositions may take place with the formation of volatile and nonvolatile decomposition products, some of which in excessive amounts are harmful to human health. A limited survey of frying fats used in commercial operations indicated that some were maintained at good quality and others were overused or abused. The volatile decomposition products produced by corn oil, hydrogenated cotton-seed oil, trilinolein, and triolein, under simulated commercial frying conditions, were collected, fractionated, and identified. A total of 211 compounds were identified. The nonvolatile decomposition products produced by trilinolein, triolein, and tristearin under simulated commercial frying conditions were collected and characterized. After being treated under deep-fat frying conditions at 185 C for 74 hr, trilinolein yielded 26.3% non-urea-adduct-forming esters, triolein yielded 10.8%, and tristearin also yielded 4.2%.     

Autoxidation of tissue lipids. I. Incorporation of dietary fatty acids and formation of monocarbonyl compounds in adipose tissue lipids of the vitamin E-deficient rat

Male weanling rats were fed vitamin E-deficient and vitamin E-supplemented diets containing 5% corn oil or cod-liver oil for 16 weeks, after which their adipose tissue lipids were extracted and analyzed in a nitrogen atmosphere for carbonyl compounds and fatty acids. The vitamin E-deficient cod-liver oil-fed rats, exhibiting incisor depigmentation and darkened adipose tissue, yielded lipids which had a lower iodine value, contained less polyunsaturated fatty acids, and contained more carbonyl compounds, particularly alkanals and alk-2-enals, than the lipids from the animals fed the vitamin E-supplemented cod-liver oil diet. The tissues of the vitamin E-deficient corn oil-fed rats contained less linoleate and more monocarbonyl compounds than those of the vitamin E-supplemented corn oil-fed animals. The results indicate that vitamin E protection is necessary for the incorporation of C₂₀ and C₂₂ fatty acids into the tissues from the diet and that in the deficiency of vitamin E, a low level of autoxidation occurs in the tissues.     

Effect of ascorbyl palmitate on the quality of frying fats for deep frying operations

The addition of 0.02% ascorbyl palmitate (AP) reduced color development of frying fat (animal fat/vegetable oil [A-V] shortening) and vegetable oil (partially hydrogenated soybean [V-S] oil) in simulation studies. It also reduced peroxide values, development of conjugated diene hydroperoxides (CDHP) and their subsequent degradation to volatile compounds, such as decanal and 2,-4 decadienal, indicating that AP has the ability to inhibit thermal oxidation/degradation of frying fats and oils. A commercial french fry fat had lower CDHP values compared to A-V fat in simulated studies, and fried chicken oil had lower CDHP values than the V-S oil. Peanut oil had higher thermal stability than the other fats and oils.     

Monitoring of 2,4‐decadienal in oils and fats used for frying in restaurants in Athens,Greece

Some frying by‐products of medium polarity, so‐called medium‐polarity materials (MPM), produced during domestic deep‐frying of French‐fried potatoes in edible vegetable oils, have recently been isolated and linearly correlated to % total polar materials and % polymerized triglycerides. The in vitro oxidation of low‐density lipoproteins in a dose‐dependent manner by MPM has also been reported. In the present study, the MPM constituents were identified after extraction of MPM from the oils, subsequent purification by RP‐HPLC, and GC‐MS analysis. The main constituent of MPM was trans,trans‐2,4‐decadienal, a compound that has previously been reported to be formed during peroxidation of linoleic and arachidonic acid. 2,4‐Decadienal was also quantified in oils and fats used for frying in restaurants in Athens, Greece, by direct injection of oil sample solutions in HPLC. For the most commonly used frying oils, 2,4‐decadienal concentration ranges were 0.3–119.7 mg/kg for sunflower oil, 13.3–92.7 mg/kg for cottonseed oil, 4.1–44.9 mg/kg for palm oil, and 2.0–11.3 mg/kg for vegetable cooking fats. Considering the common catering practices of frying, 2,4‐decadienal was more likely to be found in sunflower oil after deep‐frying of potatoes. Comparing the amounts of this aldehyde found in oils from restaurants to the amounts previously found for domestic frying (up to 30 mg/kg after the 8^th successive frying session in sunflower oil), the probability of consuming a level of 2,4‐decadienal in restaurant‐prepared food that is higher than the level in home‐fried food was determined to be approximately one third.     

Comparative Fingerprint Changes of Toxic Volatiles in Low PUFA Vegetable Oils Under Deep-Frying

The volatile fraction of three vegetable oils recommended for deep‐frying due to their high MUFA:PUFA ratios, namely extra‐virgin olive oil, peanut oil and canola oil, was compared before and after frying potatoes, with a particular focus on toxic volatiles. For the purpose, a headspace solid‐phase‐micro extraction technique coupled with gas chromatography and mass spectrometry was optimized, with semi‐quantification achieved using two internal standards. Significant qualitative and quantitative differences were observed, both before and after frying. From a total of 51 compounds, aldehydes were the main group formed after deep‐frying, their nature and abundance being highly associated with the initial fatty acid composition, particularly linoleic acid (r² = ?0.999, p ≤ 0.001). Globally, extra‐virgin olive oil revealed fewer formations of unsaturated aldehydes, including toxic ones, and correlated with lower amounts of degradation indicators, as polar compounds (r² = 0.998, p ≤ 0.001) and p‐anisidine value (r² = 0.991, p ≤ 0.001). Despite the similarities in total unsaturation degree between canola and peanut oils, the former presented lower amount of volatiles, including E,E‐2,4‐decadienal and acrolein, the more toxic ones. These results highlight for the pertinence of volatile analyses to evaluate and compare oil degradation under thermal and oxidative stress, while complementing other degradation indicators. Additionally, the optimized methodology allows a direct comparison of different oil matrices, supporting further developments into more general methods for volatiles quantification, enabling more efficient comparison of results between research teams.     

Chemical reactions involved in the deep fat frying of foods. I. A laboratory apparatus for frying under simulated restaurant conditions

A laboratory apparatus has been designed which can be used to quantitatively collect the volatile decomposition products produced during deep fat frying under simulated restaurant conditions. In order to study the chemical reactions of frying fat without any inter-reaction with the food fried, moist cotton balls were fried in corn oil. The oil used for frying was shown to differ considerably from oil which was continuously heated. The latter had a darker color and higher viscosity. It foamed significantly while the oil used for frying did not. Furthermore, the continuously heated oil had a much lower free fatty acid content than did the oil used for frying under simulated restaurant conditions. The volatile decomposition products collected during frying of cotton balls in corn oil were separated into acidic and nonacidic compounds. Each group exhibited a definite gas chromatographic pattern after only a short period of frying. Part of the degradation products, particularly those of higher boiling points, were found to remain in the frying oil. Paper of the Journal Series, New Jersey Agricultural Experiment Station, Rutgers. The State University of New Jersey, Department of Food Science, New Brunswick.     

An instrumental method for measuring the degree of reversion and rancidity of edible oils

Summary The amount of volatile carbonyl compounds diffused from 100 g. of an oil into a stream of nitrogen bubbled through the oil under specified conditions was determined by converting the carbonyl compounds into their 2,4-dinitrophenylhydrazones and then measuring the absorption of the wine-red color of the quinoidal ions at 480 mμ. From the absorbance thus obtained, a carbonyl index was calculated and assigned to the oil. The carbonyl indices of a number of edible oils, such as soybean, cottonseed, and hydrogenated vegetable, were found to correlate with the degree of reversion and rancidity of the oils as determined by organoleptic means. The reproducibility of the carbonyl index determination was 3%. This accuracy corresponded to approximately ±0.1 point in an organoleptic testing panel in which 1 point represented a very unacceptable oil and 10 points a very good oil. The carbonyl index method may also be used as a means of evaluating the flavor stability of edible oils. The carbonyl index gave a good indication of the flavor stability after aging at 60°C. for the less stable and 100°C. for the more stable oils and fats. Funds for this study were furnished by a grant-in-aid from Swift and Company, Chicago, Ill. Presented at the American Oil Chemists’ Society meeting, Minneapolis, Minn., Oct. 11–14, 1954.     

Über den Einfluß des Frittierens auf die chemische Zusammensetzung von Fisch-Frittierfetten

Effect of Frying on the Chemical Composition of Fats for Fish Frying If fats are heated upto 180° C in commercial frying plants with or without the material to be fried, corresponding chemical changes, depending on the period of heating occur. These changes are investigated by the methods commonly employed in food industry. Soybean oil and partially hydrogenated peanut oil were employed in these investigations. These two frying fats are very different as regards the degree of saturation of their fatty acids. Although these fats were subjected to more severe conditions than usually practiced in fish restaurants no detrimental physiological properties were found in both the fats after an observation period of two years.     

Quantitation and distribution of altered fatty acids in frying fats

The distribution and quantity of polar compounds and altered fatty acids in used frying oils, collected by Food Inspection Services of the Junta de Andalucía in Spain, was measured. Additional samples evaluated were sunflower oil, high-oleic sunflower oil, and palm olein that were subjected to thermoxidation and frying in laboratory experiments. A combination of adsorption and high-performance size-exclusion chromatography was applied to the oil samples both before and after transesterification. Through analysis of fatty acid methyl ester derivatives, differentiation of four groups of altered fatty acids (oxidized monomers, nonpolar dimers, oxidized dimers, and polymers) could be attained. Evaluation of real frying samples with polar compound levels around the limit for fat rejection (21.1–27.6% polar compounds) gave values of total altered fatty acids ranging from 8.1 to 11.3%, and levels higher than 20% were found in the most degradated samples. The results obtained clearly support the need for control and improvement of the quality of used fats in fried-food outlets.     

Oxidative Stability and Changes in Chemical Composition of Extra Virgin Olive Oils After Short‐Term Deep‐Frying of French Fries

We aimed at investigating oxidative stability and changes in fatty acid and tocopherol composition of extra virgin olive oil (EVOO) in comparison with refined seed oils during short‐term deep‐frying of French fries, and changes in the composition of the French fries deep‐fried in EVOO. EVOO samples from Spain, Brazil, and Portugal, and refined seed oils of soybean and sunflower were studied. Oil samples were used for deep‐frying of French fries at 180 °C, for up to 75 min of successive frying. Tocopherol and fatty acid composition were determined in fresh and spent vegetable oils. Tocopherol, fatty acid, and volatile composition (by SPME–GC–MS) were also determined in French fries deep‐fried in EVOO. Oil oxidation was monitored by peroxide, acid, and p‐anisidine values, and by Rancimat after deep‐frying. Differential scanning calorimetry (DSC) analysis was used as a proxy of the quality of the spent oils. EVOOs presented the lowest degree of oleic and linoleic acids losses, low formation of free fatty acids and carbonyl compounds, and were highly stable after deep‐frying. In addition, oleic acid, tocopherols, and flavor compounds were transferred from EVOO into the French fries. In conclusion, EVOOs were more stable than refined seed oils during short‐term deep‐frying of French fries and also contributed to enhance the nutritional value, and possibly improve the flavor, of the fries prepared in EVOO.     

n-deca-2,4-dienal,its origin from linoleate and flavor significance in fats

Summary n-Deca-2,4-dienal was identified as a major component of the carbonyl compounds in deodorization distillates from cottonseed oil, soybean oil, beef tallow, and lard. This compound also was demonstrated as a principal carbonyl component in the heat decomposition of methyl linoleate in the presence of moisture. n-Hept-2-enal and C₁₀ and C₉ 2-enals also were detected in the latter system. The flavor threshold of decadienal in water was found to be approximately 0.5 parts per billion. Flavor and odor qualities of the dienal perhaps are described best by the term “deepfried.” The compound's significance in fried and baked foods and its performance in the Kreis and 2-thiobarbituric acid tests for fat oxidation are discussed. Authorized for publication as Paper No. 2335 on January 5, 1959, in the Journal Series of the Pennsylvania Agricultural Experiment Station. Supported in part by the U. S. Public Health Service (H3632).     

Volatile compounds in oils after deep frying or stir frying and subsequent storage

Soybean oil (900 g) was heated by deep frying at 200°C for 1 h with the addition of 0, 50, 100, 150 and 200 mL water, and then stored at 55°C for 26 weeks. Soybean oil, corn oil and lard were heated by stir frying and then stored at 55°C for 30 weeks. The volatiles and peroxide values of these samples were monitored. All samples contained aldehydes as major volatiles. During heating and storage, total volatiles increased 260-1100-fold. However, aldehyde content decreased from 62–87% to 47–67%, while volatile acid content increased from 1–6% to 12–33%; especially hexanoic acid which increased to 26–350 ppm in the oils after the storage period was completed. Water addition to the oils heated by deep frying tended to retard the formation of volatile compounds. The total amount of volatile constituents of lard heated by stir frying increased more during storage than that of corn oil or soybean oil. Peroxide values did not reflect the changes of volatile content in the samples.     

Carbonyl compounds vaporize from oil with steam during deep-frying

Used frying oil recovered from food manufacturing companies in Japan and recycled often shows lower carbonyl (COV) and peroxide values (POV) than oil simply heated at 180 degrees C for 20 h does. In this study the reasons for the low COV of oil used for deep-frying were investigated by employing model experiments and actual commercial frying. The results suggested that in actual frying, the factor most influencing the low COV was vaporization of carbonyl compounds, together with steam generated from water contained in frying foodstuffs. It was also suggested that the low levels of COV were attributable partly to inhibition by protein, amino acids exuded from frying foodstuffs, and starch, and slightly to the effects of natural antioxidants in fresh oil and frying foodstuffs, oil absorption by frying foodstuffs, and dilution of oil in use by fresh oil added between uses. On the other hand, the chemical properties of oil in a fryer and in batter coatings of deep-fried foods made with the former oil were checked. Content of polar compounds (PC) and color score were obviously worse in the oil extracted from batter coatings than in that in the fryer, but there were no differences in COV or content of triacylglycerol (TG) of the two oils.     

Antioxidative properties of phenolic acids and interaction with endogenous minor components during frying

The ability of selected phenolic acids to improve the frying performance of canola oil was evaluated in a frying test. The frying performance of the oil was assessed by analysis of total polar components (TPC), level of 4‐hydroxynonenal (HNE), and the rate of formation of volatile carbonyl compounds (VCC). All the tested phenolic acids; ferulic acid (FA), caffeic acid (CA), dihydrocaffeic acid (HCA), gallic acid (GA), and vanillic acid (VA) significantly increased the frying performance of canola oil triacylglycerols (CTG). At the end of the frying test, the amount of TPC in CTG was 22.9 ± 1.0% compared to a maximum of 18.8 ± 0.8% in CTG fortified with the phenolic acids. Similarly, the level of HNE was reduced by up to 45% when it was supplemented with phenolic acids. The results showed that ethyl ferulate (EF) was a better antioxidant than FA under frying conditions; HCA offered a slightly better protection than CA; and the cinnamic acid derivative, FA was better than VA, its benzoic acid analogue. A significant synergy was observed between phenolic acids and the sterol fraction isolated from canola oil. The observed synergy was attributed to the possible formation of steryl phenolates during the frying test. Practical applications: The poor thermal stability of polyunsaturated oils limits their application for prolonged frying. PUFA offer important health benefits and can improve nutritional value of fried foods. Contrary to the commonly applied synthetic antioxidants, the phenolic acids tested in this study often are part of endogenous oil components present in oilseeds and also in some oils, and are known for their positive health benefits. Thus, the simple phenolic acids, especially the cinnamic acid derivatives may be applied as potent antioxidants to protect oils during thermal processes used for food production.