Survey of quality of used frying oils from restaurants

Commercial frying practices and frying conditions at 62 restaurants or fast-food outlets were investigated and the quality of their discarded frying oils was evaluated by several standard laboratory methods: total polar components (TPC), free fatty acids (%FFA), p-anisidine and peroxide values, color, viscosity, C18:2/C16:0, absorbance at 232 and 268 nm, and five quick test methods (Foodoil sensor, Oxifrit (RAU-Test), Fritest, Veri-Fry-TAM 150, and Veri-Fry-FFA 500). Frying techniques varied from primitive traditional practice at traditional shops to modern sophisticated frying procedures at some franchise restaurants. Discarded oils appeared to be heat-damaged to a varying extent according to the degree of quality control applied by the corresponding restaurants. Test methods were shown to possess different statistical correlations. Highly significant correlations were found between TPC and Foodoil sensor (c.f=0.93) and between Oxifrit and Fritest (c.f=0.94), each of which were also correlated relatively well to the TPC. Peroxide value followed by %FFA did not significantly (p<0.05) correlate with the TPC. Significant linear relationships (p⊕0.05) were found between the TPC and each of the other indicators but %FFA and peroxide value.     

Analysis of used frying oils

The analysis of used frying oils to ensure quality of fried foods has become a subject of great concern for health administrations, food processors and consumers, especially since some compounds formed at high temperatures impair the nutritional value of fats and have adverse health effects. At present, three lines of research on the analysis of frying oils stand out: first, development of techniques for the quantitative analysis of the new compounds formed; second, the evaluation of useful rapid methods to be applied in situ for monitoring oil quality in fried food outlets; and third, the application of the most advanced analytical techniques to delve into the structure of individual oxidized compounds present in fried foods.     

Membrane and additional adsorption processes for quality improvement of used frying oils

The main focus of our study was to improve the quality of used frying oils for recycling as edible oils. Experiments were conducted with hexane-diluted, used frying oils in a batch membrane cell using a nonporous polymeric composite membrane for improving permeate (oil) flux. The oil flux increased by 9- to 14-fold as compared to the permeate flux obtained with undiluted oil. Furthermore, the quality of processed oils was comparable with membrane-processed, undiluted oil. The maximal oil flux was obtained when the weight ratio of oil to oil-hexane retentate (feed) was 0.47 in the process stream. Examination of successive fractions of permeate revealed that membrane performance was not affected by variation in the feed quality. Although the membrane process improved the overall quality of used frying oil, it was not effective in reducing FFA and oxidation products. By combining membrane and adsorption processes, the quality of the used frying oil could be improved to the level of fresh frying oil. Silica gel along with a small amount of magnesium oxide gave the best results in the adsorption process. Combined membrane and adsorption processes seem to be the appropriate approach for the complete regeneration of used frying oils.     

Membrane processing of used frying oils

Studies were conducted with used frying oils in a flat membrane batch cell using five different types of polymeric membranes to decrease the soluble degradation products. During membrane processing, triglycerides permeated preferentially compared to the majority of the polar compounds including oxidation products, polymers, and color compounds. Two of the composite membranes, NTGS-AX and NTGS-2200, selectively rejected polar compounds and oxidation products to the extent of 25–48% and 24–44% respectively. The reduction in Lovibond color values (5R+Y) was in the range of 83–93%. The viscosity of the used frying oil was reduced to the extent of 22%. The composite membranes were effective in reducing the soluble impurities, as well as insoluble particulates, without causing any undesirable changes to the oil. The membrane process appears to improve the life of used frying oils and does not have the disadvantages associated with the active filtration systems, however, for commercial application the permeate flux needs to be improved considerably.     

Improving the quality of used frying oils by treatment with activated carbon and silica

This study was undertaken to investigate the effectiveness of activated carbon and silica in removing the degradation products present in used frying oils. Two batches of canola oil were treated with mixtures of activated carbon and silica. Three levels of activated carbon (3%, 6%, 9%, wt/wt) were blended with three levels of a silica compound (2%, 4%, 6%, wt/wt) in all nine possible combinations. Oil 1 was prepared by continuous heating in the lab, and oil 2 was obtained from a small potato-frying operation. For each treatment combination of silica and carbon, the average percent decrease for acid value, peroxide value, photometric color, polar compounds, saturated carbonyls and unsaturated carbonyls was 53.9%, 39.4%, 38.3%, 27.5%, 54.0% and 27.3%, respectively, for oil 1, and 51.1%, 53,5%, 49.7%, 16.3%, 58.9% and 39.0%, respectively, for oil 2. The levels of oleic, linoleic and linolenic acids decreased by 28%, 33% and 35%, respectively, following heat treatment but were shown to increase by 1 to 5% after adsorbent treatment. The yield of treated oil ranged from 56 to 83%.     

Regeneration of used frying oils using adsorption processing

Our study was carried out to improve the quality of used frying oils and to assess the feasibility of recycling by using an adsorption process. Experiments with used frying oils were conducted with a column method using four adsorbents, silica gel, magnesium oxide, activated clay, and aluminum hydroxide gel. Silica gel offered the most effective overall improvement in the quality of used frying oil of the four adsorbents. Although the limiting adsorption capacity of silica gel, as obtained from the Langmuir model, was 219 mg/g for total polar materials (TPM), the experimental maximal adsorption capacity of silica gel for TPM was 337 mg/g. For commercial application, silica gel used in the process must periodically be reactivated. Reactivated silica gel also was effective to some extent in the adsorption process, although the adsorption capacity decreased by 20–50%.     

Regeneration of oils used for deep frying: A comparison of active filter aids

A number of products are promoted for the purpose of regenerating used frying oils. These materials are referred to as “active” filter aids. They are purported to adsorb polar compounds, which are the products of oil degradation, and to retain them for removal by filtration. To evaluate some of these materials, portions of a used oil were treated with each of several “active” filter aids and filtered in a commercial-type recirculating oil filter. The triglycerides and any adsorbed compounds were extracted from the filter cake with a series of increasingly polar organic solvents. The composition of each of the filter cakes was quantitatively determined. The materials tested in this study were diatomaceous earth, acid-activated bleaching earth, activated aluminas, silica, carbon and synthetic magnesium silicate. Significant differences in the adsorbent characteristics of the materials were found. Adsorption of polar oil degradation compounds ranged from 2 mg of polar compounds per gram of diatomaceous earth to about 200 mg/g magnesium silicate.     

A gas chromatographic method for the assessment of used frying oils: Comparison with other methods

The application of a simple and rapid method, based on gas chromatographic measurement of dimer triglycerides, is proposed for evaluation of the quality of used frying oils. The technique involves complete conversion of the oil to its methyl esters followed by analysis on a short column packed with 3% JXR. Parameters are adjusted to provide a pattern in which the dimeric esters emerge as a doublet peak with a retention time of ca. 3 min, whereas all other monomeric esters elute with the solvent peak. The relative heights of the two components of the doublet peak appear to reverse as the frying time is increased. The test correlated well with changes in dielectric constant measurements, as well as with “altered triglycerides” in continuously heated corn oil. Presented at the IFS/AOCS World Congress, New York, April 1980.     

Alkaline cortaminant materials in used frying oils: A new quick test

The AOCS recommended practice Cc 17–79 measures titratable alkalinity (soaps) at the low parts per million level in fresh, alkalirefined vegetable oils. This method indicated that titratable alkalinity is present at levels equivalent to many parts per million of “soap” in used frying oils. A colorimetric quick test was developed to show the presence and semi-quantitative concentration of alkaline contaminant materials (ACM), such as soap, in two types of fresh and used frying oils. The quick test works with fresh and used vegetable and animal/vegetable oil blends where it is not possible to use Cc 17–79.     

Evaluation of SafTestTM methods for monitoring frying oil quality

The feasibility of applying methods developed by Safety Associates, Inc., to monitor oil degradation products, including malondialdehydes (AldeSafe^TM), FFA (FASafe^TM), and peroxides (PeroxySafe^TM), in fresh and heat-abused deep-fat frying oil was evaluated. Based on performance qualification studies, the AldeSafe method was the most suitable SafTest^TM assay for monitoring the quality of frying oil because of its high accuracy, precision, linearity, and reproducibility, and low detection/quantitation limits. A strong correlation (r=0.924) between the AldeSafe method and its counterpart, AOCS Official Method Cd 19-90, also supported the suitability of the SafTest method for monitoring oil quality. Moreover, the FASafe method had a moderately strong relationship with AOCS Official Method Ca 5a-40 (r=0.761). Our studies suggest that this test can be applied for monitoring frying oil; however, certain method performance limitations must be considered for routine analysis purposes. In contrast, the PeroxySafe method probably should not be used to monitor heat-abused oil without further development because of high variability, low accuracy, and low correlation (r=0.062) with the AOCS Official Method Cd 8-53 assay.     

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

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

Determination of optimal conditions for selected adsorbent combinations to recover used frying oils

The treatment of frying oils with adsorbents could practically extend the frying life of oils. Combined synthetic adsorbent treatment of used frying oils was studied the first time. The combinations of four commonly used filter aids: Britesorb (Br), Hubersorb 600 (HB), Frypowder (Fr), and Magnesol (Ma) were evaluated for frying oil recovery. AOCS official methods were used to evaluate their adsorptiveness, including free fatty acids (FFA), conjugated diene value, total polar components, oxidative stability index (OSI), and absorbance at 420 nm. The selected combinations HB+Ma+Fr and HB+Ma+Br exhibited consistent high recovery abilities on various used oil samples. A 3, 3, and 2% HB, Ma, and Fr, respectively, for the first combination (F), and 2, 3, and 2% HB, Ma, and Br, respectively, for the second combination (B) were the most effective. The optimal treatment duration was 6–9 min and 3–6 min for combinations F and B, respectively, which reduced FFA by 82.6–87.6%, absorbance by 26.8–32.6%, and Foodoil Sensor readings by 5.6–8.6%. Addition of antioxidant, such as 50 ppm butylated hydroxytoluene and 50 ppm propyl gallate, increased the OSI value by 48.9–80.8%. Such adsorbent combinations may be used in practical operation to extend frying life of frying oils and improve the healthy aspects of used frying oils.     

Occurrence of cyclic fatty acid monomers in frying oils used for fast foods

Cyclic fatty acid monomers were analyzed by gas chromatography in commercial frying oils, obtained in this country and in the Middle East. Samples were obtained from food outlets in California and Illinois after varying periods of usage. The samples from Egypt and Israel were collected from street vendors frying vegetable patties (known as “fallafel”) in open-air stands. The United States samples ranged from 0.1 to 0.5% cyclic monomers, and from 1 to 8% polar +noneluted thermal oxidation materials. The Middle Eastern samples showed significantly more heat abuse, with values for cyclic monomers from 0.2 to 0.7% and polar materials ranging from 2 to 22%.     

Long storage stability of biodiesel from vegetable and used frying oils

Biodiesel is defined as the mono-alkyl esters of vegetable oils. Production of biodiesel has grown tremendously in European Union in the last years. Though the commercial prospects for biodiesel have also grown, there remains some concern with respect to its resistance to oxidative degradation during storage. Due to the chemical structure of biodiesel the presence of the double bond in the molecule produce a high level of reactivity with the oxygen, especially when it placed in contact with air. Consequently, storage of biodiesel over extended periods may lead to degradation of fuel properties that can compromise fuel quality.This study used samples of biodiesel prepared by the process of transesterification from different vegetable oils: high oleic sunflower oil (HOSO), high and low erucic Brassica carinata oil (HEBO and LEBO) respectively and used frying oil (UFO). These biodiesels, produced from different sources, were used to determine the effects of long storage under different conditions on oxidation stability. Samples were stored in white (exposed) and amber (not exposed) glass containers at room temperature.The study was conducted for a period of 30-months. At regular intervals, samples were taken to measure the following physicochemical quality parameters: acid value (AV), peroxide value (PV), viscosity (ν), iodine value (IV) and insoluble impurities (II). Results showed that AV, PV, ν and II increased, while IV decreased with increasing storage time of biodiesel samples. However, slight differences were found between biodiesel samples exposed and not exposed to daylight before a storage time of 12 months. But after this period the differences were significant.     

Dielectric properties for monitoring the quality of heated oils

Dielectric properties (dielectric constant ε′ and dielectric loss ε″) were compared to conventional methods of analysis (viscosity, refractive index, iodine value, peroxide value and free fatty acids) for evaluating the frying quality of a blend of cottonseed and sunflower oils. The apparent relaxation time, τ, the activation energy H and the entropy change S for dielectric relaxation of the heated oil samples were calculated. Results indicate that dielectric constant and dielectric loss are useful tools for predicting deterioration occurring during heating of the oil.     

Tests to monitor quality of deep‐frying fats and oils

There are dozens of tests an oil chemist has at his disposal for evaluating fresh and used frying oils. These include chemical tests, instrumental methods, physical methods and rapid tests. Not all are applicable to use when evaluating oils used for deep‐fat frying. Rather than run the gamut of all the tests available to the oil chemist or quality control technician, this presentation will focus on the different rapid tests that have been developed for the marketplace. These include physical tests, tests that measure physical parameters of the oil, and chemical tests. Tests may be quite simple or complex. The presenter will provide an overview on the different rapid test methods, but will emphasize that it is up to the researcher or the processor to evaluate for themselves whether the technology is something that they can use in their own operations.     

Evaluation of nonpolar methyl esters by column and gas chromatography for the assessment of used frying olive oils

The measurement of unaltered methyl esters separated from polar methyl esters by column chromatog-raphy was used to evaluate the alteration of an olive oil that had been used 15 times to fry potatoes. Unaltered methyl ester (the nonpolar fraction) decreased significantly (94.9 ± 0.8%vs 98.2 + 0.5%; p < 0.05), while the polar fraction increased significantly (4.0 ± 0.7%vs 2.1 ± 0.7%; p < 0.05) after 15 fryings. The unrecoverable fraction also increased. In order to avoid column contamination the gas Chromatographic analysis was only done on the nonpolar fractions. Linoleic and oleic acids showed a tendency to decrease while saturated fatty acid tended to increase. The unsaturated/saturated fatty acids ratio decreased from an initial value of 7.05 to 6.40 in the last frying. Quantitative gas Chromatographic analysis using both the percentage fatty acid composition and the relative amount of unaltered methyl esters showed a significant oleic acid decrease after 15 fryings (75.8 ± 0.6vs 78.9 ± 0.2 mg/100 mg oil; p < 0.05). To whom correspondence should be addressed     

Effects of selected chemical treatments on quality of fats used for deep frying

Maintaining quality of fats and oils used for deep frying is important in food preparation. In this study, commercially used shortenings were treated with various adsorbents and/or additives with a view to extending their useful life. Quality parameters monitored were increases in dielectric constant, free fatty acids, color (absorbance at 420 nm) and total polar materials. Bleaching clay, charcoal, magnesium oxide and Celite, and their mixtures, effectively reduced one or more of these parameters in used vegetable and animal-vegetable shortenings. However, when the treated fats were used to fry more french fried potatoes in the laboratory, they often deteriorated more rapidly than the untreated control fats. The daily addition of 200 ppm ascorbyl palmitate to fresh, partially hydrogenated soybean oil shortening used to fry french fries retarded free fatty acid development but increased color development and dielectric constant. Treatments employed to extend the useful life of frying fats improved quality parameters, but continued frying after treatment led to greater deterioration than occurred in the untreated control samples. Presented in part at the AOCS Annual Meeting in Philadelphia, PA in May, 1985.     

Characterization of used frying oils. Part 2: Comparison of olestra and triglyceride

In this study, the analytical scheme presented in our previous paper [Gardner, D. R., R. A. Sanders, D. E. Henry, D. H. Tallmadge and H. W. Wharton,J. Am. Oil Chem. Soc. 69:499 (1992)] was used to provide a detailed qualitative comparison between a heated olestra and a heated triglyceride that had been used to fry potatoes. The purpose was to determine if unique components are created in olestra when it is exposed to typical frying conditions. Prior to their analysis, the heated and unheated olestra and triglyceride were converted to their corresponding fatty acid methyl esters (FAMEs) by transesterification. The FAMEs obtained were separated by degree of polarity by means of adsorption chromatography and solid-phase extraction. High-resolution capillary gas and liquid chromatography were used to profile isolated fractions, and detailed comparisons of these profiles were carried out in an effort to disclose components only present in the heated olestra. Spectroscopic data confirmed that by the methods employed, the only detectable qualitative differences between heated triglyceride and heated olestra were attributable to components also observed in unheated olestra. These species are generated during the manufacture of olestra and are not uniquely created by its use as a frying oil. In those chromatographic fractions containing altered fatty acids, no components were observed to be generated at levels ⩾5 ppm upon heating olestra that were not also generated upon heating triglyceride.     

Isolation and detection of alkaline contaminant materials (ACM) in used frying oils

Alkaline contaminant materials are formed in frying oils during cooking. The ACM can be eluted from the same International Union of Pure and Applied Chemists-Association of Official Analytical Chemists (IUPACAOAC) silica gel column used to determine polar materials in frying oils. The ACM are eluted with methanol after “non-polar” and “polar” fractions already have been eluted from the column. A residue of silica gel in the methanol eluate must be insolubilized before the ACM can be identified and quantitated. IR was used to identify sodium oleate as the major constituent of ACM from a set of restaurant generated frying oil samples.