Feasibility of recycling used frying oil using membrane process

A study was carried out to improve the quality of used frying oils and to assess the feasibility of recycling using a membrane process. Experiments were conducted with used frying oils in a flat membrane batch cell using polymeric membranes. The total polar materials and oxidation products, which normally lead to the deterioration of frying oils, were reduced by 32 to 42% and 14 to 48% during NTGS‐2200 membrane processing. Color and viscosity of the used frying oil, which are the main criteria for discontinuing use in many restaurants and homes, were improved to the extent of fresh oil. The membrane processing was effective for improving the overall quality. However, reductions of phosphatides and color compounds, presumably melanoidines, may be responsible for a deterioration of oil stability. The permeate flux values were in the range of 63 to 121 g/(m² h). The permeate flux was found to be inversely proportional to the viscosity of the oil. The stability of the processed oil as well as the permeate flux must be improved for commercial application.     

Frying Quality Characteristics of French Fries Prepared in Refined Olive Oil and Palm Olein

The objective of this study was to compare two oils with different polyunsaturated/saturated (P/S) fatty acid ratios, refined olive oil (P/S 0.75) and palm olein (P/S 0.25), in frying French fries. The chemical qualities of the oil residues extracted from the French fries were assayed for five consecutive batches fried at 1-h intervals. The levels of total polar compounds, free fatty acids, p-anisidine value and phytosterol oxidation products (POPs) were elevated in French fries fried in both oils. The level of total polar compounds increased from 4.6 in fresh refined olive oil to 7.3% in final batches of French fries. The corresponding figures for palm olein were 9.8–13.8%. The level of free fatty acid in fresh refined olive oil increased from 0.06 to 0.11% in final products. These figures for palm olein were 0.04–0.13%. The p-anisidine value increased from 3.7 to 32.8 and 2.5 to 53.4 in fresh oils and in final batches of French fries in refined olive oil and palm olein, respectively. The total amount of POPs in fresh refined olive oil increased from 5.1 to 9.6 μg/g oil in final products. These figures were 1.9 to 5.3 μg/g oil for palm olein.     

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.     

Rejection of carotenoids in oil systems by a nonporous polymeric composite membrane

Ealier studies by the authors on crude vegetable oils showed that color compounds are reduced to the extent of 74–80% during membrane processing. In the present study, attempts were made to understand the rejection mechanism of carotenoids using real and model oil systems. In case of model systems consisting of refined high-oleic sunflower (HOSF) oil, lecithin and β-carotene, the rejection of carotenes was low (11–20%). This could be explained based on the differences between the model and real systems as well as on the solution-diffusion mechanism controlling the material transport across the membrane. This study revealed that β-carotene did not have affinity for the phospholipid reverse micells present in the oil. Xanthophylls, the major carotenoids present in the crude soybean oil, were rejected to the extent of 60% in the model system (HOSF oil and lutein). High-performance liquid chromatographic analysis of crude oil revealed that there were few other xanthophylls, which are more polar than lutein. These unidentified xanthophylls would be rejected to a much greater extent by the hydrophobic membrane. The extent of color reduction would depend on the actual composition of xanthophylls present in the crude oil and their relative polarity.     

Surface modification of polyethersulfone membrane to improve its hydrophobic characteristics for waste frying oil filtration: Radio frequency plasma treatment

In this study, polyethersulfone (PES) membrane was subjected to surface modification using hexamethyldisiloxane (HMDSO) and 1,1,1,3,3,3‐hexafluoro‐2‐propanol ( HFIP) in radio frequency (RF) plasma system to improve its hydrophobic property for recovery of waste frying oil. Structural and morphological changes on the membrane surfaces were characterized by contact angle measurements, Fourier transform infrared spectroscopy‐attenuated total reflection (FTIR‐ATR) and atomic force microscopy (AFM). Permeate fluxes, physical and chemical properties of waste frying oil and waste frying oil–methanol micella (oil/methanol 1/3, 1/1, 1/3, v/v) after filtration through unmodified and modified PES membranes were investigated. The results showed that PES membranes modified with HMDSO at 75 W for 5 min (discharge power–exposure time) selectively rejected total polar compounds (TPC) and free fatty acids (FFAs) of waste frying oil to the extent of 46.9–48% and 35.3–40%, respectively. Furthermore, the viscosity of waste frying oil was reduced to the extent of 9.4–12.8%. RF‐plasma‐treated PES membranes appeared to improve the regeneration of waste frying oils and allow these oils to be used for either repeated frying operations or biodiesel production. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123:3402–3411, 2012     

Prooxidant Activity of Polar Lipid Oxidation Products in Bulk Oil and Oil-in-Water Emulsion

Lipid oxidation products can arise when oils are subjected to high temperature and exposed to oxygen. Many of these oxidation products have higher polarity than the original triacylglycerols due to the incorporation of oxygen. These polar oxidation products could have a negative impact on oxidative stability by acting as prooxidants. In this study, the influence of polar lipid oxidation products on the oxidative stability of bulk oils and oil-in-water emulsions was investigated. Polar compounds were isolated from used frying oil by silica gel column chromatography. They were added to bulk stripped corn oil (with/without reverse micelles formed by dioleoylphosphatidylcholine, DOPC) and oil-in-water (O/W) emulsion to evaluate their prooxidative activity. Polar compounds increased lipid oxidation in bulk oil with and without DOPC. The presence of DOPC reverse micelles decreased the prooxidant activity of the polar oxidation products. On the other hand, there was no significant effect of the polar compounds on oxidation of O/W emulsions. To gain a better understanding of the polar compounds responsible for the prooxidant effect, linoleic acid and linoleic hydroperoxide were added into bulk oil at the same concentration as those in the polar fraction of the frying oil. However, they did not show the same prooxidative activity compared to oil with the polar fraction.     

High Performance Liquid Chromatography–Size Exclusion Chromatography for Rapid Analysis of Total Polar Compounds in Used Frying Oils

Analysis of used frying oil samples by high performance liquid chromatography–size exclusion chromatography (HPLC–SEC or HPSEC) was compared to AOCS Official Method Cd 20-91 (silica gel column chromatography) for the purpose of developing a rapid analysis of total polar compounds (TPC). In a direct comparison of the two analytical methods using four different sets of used frying oils (21 total oil samples) ranging from fresh to discard quality (4.3 to 35.4% TPC by column chromatography), the weight percent total polar compounds (%TPC) determined by HPLC–SEC averaged 0.71% higher than the values by silica gel column chromatography. Reproducibility of the HPLC–SEC method of s _r = 0.30 and RSD_r% = 1.22 compares to the variability of s _r = 0.29 and RSD_r = 1.3 for samples of approximately the same %TPC, reported in AOCS Method Cd 20-91. Because the rapid method does not separate pure (non-polar) triacylglycerol (TAG) and polar, oxidized TAG (OX-TAG), a high concentration of OX-TAG will quantitatively affect the results. This places practical limits on the types of studies to which the method may be applied if a separate analysis for the OX-TAG is not performed. Advantages of the HPLC–SEC method include the following. It uses about 75% less solvent than standard column chromatography methods for determination of %TPC. This HPLC–SEC method is very similar to AOCS Official Method Cd 22-91, and thus, also separates and quantifies polymerized triacylglycerols. The HPLC–SEC method determines both TAG polymer concentration and %TPC of used frying oils in about 1 h.     

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.     

Physicochemical and Stability Characteristics of Flaxseed Oils During Pan-heating

Flaxseed oils are used in stir-frying in parts of China. In this study, flaxseed oils were heated at approximately 150 °C as a thin film in a frying pan for 3 and 6 min, respectively. Pan-heating caused loss of tocopherols, plastochromanol-8, phenolic acids and chlorophyll pigments. There was a significant decrease in the linolenic acid resulting in a concomitant relative increase in palmitic, stearic, oleic and linoleic acids in the oils after pan heating. Positive CIELAB “b*” color value, which indicates yellowness and levels of β-carotene and lutein in these oils showed a 42–56% and 8–53% decrease, respectively. Peroxide values, p-anisidine values, percentage of conjugated dienoic acid, specific extinction at 232 and 270 nm and food oil sensor readings of these oils showed significant increases to levels exceeding good oil quality indices. Acid values only showed one to twofold increase from fresh oil values of 0.65–2.23 mg KOH/g of sample. These results indicate that significant levels of oxidation products would be present in flaxseed oils after pan heating. The flaxseed oil with a lower amount of PUFA appeared to be more degraded suggesting that the major factor affecting the oxidative stability of the flaxseed oils during pan-heating was not the degree of unsaturation but was dependent on the complex interaction between the fatty acids and minor constituents in the oils. Presented at the American Oil Chemists’ Society 97th Annual Meeting & Expo, St. Louis, MO, 30 April–3 May, 2006.     

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.     

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.     

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.     

Review of stability measurements for frying oils and fried food flavor

Measurements of degradation in frying oils based on oil physical properties and volatile and nonvolatile decomposition products were reviewed. Rapid methods by means of test kits were also considered. Factors that affect the analysis of total polar components (TPC) in frying oils were examined. Relationships between TPC, free fatty acid (FFA) content, Food Oil Sensor readings (FOS), color change (ΔE), oil fry life and fried-food flavor were evaluated. Flavor scores for codfish, fried in fresh and discarded commercial frying oil blends, were dependent upon individuals in the consumer panel (n=77). Part (n=29) of the panel preferred the flavor of fresh fat; others (n=24) didn't; the rest (n=24) had no preference. FFA, FOS and TPC were analyzed in two soybean oils and in palm olein during a four-day period in which french fries were fried. Flavor score and volatiles of potatoes fried on days 1 and 4 in each oil were also determined. TPC, FFA and FOS significantly increased (P<0.05) in all oils during the frying period. TPC and FFA were highest in the used palm olein, and flavor of potatoes fried in palm olein on day 1 was less desirable than those fried in the soybean oils. Potatoes fried in day-1 oils had significantly higher concentrations (P<0.10) of several pyrazines and aldehydes than those fried in day-4 oils. Presented at the 84th Annual Meeting of the American Oil Chemists' Society, Anaheim, California, April 25–29, 1993.     

Continuous fractionation of used frying oil by supercritical CO<Subscript>2</Subscript>

Fractionation by supercritical carbon dioxide (SC−CO₂) might be a way to purify used frying oils, since a selective separation of the oil components based on their polarity and M.W. can be attained. In this work, we studied the purification of peanut oil used for frying by SC−CO₂ continuous fractionation in a packed column. The influence of pressure (15–35 MPa) and temperature (25–55°C) on the yield and on the composition of products was determined. The composition of the top and bottom products was evaluated by using size-exclusion chromatography and other accepted chemical methods. Process conditions were selected to separate TG from degraded compounds. Experimental results indicated that the operating conditions leading to maximal TG recovery in the extract were 35 MPa, 55°C, and a solvent-to-feed ratio of 53. By operating at these conditions, it was possible to recover 97% of the TG placed on the column and about 52% by weight of the used frying oil. The composition of the purified top stream was very similar to that of fresh frying oil.     

Addition of ferulic acid,ethyl ferulate,and feruloylated monoacyl- and diacylglycerols to salad oils and frying oils

To determine antioxidative effects of ferulic acid and esterified ferulic acids, these compounds were added to soybean oils (SBO), which were evaluated for oxidative stability and frying stability. Additives included feruloylated MAG and DAG (FMG/FDG), ferulic acid, ethyl ferulate, and TBHQ. After frying tests with potato chips, oils were analyzed for retention of additives and polar compounds. Chips were evaluated for hexanal and rancid odor. After 15 h frying, 71% of FMG/FDG was retained, whereas 55% of ethyl ferulate was retained. TBHQ and ferulic acid levels were 6% and <1%, respectively. Frying oils with ethyl ferulate or TBHQ produced significantly less polar compounds than SBO with no additives. Chips fried in SBO with TBHQ or ferulic acid had significantly lower amounts of hexanal and significantly less rancid odor after 8 d at 60°C than other samples. Oils were also aged at 60°C, and stability was analyzed by PV, hexanal, and rancid odor. Oils with TBHQ or FMG/FDG had significantly less peroxides and hexanal, and a lower rancid odor intensity than the control. FMG/FDG inhibited deterioration at 60°C, whereas ethyl ferulate inhibited the formation of polar compounds in frying oil. Ferulic acid acted as an antioxidant in aged fried food. TBHQ inhibited oil degradation at both temperatures. Presented at the 94th AOCS Meeting & Expo, Kansas City, MO, May 4–7, 2003.     

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%.     

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.     

Frying Performance of Canola Oil Triacylglycerides as Affected by Vegetable Oils Minor Components

The endogenous minor components from canola, rice bran, sesame and palm oils including selected phospholipids, and various combinations of tocopherol isomers were tested during frying using canola oil triacylglycerols as the frying medium. Thermo-oxidative degradation was assessed by measurement of the total polar components, the rate of volatile carbonyl compounds and 4-hydroxynonenal formation. All the tested minor components protected to a different extent canola triacylglycerides from thermo-oxidative degradation during frying. No significant differences were observed in the protection of the triacylglycerides among all the tested tocopherol isomers and their mixtures. Irrespective of the composition of tocopherol homologous, an increase in the added amounts above 1,000 μg/g did not improve protection. Minor components isolated from rice bran and sesame oils offered better protection during canola triacylglycerides frying than endogenous minor components isolated from canola oil. When 0.2% phosphatidylcholine or phosphatidylethanolamine was added to the canola triacylglycerides, the amount of formed polar components decreased twice as compared to the tocopherol isomers. Accordingly, by optimizing the composition and the concentration of the endogenous minor components, the frying performance of oil can be significantly enhanced.     

Studies on phytosterol oxides. II: Content in some vegetable oils and in French fries prepared in these oils

Hydrogenated rapeseed oil/palm oil blend, sunflower oil and high-oleic sunflower oil, and French fries fried in these oils were assessed for contents of sterol oxidation products. Different oxidation products of phytosterols (7α- and 7β-hydroxy-sito-and campesterol, 7-ketosito- and 7-ketocampesterol, 5α,6α-epoxy-sito- and campesterol, 5β,6β-epoxy-sito-and campesterol, dihydroxysitosterol and dihydroxycampesterol) were identified and quantiated by gas chromatography (GC) and GC-mass spectroscopy. Rapeseed oil/palm oil blend contained 41 ppm total sterol oxides before frying operations. After two days of frying, this level was increased to 60 ppm. Sunflower oil and high-oleic sunflower oil had 40 and 46 ppm sterol oxides, respectively, before frying operations. After two days of frying operations, these levels increased to 57 and 56 ppm, respectively. In addition to campesterol and sitosterol oxidation products, small amounts of 7α- and 7β-hydroxystigmasterol were detected in the oil samples. Total sterol oxides in the lipids of French fries fried at 200°C in rapeseed oil/palm oil blend, sunflower oil, and high-oleic sunflower oil were 32, 37, and 54 ppm, respectively. The levels of total oxidized sterols, calculated per g sample, ranged from 2.4 to 4.0 ppm. In addition to the content of phytosterol oxides, full scan mass spectra of several oxidation products of stigmasterol are reported for the first time. Part of these results were presented at the 86th Annual Meeting of the AOCS, May 7–11, 1995, San Antonio, TX.