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.     

What to Do with Spent Bleaching Earth? A Review

Animal fats and cold-pressed vegetable oils hardly need bleaching but when vegetable oils started to be produced by solvent extraction, the resulting oils were too dark to sell as such. That also holds for palm oil. Bleaching these oils with bleaching clay solved their color problems but led to the problem of spent earth disposal. Many ways of treating spent earth and ways of disposal have been suggested and/or developed in the past. They will be reviewed in this paper. Given the recent observation that slurrying spent bleaching earth with crude oil inhibits the deterioration of the residual oil in the earth, leads to the conclusion that, for an integrated oil mill-cum-refinery, the best way of spent bleaching earth disposal is the recycling of the earth to the solvent extractor, whereas stand-alone refineries are advised to sell their spent earth to chicken feed manufacturers. In future, a high-temperature hydrolysis process that can treat all kinds of fatty waste may become an attractive means of disposal as well.     

Palm olein quality parameter changes during industrial production of potato chips

New legislation introduced in South Africa for the quality of used frying oils has resulted in the need to identify quicker, more suitable methods that correlate well with results from two official methods, namely, total polymerized glycerides and total polar components. Oil and product samples were taken at regular intervals during a commercial frying process in palm olein. Oil samples were analyzed for a number of different quality parameters viz. tocopherol content, dielectric constant, total polymerized glycerides, total polar components, tertiary butylhydroquinone (TBHQ) content, anisidine value, Rancimat induction period, and free fatty acid content, and the results statistically compared to results from official methods. Oil was expressed from product stored under accelerated conditions and analyzed for the same quality parameters. Fried product was also subjected to sensory evaluation to measure the degree of oil deterioration and sensory preference. The frying trial was successfully executed with refined, bleached, and deodorized palm olein and the frying oil used to a free fatty acid (FFA) content of 0.41%. Oil and product sampling were done at different FFA value levels. Frying oil quality was verified at the onset of the trial and at regular intervals. The frying oil total polar component value increased to approximately half of the limit set by the official regulation. This point was reached mainly due to the high starting value of the fresh oil. Frying oil total polymerized glycerides increased from below 1% to 2.1%. This increase is negligible when compared to the general trend for polyunsaturated oils. The alternative laboratory methods used for predicting oil quality can be rated as follows: total tocopherol content >dielectric constant >FFA >TBHQ content >anisidine value >Rancimat induction period. The first three methods correlated well with total polar component levels and it is recommended that the dielectric constant and FFA measurements be applied for monitoring oil condition during frying. It is possible that viscosity changes could be used for the monitoring of polyunsaturated frying oils. Evaluation of oil extracted from product revealed a negligible effect of non-oil components on oil quality parameters. The same was observed when product was stored at −10°C and at 37°C.     

Untersuchungen zur Entölung von Bleicherde mit überkritischem Kohlendioxid

Investigations on De-oiling Bleaching Clay by Supercritical Carbon-Dioxide Refining edible fats and oils, an adsorbent accrues during the bleaching process, which contains oil up to 40% by weight. As disposal of this contaminated material is problematic, it has more and more been taken into consideration to deoil and to recycle the bleaching clay. Up to now, the methods of deposition, extraction with hexane, and scorching have been applied. Our investigations to decontaminate the bleaching clay were carried out by means of high pressure extraction using carbon dioxide as a solvent. For this purpose, bleaching clays from the refinement of rape seed oil and palm oil were investigated. During the CO₂-extraction of the earth, pressure and temperature have been varied and, during the separation process, the number of separation steps in addition. Furthermore the CO₂-mass flow has been altered. Besides of deoiling the bleaching earth as far as possible, the experiments aimed to recover the oil in good quality and to restore an operational bleaching clay. The results of the kinetic investigations revealed that the bleaching clays react differently to variation of the extraction parameters. While it is possible to determine an optimal operating point of the experimental set up for the bleaching clay of palm oil, it is not for the bleaching clay of rape seed oil. The results of the analysis of the extracted oils, palm oil and rape seed oil, are comparably good. The CO₂ extraction delivers a selectively extracted oil. On the contrary, the residual activity of the bleaching clays is a different one, i.e. at most 50% of the activity of fresh bleaching clay.     

Decolorisation of crude palm oil by acid-activated spent bleaching earth

Regeneration of spent bleaching earth by acid activation and heat treatment has been investigated. Spent bleaching earth was activated by H₂SO₄ of various concentrations (1–40%) and heat treated at 120°C–350°C. The experimental results indicate that treatment of spent bleaching earth with 10% H₂SO₄ at 350°C produced a material which was most effective in removing coloured pigments from crude palm oil. Subsequent experiments were conducted using this particular acid-treated spent bleaching earth. Various parameters which affect the sorption process were studied. They include initial crude palm oil concentration, sorbent dosage and temperature. Applicability of both the Freundlich and Langmuir isotherms to the acid-treated spent bleaching earth–palm oil hexane miscella system indicates that both physiosorption and chemisorption were involved in the sorption process. Measurements of various quality parameters of bleached and crude palm oils were carried out. They include Lovibond Colour index, carotene content, peroxide value, free fatty acid, fatty acid composition and iodine value. The results show that the bleached palm oil retained good oil quality after the decolorisation process using 10% acid-treated spent bleaching earth with a Lovibond Colour of 6·4. © 1998 SCI     

Formation of Polar Compounds During Deep‐frying—Determination by 1H NMR and ESR

The present study aims to elucidate the factors influencing the generation of polar compounds in oils during deep‐frying. Oils with different fatty acid compositions, including palm oil (PO), refined palm kernel oil (RPKO), and refined coconut oil (RCO), are applied in successive frying processes. ¹H NMR spectra reveal that heated PO has a higher percentage of allyl acyl group and is more prone to formation of non‐polar dimeric triglycerides as compared to other types of oils. In addition, electron spin resonance (ESR) spectra indicate that alkyl radicals are more predominant than alkoxy radicals in heated PO. In contrast, RPKO and RCO are inclined to generation of alkoxy radicals during the thermal treatment. The results reveal that oils with high unsaturated fatty acid content are more prone to generation and oxidation of non‐polar dimeric triglycerides. Practical Applications: The change in free radical profile and concentration is one of the indicators of lipid oxidation and polymerization. Alterations in the levels of alkyl and alkoxyl radicals, revealed by ESR, can be used to illustrate the formation of polar compounds in deep‐fried oils with different fatty acid compositions. The percentage of allyl acyl group, revealed by ¹H NMR, can be used to predict the generation of polar compounds. Therefore, this study provides useful information for the development of different methods to reduce polar compound formation in oils during thermal processing depending on the fatty acid composition of different deep‐fried oils.     

Quality assessment of industrial prefried french fries

An industrial production of prefried french fries using palm oil as a frying medium was studied over a period of 12 days. Samples of oils and french fries were withdrawn once a day. The quality of both the oil and the french fries was assessed using two types of tests. Some tests, such as the determination of free fatty acid (FFA) and the determination of thiobarbituric acid value (TBA), oxifritest and Food Oil Sensor correspond to what was used by a quality control laboratory. More elaborate techniques such as the determination of polar components, polymers and cyclic fatty acid monomers (CFAM) were also used. Only small increases of FFA, TBA, polar components and polymers were observed. However, in the case of palm oil, which contains a high percentage of diglycerides, it is more reliable to determine the quality of the oil using the amount of polymers instead of polar components which may include some diglycerides. Thus a high “polar components” value (up to 20–25%) would not necessarily reflect an altered sample. The maximum amount of CFAM detected was 0.1% and they did not seem to be preferentially adsorbed on the french fries. These results, along with the sensory evaluations, showed that the french fries obtained in these production conditions were of good quality as far as the fat was concerned.     

Evaluation of physicochemical changes in cooking oil during heating

Rice bran oil and double fractionated palm olein (DF palm olein) were heated at 180 C for 50 hr to measure lipid deterioration in the oils. Free fatty acid content of both oils increased during heating; however, iodine value and smoke point decreased. Solid fat contents of both oils were unaffected by heating time. Cloud point of rice bran oil was much lower than that of palm olein. Color of oils changed gradually to dark brown from light yellow with increased heating time. Absolute content of polyunsaturated fatty acid, such as linoleic acid, reduced more than that of monounsaturated fatty acid, such as oleic acid, in both oils. In both oils, iodine value correlated very well with linoleic acid content, with correlation coefficient higher than 0.96.     

Silica refining of palm oil

The amount of bleaching earth required in the physical refining process of palm oil depends on the activity of the earth, quality of the oil and final color specification of the refined products. The use of silica (Trisyl) in combination with bleaching clay in palm oil refining has been investigated. The optimum conditions required for Trisyl and bleaching clay are 95–105°C for a period of 30–40 min. Improvements in color performance for palm oil products are noted with the addition of small quantities of Trisyl (0.06–0.24%) to the bleaching clay. Addition of 0.12% Trisyl to 0.4% bleaching clay improved the color of the refined oil by as much as 1.7 Red Lovibond units. Lower phosphorus levels (18.4 and 16.9 ppm) were obtained in the refined oils with an addition of 0.12 and 0.24% Trisyl, respectively, as compared to a level of 36.2 ppm of phosphorus when no silica was added to the earth. Better color stability was also obtained with oils treated with Trisyl. An additional advantage was the reduction in filtration time, leading to possible higher throughput in refining.     

应对月桂油的短缺

随着油脂化工生产的扩张，价格和基本原材料的获得是其成功的关键。月桂油（棕榈仁油和椰子油）对于油脂化学品的需求而言，在价格方面是最敏感的油品。目前月桂油主要用于食品方面，因此，油脂化工需求的增加必然导致月桂油的供应面临困难。月桂油的供应取决于椰子油和棕榈仁油的产量。油棕的产油率比椰棕高。很明显，将来月桂油供应主要取决于棕榈仁油生产的增加，这是棕榈油产量提高的结果。因此，为了提供充足的月桂油以满足油脂化学工业的需要，其用于食用方面的量将不得不减少。     

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

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

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.     

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.     

The Effect of Type of Oil and Degree of Degradation on Glycidyl Esters Content During the Frying of French Fries

The aim of the study was to determine the effect of oil degradation on the content of glycidyl esters (GEs) in oils used for the frying of French fries. As frying media, refined oils such as rapeseed, palm, palm olein and blend were used. French fries were fried for 40 h in oils heated to 180 °C in 30‐min cycles. After every 8 h of frying, fresh oil and samples were analyzed for acid and anisidine values, color, refractive index, fatty acid composition, and content and composition of the polar fraction. GEs were determined by LC–MS. Hydrolysis and polymerization occurred most intensively in palm olein, while oxidation was reported for rapeseed oil. The degradation of oil caused increased changes in the RI of frying oils. Losses of mono‐ and polyunsaturated fatty acids were observed in all samples, with the largest share in blend. The highest content of GE found in fresh oil was in palm olein (25 mg kg^?1) and the lowest content of GE was found in rapeseed oil (0.8 mg kg^?1). The palm oil, palm olein and blend were dominated by GEs of palmitic and oleic acids, while rapeseed oil was dominated by GE of oleic acid. With increasing frying time, the content of GEs decreased with losses from 47 % in rapeseed oil to 78 % in palm oil after finishing frying.     

Purification of Waste Cooking Oils via Supercritical Carbon Dioxide Extraction

Purification of waste cooking oils (palm oil and soybean oil) using supercritical carbon dioxide (scCO₂) extraction has been investigated. The purified oils were characterized by their acid value, conjugated diene value, total polar compound measurements, and high-performance size exclusion chromatography. Using optimal extractions conditions of 353.15 K, 20 MPa, and CO₂ flow rate of 40 g/min, 80% of the oil was recovered and the purified oil compositions and properties were very close to those of the fresh oils. At higher pressures or lower temperatures, the separation efficiency of the scCO₂ extraction was significantly reduced.     

Triterpene alcohols and sterols of vegetable oils

Triterpene alcohols and sterols were separated by thin-layer chromatography and gas-liquid chromatography from the unsaponifiable fractions of the following 18 vegetable oils: linseed, peanut, olive, rice bran, palm kernel, corn, sesame, oiticica, palm, coconut, rapeseed, grape seed, sunflower, poppy seed, castor, tea seed, cocoa butter and soybean. Two triterpene alcohols, cycloartenol and 24-methylene cycloartanol, were found in all of the oils except soybean oil, which contained only cycloartenol. Triterpene alcohols such as α- and β-amyrin, euphorbol, butyrospermol and cyclolaudenol also were encountered occasionally. Three sterols, β-sitosterol, stigmasterol and campesterol were present in all of the oils. In addition a fourth sterol, not yet idenfified, was found in oils of palm, palm kernel and sunflower in varying amounts. This unknown sterol and brassicasterol were found in rapeseed oil in addition to the three sterols that were common to all of the oils studied. Experiment Station for Fats and Oils, National Center for Lipochemistry of National Research Council, Milan, Italy.     

Comparative study of physical methods for lipid oxidation measurement in oils

For the determination of lipid oxidation in oils, electrical, optical, spectroscopic and extraction methods were investigated using rice bran oil and doubly-fractionated palm olein as model systems. The oxidized polar components and dielectric constant of rice bran oil increased very similarly with those of double-fractionated palm olein. In the case of rice bran oil, all of the test methods were shown to possess good statistical correlations. Polar components, dielectric constant, refractive index and polymer content showed relatively better correlations. It was shown that diene and triene content determined by spectroscopic methods was not suitable for more saturated oils such as palm olein.     

Utilization of high‐oleic rapeseed oil for deep‐fat frying of French fries compared to other commonly used edible oils

Changes in chemical, physical and sensory parameters of high‐oleic rapeseed oil (HORO) (NATREON?) during 72 h of deep‐fat frying of potatoes were compared with those of commonly used frying oils, palm olein (PO), high‐oleic sunflower oil (HOSO) and partially hydrogenated rapeseed oil (PHRO). In addition to the sensory evaluation of the oils and the potatoes, the content of polar compounds, oligomer triacylglycerols and free fatty acids, the oxidative stability by Rancimat, the smoke point and the anisidine value were determined. French fries obtained with HORO, PO and HOSO were still suitable for human consumption after 66 h of deep‐fat frying, while French fries fried in PHRO were inedible after 30 h. During the frying period, none of the oils exceeded the limit for the amount of polar compounds, oligomer triacylglycerols and free fatty acids recommended by the German Society of Fat Science (DGF) as criteria for rejection of used frying oils. After 72 h, the smoke point of all oils was below 150 °C, and the amount of tocopherols was reduced to 5 mg/100 g for PHRO and 15 mg/100 g for HORO and HOSO. Remarkable was the decrease of the oxidative stability of HOSO measured by Rancimat. During frying, the oxidative stability of this oil was reduced from 32 h for the fresh oil to below 1 h after 72 h of frying. Only HORO showed still an oxidative stability of more than 2 h. From the results, it can be concluded that the use of HORO for deep‐fat frying is comparable to other commonly used oils.     

Activated Kaolin Minerals as Bleaching Clays for Prolonging the Useful Life of Palm Oil in Industrial Frying Operations

During frying operations, vegetable oils break down and compounds with undesirable flavors are produced. Various procedures have been developed to extend the useful life of frying oils, including treatment with bleaching clays. In this article, we describe the activation of kaolin minerals by a combination of grinding and chemical treatments, and report their performance in removing breakdown products generated in palm oil that had been used for 20 hours continuous frying. There was little influence of the original kaolin mineral form on the ability to reduce the free fatty acid (FFA) contents, and grinding only changed FFA reduction from ∼32% to ∼36%. However, much greater FFA reductions were obtained after chemical treatment of the ground clays, and the best performing kaolin product gave similar FFA reduction to a commercial bleaching clay (∼76% and ∼77%, respectively). This activated kaolin also produced a reduction in viscosity at 40 °C from ∼73 to 48.4 cSt (compared to 45.5 cSt in the unused oil), and in the peroxide value from 30.0 to 22.0 meq/kg (compared to 10.0 meq/kg in the initial oil). Thus, activated kaolin samples represent a cheap and convenient alternative to conventional bleaching clay for improving common quality parameters in used palm oil, although we also found that the optimum kaolin preparation conditions were different from those that have been reported for raw rice bran oil refining.