Determination of trace elements in edible vegetable oils by atomic absorption spectrophotometry

Methods are described for the direct determination of Al, Cr, Cu, Fe, Ni and Pb by atomic absorption spectrophotometry in sunflower oil and olive oils. Metallic contamination after storage under different controlled conditions and in contact with carbon steel, austenitic steel, ferritic steel and aluminum was examined. Additionally, chemical characteristics related to quality (acid value, peroxide value, K₂₇₀ and oxidative stability) and some physical parameters of interest were evaluated after storage. When the samples were atomized directly off the tube wall, matrix interferences were not observed in the determination of Cr, Cu, Ni and Pb, whereas such interferences were noted when Al and Fe were determined. The results indicated that the L'Vov platform for Al and Fe determinations eliminates the matrix effects. Contamination with Fe was only detected in the olive oil that had been in contact with carbon steel, Fe concentration increased from 120±12 to 3,520±157 ppb. The physicochemical characteristics were affected only by the storage conditions, regardless of the metal sheets with which they were in contact. Virgin olive oil showed lowered stability after storage in contact with a carbon-steel sheet than when stored in absence of metal.     

Bleaching of edible fats and oils

Almost all fats and oils are subjected to so‐called bleaching during processing. Originally bleaching was only used to reduce the colour. Today, however, the bleaching step is used mainly to remove or convert undesired by‐products to harmless ones from fats and oils. This will guarantee that such compounds do not interfere with the processing and that the requirements for human food are being met.     

Copper in edible oils: Trace amounts determined by atomic absorption spectroscopy

The application of atomic absorption spectroscopy to trace copper analysis in fats was investigated. Copper spectral resonance lines 3248 A and 3274 A possess the most useful sensitivities applicable to fats and vegetable oils. These sensitivities were determined to be 0.15 and 0.25 ppm in oil, respectively, with detection limits about tenfold lower. Data show that copper absorption depends strictly on the amount of oil aspirated into the flame. In soybean oil-solvent mixtures, aspiration rates decrease logarithmically and linearly with increasing fat content. The accuracy and precision of atomic absorption techniques were established by comparing results from atomic absorption with those from two independent techniques. Error estimates were determined by analysis of copper-hydrogenated soybean oil diluted to low copper levels. The standard deviation, relative standard deviation and analytical error were 0.0108 ppm, 5% and 2.5%, respectively, over a range of 0.05–0.40 ppm copper in soybean oil. One of 28 papers presented at the Symposium, “Metal-Catalyzed Lipid Oxidation,” ISF-AOCS World Congress, Chicago, September 1970. Biometrical Services, ARS, USDA, stationed at Northern Laboratory. No. Market. Nutr. Res. Div., ARS, USDA.     

Applied ultraviolet spectrophotometry of fats and oils

Conclusion The chemist has in the spectrophotometric method a rapid and simple means of studying changes in the double bond systems of fatty acids. The method is highly sensitive. It has found application in studies of processing of oils, improvement of drying oils, catalytic hydrogenation, routine analytical work, soap and tallow control, as well as in nutrition studies in which the composition of depot and ingested fats are of interest. It also finds application in strictly academic studies which have as their purpose an extension of our knowledge concerning the composition of naturally occurring fats and oils. In general, it is especially valuable in cases in which thiocyanometric procedures are not sufficiently sensitive or in which conjugated as well as non-conjugated constituents occur. Presented at the 19th annual fall meeting of the American Oil Chemists' Society, Nov. 7–9, 1945, in Chicago     

The purification of edible oils and fats

Originally, oils were not refined but with the introduction of solvent extraction, refining became necessary. Crude cottonseed oil was refined by treating the oil with caustic soda and the same process was used for all other oils that needed refining. The subsequent introduction of centrifugal separators converted the original batch process into a continuous process. Degumming was introduced to obtain lecithin but limited to soya bean oil. Physical refining was introduced for high acidity oils like palm oil after the oil had been degummed to low residual phosphorus levels in the dry degumming process, in which the oil is first of all treated with an acid and then with bleaching earth. In Europe, further degumming processes were developed that allowed seed oil to be physically refined and later phospholipase enzymes were introduced to reduce oil retention by the gums and improve oil yield. Given these various oil purification processes, the refiner must decide which process to use for which oil in which circumstances. The paper provides a survey of what to do and when. It also discusses several topics that require further investigation and development.     

Fractionation and winterization of edible fats and oils

Processes for fractionation and winterization are reviewed. Properties of solid and liquid fractions from various fats and oils are discussed. In view of the current interest in palm oil, the operation and economics of a small palm oil processing plant including fractionation are described.     

Analysis of fats and oils by gas-liquid chromatography and by ultraviolet spectrophotometry

1. Known mixtures of pure fatty acid methyl esters and a number of fats and oils as their methyl esters have been analyzed by conventional GLC with thermal conductivity detectors.
2. Percentage of fatty acid distribution determined by GLC agreed well with known percentages in model mixtures and with analysis by the spectrophotometric method for fats and oils.
3. Determination of very small amounts of arachidonic and pentacnoic acids in lard by GLC was not successful.

     

The solubility of water in edible oils and fats

The solubilities of water in rapeseed oil, coconut oil, and a palm-coconut oil mixture were determined at temperatures of 60C, 80C, and 100C. Oil samples were equilibrated with water vapor under conditions of constant temperature and humidity. The equilibrium water content was determined by means of the Karl Fischer titration method. The solubility was found to be independent of the type of oil when expressed in terms of the mole fraction. An equation relating solubility and temperature is given.     

Estimation of conjugated octadecatrienes in edible fats and oils

Interest in conjugated-diene fatty acids in foods has recently been increased by discovery of their antioxidant and anticarcinogenic properties. Conjugated octadecatrienes (COTs), members of another group of fatty acids, are also present in foods. COTs are formed during the processing of vegetable oils as the result of the dehydration of secondary oxidation products of linoleic acid. Little information is available concerning the occurrence and nutritional properties of COTs in edible oils. Levels of COTs, determined in 27 vegetable oils by ultraviolet (UV) spectroscopy, ranged from not detected (<0.001) to 0.2%. Determination of COTs by gas chromatography of the methyl esters, obtained by transesterification at room temperature with sodium methoxide/methanol, gave lower levels (not detected, 0.051%) than did determination by UV spectroscopy. Methylation with boron trifluoride produced COTs from naturally occurring moieties in the oils and, therefore, is not recommended.     

Spectrophotometric determination of BHA in edible fats and oils

A method is described for the analysis of 2- and 3-tert-butyl-4-hydroxyanisole (BHA) in edible fats and oils. The method is based on measurement of a specific color developed from the reaction of BHA with NN-dimethyl-p-phenylenediamine in the presence of a mild oxidizing agent in alkaline solution. The purple-violet color developed is extracted in carbon tetrachloride, and the absorbance is measured at 550 nm. The presence of other antioxidants also was examined, and the method was applied to various foodstuffs (e.g. olive oil, lard and butter). It gave satisfactory results. The detection limit was 0.4 ppm of BHA.     

Refining and degumming systems for edible fats and oils

Crude edible fats and oils contain variable amounts of nonglyceride impurities, such as free fatty acids, non-fatty materials generally classified as “gums”, and color pigments. Most of these impurities are detrimental to end product fresh and aged quality characteristics, hence must be eliminated by a purification process before the finished fats and oils are suitable for human consumption. The object of this process is to remove these objectionable impurities with the least possible loss of neutral oil and tocopherals. Key theoretical and practical factors for degumming and refining crude edible oils are discussed with particular reference to processes, flow charts, control systems and analytical testing requirements. In addition to typical large volume oils, such as soya and cotton, techniques are also reviewed for smaller volume oils, including palm, lauric and corm.     

Stability of edible oils and fats to fluorescent light irradiation

Butter, butterfat, and corn, coconut, rapeseed, and soybean oils were exposed to 500 ft-c of fluorescent light at varying time-temperature conditions. Oxidation rates were measured by the peroxide values. Vitamin A and β-carotene content of butterfat were estimated. The effect of wavelength on the relative rates of oxidation was determined. The light transmitting properties of the samples at 15 and 30 C over a spectral range of 380–750 nm were measured. It was observed that there was no increase in oxidation rate when the light was switched off. The stability of the oils as shown by the oxidation rates did not correlate well with the ratios of C18:2 to C18:1 or C18:3 to C18:2 nor with the degree of unsaturation. Increase in temperature alone had minimal effect; however, in the presence of light the rate of oxidation increased considerably with a corresponding decrease in the content of Vitamin A and β-carotene. β-Carotene provided strong protective properties. After the photobleaching of β-carotene in butterfat, there was a rapid increase in peroxide values. With coconut oil, the oxidation rate was greater at 15 C than at 30 C due to greater light absorption at 15 C over the entire spectrum. The rate of oxidation decreased at higher wavelengths, and this effect was more pronounced in the vegetable oils than in butterfat, where the β-carotene was considered to serve as a filter for light of low wavelength. Presented at the AOCS meeting, Dallas, April 1975.     

Characterization of semisolid fats and edible oils by Fourier transform infrared photoacoustic spectroscopy

The potential of Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) for the analysis of semisolid fat and edible oil was demonstrated with butter, soybean oil, and lard as representative materials. Results of Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy analysis was compared with FTIR-PAS results. The PAS technique is simple and requires no sample preparation unlike ATR. Optimal PAS instrumental parameters for obtaining quality spectra are a scanning speed of 5 kHz, number of scans of 256 scans/sample, and a resolution of 4 cm⁻¹. The PAS spectra of soybean oil and lard are similar because they have similar functional groups. Results for soybean oil compare well with those available in the literature. The ATR spectra of butter were better than those from its PAS counterpart. Functional groups corresponding to vibration mode and intensity are provided for soybean oil and lard.     

Determination of Fe,Cu, Ni,and Mn in fats and oils by flameless atomic absorption spectroscopy

The determination of Fe, Cu, Ni, and Mn is performed by flameless atomic absorption spectroscopy. The procedure is applicable to oils, including crudes, and also fats such as hydrogenated oils. A solution of the sample is analyzed directly, and absorbance values are compared to standards. Matrix-matching is not required, and virtually any concentration of metal can be analyzed since both sample concentration and aliquot volume are variable. Solvent selection is based on the results of a stability study and includes methyl isobutyl ketone (MIBK) for Cu and Mn determinations and MIBK with a small amount of HNO₃ added for Fe and Ni. Sensitivity approximated 80 picograms for Fe, 100 pg for Cu, 525 pg for Ni, and 30 pg for Mn. Absorbance values of appropriate aliquots, each having the equivalent of 0.10 ppm Fe, are compared to determine the effect of variation of pipette size on response. Pipette size ranged from 5 μl to 50 μl. Responses using multiple 50 μl volumes are also recorded. Sources of error in procedure, error analyses, and proper pipetting technique are given.     

Accurate determination oftrans isomers in shortenings and edible oils by infrared spectrophotometry

A procedure is described for the accurate analysis of thetrans content of fats and oils in the range of 0.5–36%trans. This procedure is shown to be more accurate than other current infrared spectrophotomeny methods. This increased accuracy is obtained by using a 2-component calibration standard mixture and measuring the samples as methyl esters. Good agreement between this method and the measurement oftrans content by a gas chromatography procedure is demonstrated.