No abstract

A new method is described for detecting the adulteration of maize oil based on differences in <$13C values of oils arising through biosynthetic differences in the mode of carbon dioxide fixation in C4 and C3 plants. Fatty acid composition data are obtained on whole vegetable oils (following saponification and methylation) by GC using a capillary column with a polar stationary phase. 513C values for the individual fatty acids, determined by GCC/ IRMS, are then used together with the compositional data to construct theoretical mixing curves. The validity of these mixing curves was verified experimentally. The ability of the method to detect adulteration of maize oil with C3 oils at low concentrations was tested in a "blind" trial. The technique was readily able to detect adulteration

The fatty acid compositions and δ 13 C values of the major fatty acids of more than 150 vegetable oils were determined to provide a database of isotopic information for use in the authentication of commercial maize oil. After extraction of oils from seeds, nuts or kernels, and methylation, fatty acid compositions were determined by capillary gas chromatography. All compositions were within the ranges specified by the Codex Alimentarius. Gas chromatography combustion-isotope ratio mass spectrometry was employed to determine the δ 13 C values of the major fatty acids of the oils. A large number of pure maize oils and potential adulterant oils from various parts of the world were studied to assess the sources of variability in δ 13 C values. Such information is vital to establishing the compound specific isotope technique as a reliable means of assessing vegetable oil purity. Variability in δ 13 C values was related to the geographical origin of the oil, year of harvest, and the particular variety of oil. This suggests that the ultimate δ 13 C values of fatty acids are determined by a combination of environmental and genetic factors. JAOCS 75, 371-379 (1998) . FIG. 6. A summary of the variations in δ 13 C values of oleic acid due to plant type, origin, and year of harvest. a Year-to-year variability is shown for variety Idol (winter oilseed rape) grown in five consecutive years. b Variability between varieties for 1992/1993.

International trade in palm oil has increased considerably over the last ten years, and so too has the trade in processed palm oil products, especially palm fractions. It is important to establish reliable purity criteria for palm oil, not only because of the commercial need to verify oil authenticity, but also to comply with foodstuff labelling legislation in many countries.Palm kernel and coconut oils both contain about 47% lauric acid. This gives the oils close similarities in physical and chemical properties. The oils do differ, however, and it is important to be able to distinguish between them.Purity problems can arise as a result of commingling of oils with one another, or as a result of fractionation perhaps coupled with subsequent blending. A research program jointly funded by the (U.K.) Ministry of Agriculture, Fisheries and Foods, the Federation of Oils, Fats & Seeds Associations Ltd (FOSFA International), and the Leatherhead Food RA, was established to study purity characteristics of the major edible vegetable oils.Forty‐seven samples of crude palm oil were obtained from reliable sources, often plantation managers, together with five samples of palm olein and eight samples of palm stearin. Fifty‐four palm kernel and 23 coconut oils were obtained in the laboratory from seed samples of known geographical origins and authenticities.These oil samples were analyzed for fatty acid, triglyceride, sterol and tocopherol compositions; the melting properties were also determined, and in the case of palm oil the compositions of the acids at the triglyceride 2‐positions were measured. Compositional ranges will be presented for the different geographical production areas in each case and related to existing data, e.g., of PORIM and Codex. An initial statistical analysis of the results has shown that a combination of values from the carbon number analysis differentiates palm kernel and coconut oils, and can be used to decide on the proportion of each in a blend. In the case of palm oil samples suspected to be contaminated with palm fractions, it was found useful to plot melting point against iodine value, and to compute the product of the C48 triglyceride content and the palmitic acid enrichment factor.

A program of work is in progress to establish the levels and ranges of fatty acids and other components present in the major edible vegetable oils. Authentic samples from the major producing areas for such oil have been obtained and analyzed. In the case of palm oil, ranges of the fatty acid composition and of the acids at the triglyceride 2‐position, have been obtained for about 40 samples. These data were used to calculate enrichment factors, and triglyceride carbon number compositions, using a small computer program. Comparison with experimentally determined carbon number compositions were then made. Good correlations were found for whole unadulterated oils, but not for oil fractions. Unfortunately, these differences were insufficient to detect contamination of palm oil by 10 or 20% levels of other oils, or of palm fractions. Compositional ranges of sterols and tocopherols have also been determined on a selection from the original set of palm samples. Work on sunflower seed and groundnut oils has followed the same lines, particular attention having been paid to linolenic acid and, in the case of groundnut oil, also erucic acid, levels. Some groundnut kernels were found to have an oil with a component which cochromatographed with methyl erucate during fatty acid determination. This unknown constituent was studied by gas chromatography‐mass spectrometry, and is thought to comprise a mixture of epoxy fatty acids. Analysis of the triglyceride fraction isolated from groundnut oil by thin layer chromatography removes this unknown constituent, and simplifies interpretation of the fatty acid composition of groundnut oil.