High‐resolution detection of adulteration of maize oil using multi‐component compound‐specificδ¹³C values of major and minor components and discriminant analysis

Abstract

Maize oil commands a premium price and is thus a target for adulteration with cheaper vegetable oils. Detection of this activity presents a particular challenge to the analyst because of the natural variability in the fatty acid composition of maize oils and because of their high sterol and tocopherol contents. This paper describes a method that allows detection of adulteration at concentrations of just 5% (m/m), based on the Mahalanobis distances of the principal component scores of the delta(13)C values of major and minor vegetable oil components. The method makes use of a database consisting of delta(13)C values and relative abundances of the major fatty acyl components of over 150 vegetable oils. The sterols and tocopherols of 16 maize oils and 6 potential adulterant oils were found to be depleted in (13)C by a constant amount relative to the bulk oil. Moreover, since maize oil contains particularly high levels of sterols and tocopherols, their delta(13)C values were not significantly altered when groundnut oil was added up to 20% (m/m) and it is possible to use the values for the minor components to predict the values that would be expected in a pure oil; therefore, comparison of the predicted values with those obtained experimentally allows adulteration to be detected. A refinement involved performing a discriminant analysis on the delta(13)C values of the bulk oil and the major fatty acids (16:0, 18:1 and 18:2) and using the Mahalanobis distances to determine the percentage of adulterant oil present. This approach may be refined further by including the delta(13)C values of the minor components in the discriminant analysis thereby increasing the sensitivity of the approach to concentrations at which adulteration would not be attractive economically.

Single‐step transesterification with simultaneous concentration and stable isotope analysis of fatty acid methyl esters by gas chromatography‐combustion‐isotope ratio mass spectrometry

“…The δ 13 C values increased with reaction yield, by an average of 0.73 ± 0.04‰, from the mildest (2% AcCl for 10 min) to the strongest (10% AcCl for 60 min) conditions for the three FAMEs measured. As with the compositional profile data, the relative δ 13 C value of each fatty acid remained consistent (Table ), with the order (from heaviest to lightest) being 18:2 > 18:1 > 16:0, as reported for some maize oils . These results indicated that the optimal reaction conditions for FAME sample preparation prior to GC‐C‐IRMS analysis depends largely on the goals of the experiment; i.e., whether the study requires accuracy for an ‘absolute’ δ 13 C value (e.g., authentication) or if the aim is to obtain a relative enrichment factor (e.g., tracer flux studies).…”

“…GC‐C‐IRMS is useful in the study of fatty acid methyl esters (FAMEs) and has been exploited in the food sciences to determine the adulteration of fine vegetable oils by natural abundance δ 13 C measurements, and, in combination with other stable isotopes, the geographic origin of oils . GC‐C‐IRMS has also garnered attention for its ability to quantify very low‐level enrichment in 13 C‐labelling studies to understand in vivo fatty acid metabolism in animal models, and it is more precise and accurate than other MS‐based methods for this purpose …”

“…Stable isotope values are often coupled with quantitative data (e.g., fatty acid composition profiles) thus providing a very powerful assessment of partitioning in mixed systems (e.g., ). Such studies typically obtain the analyte concentration data independent of the δ 13 C data via GC‐flame ionization detection (FID).…”

Gas Chromatography‐Combustion‐Isotope Ratio Mass Spectrometry for Traceability and Authenticity in Foods and Beverages

“…Edible oils such as olive oil and vegetable oils are often adulterated with cheaper oils (Angerosa and others ; Lees ; Mottram and others ) and IRMS on bulk oil (the “whole” oil) was proposed as a possible means to detect this type of fraud over 30 y ago (Gaffney and others ). As well as bulk oil, various fractions of oil, such as aliphatic alcohols (Angerosa and others ), can be submitted to isotope ratio analysis to determine whether adulteration has occurred.…”

Differentiation of Lard, Chicken Fat, Beef Fat and Mutton Fat by GCMS and EA-IRMS Techniques

“…Many past reports highlighted the use of stable isotope ratio in the detection of adulteration in edible oils. For instance, the potential use of SIRMS has been investigated for detection of sesame oil adulteration with corn oil 15 , adulteration of ganoderma spore lipid with cheaper vegetable oils 16 as well as adulteration of maize oil with other vegetable oils 17 . According to our literature search, the carbon isotope ratios determination has been scantily used for differentiation of animal fats such as lard, chicken fat, mutton fat and beef fat.…”