High‐resolution detection of adulteration of maize oil using multi‐component compound‐specific δ 13 C values of major and minor components and discriminant analysis

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 δ 13 C values of major and minor vegetable oil components. The method makes use of a database consisting of δ 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 δ 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 δ 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 δ 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. Copyright © 2003 John Wiley & Sons, Ltd.