Bulk C, H, O, and fatty acid C stable isotope analyses for purity assessment of vegetable oils from the southern and northern hemispheres.
BACKGROUND
The carbon, hydrogen and oxygen stable isotope composition (δ13C, δ2H, and δ18O values) of plants and their products is linked to photosynthetic fractionation, environmental factors and agricultural practices. Therefore, they contribute to determining the purity of commercial vegetable oils and may provide information on their geographical origin.


METHODS
Maize, olive, sunflower, groundnut, soybean and rice oils differing in sites of growth in the southern and northern hemispheres were characterized by bulk oil stable isotope ratios (δ13Cbulk, δ2Hbulk, and δ18Obulkvalues), fatty acid (FA) concentrations and δ13CFAvalues using elemental analysis/isotope ratio mass spectrometry, gas chromatography/mass spectrometry, gas chromatography/flame ionization detection and gas chromatography/combustion/isotope ratio mass spectrometry. Principal component analysis was applied to examine the inherent structure of the data.


RESULTS
The δ13Cbulkvalues of maize oils (-18.4 to -14.9 ‰) are typical for C4plants, and those of olive (-30.2 to -28.2 ‰), sunflower (-30.2 to -29.2 ‰), groundnut (-29.3 ‰), soybean (-30.6 ‰), and rice (-34.5 ‰) oils are typical for C3plants. The δ2Hbulkvalues vary from -161 to -132 ‰ for maize oils and -171 to -109 ‰ for C3oils. The δ18Obulkvalues of all oils vary between 15.2 and 38.9 ‰. The major δ13CFAdifferences (>5 ‰) within plant species render the inter-C3-species comparison difficult. These differences are explained in terms of variations in the lipid biosynthetic pathways and blend of vegetable oils of different FA composition and δ13CFAvalues. The samples from the southern hemisphere are generally enriched in13C compared with those from the northern hemisphere. Differences between the southern and northern hemispheres were observed in δ2H (p < 0.001) and δ18Obulk(p = 0.129) values for all C3oils, and in δ13C18:1(p = 0.026) and δ18Obulk(p = 0.160) values for maize oils.


CONCLUSIONS
The results of this study show that combining bulk and molecular stable isotope ratios, FA compositions and their statistical analysis helps the characterization of the geographic origin of oils. This methodology can be used to detect and source impurities in valuable vegetable oils commercialized worldwide. Copyright © 2016 John Wiley & Sons, Ltd.
Institute of Earth Surface Dynamics, University of Lausanne, Geopolis Building, CH-1015, Lausanne, Switzerland.
Jorge E
JE
Spangenberg