Researchers have developed a novel approach to the rapid assignment of carbon-13 NMR spectra to the major components of vegetable oils such as avocado, mango kernel and macadamia nut oils.

Quality control of vegetable oils is an important facet of the food industry. This is particularly true from the regulatory point of view where consumers must be protected from the illicit adulteration of products labelled as expensive single source oils such as fruit and nut oils. Without the analytical capacity to check that products have not been adulterated with less expensive oils, such as sunflower oil, there would be only limited means to verify what was being purchased as genuine.

Proton NMR spectroscopy has already been used widely in determining the major fatty acids present in expensive products such as olive oil. The big advantage of this analytical technique over others is that no extraction, separation or sample preparation is needed to allow a vegetable oil to be analysed. In contrast, gas chromatography (GC), which is widely used in the food industry usually requires derivitisation of the components of interest in a sample prior to analysis.

As NMR spectroscopists know all too well, however, it has a relatively low sensitivity. So, while it is perfectly suited to analysing olive oil, it only really works well in detecting major and minor components as opposed to the trace components that distinguish even more specialist oils.

Liezel Retief and Klaus Koch of the Department of Chemistry and Polymer Science, at Stellenbosch University, in Matieland, and Jean McKenzie of DISA Vascular (Pty) Ltd, in Mowbray, South Africa, have turned to carbon-13 NMR for their vegetable oil analysis.

Initially, the team attempted assignment of ¹³C NMR spectra of the major fatty acid components of South African produced vegetable oils using a method in which the vegetable oil was spiked with a standard triacylglycerol. However, this quickly proved inadequate and so the team had to develop their own rapid graphical linear correlation method for assigning the spectra of apricot kernel, avocado pear, grapeseed, macadamia nut, mango kernel and marula vegetable oils. They point out that their approach is potentially generic and could have wide utility in the industry.

"In this graphical correlation method, chemical shifts of fatty acids present in a known standard triacylglycerol is plotted against the corresponding chemical shifts of fatty acids present in the vegetable oils," the team explains, "This new approach (under carefully defined conditions and concentrations) was found especially useful for spectrally crowded regions where significant peak overlap occurs and was validated with the well-known ¹³C NMR spectrum of olive oil."

Using this approach, they were able to undertake a complete assignment of the ¹³C{¹H}NMR spectra of each vegetable oil sample. They could also assign tripalmitolein and the resonances belonging to the palmitoleic acid component of the triacylglycerols in the case of macadamia and avocado oil. This is the first assignment of those peaks for these two products, they add.

The team observed a striking, but superficial, resemblance between the spectra for macadamia and olive oils. This suggests that similar triacylglycerols are present in both. However, a more detailed assessment of these and the other vegetable oil spectra using their novel approach to assignment revealed important differences between each.

"This method is reasonably robust and rapidly leads to accurate ¹³C NMR assignments in mixtures of triacylglycerols at reasonable concentrations in similar vegetable oils not previously examined, without having to resort to time-consuming additions of pure standards," the team concludes.

Related links:

• Magn Reson Chem, 2009, in press
• Koch Page

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.