Comparison of Regular, Pure Shift, and Fast 2D NMR Experiments for Determination of the Geographical Origin of Walnuts

H-1 NMR spectroscopy, in combination with chemometric methods, was used to analyze the methanol/acetonitrile (1:1) extract of walnut (Juglans Regia L.) regarding the geographical origin of 128 authentic samples from different countries (France, Germany, China) and harvest years (2016-2019). Due to the large number of different metabolites within the acetonitrile/methanol extract, the one-dimensional (1D) H-1 NOESY (nuclear Overhauser effect spectroscopy) spectra suffer from strongly overlapping signals. The identification of specific metabolites and statistical analysis are complicated. The use of pure shift H-1 NMR spectra such as PSYCHE (pure shift yielded by chirp excitation) or two-dimensional ASAP-HSQC (acceleration by sharing adjacent polarization-heteronuclear single quantum correlation) spectra for multivariate analysis to determine the geographical origin of foods may be a promising method. Different types of NMR spectra (1D H-1 NOESY, PSYCHE, and ASAP-HSQC) were acquired for each of the 128 walnut samples and the results of the statistical analysis were compared. A support vector machine classifier was applied for differentiation of samples from Germany/China, France/Germany, and France/China. The models obtained by conduction of a repeated nested cross-validation showed accuracies from 58.9% (+/- 1.3%) to 95.9% (+/- 0.8%). The potential of the H-1-C-13 HSQC as a 2D NMR experiment for metabolomics studies was shown.

Automated summary

H-1 NMR spectroscopy combined with chemometric methods was used to analyze the geographical origin of walnut samples. Different NMR spectra were compared for multivariate analysis and a support vector machine classifier was applied for sample differentiation. The study showed the potential of H-1-C-13 HSQC as a 2D NMR experiment for metabolomics studies.