1H-NMR-based Untargeted Metabolomics to assess the impact of soil type on the chemical composition of Mediterranean red wines

Abstract

Untargeted metabolomics has proven to be an effective method to study the impact of the terroir on metabolic profile of wines. In this context, the aim of this study was to evaluate the effects of different soil types on the chemical composition of Mediterranean red wines, through 1H-NMR metabolomics combined with chemometrics.Grapes from Nero d’Avola L. red cultivar cultivated on four different soil types were separately vinified to obtain four different red wines.One milliliter of raw wine was analyzed by means of a Bruker Avance II 400 spectrometer operating at 400.15 MHz. The 1H-NMR spectra were recorded at 298.8 K by applying the NOESYGPPS1D pulse sequency, to achieve water and ethanol signals suppression.The free induction decay (FID) was collected into a time domain of 65536 real data points (64 k), with a spectral width of 8012.82 Hz, a relaxation delay of 4 s and acquisition time of 4 s per scan.The solvent used was D2O, which provided a field frequency lock and the chemical shift reference. No quantitative internal standard was used, and no modification of the pH was performed, to avoid any chemical alteration of the matrix. Signal assignment was performed by comparison to pure compounds spectra by means of Simple Mixture Analysis (SMA) plug-in of MNova 14.2.3 software.The generation of input variables was done via bucketing the spectra within the range 0.50-9.50 ppm. The NMR spectral data were reduced into 0.01 ppm spectral buckets. The resulting dataset was log transformed and scaled to Pareto variance prior to perform unsupervised PCA, by means of MetaboAnalyst web-based tool suite.The PCA reduced the number of original variables (890) to three Principal Components that, combined, accounted for 100 % of the total variance. The 3D PCA scores plot revealed a clear differentiation among the wines. The 3D PCA loadings plot revealed the fragments of the spectra contributing mostly to the separation, that were attributed to flavonoids, aroma compounds and amino acids. The results were related to soils physical-chemical analysis and showed that: 1) high concentrations of flavan-3-ols and flavonols are correlated with high clay content in soils; 2) high concentrations of anthocyanins, amino acids, and aroma compounds are correlated with neutral and moderately alkaline soil pH; 3) low concentrations of flavonoids and aroma compounds are correlated with high soil organic matter content and acidic pH.The 1H-NMR metabolomic analysis combined with chemometrics proved to be an excellent tool to discriminate between wines originating from grapes grown on different soil types and revealed that soils in the Mediterranean area exert a strong impact on the chemical composition of the wines.