1H NMR-based metabolomics of cacao pod husk: profile comparison of three drying treatments

Aim: The aim of this study is to apply untargeted proton nuclear magnetic resonance (1H NMR) and chemometric analysis to obtain the cacao pod husk (CPH) fingerprint and evaluate the effect of dehydration in the CPH metabolome. Methods: Phosphate buffer extracts (pH 6.5) were obtained and measured using a one-dimension (1D) 1H NMR spectrometry. The 1D 1H NMR spectra were recorded without spinning and using the presaturation (PRESAT) pulse sequence to suppress the residual H2O signal. The 3-(trimethylsilyl) propionic-2,2,3,3-d4 acid sodium salt (TSP) was used as an internal reference. Analysis of processed data, applying an orthogonal projection on latent structure-discriminant analysis (OPLS-DA) model was used to highlight significant differences between the three dehydration treatments. Signal assignment of CPH metabolites was carried out based on the coupling constant, software simulation prediction, published data comparison, and metabolomics databases. Results: A total of 25 compounds were detected by 1H NMR, methylxanthines, sugars, some amino acids, fatty acids, and organic acids were found among the identified compounds. The fingerprint spectra of the three dehydration methods were clustered separately discriminating the metabolome profile of each of the dehydration treatments, finding that metabolome remarkably differed in theanine, myristic acid, fumaric acid, and aspartic acid composition. Conclusions: An untargeted metabolomics to obtain the fingerprint of CPH was successfully established. A 1H NMR spectra with a detailed signal assignment aided to identify 25 metabolites present in CPH fresh and dried by different methods. The results complement the information about CPH composition and how it is affected by the temperature used during the dehydration process. The multivariate analysis points out that freeze drying (FD) preserves the metabolites better than microwave drying (MWD) or hot air drying (HAD). FD and MWD are similar in composition maintaining most of the compounds after drying.