An Integrated Approach Based on NMR and HPLC–UV-ESI–MS/MS to Characterize Apple Juices and Their Nanofiltration (NF) Bioactive Extracts

In the present work, Calabrian apple juices belonging to four different cultivars, Royal Gala, Pink Lady, Golden Delicious, and Fuji, were studied. One of the objectives of this work was the characterization of metabolites present in the food matrix and at the same time the possibility of using innovative methodologies based on membrane operations, such as ultrafiltration (UF) and nanofiltration (NF), to obtain from the squeezed apple juice, extracts rich in antioxidants. To improve the overall quality of the study and enhance the coverage of the metabolome two analytical tools were used: NMR and HPLC–UV-ESI–MS/MS. One- and two-dimensional multinuclear NMR spectra combined with chemometric analysis (PCA) were used to identify the metabolic profile of different juice varieties and variations in metabolic composition. On the basis of its higher content of phenolic compounds, the Golden Delicious variety was selected to carry out membrane filtration tests. Specifically, the fresh juice was clarified by UF and then concentrated by NF up to a volume reduction factor (VRF) of 3.3. Permeate and retentate fractions of both processes were analyzed by HPLC–UV and NMR in order to evaluate the membrane selectivity of the selected membranes towards target compounds.