Nanofiltration of Calabrian apple juice: characterization of bioactive fractions and sugar reduction

Apple is certainly one of the most commonly consumed fruit in worldwide, not only due to its pleasant taste, but also because it has been validated as exerting health-promoting effects in preventing chronic heart and vascular diseases, diabetes, respiratory and pulmonary dysfunctions, obesity, or cancer, among others. In the present work, one- and two-dimensional multinuclear NMR spectra combined with chemometric analysis (PCA) were used to identify the metabolic profile of Calabrian apple juices belonging to four different cultivars (Royal Gala, Pink Lady, Golden Delicious and Fuji) and variations in the metabolic composition. 12 more abundant compounds with low molecular weight in the fresh juices were identified by NMR analysis while a clear separation between the four varieties was established by multivariate analysis. Nanofiltration (NF) was investigated to preserve the biofunctional properties of the original juice and to reduce the sugar content in clarified juices of Golden Delicious and Fuji varieties. In a first approach, the ultrafiltered Golden Delicious juice was concentrated by using a spiral-wound NF membrane up to a volume reduction factor (VRF) of 3.3. Permeate and retentate samples at different VRF were characterized by NMR and by HPLC-UV. Rejection indexes of bioactive compounds, such as chlorogenic acid and caffeic acid, resulted of 94.2% and 69.6%, respectively, in agreement with the molecular weight cut-off (MWCO) of the NF membrane (150-300 Da). A combination of NF in batch concentration mode and diafiltration (DF) was also investigated to reduce the sugar content of the Fuji variety. The clarified juice was concentrated up to a VRF 5 by using a 300-500 Da membrane in spiral-wound configuration. After the NF, a diafiltration step was conducted according to a constant-volume dilution mode by adding distilled water to the NF retentate. In the first step of the process 45.9% of glucose and 50.8% of fructose were recovered in the permeate stream. On the other hand, the rejection of total phenolic compounds resulted of 74%. Nearly 48% and 50% of glucose and fructose, respectively, were removed in the diafiltration step at a diafiltration volume of 1.92. Chlorogenic acid was totally retained during the diafiltration step, while losses of phlorizin were of the order of 29% (Figure 1). These preliminary results confirm the potential of the NF process as a promising technique to produce apple juices with reduced content of sugars and enriched in bioactive compounds.