Towards a green process for recovering phenolic compounds from fennel wastes (Foeniculum vulgare Mill.): Identifying target molecules and assessing membrane nanofiltration for their separation

Fennel (Foeniculum vulgare Mill.) wastes represent a source of phenolic compounds with multiple healthy properties. A green biorefinery strategy, based on an eco-friendly extraction of fennel wastes followed by sequential microfiltration (MF) and nanofiltration (NF) processes, to purify and concentrate low molecular weight phenolic compounds (caffeic, chlorogenic and ellagic acids, catechin) from a fennel waste extract was investigated for the first time in this study. A detailed analysis of the effects of operating conditions, such as transmembrane pressure (TMP) and cross flow velocity (CFV), on permeate flux and separation efficiency of the NF membrane for target molecules was undertaken. The MF process enabled the complete recovery of phenolic compounds in the clarified extract and their purification from suspended solids particles. An increase of both TMP and CFV resulted in higher permeate fluxes, as well as in higher rejection values and separation efficiency of the NF membrane towards all analyzed compounds. Specifically, at a TMP of 20 bar and CFV of 4.43 m/s, the separation factor between caffeic acid and chlorogenic acid, ellagic acid and catechin was close to 280, and much lower (11–15) with respect to other compounds such as carbohydrates and total polyphenols. The concentrated fraction, enriched in chlorogenic acid (148.2 mg/L), ellagic acid (73.6 m/L), catechin (2.56 mg/L) and flavonoids (722.6 mg/L), exhibited a total content of polyphenols of about 5000 mg GAE/L. On the other hand, about 60 % of caffeic acid was recovered in the NF permeate. The investigated process can be considered a promising approach for an environmentally sustainable valorization of fennel wastes and could be recommended for largescale industrial applications.