Toward a green integrated membrane process for the separation, concentration and purification of bioactive compounds from chestnut shells

The chestnut processing industry is characterized by the production of a considerable amount of byproducts including leaves, burs and shells. Different studies have been performed to valorise these wastes, particularly shells, for their exploitation in different fields. Indeed, these wastes are an excellent source of tannins (condensed and hydrolysable), phenolic acids (ellagic and gallic acids) and flavonoids (catechin, epicatechin, apigenin, quercetin and rutin) well recognized for their biological activities, including antioxidant, anti-inflammatory and anticancer activities. The richess in bioactive compounds reinforced the potential of shell valorisation as natural antioxidants and prebiotics and emphasized the possible benefits of designing high-added value products with attractive applications in food and nutraceutical fields. Furthermore, the valorisation of these wastes, through the implementation of efficient and eco-friendly extraction processes, represents a great challenge for the chestnut processing industries. This work has been focused, for the first time, on the development of a sustainable integrated membrane process for separating and purifying different classes of bioactive compounds from dried chestnut shell extracts. The experimental process, investigated on lab scale, is based on a preliminary extraction of dried chestnut shells followed by clarification, fractionation and concentration of the extract with ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes in a sequential design. The operating parameters of the extraction process (as temperature and type of extracting solvent) have been optimized for maximizing the polyphenols yield. The obtained results showed a maximum polyphenol yield of about 6.6 g/L at an operating temperature of 50 °C and a water-to-solid ratio of 9 mL/g. The aqueous extract has been previously clarified with hollow fiber UF membranes of 500 kDa in order to remove macromolecules and suspended solids. Then, the UF permeate was treated by a spiral-wound NF membrane (MWCO, 300-500 Da): an average permeate flux of about 50 L/m2h was measured in selected operating conditions. Ellagic and gallic acids have been totally recovered in the NF retentate stream (Figure 1). In addition, most of gallic acid has been recovered in the NF permeate and concentrated through a final RO step with the production of fresh water (RO permeate). Results of the investigated process indicate the effective valorization of the chestnut shells and the reduction of the environmental impact by supporting the transition towards a circular economy for meeting the "zero waste" goals proposed by the 2030 agenda.