An integrated membrane-assisted biorefinery process for the recovery of biophenols from Castanea sativa shell

Chestnut shell (Castanea sativa Mill.), an abundant agro-industrial by-product generated during chestnut peeling process, is usually discarded or used as fuel. However, this by-product has been recognized as a remarkable source of biophenols, such as gallic and ellagic acids, well recognized for their protective activities as antioxidants and the prevention of several types of human diseases [1]. The growing interest of both consumers and producers towards these substances has intensified research efforts to develop novel and sustainable procedures for their extraction, separation, and purification in an efficient and environmentally friendly manner without affecting their stability [2]. In this contest, this work was aimed at investigating, for the first time, an integrated biorefinery strategy for the extraction, separation and concentration of bioactive compounds (mainly gallic and ellagic acids) from dried chestnut shell. The process is based on a preliminary extraction of dried chestnut shell followed by clarification, fractionation and concentration of the extract with ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes in a sequential design. The extraction process has been optimized for obtaining the maximum yield of polyphenols by following an eco-friendly route. At this purpose, different parameters such as extraction temperature and type of solvent (water and alcohol/water mixture) have been optimized. 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 was previously clarified with hollow fiber UF membranes of 500 kDa in order to remove macromolecules and suspended solids. Then, the UF permeate has been 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 acid was totally recovered in the retentate stream, while most of gallic acid was recovered in the permeate and concentrated through a final RO step with the production of clean water (RO permeate). The proposed process, based on circular economy approach, allows (i) the recovery of valuable compounds for the formulation of functional ingredients and (ii) minimise the highly expensive and environmentally risky disposal of chestnut shell.