Valorization of chestnut processing by-products: A membrane-assisted green strategy for purifying valuable compounds from shells

The recovery of value-added compounds from chestnut by-products (shells, leaves, burs) contributes to enhance the sustainability of chestnut transformation processes. In this work, for the first time, a green strategy for the valorization of chestnut shells, was achieved. In particular, the efficiency of an extraction process followed by a combination of membrane operations including ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) in a sequential design, was validated. This integrated process permitted different classes of bioactive compounds (gallic, ellagic and tannic acids) to be separated and purified from dried chestnut shell extracts. The extraction process was studied in order to obtain a high concentration level of polyphenols through an eco-friendly route. To this purpose, water was selected for the extraction process being a green solvent, bio-renewable, cheap and readily recyclable. The maximum content of total phenols, under optimized extraction temperature conditions (50 oC) at fixed water-to-solid ratio of 9 mL/g, was 6.6 g of Gallic Acid Equivalents (GAE)/L of extract. The UF process allowed the complete removal of suspended solids producing a clear aqueous extract. An NF retentate containing 30.16 g/L of total polyphenols and 6.2 g/L total flavonoids and enriched in ellagic and tannic acids (with average concentration of 3.83 g/L and 1.49 g/L, respectively), was obtained. The recovered NF permeate, enriched in gallic acid, was concentrated through an RO process with the production of a purified retentate fraction containing 118.37 mg/L gallic acid and a fresh water stream (as permeate). The results indicated that the proposed process, based on a circular economy approach, is promising for an effective industrial valorization of chestnut shells through the recovery of valuable compounds and to minimize their risk disposal.