Valorization of hazelnut shells into natural antioxidants by ultrasound-assisted extraction: Process optimization and phenolic composition identification

The objectives of this study were to optimize the ultrasound-assisted extraction for phenolic compounds from hazelnut shells and to quantify the individual phenolics. A central composite design was used to optimize extraction time, acetone concentration, and solid to liquid ratio (S/L), for the maximal yield of phenolic compounds and antioxidant capacities. The optimal conditions were 4.0 hr extraction time, 61% acetone, and 10 g/L S/L for the maximal total phenolic content (TPC, 11.23 mg gallic acid equivalents/g shell); 3.7 hr, 60.5% acetone, and 12.9 g/L for the highest ferric reducing antioxidant power (FRAP, 106.3 µmol Trolox equivalents/g shell); and 4.7 hr, 58.3% acetone, and 10 g/L for the highest DPPH radical scavenging capacity (DRSC, 101.83 µmol Trolox equivalents/g shell). Catechin, epicatechin gallate, and gallic acid were the most abundant phenolic compounds in the hazelnut shells. Ultrasound could be a good tool for accelerated recovery of polyphenols from hazelnut shells. Practical applications: There has been an increasing production and application of hazelnuts globally. As the main byproduct of hazelnut industry, hazelnut hard shells are a good source of high value polyphenols. The optimized process presented here will allow a better understanding in the extraction of polyphenols from hazelnut shells. The results of this study are expected to improve the extraction efficiency of phenolic compounds and to help with the development of industrial extraction processes of natural antioxidants from hazelnut shells and other hard shells of tree nuts.