Optimization of Phenolic Compounds Recovery from Pistachio Hull Using Accelerated Solvent Extraction

Pistachio hull (PH) is the largest by-product of the pistachio industry. It contains high levels of phenolic compounds, which have antioxidant properties and promote health. However, PH can accumulate during harvesting, potentially resulting in environmental pollution. This study aimed to optimize the operational conditions for conventional solvent extraction (CSE) and accelerated solvent extraction (ASE) of phenolic compounds from PH using response surface methodology (RSM). The extracts obtained under optimal conditions for the two extraction techniques were compared. The highest values of phenolic recovery (9.92 ± 0.09 g gallic acid equivalent (GAE)/100 g dried matter (DM)) and radical-scavenging activity for α,α-diphenyl-β-picrylhydrazyl (DPPH) (IC50 = 0.16 ± 0.00 mg/mL) were achieved by the extract obtained by ASE (23% ethanol in water, 180 °C, 15 min). Regarding individual phenolic compounds, gallic acid accounted for 35.7-48.1% of the total phenolic compounds contained in the PH extracts, followed by 3,4-dihydroxybenzoic acid, 4-hydroxybenzoic and 2,5-dihydroxybenzoic acid. The findings of this study demonstrate for the first time that PH can be valorized by ASE using eco-friendly solvents, obtaining extracts with a high phenolic content, reducing waste and promoting the bioeconomy development.