Pistachio (Pistacia vera L.) Hull as a Potential Source of Phenolic Compounds: Evaluation of Ethanol–Water Binary Solvent Extraction on Antioxidant Activity and Phenolic Content of Pistachio Hull Extracts

Abstract: Pistachio (Pistacia vera L.) hull is the largest by-product of pistachio industry which is accumulated every year at harvest time and can lead to environmental pollution. This study investigated the microwave-assisted ethanol–water binary solvent extraction of phenolic compounds from pistachio hull. The effect of different ethanol/water ratios was investigated on the changes in extraction yield, total phenolic content and antioxidant activity. The obtained results indicated relationships between the tested parameters, i.e. ethanol concentration and extraction yield. The maximum yield was obtained with 50% ethanol (32.9 g dry extract/100 g dry matter). Total phenolic content of the extracts was found in the range of 21.3–39.3 mg/g extract as gallic acid equivalent. Antioxidant activity was determined by using three different tests. Using the DPPH test, the best antioxidant activity with the lowest IC₅₀ value (0.70 mg/mL) was obtained for 40% ethanol and the least antioxidant activity was obtained for 100% ethanol with the highest IC₅₀ value of 2.73 mg/mL. Using the β-carotene bleaching assay, the pistachio hull extracts showed antioxidant activity at 5000 mg/L in the range of 44–97%. The ORAC test showed that 50% ethanol extract had the highest antioxidant activity (260.9 µmol Trolox equivalents/g extract). Using HPLC assay, the gallic acid was the main phenolic compound of hull extracts which ranged between 1.18 and 19.83 mg/g crude extract. These findings propose that pistachio hull extracts can be a valuable source of bioactive compounds. Graphical Abstract: [Figure not available: see fulltext.].