Green extraction of polyphenolic compounds from pistachio hulls: antioxidant potential and identification by HPLC/ESI/MS

Pistachio (Pistacia vera L.), a Middle Eastern crop, is a globally popular nut due to its high nutritional value. Pistachio hulls, typically discarded as low-value byproducts during nut processing, represent approximately 60% of total production and pose environmental concerns. This study aimed to evaluate the extraction efficiency of phenolic compounds and their antioxidant potential from pistachio hull extracts using green technologies. Three extraction methods were evaluated: ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and a combined ultrasound–microwave method (UMAE). Extracts were purified via column chromatography to isolate phenolic-rich fractions. The highest extraction efficiency was achieved with MAE, yielding 263.77 ± 0.71 mg of total phenols per g of dry matter (dm), comprising 89.89 ± 2.97 mg EC/g dm of condensed phenols and 173.87 ± 2.39 mg GAE/g dm of hydrolysable phenols. The MAE extract exhibited the highest antioxidant capacity, reaching 520.73 ± 7.91 mg TE/g dm (DPPH•) and 747.33 ± 13.48 mg TE/g dm (ABTS). RP-HPLC–ESI-MS analysis identified six bioactive compounds, with geraniin as the major constituent. FTIR confirmed the presence of hydroxylated, glycosylated, and esterified phenolics. These findings demonstrate the effectiveness of green technologies in recovering high-value bioactive compounds, supporting the use of pistachio hulls as a sustainable source of natural antioxidants with potential applications in the industry.