Utilization of pistachio by-products for the recovery of phenolic antioxidants

Pistachio processing accrues high amounts of by-products. Pistachio hulls, the main byproduct of pistachio processing, has yet no commercial value. The main objective of this study was utilization of pistachio hull focusing on the recovery of the phenolic antioxidant compounds. First, detailed analysis of phenolic constituents of the pistachio hull was assessed using high-performance liquid chromatography coupled with diode array detector and electrospray ionization mass spectrometry (HPLC-DAD-ESI-MSn) as well as by highresolution mass spectrometry (HR-MS). Then, a rapid method was developed and validated for the simultaneous extraction and quantitation of phenolic compounds from pistachio hull after screening of various extraction parameters, and solvent systems with variable polarity and acidity. Moreover, a method using ultra-high-performance liquid chromatography coupled with diode array detector and evaporative light scattering detector (UHPLC-DADELSD) was developed for faster separation of pistachio hull phenolics. Optimized method was then applied for the quantitation of pistachio hull phenolics from different varieties. Finally, an environmentally friendly extraction process using subcritical water (SCW) was developed for the recovery of pistachio hull phenolics. A total of 66 individual phenolic compounds were identified in the aqueous methanolic extracts in three different classes, namely gallotannins, flavonoids, and anacardic acids. The total amount of individual phenolics were ranged from 61.2 to 100.7 g/kg dry matter, DM for different varieties where anacardic acids predominated (64.6-80.4 per cent), followed by gallotannins (13.4-21.2 per cent), and flavonol glycosides (5.7-16.3 per cent). By using SCW extraction at 110-150 °C,extracts rich in gallic acid (22.2 g/kg DM), penta-O-galloyl-β-D-glucose (9.77 g/kg DM),and flavonols (4.37-5.65 g/kg DM) were obtained, while anacardic acids (up to 50.7 g/kg DM) were retained in the extraction residue. Moreover, SCW extracts showed superior antioxidant properties compared to those obtained for aqueous methanol extracts. In conclusion, pistachio hulls were shown to be a rich source of bioactive phenolic compounds.