Valorization of Pistachio Green Hull: Advances in Extraction and Characterization of Phenolic Compounds

Substantial agro-industrial waste is generated by the food industry, including pistachio green hulls (PGH), which can constitute 40% to 60% of the fresh fruit weight. This by-product contains bioactive functional components, especially phenolic compounds (PCs). An overview of research focused on PCs extracted from PGH is presented, highlighting their chemical composition, extraction methods, compound identification, and antioxidant and antibacterial activities. Extraction techniques such as ultrasound, microwave-assisted extraction, and solid-state fermentation are utilized, with mild organic solvents like water, ethanol, methanol, or their mixtures employed. The quantification of PCs is commonly performed using the Folin–Ciocalteu assay, HCl-Butanol technique, and aluminum chloride colorimetric assays. Furthermore, identification of compounds is generally accomplished through high-performance liquid chromatography (HPLC) or gas chromatography (GC), often coupled with mass spectrometry or photodiode-array detectors to enhance accuracy and reliability. Gallic acid, kaempferol, quercetin, cyanidin, and catechin are the main PCs identified, with their antioxidant activity validated by ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), DPPH (2,2-diphenyl-1-picrylhydrazyl), and FRAP (ferric-reducing antioxidant power) assays. Antibacterial effectiveness has been demonstrated against bacteria using disk diffusion and minimum inhibitory concentration methods. These findings indicate potential uses of PGH by-products in the food, cosmetic, and pharmaceutical industries, contributing to a circular economy and enhancing agro-industrial waste management.