High-intensity ultrasound-assisted extraction for functionalized pistachio meal protein concentrate

The residues left over from cold-pressing oilseeds and fruits offer a promising source of plant proteins, serving as an alternative resource within the circular economy framework. This research aims to develop protein concentrates (PCs) from pistachio meal with enhanced functional properties using ultrasound -assisted extraction (UAE). A response surface methodology was employed to optimize UAE considering power (200-600 W), time (5-25 min), and pH (8-11). Through a Box-Behnken experimental design, the optimal extraction conditions were determined as 595 W, pH 11, and 19 min, targeting maximal protein extraction yield (R²: 0.9359, desirability of 0.993). Under these ideal conditions, UAE achieved higher protein extraction yield (51%), compared to the conventional alkaline extraction (AE) (39%). Protein recovery rate increased from 33.1% to 44% by UAE. PCs obtained through the AE (PC-AE) and optimal UAE (PC-UAE) were evaluated for their functional, structural, and morphological properties. UAE significantly improved the emulsion activity index of the PC-UAE by 34.9% (p < 0.05), whereas solubility and foaming activity showed slight increases compared to PC-AE (p > 0.05). US-induced size reduction as observed by smaller and irregular particles in morphological analysis. The Fourier-transform infrared spectroscopy spectra, analyzed using a curve-fitting approach on the second derivative, revealed hidden peaks that highlighted changes in the secondary structure of the PCs, including slight shifts in characteristic wavenumbers and variations in the proportional amounts of β-sheet, α-helix, and random coil structures. This study demonstrated that UAE achieved similar protein purity as AE while producing larger quantities through higher recovery of proteins with improved functional properties. PRACTICAL APPLICATION: Ultrasound-assisted alkaline extraction significantly boosts protein extraction yield and recovery from pistachio meal and enhances functional properties (especially emulsion activity) of the extracted proteins. These improvements make the UAE protein concentrates suitable for applications in food formulations, such as plant-based beverages and emulsified products. UAE offers a sustainable approach to valorize oilseed residues in the context of circular economy.