Physicochemical, Structural, and Techno-functional Characterization of Solanum scabrum Leaf Protein Concentrates Obtained by Alkaline Extraction, Ultrasound-assisted Extraction, and Ultrafiltration

Abstract Global demand for plant-based proteins is rapidly increasing, driven by sustainability concerns and health-conscious consumer preferences. In this context, African nightshade ( Solanum scabrum ), an underutilized leafy biomass rich in protein, represents a promising alternative protein source. This study investigated the influence of alkaline extraction followed by isoelectric precipitation (AE-IP), ultrasound-assisted extraction (UAE), and ultrafiltration (UF), on the yield, composition, structural and functional characteristics of Solanum scabrum protein concentrates (AE‑IP_SPC, UAE_SPC, UF_SPC). UAE significantly enhanced protein yield (48.71%) compared to AE-IP (34.02%), while UF produced concentrates with lowest yield (26.45%) but highest protein content (71.43%). UF_SPC exhibited a lighter color (L* = 71.53 ± 0.22) and lower browning index (20.76 ± 0.11%) than AE-IP_SPC (L* = 57.89 ± 0.17; browning index = 42.90 ± 0.54%), indicating reduced pigment co-extraction. Scanning electron microscopy revealed dense, fibrous structures with irregular cracks in AE-IP_SPC, smooth, layered structures with interconnected channels and cavities in UAE_SPC, and irregular wavy patterns and shallow depressions in UF_SPC. UF_SPC displayed the highest absolute zeta potential values at both acidic (36 mV at pH 2) and alkaline pH (-39 mV at pH 10), indicating enhanced electrostatic repulsion and improved colloidal stability. Functionally, UF_SPC and UAE_SPC exhibited superior solubility at alkaline pH, along with enhanced emulsifying and foaming properties. Spectroscopic analysis suggested extraction-dependent alterations in protein conformation associated with improved hydration behavior. Overall, ultrasound-assisted and ultrafiltration approaches effectively improved the functional and nutritional performance of Solanum scabrum protein concentrates, supporting their potential application as sustainable plant-based protein ingredients and establishing clear process-structure–function relationships.