Engineering Protein-Anthocyanin Complexes from Purple Sweet Potato (Dioscorea alata): Structural, Physicochemical, and Functional Characterization

This study investigated the ability of three protein matrices, including α-lactalbumin (α-L), whey protein isolate (WPI), and soy protein isolate (SPI), to encapsulate and stabilize anthocyanins (ACNs) isolated from purple sweet potato (PSP) through electrostatic precipitation, emphasizing their effects on entrapment efficiency, viscoelastic behavior, and particle morphology. PSP-ACNs were extracted, standardized as cyanidin-3-glucoside equivalents, and incorporated into protein solutions at concentrations ranging from 2 to 25 mg/L. Entrapment efficiency (EE%) and ACN-to-protein ratios increased significantly (p < 0.05) with rising anthocyanin concentrations for all proteins, with α-L consistently achieving the highest EE (up to 97.29%) due to its flexible conformation and abundance of reactive amino acid residues. WPI demonstrated similar performance at higher concentrations, whereas SPI exhibited lower EE at low ACN levels but approached comparable encapsulation efficiency near saturation. Rheological analyses revealed contrasting microstructural behaviors: α-L and WPI formed weak, dynamic viscoelastic fluids with G′–G″ crossover points, while SPI generated strong, elastic networks (G′ > G″) across all concentrations. Increasing ACN concentration caused reductions in storage and loss moduli and lowered critical frequency values, indicating weakened structural integrity under oscillatory stress. Transmission electron microscopy showed predominantly spherical nanoparticles (12–45 nm), with α-L exhibiting the most uniform and densely packed ACN distribution, WPI forming slightly larger aggregates, and SPI displaying irregular ellipsoids under high ACN loading. Collectively, these results demonstrate that protein type and ACN concentration govern molecular interactions, mechanical stability, and encapsulation behavior within PSP-ACN complexes, highlighting α-L and WPI as promising carriers for stabilizing anthocyanins in functional foods and nutraceutical formulations, while SPI offers potential for plant-based systems when properly optimized.

Keywords: Anthocyanins, α-Lactalbumin, Soy Protein, Whey Protein, Isolate.