Encapsulation of Oat Bran Polyphenols within the Zein–Whey Protein Isolate Complex, Characterization, In Vitro Release, and Bioaccessibility of Avenanthramides

Avenanthramides (AVNs), polyphenols unique to oats, exhibit notable biological activities but are unstable during digestion, limiting their use in food formulations. To overcome this, AVNs were encapsulated in a zein–whey protein isolate (WPI) complex using a pH-driven method. The optimal WPI–zein ratio (8:2) produced nanoparticles with a small size (108.6 ± 4.3 nm), high encapsulation efficiency (94.2 ± 0.2%), and zeta potential (−18.8 ± 0.4 mV). Chromatographic, spectroscopic, and dissociation analyses revealed that the structure was maintained by hydrophobic, electrostatic, and hydrogen bonding interactions. Encapsulation significantly improved AVN2c, AVN2p, and AVN2f bioaccessibility, preserving up to 73.3% of their content compared to ≤34% without encapsulation. Antioxidant assays showed synergistic ROO• scavenging and an additive effect for ABTS• + scavenging activity. The complex demonstrated excellent stability across pH, salt, and thermal conditions. These results confirm successful AVN encapsulation in zein–WPI biopolymers, offering a promising strategy for developing foods with enhanced bioaccessibility and antioxidant functionality.