Upcycling brewers’ spent grain into antimicrobial, UV-shielding PVA/ZnO biocomposite films for active food packaging

A bioactive and biodegradable packaging film was fabricated via a solvent-casting method using poly(vinyl alcohol) (PVA) reinforced with alkaline–peroxide treated brewers’ spent grain (BSG-M) and zinc oxide nanoparticles (ZnO-NPs). Alkaline–peroxide treatment was applied to BSG, which is reported to disrupt lignin–carbohydrate associations and promote microfibrillation, yielding a modified lignocellulosic filler containing residual proteins that dispersed uniformly within the PVA matrix, thereby enhancing the tensile performance and soil degradability of the composite films. The incorporation of an optimal ZnO loading (2–3 wt%) facilitated the formation of dense hydrogen-bond networks and increased the tortuosity of the diffusion path. Consequently, the composite films exhibited a UV-B blocking efficiency exceeding 96%, antibacterial inhibition rates up to ∼100% (depending on ZnO loading) against Escherichia coli and Staphylococcus aureus , and a moderate DPPH-radical scavenging activity (approximately 20%). Specifically, the PBZ-3 film (containing 3 wt% ZnO) significantly enhanced the preservation of strawberries during 8 days of storage at 4 °C. Compared with commercial polyethylene (PE) packaging, the PBZ-3 film minimized weight loss to 12.84% and provided superior vitamin C retention and improved visual quality, while maintaining the critical advantage of biodegradability. Furthermore, soil burial tests demonstrated a 54% mass loss within 30 days, confirming a favorable balance between functional performance and biodegradability. These results indicate that the PVA/BSG/ZnO ternary composite film is a promising biodegradable active packaging material with moisture-barrier, UV-shielding, antibacterial, and extract-based antioxidant functions for fresh produce.