Reapplying native defenses: walnut kernel pellicle extract-loaded sodium alginate edible films for peeled fresh walnut kernel preservation

Walnut kernel pellicle is valorized as a functional ingredient in edible films. • Walnut kernel pellicle extract exhibits antioxidant and antibacterial activity. • Chitosan nanoparticles achieve 89.79% extract encapsulation. • Active coatings delay the oxidation and microbial growth in fresh walnut kernels. This study developed an active edible film based on sodium alginate, incorporating chitosan nanoparticles loaded with walnut kernel pellicle extract to enhance the storage stability of fresh walnut kernels. The bioactivities of walnut kernel pellicle extracts obtained using methanol, ethyl acetate, n-butanol, and petroleum ether were compared. Based on antioxidant and antibacterial activities, the methanol and n-butanol extracts (16 mg/mL) were selected for subsequent experiments and further characterized by non-targeted metabolomics. Chitosan nanoparticles were prepared via ionic gelation, and at a chitosan-to-sodium tripolyphosphate (TPP) ratio of 5:1, the nanoparticles encapsulating the MeOH extract exhibited the highest encapsulation efficiency, together with a relatively small particle size (475.17 nm) and a positive zeta potential (+34.27 mV), indicating good stability. Incorporating the extract-loaded nanoparticles into sodium alginate at a polymer-to-nanoparticle ratio of 10:1 enhanced the bioactivities, structural integrity, thermal stability, and mechanical properties of the film. When applied to peeled fresh walnut kernels, the composite film significantly delayed lipid oxidation and microbial growth. The 10 g/L sodium alginate formulation exhibited a higher oxidative induction time during storage, indicating better oxidative stability and preservation performance. This study provides an effective strategy for preserving fresh walnut kernels and an approach for valorizing walnut by-products.