Carboxymethyl cellulose/sodium alginate bilayer films doped with citric acid and nano-ZnO/powdered activated carbon for walnut kernel preservation

Walnut kernels are highly susceptible to oxidative rancidity due to their elevated polyunsaturated fatty acid content; however, existing functional packaging predominantly provide single-mode protection, lacking hierarchical barrier systems specifically constructed for the oxidative characteristics of high-fat nuts. To bridge this gap, an eco-friendly bilayer composite film featuring an “outer oxygen-blocking/inner absorbing” architecture was designed. The outer layer was developed from a carboxymethyl cellulose–sodium alginate matrix cross-linked with 0.3% (w/w) citric acid to maximize oxygen impermeability, whereas the inner functional layer integrated 0.4% (w/w) nano-ZnO with 0.015% (w/w) powdered activated carbon (PAC) to endow UV-shielding capacity and to adsorb volatile lipid-oxidation products. At an optimized outer-to-inner mass ratio of 3:7, the bilayer film exhibited a tensile strength of 18.99 MPa, near-complete UV attenuation (>99%), water vapor permeability of 5.33 g m⁻² /(cm 2 48 h Pa), and oxygen transmission rate of 0.65 cm³ /(m²·day·bar). Additionally, the film demonstrated potent radical-scavenging activity against both DPPH (26%-71%) and •OH (83%-100%). Accelerated oxidation tests revealed that, relative to conventional polyethylene (PE) packaging, the composite film reduced acid value, peroxide value, and malondialdehyde content of walnut kernels by 28.18%, 16.53%, and 17.37%, respectively, during late storage, while markedly suppressing the formation of volatile aldehydes and acids. These findings establish a green and effective strategy for prolonging the shelf life of high-fat nuts.