Double-layer coating for nuts preservation: covalently grafted resveratrol for antioxidant activity and protein-polysaccharide complexes for high barrier properties.

High-fat nuts are highly susceptible to quality deterioration due to oxidative rancidity during storage. Existing single-function edible film coatings often struggle to simultaneously achieve efficient antioxidant activity and high barrier properties, limiting their practical effectiveness in nut preservation. This study developed a double-layer active coating that integrates active antioxidant capability with efficient physical barrier functionality. A pH-shift alkaline treatment method was employed to covalently bind the natural antioxidant Resveratrol (RES) to Egg White Protein (EWP), forming an active composite that served as the functional inner layer. Structural characterization of the covalent complex revealed that pH 9 treatment promoted the formation of a stable covalent complex between RES and EWP, increasing its DPPH radical scavenging rate by 39.8% compared to untreated EWP. Meanwhile, a blended system of zein and xanthan gum constituted a high-barrier outer layer, resulting in the double-layer coating-DL. The DL coating exhibited outstanding comprehensive performance, with water vapor transmission rate and oxygen permeability reaching 47.6% and 46.7% of those of the single-layer ZEIN coating, respectively. Its DSC profile also demonstrated improved thermal stability. In preservation studies on cashews and almonds, after 180 days of storage, the peroxide value (POV) and acid value (AV) of the DL coating treatment group were lower than those of other single-layer coating groups and the blank control group, effectively retarding the lipid oxidation process. This study confirms that optimizing the antioxidant activity of the active layer through pH-modulated covalent binding, combined with a double-layer structural design to achieve functional synergy, can significantly enhance the overall preservation performance of protein-based coatings. This strategy provides a new approach with both theoretical foundation and process feasibility for developing green packaging materials specifically designed for high-fat nuts.