Lipid phase behavior governs microstructural retention and sustained α-tocopherol release in pullulan composite films for walnut kernel preservation.

The entrapment of lipophilic bioactives within hydrophilic polymer matrices presents physical challenges regarding phase separation and uncontrolled release. This study evaluates a strategy based on tuning lipid phase behavior to modulate the microstructural evolution of α-tocopherol-loaded nanoemulsions and derived pullulan films. A 40% coconut oil proportion induced a transition from a disordered supercooled liquid to an ordered viscoelastic fluid. This state facilitated the formation of stable nanodroplets with a mean diameter of 18.59 nm and a polydispersity index of 0.24 by reducing the interfacial tension to 5.07 mN·m-1 and the critical micelle concentration to 0.01 g·L-1. Conversely, a 60% coconut oil proportion caused microstructural crowding and interfacial bridging. This highly homogeneous architecture was preserved within the solid film, yielding a tensile strength of 26.51 MPa, an elongation at break of 15.14%, and reduced permeabilities for oxygen (0.14 × 10-4 g·m-1·s-1) and water vapor (1.04 × 10-13 kg·m-1·s-1·Pa-1). Kinetic modeling confirmed a quasi-Fickian diffusion profile in which the embedded nanodomains functioned as steric obstacles. This tortuous diffusion path restricted solute mobility, suppressing the initial release and achieving an 88.88% retention of tocopherol activity. Application on walnut kernels demonstrated preservation efficacy, evidenced by a 39.94% reduction in peroxide value and a 38.56% decrease in thiobarbituric acid reactive substances compared to the control group. Regulating internal lipid phase behavior provides a predictable microstructural framework for developing active packaging systems to extend the shelf life of lipid-rich foods.