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

Lipid phase behavior governs microstructural evolution of nanoscale emulsions. • Ordered viscoelastic fluid states enable nanodroplet stabilization. • Nanoscale emulsion architecture is preserved within solid pullulan matrices. • Embedded lipid nanodomains function as steric obstacles to retard solute diffusion. • Phase-engineered composite films intercept oxidative cascades in walnut kernels. 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. Download: Download high-res image (205KB) Download: Download full-size image