Phenolic-Enriched Pullulan Coatings: Molecular Interactions and Functional Properties for Active Food Packaging Applications

Sustainable active coatings based on renewable polymers are increasingly sought for food-packaging applications; however, surface-applicable colloidal coating systems remain markedly underexplored compared to conventional bulk films. In practical applications, coatings are applied as liquid colloidal dispersions, which subsequently form solid films at the food-material interface, where their functionality is ultimately expressed. A predictive understanding of coating performance, therefore, critically depends on a comprehensive characterization of both the colloidal state and the resulting film, an aspect that is often underestimated in current formulation-driven approaches. In this study, we report pullulan-based colloidal coatings functionalized with polyphenol-rich yerba mate (YE) and chestnut wood (WE) extracts, obtained via green ultrasound-assisted aqueous extraction. Distinct from conventional cast-film-centric studies, this work adopts a structure-property-driven strategy, systematically linking the physicochemical and colloidal properties of the liquid formulations to the interfacial, structural, and functional properties of the formed films. Such an integrated approach enables informed optimization and rational manipulation of coating performance already at the formulation stage, rather than relying on empirical surface deposition alone. HPLC analysis of the extracts identified chlorogenic, caffeic, rutin, and ellagic acids as the dominant phenolics governing bioactivity. The incorporation of YE and WE into pullulan significantly enhanced colloidal stability (ζ ≈ -25 mV; PDI ≈ 0.16) in dispersion, while, upon film formation, it reduced the water contact angle (54.6° vs 65° for neat pullulan) and increased surface free energy by 26.3%, indicating improved interfacial performance. ATR-FTIR and XRD analyses confirmed noncovalent pullulan-polyphenol interactions while preserving the amorphous polymer structure. The resulting coatings exhibited effective UV shielding, strong antioxidant activity (near-complete radical scavenging within 60 min), and antibacterial efficacy against <i>Staphylococcus aureus</i> (12 ± 2.8 mm inhibition zone). Overall, this work demonstrates that a combined colloidal-film characterization framework is essential for the rational design of functional biopolymer coatings and highlights pullulan-polyphenol dispersions as promising, biodegradable, and multifunctional active layers for sustainable food-packaging applications.