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

## TL;DR

This paper explores how adding plant-based polyphenols to pullulan coatings improves their performance for sustainable food packaging.

## Contribution

The study introduces a structure–property-driven strategy linking colloidal and film properties for optimizing active food coatings.

## Key findings

- Pullulan-polyphenol coatings showed improved colloidal stability and interfacial performance.
- The coatings exhibited strong antioxidant and antibacterial properties.
- Noncovalent interactions between pullulan and polyphenols were confirmed without altering polymer structure.

## Abstract

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 Staphylococcus aureus (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.