Improving the Active Function of Polylactic Acid (PLA) Packaging Films Coated by Pectin Layers Enhanced with Gallic Acid

This study investigated the feasibility of imparting active functions to commercial polylactic acid (PLA) packaging films through a coating process and the resulting impact on pasteurized milk quality. PLA films were coated with citrus or apple pectin film-forming solutions, plasticized with glycerol, and enriched with gallic acid. The effects of this coating on the thickness, optical, barrier, structural, wetting, and antioxidant properties of the PLA films were examined. The thickness of the coated films increased from 30.0 to 61.3 µm and their visual surface roughness increased, while the addition of gallic acid significantly improved their UV-VIS light-barrier capacity and antioxidant activity against ABTS and DPPH radicals. Color analysis revealed a decrease in film lightness (the L* parameter decreased from 91.0 to 90.2 after modification with apple pectin-based coatings) and a color shift toward green (negative a* parameter) and yellow (positive b* parameter). The total color difference measured using the control PLA film (uncoated PLA) as a reference was significantly lower for coated samples based on citrus pectin (0.3–0.5) than those based on apple pectin (1.6–1.7), with no significant effect of the addition of phenolic acid. The water vapor permeability of the PLA films increased from 6.8 ∙ 10⁻¹¹ g∙m⁻¹∙s⁻¹∙Pa⁻¹ to 8.2–9.4 ∙ 10⁻¹¹ g∙m⁻¹∙s⁻¹∙Pa⁻¹ and 10.1–10.6 ∙ 10⁻¹¹ g∙m⁻¹∙s⁻¹∙Pa⁻¹ for samples coated with apple and citrus pectin without and with gallic acid, respectively. However, the oxygen permeability of PLA films significantly decreased from 30.7 ∙ 10⁻¹⁶ g∙m⁻¹∙s⁻¹∙Pa⁻¹ to 0.4–0.8 ∙ 10⁻¹⁶ g∙m⁻¹∙s⁻¹∙Pa⁻¹ for coated samples. This tendency aligned with the results of carbon dioxide permeability, which was reduced from 39.1 ∙ 10⁻¹⁶ g∙m⁻¹∙s⁻¹∙Pa⁻¹ to 0.7–0.9 ∙ 10⁻¹⁶ g∙m⁻¹∙s⁻¹∙Pa⁻¹ for coated films. Water contact angle measurement showed the hydrophilic nature of all PLA samples (contact angles less than 90°). The application of analyzed control and coated PLA packaging films slowed milk souring during storage, as evidenced by reduced quality changes and delayed curd formation. The application of PLA coated with apple pectin containing gallic acid, as well as citrus pectin with or without gallic acid, resulted in the lowest pH changes after 8 days of storage (6.6 and 6.7), demonstrating the superior efficacy of these films in maintaining milk quality.