Fabrication, characterization, and application of complex essential oils loaded bilayer films using PLA and pickering emulsions with SPI-chitosan nanoparticles as solid support

Chitosan (CS) and polylactic acid (PLA) are both commonly used packaging materials in the food industry. However, a bilayer film made from CS and PLA, simultaneously incorporated with a complex essential oil-loaded Pickering emulsion (PE) system, have not been widely investigated. Therefore, this study designed bilayer films consisting of PLA layer and a PE layer. In the PE layer, binary nanoparticles composed of soybean protein isolate (SPI) and CS were used as solid supports to load increasing amounts (0 %, 2 %, 4 %, 6 %, 8 %, v/v) of complex essential oils (carvacrol and thymol, CT). The films were labeled BF-CT0, BF-CT2, BF-CT4, BF-CT6, and BF-CT8. The characteristics and functional properties were evaluated, along with the quality changes in refrigerated pork wrapped with the films (BF-CT0 or BF-CT8). Results indicated that CT addition significantly enhanced the stability of the PE. The PE layer with higher CT doses exhibited a denser microstructure, more hydrophobic surface, improved elongation at break, and greater water resistance. CT addition significantly improved light and gas barrier functions. Notably, the BF-CT8 film demonstrated extremely low water vapor and oxygen permeability (1.25 ± 0.06 kg•m⁻¹•s⁻¹•Pa⁻¹ and 0.24 ± 0.01 kg•m⁻¹•s⁻¹•bar⁻¹, respectively). All films showed a similar semi-crystalline structure, and CT addition decreased crystallinity. As expected, BF-CT8 displayed enhanced antioxidant and antibacterial performance. Finally, BF-CT8 extended the shelf life of refrigerated pork from 3 days to 6 days compared to BF-CT0 and unwrapped pork. In conclusion, the bilayer film shows great potential for use in pork preservation.