pH-responsive active packaging based on chitosan films embedded with grape skin anthocyanins/ thymol nanoparticles:...

In response to the environmental pollution caused by traditional plastic packaging and the need for real-time monitoring of food freshness, we developed a pH-responsive active packaging film loaded with nanoparticles. Using an antisolvent precipitation method, grape skin anthocyanin (GSA) was entrapped in the hydrophobic core of zein and thymol was adsorbed onto zein surface, followed by the coating of a protective outer layer of pullulan, successfully preparing GSA-zein-thymol-pullulan (GZTP) nanoparticles. Upon incorporation of the GZTP nanoparticles into a chitosan (CS) matrix, the performance of the resulting composite films (GZTP-CS) was significantly enhanced. The nanoparticles exhibited good interfacial compatibility with CS via intermolecular hydrogen bonding, and the mechanical properties were significantly enhanced (tensile strength increased from 7.48 MPa to 27.57 MPa, and elongation at break increased from 35.05% to 112.05%), whilst simultaneously improving the films' hydrophobicity, barrier properties and thermal stability. The composite films exhibited excellent antioxidant activity (57.13% DPPH and 85.02% ABTS⁺ radical scavenging), coupled with potent antibacterial effects (nearly 100% inhibition against Escherichia coli and 93.44% against Staphylococcus aureus). The films exhibited visually discernible color changes in response to pH, ammonia and acetic acid, turning from grey to blue-green in alkaline environments and from grey-purple to bright purple-red in acidic environments. In applications for preserving pork at 25 °C, the GZTP-CS films not only extended the shelf life by more than 24 h but also provided a real-time, intuitive indication of pork freshness through a color change, thereby demonstrating their potential for application in the field of smart food packaging.