A biodegradable dual pH/humidity-responsive film: gelatin-poly(lipoic acid) loaded with thymol for enhanced fruit preservation.

To mitigate postharvest fruit deterioration caused by oxidative browning and microbial spoilage, we developed an intelligent, eco-friendly thymol/gelatin-poly(lipoic acid) (TGP) composite film. The porous gelatin-poly(lipoic acid) (GP) matrix was fabricated by grafting gelatin (Gel) with poly(lipoic acid) (PLA) synthesized through ring-opening polymerization of lipoic acid (LA), achieving a grafting efficiency of 25.46%. Structural characterization via Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analysis validated the successful grafting and the formation of a porous architecture. Additionally, thymol (THY) was encapsulated within the GP matrix with an efficiency of 82.82%. Film performance tests demonstrated that the TGP composite film exhibited dose-dependent improvements in both functional properties (e.g., antioxidant and antibacterial activities) and physical characteristics (including barrier and mechanical performance), among which the TGP-0.9 film delivered the most outstanding overall performance. Notably, the release of THY was pH- and humidity-responsive, accelerating under slightly acidic and high-humidity conditions that simulated decaying fruit microenvironments. In preservation experiments, the TGP-0.9 film prolonged fruit freshness for at least 6 days. Collectively, this study presented a sustainable TGP composite film with dual functionality (preservation efficacy and environmental compatibility), offering a promising solution for postharvest fruit management.