Intelligent double-layer films based on gellan gum/mica nanosheets/anthocyanin/konjac glucomannan/carrageenan for food real-time freshness monitoring

In this work, double-layer intelligent films were designed and synthesized based on Broussonetia papyrifera fruit anthocyanin (BPFA), mica nanosheets (MN), konjac glucomannan (KG), carrageenan (CA) and gellan gum (GG) using layer-by-layer casting strategy for real-time and in situ food freshness monitoring. The KG/CA/BPFA layer served as the indicator layer, while the GG/MN layer acted as the protective outer layer. It was observed that the water vapor permeability value of the GG-MN/KG-CA-BPFA double-layer film was reduced by 4.7 times, and the tensile strength was increased by 4.1 times compared with the GG film, which could be related to the reinforcing effect and proper barrier property of MN. Notably, the double-layer film blocked most of UV-light and oxygen, and its water contact angle reached 102.4° ± 1.18°. In particular, the films showed favorable visually detectable colorimetric responses to different pH buffer solutions and volatile ammonia. More importantly, visual assessment using shrimps as test specimens demonstrated distinct color alteration from red to blue-grey on spoilage. The color change of the film was also highly correlated with TVB-N (R² = 0.964) and pH (R² = 0.999). In conclusion, the newly designed double-layer film exhibited excellent performance, which guides an innovative approach for advanced food-packing material development.