Preparation and characterization of a pH-responsive smart film based on soy lipophilic protein/hydroxypropyl methylcellulose for salmon freshness monitoring and packaging

Spoilage of aquatic products can lead to social problems such as environmental problems and health problems, and timely preparation of smart active packaging that can maintain and detect the freshness of aquatic products is crucial in the food field. For the first time, we developed a bilayer active packaging with soy lipophilic protein-hydroxypropyl methylcellulose (LP-HPMC) as the main film matrix and nanoemulsion (NE) prepared by LP (thyme essential oil and shikonin (SKN) as the oil phase) as the functional layer. The results have proven that adding NE enhances the water-blocking performance, thermal stability, antioxidant, and antibacterial properties of the LP-HPMC film. In addition, SKN and thyme essential oil had synergistic antioxidant and antibacterial effects. Compared with simply adding SKN, the NE film showed excellent indicating stability for pH and ammonia. When applied to salmon, the bilayer film had a better antimicrobial effect than the single-layer film due to its unidirectional release, which reduced microbial growth more effectively and monitored the freshness of the salmon through the discoloration effect. The color change process of the bilayer film attached to salmon of different freshness was recorded, and deep machine learning was performed using the VGG-16 model to quickly identify the "fresh" and "spoiled" salmon. This kind of multi-functional active packaging prepared based on pure natural substances has excellent prospects for preserving the freshness of aquatic products.