Preparation of Double-Layered Nanofiber Film Loaded with Bayberry Pomace Anthocyanin Extract and Its Application in Shrimp Freshness Monitoring

Pullulan (PUL) and polyvinyl alcohol (PVA)-based nanofiber film (PUL-PVA-BAE) loaded with bayberry pomace anthocyanin extract (BAE) has good pH response. However, due to the high hydrophilicity of PUL-PVA-BAE, its stability needs to be improved for its application as a freshness indicator label in high humidity environments. In this study, a double-layered nanofiber film (ZG/PUL-PVA-BAE) was prepared by incorporating a hydrophobic layer consisting of zein and gelatin (ZG) onto PUL-PVA-BAE, and its physical properties, structure, color response, stability, and application were studied. The results showed that the surface of the double-layered film was smooth with uniform fiber distribution, in which PUL-PVA-BAE was successfully fixed on the hydrophobic layer through intermolecular hydrogen bonding. After the addition of the hydrophobic layer, the water resistance and mechanical properties of the nanofiber film were improved. Specifically, the water solubility decreased from 84.01% to 63.62%, and water vapor permeability from 2.52 to 2.01 g·mm/(m²·Pa·s·10⁵); the tensile strength increased from 1.74 to 2.64 MPa, and elongation at break from 15.08% to 23.29%. There were no significant changes in pH sensitivity, color response, reversibility, or stability between the single-layered and double-layered films. The double-layered film could discriminate the freshness state (fresh, sub-fresh, and spoiled) of Penaeus vannamei stored at 4 ℃ based on color changes (purple red, gray purple or gray green and green), which showed similar color changes to the single-layered film. In conclusion, the double-layered nanofiber film still functions as a freshness indicator while showing improved morphological stability in high humidity environments, offering technical support for aquatic product freshness monitoring.