Preparation of Soy Isolate Protein-Based pH-Sensitive Intelligent Indicator Film and Its Application in Monitoring Milk Freshness

In this study, a novel pH-sensitive intelligent indicator film (SPI/PVA-BA film) composed of soybean protein isolate (SPI), polyvinyl alcohol (PVA), and blueberry anthocyanin (BA) was prepared via a casting method for freshness monitoring of pasteurized milk. The microstructure, mechanical properties, moisture content, water solubility, ultraviolet (UV) transmittance, water vapor permeability (WVP), and pH sensitivity of the indicator film with different BA levels were characterized. The results showed that the film containing 12% BA exhibited the highest elongation at break (105.12%), while that containing 8% BA had the highest tensile strength (11.58 MPa) and showed the best color sensitivity to pH changes. The pH decline in pasteurized milk during storage for 72 hours at (22 ± 1) ℃ caused color changes in the film containing 8% BA, showing that this film could reflect the freshness of pasteurized milk. Additionally, the indicator film was almost completely degraded after being buried in soil for 10 days, demonstrating its good biodegradability. Therefore, this intelligent packaging film not only holds potential as a freshness indicator, but also contributes to reducing environmental pollution associated with bio-based plastics.