Development and Application of a Novel pH-Responsive Bilayer Indicator Film for Yellowfin Seabream Preservation and Freshness Monitoring

A pH-responsive bilayer film was developed for real-time freshness monitoring and preservation of yellowfin seabream. The emulsified layer contained chitosan (CS) and flaxseed oil (FO), while the indicator layer comprised carrageenan (CAR), gelatin (GEL), grape seed anthocyanins (GSA), and curcumin (CUR). Optimization via response surface methodology determined the ideal formulation: CAR/GEL mass ratio 1.11:1, CS concentration 1.70%, and GSA/CUR dosage 53.99 mg/100 mL. The optimized film demonstrated superior mechanical properties (TS = 12.74 MPa, EAB = 68.24%), enhanced hydrophobicity (WVP = 1.21 × 10⁻¹¹ g·m⁻¹·s⁻¹·Pa⁻¹), and potent antioxidant activity (HRC = 92.35%). FTIR and SEM confirmed stable cross-linking and bilayer compatibility. Distinct color transitions (yellow → reddish-brown) occurred at different pH levels, correlating with fish spoilage indicators. During 25°C storage, the film effectively inhibited quality deterioration (TVB-N, TBARS, moisture loss, lipid oxidation) while extending shelf-life. Strong correlations were observed among TVB-N, TBARS, moisture (|r| > 0.97), and PUFAs’ spoilage contribution (r ≈ −0.99). This intelligent film enables dual-functionality: active preservation and visual freshness monitoring.