Abstract: To address increasingly severe plastic pollution, this study aimed to develop eco-friendly bio-based packaging materials. Carboxymethyl chitosan (CMCS) and soy protein isolate (SPI) were selected as film-forming substrates, and bioactive berberine (Ber) was incorporated to fabricate biodegradable and functionally active SPI-CMCSn-Berm composite films (n = 1, 2, 3; m = 0.5, 1.0, 1.5). The effects of different mass ratios of SPI to CMCS (1:1, 1:2, and 1:3) and various Ber contents on the properties of the composite materials were systematically investigated. The films were prepared via solution blending, and their chemical structure, crystalline morphology, thermal stability, appearance, and barrier properties were comprehensively characterized and analyzed. The results showed that the tensile strength and elongation at break of the SPI-CMCS1 group reached 1.8 MPa and 60%, respectively. After Ber incorporation, the mechanical properties of the film slightly decreased, while its overall stability was enhanced. X-ray diffraction and Fourier-transform infrared spectroscopy confirmed the formation of effective molecular cross-linking between Ber and the SPI/CMCS matrix. The SPI-CMCS1-Ber1.0 group exhibited the optimal light barrier performance with a transparency value of 3.17. Appropriate Ber addition reduced the water solubility and swelling degree of the film and improved its 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity. During storage at 4 ℃ for 10 d, the total volatile basic nitrogen content in tilapia fillets treated with the composite film remained below the safe limit of 20 mg/100 g. The total color difference of the SPI-CMCS1-Ber1.5 group was only 12.81 with a low browning index. Low-field nuclear magnetic resonance results revealed that the SPI-CMCS1-Ber1.0 group maintained a stable proportion of immobile water, with free water at only 5.5%, which effectively reduced the centrifugal and cooking loss rates of fish muscle. At the end of storage, the relative content of α-helix in myofibrillar proteins in the coated groups reached up to 74.40%, while the relative content of random coils decreased to 7.60%, indicating effective inhibition of the order-disorder transformation of protein secondary structure. Meanwhile, the fluorescence intensity of protein tertiary structure was maintained, delaying conformational deterioration. In conclusion, Ber-modified SPI-CMCS composite film can retard protein denaturation, water loss, and quality deterioration of tilapia fillets during refrigeration, rendering it an excellent biodegradable active packaging material for aquatic products. This active composite film has great potential to replace conventional plastic packaging in aquatic product preservation, providing a theoretical basis for the research and development of green packaging materials.

Key words: soy protein isolate; carboxymethyl chitosan; berberine; tilapia; biodegradable film