Preparation and Properties of Deep Eutectic Solvent-Modified Soy Protein and Soy Protein–Chitosan Composite Films

Conventional non-biodegradable plastic packaging causes severe environmental pollution, driving the demand for degradable biomass food packaging. Chitosan (CS) is an ideal green film material but suffers weak mechanical, water-resistance and antioxidant properties. Soy protein isolate (SPI) is cheap and abundant; however, its compact molecular structure leads to poor miscibility with pure CS. Herein, ultrasonic-assisted deep eutectic solvent (DES) pretreatment was used to unfold SPI’s protein structure and release active sites for stronger SPI–CS interfacial interactions. Citric acid and transglutaminase (TG) served as dual green crosslinkers, while carvacrol (Car) and gallic acid (GA) were added as natural polyphenol antioxidants. Single-factor and Box–Behnken response surface experiments determined the optimal film-forming parameters: a citric acid mass ratio of 0.84, a TG dosage of 2.60 U∙g−1, and a drying temperature of 65 °C. The optimized DES–SPI–CS film possessed a tensile strength of 18.79 MPa and an elongation at break of 41.39%, with greatly enhanced water resistance and thermal stability. After co-loading Car and GA, the composite film reached 79.33% DPPH radical-scavenging activity. Twelve-day cherry tomato preservation tests confirmed its capacity to slow vitamin C loss and fruit decay. This work provides a sustainable integrated strategy combining DES modification, dual crosslinking and natural antioxidant doping to fabricate multifunctional bio-based packaging, offering theoretical support for biodegradable film substitution of disposable plastics.