Intelligent colorimetric soy protein isolate-based films incorporated with curcumin through an organic solvent-free pH-driven method: Properties, molecular interactions, and application

The preparation of intelligent colorimetric films based on soy protein isolate and curcumin at different final pHs was investigated by using blending and pH-driven methods with curcumin-free films as blank controls. It was found that the films loaded with curcumin using the pH-driven method exhibited enhanced ductility and water resistance with an elongation at break of 122 ± 6% and water vapor permeability of 1.70 ± 0.01 × 10−10 g·m/m2·Pa·s. The Fourier transform infrared (FTIR) spectroscopy results showed that curcumin was loaded in the film network mainly through hydrogen bonds and hydrophobic interactions. The curcumin encapsulated using the pH-driven method was present in an amorphous form, which effectively suppressed the adverse effect of curcumin blending on film opacity and increased the UV blocking ability of the films. The high encapsulation efficiency and reduced thermal degradation of curcumin resulted in enhanced antioxidant activity of the films with high 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging activities of 102.91 ± 0.83 Trolox equivalent (TE)/g and 211.47 ± 5.20 μg TE/g, respectively. Furthermore, films showed better color responsiveness to volatile amines than curcumin-blended films. In particular, the films generated with a film-forming solution at pH 8 had the strongest color sensitivity in the presence of low concentrations of volatile amines. In shrimp storage experiments, only the films prepared using the pH-driven method identified the freshness of the shrimp based on visual observation. Hence, the pH-driven method might represent a promising organic solvent-free method for the preparation of intelligent packaging films.