Insight into the formation mechanism of soy protein isolate films improved by dialdehyde starch with different degrees of aldehyde substitution

Protein and polysaccharide materials are gaining attention as potential solutions for environmental pollution and reducing dependence on fossil resources. In this research, films were formed by crosslinking soy protein isolate (SPI) with dialdehyde starch (DAS) of varying aldehyde group contents. The aim was to evaluate the influence of the content/degree of aldehyde groups of DAS upon the basic properties of SPI-based films and propose a formation mechanism. Results showed that adding DAS improved the hydrophobicity, UV-blocking properties, and thermal stability of the films. The tensile strength of the film was increased from 2.95 to 7.94 MPa. The reaction between DAS and SPI formed Schiff base reactions and covalent imine bonds (CN), along with a network of hydrogen bonds. In addition, atomic force microscopy indicated that the addition of DAS resulted in an increase in the surface roughness of the films to 19.2 nm, favoring hydrophobicity and UV-barrier properties. These results provide a solid theoretical basis for further research on SPI-DAS composites.