Fabrication and characterization of kidney bean (Phaseolus vulgaris L.) protein isolate–chitosan composite films at acidic pH

The kidney bean protein isolate-chitosan (KPI/CH) composite films were fabricated at acidic pH to design potential carriers for antimicrobials (e.g., nisin). Tensile and thermo-mechanical properties, surface hydrophobicity and surface free energy (γ) as well as microstructure of the composite films were evaluated. A peelable and self-supported film was obtained for the KPI alone, exhibiting the typical stiff behavior. In contrast, the complex between KPI and CH produced less rigid and much more flexible films, with decreased elastic modulus (EM), storage modulus (E′) and glass transition temperature (Tg). The inclusion of CH enhanced surface hydrophobicity of the films. Concomitantly, the surface free energy (γ) of the films exhibited a reduced tendency. Upon increasing CH-to-KPI ratios, the microstructures of the films changed gradually from protein continuous to bi-continuous, CH continuous pattern. Moreover, the KPI/CH blend films exhibited a visible compositional gradient, reflecting that the CH was rich in near-surface of the films, especially air side. Antimicrobial activity of the films was also analyzed, indicating the test bacteria, Bacillus subtilis, Escherichia coli and Salmonella were sensitive to KPI/CH composite films, in a CH-to-KPI dependent manner. The results presented here supported that the KPI/CH blend films are expected to serve as antimicrobial packaging for food or the carrier of antimicrobials (e.g., nisin).