Development and characterization of antimicrobial protein films based on soybean protein isolate incorporating diatomite/thymol complex

Abstract Diatomite was used as an effective carrier of thymol to obtain diatomite/thymol (D/T) complex, which was further incorporated into soybean protein isolate (SPI) to develop antimicrobial protein films. The as-prepared D/T complex was characterized by scanning electron microscopy (SEM), Fourier transformed infrared (FTIR) spectroscopy, laser particle size analyzer and thermogravimetric analyzer (TG). It was demonstrated that thymol was adsorbed into the pores and loaded onto the surface of diatomite, and the content of thymol in the D/T complex was estimated to be about 39%. Then, the D/T complex was incorporated into SPI, and the morphological, physical and functional properties of antimicrobial protein films were investigated in comparison with those of SPI films incorporating diatomite or thymol alone. The FTIR and SEM analysis indicated the improved dispersibility and compatibility of D/T complex in the SPI matrix. Consequently, the film containing D/T complex had moderate mechanical and water barrier properties among all the films. For instance, its tensile strength was 1.53 MPa higher than that of film containing thymol, and elongation at break was 39.98% higher than that of film containing diatomite only. More importantly, compared to the SPI film incorporating thymol alone, diatomite was found to reduce the loss of thymol during the film formation and storage, and a sustained release of thymol from D/T complex in film was also observed, thus noticeably prolonging its antimicrobial effect against on Escherichia coli even until 100 h. In conclusion, the obtained antimicrobial SPI film containing D/T complex with tunable physical properties and antibacterial activity showed great potential for active food packaging.