Impacts of tannin inclusion on fabrication, characterization and antioxidant activity of sodium alginate-silk fibroin-tannin films and their application on fresh-cut apple packaging

The current work aimed to formulate sodium alginate-silk fibroin-tannin films to preserve fresh-cut apples. In this experiment, tannins (0.1, 0.2, 0.3, 0.4, 0.5%) were added to the sodium alginate (1.05%)-silk fibroin (3.38%) films by casting method. Fourier transform infrared spectroscopy, molecular docking was employed to investigate the presence of hydrogen bonding among the sodium alginate, silk fibroin and tannins. Scanning electron microscopy showed surfaces and cross-sections became porous and rough with increasing tannins concentration. The films were characterized by mechanical properties, light barrier properties, water solubility and water vapor permeability. The value of maximum tensile strength and minimum elongation at break was 0.99 MPa and 91.38% respectively for the sodium alginate-silk fibroin-tannin films with 0.3% (w/v) tannins. The percentage of light transmittance of sodium alginate-silk fibroin-0.3% tannin films was 54.45% at 500 nm. The water solubility rate of sodium alginate-silk fibroin-0.3% tannin films at pH 8.0 was improved 54.42% compared with that at pH 1.2. The value of minimum water vapor permeability of the sodium alginate-silk fibroin-0.3% tannin films was 1.49 × 104 g mm/cm2∙s∙Pa. The sodium alginate-silk fibroin-tannin films showed higher scavenging ability of 2,2-diphenyl-1-picrylhydrazyl and superoxide anion radical than sodium alginate-silk fibroin films. The sodium alginate-silk fibroin-tannin films had antibacterial activity against Staphylococcus aureus and Escherichia coli. The apples packed with the sodium alginate-silk fibroin-0.3% tannin films did not rot during seven days. The fabricated films of sodium alginate-silk fibroin-tannin in this study are biodegradable and they may be a promising food packaging material.