Preparation and characterization of chitosan/gelatin composite films incorporated with four natural pigments

Abstract: In order to obtain antioxidant active packaging materials, chitosan/gelatin (CG) composite films incorporated with 4 kinds of natural pigments were developed. The 4 kinds of natural pigments were extracted from red cabbage, black rice, rose and blueberry, respectively, and the anthocyanins content in each pigment was measured. The films were prepared by casting and solvent-evaporation method. The composite film-forming solutions were prepared by dispersing chitosan (1%) in the aqueous solution of 0.5% glacial acetic acid mixed with 1% gelatin at the volume ratio of 7:3. As for the film, the final concentration of anthocyanins in the film was established as 25% of the total volume of the mixture of hydrogels. The physical, mechanical, antioxidant and structural properties of the composite films were also investigated. Physical and mechanical properties of the films showed that the CG film had a lower water-vapor permeability (10.69×10-11 g/(m·s·Pa)) when not incorporated with natural pigments. The incorporation of rose pigments led to the reduction of the water-vapor permeability while the other extracts did inversely. The lowest water-vapor permeability value was 10.05×10-11 g/(m·s·Pa) and the incorporation of blueberry pigment reached the maximum level of 14.52×10-11 g/(m·s·Pa). Furthermore, the addition of rose pigment led to a reduction of elongation at break but an increase of tensile strength, with the lowest elongation at break of 37.66% and the greatest tensile strength of 27.03 MPa. However, black rice pigment extract led to an increase of elongation at break from 52.59% to 57.67% compared with CG film without incorporating natural pigments. Fourier transform infrared spectroscopy (FTIR) showed that the incorporation of natural pigments brought about the interactions between its hydroxyl groups and the amino groups of chitosan. Significant structural transformation could be observed from scanning electron microscope (SEM), and each film showed homogeneous surfaces without pores and with excellent structural integrity, indicating a high compatibility between the components. DPPH (1,1-Diphenyl-2-picrylhydrazyl radical 2,2-Diphenyl-1- (2,4,6-trinitrophenyl)hydrazyl) radical scavenging activity assay was selected to evaluate the antioxidant activity of the films. The antioxidant activity of the films increased significantly with the addition of natural pigments. The film incorporated with rose pigment showed a higher antioxidant activity whose DPPH radical scavenging activity reached 95.47%, while the incorporation of black rice pigment led to a lower antioxidant activity of 39.98%. Moreover, the addition of natural pigments caused significant changes in moisture content, water solubility, thickness and color. The incorporation of red cabbage extract increased the film's moisture content and water solubility compared with CG film without incorporating natural pigments, which reached their maximum levels of 23.56% and 28.42%, respectively. Natural pigments exist in a wide variety of flowers, vegetables and fruits, it is available and inexpensive. The results indicate that incorporating natural pigments with CG film is a feasible way to develop new composite film for active packaging.