Alginate and pH Improve Properties of Soy Protein-based Films

Soy protein isolate (SPI) and its mixture with other biopolymers, such as linear polysaccharides, have been considered as potential components to form biodegradable packaging. Films based on SPI (5% m/m film-forming solution) were prepared with the addition of sodium alginate (ALG) at different concentrations and pH values. The mechanical properties and oxygen permeability were studied and correlated with the crystallinity (X-ray), Fourier transform infrared (FT-IR) spectroscopy and the microstructure (scanning electron microscopy - SEM) of the films. The addition of ALG caused expressive changes in the structural arrangement of the SPI films, especially at pH 6 and 8. The FT-IR spectra of the films also showed the effects of the pH and ALG concentration on the relative intensity of the protein absorption bands, indicating stronger interaction between SPI and ALG at pH 11 than at pH 6 and 8. The resistance of the films was improved with the increasing addition of the polysaccharide, and with the highest alkalinity. Oxygen permeability evaluation of the films formed at pH 11 showed a significant decrease in permeability with increasing incorporation of ALG into the film matrix. The results indicate that the structure and the properties of the SPI-based films were modified and improved with ALG. It was concluded that alginate incorporation in SPI-based films with pH 11 improves mechanical and oxygen barrier properties.