Films based on proteins and polysaccharides: Preparation and physical–chemical characterization

Soy protein isolate (SPI) and agar (AG) obtained from Gelidium sesquipedale (Rhodophyta) were used to prepare films by using a thermo-molding method. In all cases, flexible and transparent films were obtained and physico-chemically characterized. On one hand, Fourier Transform Infrared (FTIR) analysis provided a detailed description of the binding groups present in both polymers. The second derivatives of the bands in FTIR spectra showed changes in peak positions and shapes, which indicate changes in the secondary structure of SPI due to its interaction with agar. On the other hand, the surface composition of the films was examined by X-ray photoelectron spectroscopy (XPS). By comparison of the percentages of C, O, and N, it could be observed that the surface composition of the films was different. Moreover, it was observed that hydrophobic character of the surface increased with the incorporation of agar.