The structure-function relationship of composite films obtained from soy protein isolate (SPI) and cod gelatin was studied. Films with different ratios of SPI:gelatin (0, 25, 50, 75, 100% w/w) plasticized by a mixture of glycerol and sorbitol, were prepared by casting. Regardless of the soy protein concentration, the thickness and water vapor permeability of the composite films diminished significantly as compared to pure gelatin films. The formulation containing 25% SPI: 75% cod skin gelatin had the maximum force at the breaking point which was 1.8-fold and 2.8-fold of those of 100% gelatin and 100% SPI films, respectively. Moreover, this formulation offered high percent deformation values as the 100% gelatin film and relatively low water vapor permeability as the 100% SPI film. While all the films exhibited high water solubility, a slight reduction of the protein released in water was observed with increasing soy contents. DSC analyses revealed that gelatin was completely denatured in all the films, while the soy proteins maintained most of their native conformation. The FTIR analysis revealed the presence of 25% SPI to produce gelatin conformational changes, self aggregation of gelatin α-chains as well as intermolecular associations through C=O bonds between gelatin and SPI proteins.. Regarding appearance, all the films were translucent, but the yellowish color (b*) increased with increasing proportions of soy proteins.

This work studied the preparation of edible smart films based on gelatin and curcumin. Films were prepared by casting using water and an ethanol-water mixture as solvents. The addition of curcumin, besides affecting the physicochemical properties of gelatin films, colored them depending on the pH of the film-forming dispersion (yellow at pH=6 and red at pH=11). It also provided films with important antioxidant properties, but no antimicrobial activity. The response of these materials against pH changes was evaluated simulating their contact with liquid and semisolid foods, and with a container headspace at acid and alkaline pH. In all tests, gelatin films with curcumin added could modify their color after being in contact with media of different pH. The use of an ethanol-water mixture as solvent was a good alternative to intensify film color and the visualization of their response capacity against pH changes, as well as to increase the antioxidant properties and hydrophobicity of films. These edible films could be used as smart food packaging, since they could inform consumers if the product was suitable for consumption through their capacity to sense pH changes.