Physical and mechanical properties of compression molded and solution casting soybean protein concentrate based films

Soybean protein concentrate-based films plasticized by glycerol were obtained by two processing methods: intensive mixing followed by compression molding and solution-casting. Film forming conditions such as molding temperature, molding pressure, drying conditions as well as glycerol level were determined. The effect of the forming method on the physical and mechanical properties of the resultant films was analyzed in terms of color, light transmission, tensile properties, water solubility and water vapor and oxygen barrier properties. Thermo-pressed soy protein concentrate films were significantly more transparent, less soluble, more stretchable and had lower water vapor permeability but greater oxygen permeability coefficient than solution casting films at the same plasticizer level. These results were associated with the intermolecular forces involved in the formation of the films. Hydrophobic interactions and hydrogen bonding dominated the formation solution-casting films, whereas disulphide bonding played a more important role in the formation of compression molded films, as revealed by solubility of obtained films in denaturing solutions and infrared spectroscopy. This study demonstrates that forming process plays a major role in determining the final properties of soy protein concentrate-based films and reveals the possibility of soy protein concentrate-glycerol mixtures to be transformed through thermo-mechanical processing into biodegradable films with potential application in food packaging.