Abstract:
The objective of this research was to investigate the effects of soy protein isolate-to-gelatin ratio, transglutaminase treatment, as well as glycerol concentration on properties of composite film. The effects of soy protein isolate-to-gelatin ratio and transglutaminase treatment on film properties were monitored in the first part of this study. Five soy protein isolate-to-gelatin ratios (100:0, 75:25, 50:50, 25:75 and 0:100) and three levels transglutaminase (0, 4 and 8 units/g protein) were used in film preparation. Tensile strength and elongation at break were found to increase with increasing gelatin concentration (p≤0.05). The composite films exhibited increased transparency as gelatin concentration increased. However, all composite film samples were less transparent than those films made from single type of protein. Reduced moisture content was demonstrated as gelatin concentration increased (p≤0.05). On the other hand, soy protein isolate-to-gelatin ratio was shown to have no significant effect on film thickness and water vapor permeability (p>0.05). Transglutaminase treatment resulted in films with increased thickness, tensile strength, elongation at break and water vapor permeability (p≤0.05), with a decrease in moisture content (p≤0.05). Transglutaminase-treated and untreated films were similar in transparency. Different enzyme concentrations (4 and 8 units/g protein) did not significantly affect film properties. Cross-linking of proteins resulted from transglutaminase treatment was confirmed by SDS-PAGE protein pattern. Decreases in band intensity of 84, 66 and 54 kDa proteins, which correspond to the α-, α- and β-subunits of soy β-conglycinin, were observed in the enzyme-treated films, with a concomitant increase in band intensity of proteins with higher molecular weight. The second part of this research was the study of the effect of glycerol concentration on film properties. Glycerol concentration was varied at the levels of 55, 60 and 65% by weight of protein. As glycerol concentration increased, film thickness, moisture content and elongation at break were found to increase significantly (p≤0.05) while tensile strength decreased with increasing glycerol concentration (p≤0.05). Glycerol concentration had no effect on water vapor permeability and transparency. From this study, it was revealed that the composite film with soy protein isolate-to-gelatin ratio of 25:75 and 60% glycerol that was treated by 8 units of transglutaminase/g protein was high in both tensile strength (2.59 MPa) and elongation at break (217.59%). This film sample possessed an S-shaped or type II moisture sorption isotherm, with reduced rate of moisture change in the 0.50-0.85 water activity range.