Formation, physicochemical properties, and comparison of heat- and enzyme-induced whey protein-gelatin composite hydrogels

Hydrogels are widely used in the food industry due to their desirable physicochemical and functional attributes. The combination of two or more biopolymers can be used to formulate hydrogels with novel or improved attributes. In this study, whey protein isolate (WPI) and gelatin were used to prepare composite hydrogels using either heat- or enzymatic-crosslinking methods. For heat-induced gelation, gelatin-WPI mixtures were heated above the thermal denaturation temperature of the whey proteins to unfold and crosslink them. For enzyme-induced gelation, transglutaminase was added to crosslink both the gelatin and heat-denatured whey proteins in the gelatin-WPI mixtures. The effects of pH and crosslinking methods on the formation and properties of the hydrogels were investigated. The initial pH of the WPI solution influenced the thermal aggregation of the proteins by affecting the electrostatic interactions between them, thereby influencing the macroscopic properties of the hydrogels formed. For instance, heat-induced hydrogels with a higher elastic modulus were formed at pH 6 (1767 Pa) than at pH 7 (866 Pa) or pH 8 (735 Pa). The enzyme-induced hydrogels set more rapidly and had a considerably higher elastic modulus at pH 7 (2566 Pa) and pH 8 (1566 Pa) than the heat-induced ones. The enzyme-induced hydrogels also exhibited greater hardness and swelling, but the heat-induced ones exhibited better freeze-thaw stability. By changing the pH and crosslinking methods, the structure and texture of the composite hydrogels could be controlled, which may allow these systems to be used for a broad range of commercial applications in food and other industries.