Characterization of hydrogels formed by non-toxic chemical cross-linking of mixed nanofibrillated/heat-denatured whey proteins

In the current research, whey protein isolate (WPI) solution was nanofibrillated or denatured by heating at pH 2.0 or 8.0, respectively. The formation of whey protein nanofibrils with a nanometric thickness and micrometric length was confirmed by atomic force microscopy. Subsequently, different concentrations of citric acid (0–200 mM) as a gelling agent was used to fabricate cold-set hydrogels from heat-denatured, fibrillated, and mixed fibrillated/heat-denatured protein solutions at pH value of 8.0. The fibrillated and mixed fibrillated/heat-denatured solutions required lower concentrations of citric acid to form self-supporting gels compared to the heat-denatured WPI. The formation of covalent bonds between the network-building protein units through the citric acid-induced gelation was confirmed by gel electrophoresis and Fourier transform infrared spectroscopy. Hydrogels made of nanofibrillated and mixed solutions were firmer, had a lower water holding capacity, and showed more degradation at the simulated gastric fluid than the gel from heat-denatured WPI. The increase in citric acid concentration also increased the firmness and gastric degradation stability of the hydrogel samples. The results also showed that the hydrogels in the presence of nanofibrils were degraded much more than the fibril-free gel samples in the simulated gastric condition. Generally, these findings suggested that the combination of nanofibrillation and citric acid-mediated cross-linking could be employed to fabricate hydrogels with excellent techno-functional attributes.