Swelling behaviour, charge and mesh size of thermal protein hydrogels as influenced by pH during gelation

We provide new information on the structural properties of thermal protein hydrogels. It is shown that the pH value during gelation strongly affects the pH-dependent charge and the isoelectric point of the generated hydrogels and thus their pH-dependent swelling behaviour and mesh size. Whey protein hydrogels were thermally generated at various pH values and their swelling kinetics, equilibrium swelling ratio and charge were determined as a function of pH. The apparent mesh size was calculated based on the equilibrium swelling ratio of the hydrogels using a variation of the Flory-Rehner method and was dependent on both the pH during gelation and the pH of the swelling medium. The lowest equilibrium swelling ratio and thus the mesh size of the hydrogels coincided with their pI, which was dependent on the pH during gelation. Those hydrogels prepared at a pH value above the pI of the native whey proteins (pH 5.1) showed a shift of their pI towards acidic values below pH 5.1. This indicates that the pH during gelation has a strong impact not only on the resulting network structure like density and nature of cross-links but also on the pH-dependent hydrogel charge. Thus, the pI of the hydrogel, and not the pI of the native protein solution, has to be considered to estimate the pH-dependent swelling behaviour of a protein hydrogel. The results of this study are of relevance for the basic understanding of the protein hydrogel structure as well as for engineering protein hydrogels as biopolymer-based drug carriers with specific drug release properties.