Contrasting Effects of Chitosan Molecular Modifications on the Properties of Fish Gelatin Hydrogels

The limited water solubility of chitosan, which necessitates acidic conditions for its dissolution, restricts its application in food and pharmaceutical systems. To overcome this limitation, we chemically modified a chitosan molecule by introducing hydrophilic groups and compared three chitosan forms—unmodified chitosan, its quaternary ammonium derivative, and a graft copolymer with poly-N-vinylpyrrolidone, with respect to their effect on the structural and mechanical properties of fish gelatin hydrogels. An experimental study combining NMR relaxometry and water self-diffusion measurements, scanning electron microscopy, and rheology revealed a distinct supramolecular architecture of gelatin–chitosan hydrogel network for different chitosan modifications. The NMR data gave information on the molecular mobility of water as a marker of hydrogel structure to elucidate molecular basis for the observed rheological properties. Unmodified chitosan maximally reinforces gelatin network and enhances its thermorheological stability. Chitosan grafting with poly-N-vinylpyrrolidone produces a fine-pored xerogel structure with higher thermorheological stability, possibly through a non-cooperative reinforcement mechanism. The quaternary ammonium derivative results in a heterogeneous xerogel morphology with weak mechanical strength and low gel–sol transition temperature. These findings establish the structure–property correlation, demonstrating that chitosan modification governs the molecular network architecture and macroscopic performance of gelatin-based hydrogels, providing molecular-level insights into the behavior of fish gelatin–chitosan systems.