Hydrogels based on gelatin–chitosan mixtures have great potential for practical application in the development of new materials in food technology and biomedicine. This study examines the effect of chitosan on the gelling properties, rheological, and structural characteristics of fish gelatin type A hydrogels in the acidic pH range of 3.2–3.9. It was shown that an increase in the chitosan-to-gelatin mass ratio up to 0.15 resulted in a growth in the hydrogel thermal stability and an increase in the elastic modulus, hardness, and yield stress. The structural strength of the fish gelatin–chitosan hydrogel increased due to the strengthening of the binding zones in the fish gelatin gel network in the presence of chitosan. According to scanning electron microscopy, the supramolecular microstructure of the gels demonstrated a significant compaction upon the addition of chitosan to fish gelatin. UV and IR spectroscopy data, as well as changes in zeta potential, showed the formation of supramolecular complexes of fish gelatin with chitosan as a result of hydrophobic interactions between biomacromolecules and the establishment of hydrogen bonds; in this case, electrostatic interactions between macromolecules of fish gelatin and chitosan are practically absent in the acidic pH region. The ability to form supramolecular complexes of different compositions at different mass ratios of polysaccharide-to-fish gelatin makes it possible to obtain hydrogels with high gelling properties, strength, elasticity, and thermal stability comparable to hydrogels of mammalian gelatin.

Food gels were obtained using fish protein isolate recovered from Atlantic cod (Gadus morhua) by-products using the isoelectric solubilisation/precipitation method. The use of low temperatures (not exceeding 10 °C) at the alkaline solubilisation stage resulted in the production of a fish protein isolate with high-molecular mass (FPI-1), while the use of high temperatures (24 °C) resulted in the production of a fish protein isolate with low-molecular mass (FPI-2). The isolates demonstrated excellent gelling and nutritional properties based on their amino acid profiles. The denaturation temperatures of FPI-1 and FPI-2 determined by DSC were 163.0 and 158.5 °C. The secondary structure of FPI-1 demonstrated a high α-helix content and a low random coil content compared to FPI-2. The high-molecular isolate formed stronger gels than the low-molecular isolate, which is explained by the formation of a dense gel network with small pores of about 250 nm. The recovered cod protein isolates can be successfully used as food ingredients or food additives in the production of gel-like/enriched products.