Gelatin is a collagen-derived biopolymer widely used in food, pharmaceutical and biomedical applications due to its biocompatibility and gelling ability. However, gelatin hydrogels suffer from unstable mechanical strength, limited thermal resistance and susceptibility to microbial contamination. The main aim of the present study is to investigate the influence of gelatin cryostructuring followed by photo-induced menadione sodium bisulfite (MSB) chemical crosslinking on the structural and functional characteristics of mammalian and fish gelatin hydrogels. The integration of scanning electron microscopy, dielectric spectroscopy and rheological experiments provides a comprehensive view of the of molecular, morphological and mechanical properties of gelatin hydrogels under photo-induced chemical crosslinking. The SEM results revealed that crosslinked hydrogels are characterized by enlarged pores compared to non-crosslinked systems. For mammalian gelatin, multiple pores with thin partitions are formed, giving a dense and stable polymer network. For fish gelatin, large oval pores with thickened partitions are formed, preserving a less stable ordered architecture. Rheological data show strong reinforcement of the elastic and thermal stability of mammalian gelatin. The crosslinked mammalian system maintains the gel state at higher temperatures. Fish gelatin exhibits reduced elasticity retention even after crosslinking because of a different amino acid composition. Dielectric results show that crosslinking increases the portion of bound water in hydrogels considerably, but for fish gelatin, bound water is more mobile, which may explain weaker mechanical properties.

The effect of agar concentration C_A on the colloidal chemical properties of complex fish gelatin–agar hydrogels was studied using DSC, SEM, and 3D rheometry. The hydrogels were formed at constant gelatin content (10%) over a biopolymers ratio (weight/weight) Z ranging from 0.02 to 0.10 g_A/g_CFG. The addition of agar was shown to accelerate the gelation of fish gelatin by approximately 2 times. Gelatin increases the gelation temperature but does not affect the melting point of the agar component of the gel. Agar increases the melting heat by 13–14 times without affecting the melting point of the gelatin component of the gel. The complex hydrogel is characterized by a compact structure with a reduced mesh size compared to the gels of the individual biopolymer components. The microstructure of the gel consists of a cellular gelatin network reinforced by fibers of agar network. The strength and elasticity of the complex gel increase significantly with increasing C_A, while syneresis decreases. The optimal biopolymer ratio of Z = 0.08 g_A/g_CFG was determined at which a fish gelatin–agar hydrogel exhibits elastic-strength characteristics comparable to those of gelatin from porcine skin, and is also resistant to melting and syneresis during storage at room temperature.

Gelatin-based hydrogels are promising matrices for wound management, but their direct application is constrained by insufficient structural stability and lack of intrinsic antimicrobial activity. Porcine and fish gelatin hydrogels were photochemically crosslinked with menadione sodium bisulfite (MSB), a water-soluble hemostatic derivative of vitamin K3, and characterized by ATR-FTIR and ¹H NMR spectroscopy, together with antimicrobial testing against Staphylococcus aureus, Candida albicans, Escherichia coli, and Salmonella enterica. FTIR analysis showed that MSB crosslinking retards the thermal disruption of collagen-like triple helices in both gelatins, with the effect being more pronounced in porcine gelatin owing to its higher imino acid content and more developed collagen-like network. NMR measurements confirmed that crosslinking increases the bound-water fraction approximately threefold in porcine and twofold in fish gelatin, while the bulk water mobility stays unchanged. MSB crosslinking enhanced antimicrobial activity against S. aureus and C. albicans by up to 5.6-fold relative to non-crosslinked controls and additionally conferred activity against E. coli, while S. enterica remained resistant in all variants. MSB, thus, simultaneously serves as a structural crosslinker and imparts intrinsic antimicrobial activity to the resulting hydrogels, making them a promising basis for multifunctional wound-healing materials.

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.

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.