Co-assembly of ι-, κ-, and λ-carrageenan with fish gelatin: Modulating physicochemical and gelling properties

The gelling properties of fish gelatin (FG), a promising mammalian alternative, could be enhanced by carrageenan (CG), yet the specific role of different CG types is unclear. Therefore, this study investigated the effects of three CG types (ι-, κ-, and λ-) on the gelling, thermal, and microstructural properties of FG, as well as their molecular interactions using rheometry, scanning electron microscopy, circular dichroism, etc. At a total polymer concentration of 6.0 % (FG:CG = 75:25, w/w), the λ-CG-FG composite gel exhibited the highest elastic modulus (7763.0 Pa) but the lowest water-holding capacity (WHC, 94.78 ± 4.43 %). The κ-CG-FG system exhibited the highest gel strength (134.6 ± 4.4 g), while the lower strength of the λ-CG-FG likely due to the high sulfate esterification of λ-CG, which promotes the intermolecular electrostatic repulsion. The WHC of both ι-CG-FG (99.87 ± 0.06 %) and κ-CG-FG (99.88 ± 0.08 %) composite gels was comparable to that of pure FG gel (98.89 ± 0.28 %). Notably, tightly bound water was observed only within κ-CG-FG composites. Co-assembling CGs with FG formed a collagen-like triple helix structure, which is the most prominent in λ-CG-FG complex. While both CG-FG complexes involved hydrogen bonding, electrostatic, and hydrophobic interactions, κ-CG-FG exhibited stronger electrostatic and hydrogen bonding, contrasting with the electrostatic shielding and consequent SO bond strengthening observed in λ-CG-FG. These findings highlight that CG type significantly influences the physicochemical and gelling properties of FG hydrogels, providing a theoretical foundation for modifying protein-based gels using ionic polysaccharides.