Effects of κ-Carrageenan on Gel Properties and Microstructure of Unrinsed Nile Tilapia Surimi Gels During Heat-Induced Gelation

Unrinsed surimi production has emerged as a sustainable alternative to conventional rinsed surimi due to its substantial water savings, reduced nutrient loss, and lower environmental footprint. However, poor gel strength and water-holding capacity remain major bottlenecks limiting its industrial application. This study investigated the effects of κ-carrageenan (κ-CG) on the rheological properties, textural characteristics, water distribution, intermolecular interactions, and microstructure of unrinsed surimi gels. The results showed that κ-CG enhanced gel properties in a concentration-dependent manner. At 0.75% κ-CG, gel strength increased to 47.0 g·cm (a 2.2-fold increase vs. control), water holding capacity (WHC) reached 97.9%, and cooking loss decreased to 4.0%. In contrast, at 1.0% κ-CG, gel strength and hardness further increased, but WHC declined slightly. Low-field nuclear magnetic resonance analysis revealed that κ-CG promoted the conversion of free water to immobilized water, with the relative peak area of T22 (immobilized water) increasing from 85.9% to 88.9% at 0.75% κ-CG. Protein solubility measurements indicated that hydrophobic interactions and disulfide bonds appeared to be the major contributors stabilizing the gel network. FTIR revealed enhanced hydrogen bonding via an O–H red shift and intensification with κ-CG, and amide I deconvolution showed that 0.75% κ-CG maximized β-sheet content while reducing α-helix, indicating ordered protein reorganization during heating. Quantitative SEM analysis further verified that 0.75% κ-CG produced the densest protein network with the maximum fractal dimension and minimum lacunarity, alongside reduced average pore area and porosity. These findings demonstrate that κ-CG at an appropriate level effectively improves the gel properties of unrinsed tilapia surimi by strengthening intermolecular interactions and optimizing water distribution, offering a practical approach for developing high-quality surimi products.