Cationic and Anionic Peptides from Sheepskin Collagen as Effective Salt Substitutes for Improving the Quality of Low-Sodium Surimi Gels

Growing demand for low-sodium surimi products has driven the search for safe salt alternatives. Anionic peptides (APPs) and cationic peptides (CPPs) were isolated from sheepskin collagen via Diethylaminoethyl (DEAE) chromatography. CPPs contained higher arginine (46.11%) and lysine (4.64%) than APPs (40.57% and 3.99%, respectively), while APPs were enriched in acidic amino acids like glutamic acid (3.88%). Comprehensive evaluations of low-salt silver carp surimi gels showed both peptides significantly improved gel strength and water-holding capacity (WHC). The water-holding capacity increased from 60.68% in the blank control group to 74.31% in the CPP-treated group, while that in the APP-treated group was 66.86%. Cooking loss was significantly reduced, decreasing from 40.64% in the blank control group to 28.57% in the CPP-treated group and 34.52% in the APP-treated group. The samples achieved a quality comparable to that of the NaCl-supplemented group, with CPP outperforming APP in terms of hardness and gel network density. The LF-NMR confirmed enhanced water retention by reducing free water (T₂₂) and increasing bound water (T₂b). The FTIR indicated a conformational shift from α-helix to β-sheet, and the SEM revealed denser networks with fewer large voids. The SDS-PAGE demonstrated enhanced myosin heavy chain (MHC) cross-linking, more pronounced in the CPP-treated samples. CPPs exerted stronger electrostatic attraction with negatively charged surimi proteins (isoelectric point 5.5), while APPs chelated Ca²⁺ to activate transglutaminase. These findings validate APPs and CPPs as promising salt substitutes, enabling low-sodium surimi production and high-value utilization of sheepskin by-products.