Preparation of antioxidative agar hydrolysate from Gracilaria tenuistipitata using H2O2 scission to improve surimi gel stability through multiple freeze–thaw cycles

Agar hydrolysate (AH) from Gracilaria tenuistipitata was prepared using hydrogen peroxide (H₂O₂) scission and assessed as an antioxidative gel enhancer in surimi. Hydrolysis conditions were optimized via response surface methodology, considering time (0–3 h) and H₂O₂ concentration (2–4% w/w). Optimal hydrolysis (2.64% H₂O₂, 3 h) yielded a degree of hydrolysis (DH) of 30.49 ± 3.18% with high antioxidant activity. Both variables positively affected DH and antioxidant properties (P < 0.05). To test the potential of AH, agar/AH blends (100:0 to 50:50) were incorporated as 3% supplements in surimi gels, subjected to repeated freeze-thaw (F-T) cycles (0, 3, and 6 cycles). The 60:40 ratio significantly improved gel strength, reduced expressible moisture content, and prevented F-T-induced lipid oxidation (P < 0.05), while whiteness remained unchanged. Microstructural analysis confirmed the presence of finer gel networks and lower TBARS values (P < 0.05), indicating enhanced oxidative stability. These findings demonstrate that H₂O₂-generated agar hydrolysate, combined appropriately with native agar, serves as an effective natural multifunctional ingredient that simultaneously enhances gel properties and stabilities as well as provides antioxidative protection in surimi-based products. This approach advances sustainable production processes and enhances the utilization of fisheries resources.