Role of Skipjack Tuna (Katsuwonus pelamis) Blood Protein Hydrolysate as a Natural Cryoprotectant on Threadfin Bream Surimi Protein During Repeated Freeze-Thaw Stability

This study investigated the effects of various cryoprotectants including sucrose (8%, w/w), sucrose: sorbitol (1:1) for 8%, tuna blood protein hydrolysates (TBPH) at 6 and 8% on protein structural changes in threadfin bream surimi after repeated four freeze-thaw cycles. Changes in salt solubility, total sulfhydryl content, disulfide bond content, and protein secondary structure were used to assess structural alterations of proteins in surimi with and without various cryoprotectant. Across all samples, protein solubility and total sulfhydryl content decreased with an increasing number of freeze-thaw cycles, while disulfide bond content increased. Notably, threadfin bream natural actomyosin (NAM) without cryoprotectants showed a significant decrease in protein solubility, indicating protein denaturation due to frozen storage and freeze-thaw processes. Interestingly, 6% TBPH was most effective at slowing the reduction in surimi protein solubility. Furthermore, NAM without cryoprotectants exhibited the highest disulfide bond content after freeze-thaw cycles, whereas NAM with TBPH and the sucrose:sorbitol (1:1) showed the lowest. TBPH most effectively retarded disulfide bond formation in NAM extracted from threadfin bream muscle, which correlated with increased water-holding capacity. TBPH induced the transformation of alpha-helix content into beta-sheet and beta-turn structures, thereby contributing to the formation of a strong, well-organized protein network in surimi. Additionally, TBPH displayed antimicrobial activity in surimi. Therefore, TBPH, particularly at 6%, effectively mitigated protein changes associated with freezing and thawing, demonstrating its potential as a natural cryoprotectant and antimicrobial agent for frozen surimi and related products.