Effects of Freeze-Thaw Cycles on Gel-Forming Ability and Protein Denaturation in Alaska Pollock Frozen Surimi

Frozen surimi may be damaged by freeze-thaw cycles (refreezing) under various conditions. However, few studies have examined the deterioration of the quality of refrozen surimi. The objective of this study was to determine the deterioration mechanism of refrozen surimi. We used Alaska pollock frozen surimi, which has been studied extensively for gel formation. Refreezing decreased the breaking strength and breaking strain of the heated gel. The length of the line in the diagram between breaking strength and gel stiffness ( L value, which indicates the level of change in the breaking strength and gel stiffness with setting time) was also decreased. In contrast, the effect of refreezing on the texture of the gel without the setting process was small. The polymerization rate of myosin heavy chain in refrozen surimi during the setting process was slower than that in nonrefrozen surimi. Additionally, the Ca-ATPase activity of surimi was reduced by approximately 30% with each refreezing. These results demonstrate that the reduction in gel-forming ability by refreezing was caused by the decreased polymerization rate of myosin heavy chain because of myosin head denaturation. We also found that quality deterioration including myofibrillar protein denaturation of frozen surimi occurred mostly during first refreezing rather than during second refreezing. Overall, refreezing and/or repeated refreezing of surimi simply decreased the suwari gel-forming ability without changing the characteristics of surimi. The primary cause of the decrease in gel-forming ability induced by refreezing is considered to be the suppression of myosin heavy-chain polymerization during setting.