Effect of proteolytic squid protein hydrolysate on the state of water and denaturation of lizardfish (Saurida wanieso) myofibrillar protein during freezing

The concentration dependent (2.5–10% of dry weight/wet weight) protective effect of squid protein hydrolysate (SPH), extracted from Japanese flying squid and swordtip squid by protease treatment, on the state of water and denaturation of frozen lizardfish (Saurida wanieso) myofibrillar protein (Mf) were assessed on the basis of the amount of unfrozen water in Mf by differential scanning calorimetry and Mf Ca-ATPase inactivation during freezing at −25 °C for 90 days; the effects were compared with those of sodium glutamate. The Mf showed a higher amount of unfrozen water upon addition of SPH, regardless of level of addition and species differences, resulting in a markedly decreased inactivation of Mf Ca-ATPase throughout the freezing period. The Ca-ATPase activity in the Mf without SPH (control) dropped drastically from the beginning of the freezing. These findings suggest that the functional side chains of the peptides of SPH produce bound water in the Mf structure, which provides a structural alteration of the hydrate water that has a capacity to suppress the freeze-induced denaturation of Mf. An addition of 5.0–7.5% concentration of SPH is found to be suitable to increase the amount of unfrozen water and to prevent the freeze-induced denaturation of Mf.