Effects of Freezing Methods on Water-Holding Capacity and Structural Properties of Myofibrillar Proteins from Hind Leg Muscle of Rana nigromaculata

In this study, the effects of different freezing methods and temperatures (freezer at –20 and –50 ℃, and liquid nitrogen at –50 and –80 ℃) on the water-holding capacity and structural properties of myofibrillar proteins from the hind leg muscle of Rana nigromaculata were investigated. The results showed that the time required for hind legs frozen in freezer at –20 and –50 ℃ and with liquid nitrogen at –50 and –80 ℃ to pass through the zone of maximum ice crystal formation temperature (–1 to –5 ℃) were 91.5, 57.0, 1.5 and 0.5 min, respectively. Compared with the freezer freezing groups, the liquid nitrogen freezing groups exhibited reduced thawing loss, increased content of immobilized water, and decreased content of free water. Compared with fresh samples, the water-holding capacity, solubility, total sulfhydryl content and endogenous fluorescence intensity of myofibrillar proteins decreased significantly after freezing, and the particle size, surface hydrophobicity and relative content of random coil showed an increasing trend. In the –50 ℃ liquid nitrogen freezing group, the water-holding capacity, protein solubility, total sulfhydryl content and endogenous fluorescence intensity decreased by 8.10%, 9.57%, 10.50% and 67.36%, respectively, and the volume average particle size, surface hydrophobicity and random coil content increased by 100.05%, 82.24% and 13.65%, respectively; these changes were significantly lower than those in the other freezing groups (P < 0.05). The degree of myofibrillar protein denaturation in the –80 ℃ liquid nitrogen freezing group was significantly lower than that in the –20 ℃ freezer freezing group, but higher than that in the –50 ℃liquid nitrogen freezing group (P < 0.05). According to the results of scanning electron microscopy (SEM), muscle fibers were more tightly and regularly arranged in the –50 ℃ freezer freezing and –50 ℃ liquid nitrogen freezing groups, while those in the –80 ℃ liquid nitrogen freezing group were arranged regularly but slightly cracked, and those frozen in the –20 ℃ freezer freezing group were obviously broken. These findings indicate that liquid nitrogen freezing can slow down the migration of water in the hind leg muscle of R. nigromaculata and effectively inhibit the cold denaturation of myofibrillar proteins, the effect being more pronounced at –50 ℃ than at –80 ℃.