Abstract: In order to explore the effects of freeze-thaw cycles on texture properties, microstructure and protein properties in big-eye tuna (Thunnus obesus), fish samples were subjected to 1-5 freeze-thaw cycles under conditions simulating temperature fluctuations encountered in a supermarket (F1) and those encountered in a household (F2). Changes in physicochemical indexes (thaw drip loss rate, metmyoglobin content, myofibrillar protein content, total sulfhydryl content and Ca2+-ATPase activity) and texture profile analysis (TPA) parameters were measured during freeze-thaw cycles. Quality changes were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI). The results showed that with increasing freeze-thaw cycles, thaw drip loss rate was increased in both groups, hardness, chewiness and springiness were significantly decreased (P < 0.05), and adhesiveness was increased; myofibrillar protein content, total sulfhydryl content and Ca2+-ATPase activity were decreased, and metmyoglobin content were increased. The thaw drip loss rate in group F1 was higher than that in group F2 at the same freeze-thaw cycles, which reached the maximum value after the fourth freeze-thaw cycle, accompanied by a significant decrease in hardness. A slower increase in metmyoglobin content, and slower decreases in myofibrillar protein content and Ca2+-ATPase activity in group F1 were observed compared with group F2. The results of SDS-PAGE showed that repeated freeze-thaw cycles caused partial protein degradation. The degradation rate was accelerated with increasing number of freeze-thaw cycles and freezing temperature. The results of LF-NMR revealed that repeated freeze-thaw cycles could result in the conversion of immobile water in fish myofibrils into free water. The relative content of free water as well as thaw drip loss rate in group F1 was increased with increasing freeze-thaw cycles, which was consistent with the results of MRI analysis. Accordingly, compared with F2, F1 was more effective in inhibiting the increase in metmyoglobin content and maintaining Ca2＋-ATPase activity despite increasing thaw drip loss rate and deteriorating texture properties. Increase the number of freeze-thaw cycles could aggravate the deterioration of functional characteristics in fish tissues and proteins, leading us to conclude that the number of freeze-thaw cycles during circulation should be as small as possible.

Key words: big-eye tuna; freeze-thaw cycles; texture propertites; protein properties