Effect of ultrasonic-assisted immersion freezing and quick-freezing on quality of sea bass during frozen storage

Freezing is an effective way to maintain seafood quality. In this study, sea bass fillets were frozen at −20 °C and −40 °C via ultrasonic-assisted immersion freezing (UIF) and quick-freezing (QF). The ice crystal morphology, thawing loss, cooking loss, texture, water distribution, tissue structure, and protein structure of the fish samples were then determined after 2, 4, 6, 8, 10, and 12 weeks of storage. Compared with the QF treated samples, the UIF samples frozen at −40 °C exhibited better quality parameters. Particularly, the ice crystals in the UIF samples were significantly smaller, rounder, and more regular. Moreover, the thawing and cooking loss, hardness, elasticity, and intrinsic fluorescence intensity of the samples decreased steadily throughout the 12 weeks experiment; however, these indices remained higher in the −40 °C UIF treatment than in the −40 °C QF treatment. Additionally, the muscle fibers of the samples were smoother in the −40 °C UIF treatment and the content of α-helices and β-turns was significantly higher (P < 0.05), indicating that −40 °C UIF treatment preserves protein stability more effectively than other treatments. Taken together, our findings provide novel insights into the effect of different freezing methods on the quality of aquatic products.