Objective The present study aims to address the issue of decreased texture and quality of frozen-cooked grass carp (Ctenopharyngodon idella) products such as pre-fried fish.

Methods C. idella was selected as the research subject and subjected to three different processes: deep-frying, freezing, and reheating. Indicators including water content, myofibrillar fragmentation index, shear force, degree of lipid oxidation, microstructure, protein secondary structure, and color difference were used to investigate the impact of cooking degree on the quality of frozen-cooked C. idella chunks. Based on these observations, the study explored the effects of different deep-frying temperatures (160, 180, 200 °C) on the quality of frozen-cooked C. idella chunks by using water content, myofibrillar fragmentation index, and color difference as indicators to achieve the same level of maturation.

Results As the cooking degree increased, the water content and water-holding capacity of the fish chunks gradually decreased, with bound water transforming into free water. After two cycles of freeze-thawing, the shear force of fish chunks at various cooking degrees decreased compared to before freezing. Specifically, the shear force values and myofibrillar fragmentation index of fish chunks with a center temperature of 75 °C were respectively 1.33 times and 1.22 times higher than those of fish chunks with a center temperature of 45 °C, indicating severe damage to the myofibrillar structure and reduced fish tenderness. Additionally, freeze-thawing treatment reduced the relative content of protein α-helices and β-sheets, leading to increased protein disorder. After two cycles of freeze-thawing treatment, then reheating, the water content, water-holding capacity, shear force, degree of lipid oxidation, microstructure, sensory scores, and other parameters of fish chunks precooked to a center temperature of 45 °C before freezing were superior to those of fish chunks cooked to 60 °C and 75 °C. Furthermore, using a high-temperature short-time deep-frying process at 200 °C for frozen-cooked C. idella chunks resulted in improved water content, myofibrillar fragmentation index, and a golden-yellow appearance on the surface of the fish chunks.

Conclusion Deep-frying C. idella chunks at 200 °C to achieve a center temperature of 45 °C, not only enhances the color of frozen-cooked C. idella chunks, but also effectively enhances the quality of reheated frozen-cooked fish.

Significance By adjusting the cooking degree and optimizing the deep-frying process, this study markedly enhances the flavor of reheated frozen-cooked fish products. This study provides valuable insights for enhancing the quality of pre-cooked fish dishes and optimizing the processing of large-scale farmed fish species.