To study the effect of magnetic field‐assisted freezing on tilapia quality changes, this experiment used the application of a magnetic field as an auxiliary condition to freeze tilapia, and the static magnetic field strength was 0 G, 5 G, 10 G, 15 G, 20 G, 30 G. The results show that magnetic field‐assisted freezing has a positive effect on the freezing rate, water migration, microstructure, and other aspects. Among the strengths applied, the effect of 15 G is the most significant, and the effect of 20 G and 30 G is lower than that of 15 G. Compared with the control group, the magnetic field intensity of the other groups showed no significant difference in the effect of each index. Under the experimental conditions, the magnetic field has a significant promotion effect on the sample quality, but the effect does not increase linearly with the increase of the magnetic field intensity. Novelty impact statement The use of magnetic field for auxiliary freezing provides a new theoretical method for the freezing quality of tilapia. Using fractal dimension to quantitatively analyze the quality of frozen tilapia and verify the applicability of magnetic field in frozen tilapia.

Tilapia, also known as the African crucian carp, is a freshwater farmed fish cultivated worldwide by the aquaculture industry. It has high economic value, and its data are supported by key scientific research. China is the world's largest tilapia farming and supplying country (China Agriculture Press, 2020). Magnetic resonance imaging technology refers to the use of nuclear magnetic resonance principles. Through spatial coding technology, the radio frequency signal emitted from hydrogen atoms is processed by a computer to form an image. This technology has been widely used in food and agriculture, petroleum surveying, porous media, polymer materials, and other fields and has become an increasingly popular scientific research method (Zhao et al., 2017). Cui Zhiyong analyzed the relaxation characteristics of three kinds of pork to establish prediction models of moisture content based on the LF-NMR technology.According to the differential moisture-binding strength of bound water, nonflowing water, and free water, the relaxation time can be recorded. The comparison of this relaxation time, in combination with the least-squares method, partial least-squares method, and

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To study the effects of freezing and thawing times and freezing temperatures on the water state and microstructure of tilapia fillets, experiments on tilapia fillets were carried out at –4 and –18°C with one to four freezing and thawing cycles (FTCs). Low‐field nuclear magnetic resonance (LF‐NMR) and nuclear magnetic resonance imaging were used to observe the water state after different treatments, and scanning electron microscopy (SEM) and frozen sections were used to observe the microstructure changes. Fractal dimension (FD) was used to quantitatively characterize the microstructure of the fish tissue, and the correlation between FD and fish fillet quality parameters was studied by principal component analysis (PCA). The findings showed that with the increase of FTCs, the thawing loss increased, and the water holding capacity (WHC) fell. FTCs cause a decrease in immobilized water and an increase in free water in the fillet. This indicates the migration of immobilized water to free water. SEM and frozen slice images showed that the growth of ice crystals led to the destruction of myogenic fibers. A decrease in freezing temperature inhibited ice crystal growth. The FD value dropped in accordance with an increase in FTCs. PCA demonstrated that the WHC, NMR data, and FD value had a strong correlation with the quality changes in the tilapia fillets. Therefore, FD and water state can reflect the quality characteristics of tilapia fillets. Practical Application The water migration in tilapia fillets is detected with LF‐NMR, and the microscopic image may be quantified using the FD value. Both approaches can offer fresh perspectives on how to assess the quality of tilapia fillets and reflect changes in their quality.