Surimi gel products (such as kamaboko, fish sausage, and fish ball) are popular with consumers around the world, especially in Asia due to their high protein, low-fat content, and appreciable sensory properties (Mi et al., 2021). Traditionally, marine fish was used as raw material for preparing surimi gel products. However, the growing market demand for marine fish as a raw material is not possible to meet due to the gradual shortage of marine resources and the high cost of harvesting (Liu et al., 2021; Yi, Huo, Qiao, Wang, & Li, 2019). Therefore, freshwater fish especially grass carp (Ctenopharyngodon idella) have gained significant attention as a raw material for surimi products due to their fast growth, high yield, low price, and high commercial value (Xiong et al., 2009). Unfortunately, surimi gels made from freshwater fish exhibit poor color and gel-forming properties compared to the surimi gel prepared from marine fish, which limits their application for developing surimi gel-based products (Sun, Huang, Hu, Xiong, & Zhao, 2014). In addition, freshwater fish also contains high levels of proteases that induce degradation of muscle fibrin which in turn associated with the gel weakening (Zhang et al., 2019).

Nowadays, fish is mostly consumed in the form of frozen fish/surimi (Jia, Roy, Pan, & Mraz, 2022). However, frozen storage lead to a significant deterioration of surimi gel products, resulting in the quality of surimi gel not being as good as fresh surimi (Zhang & Ertbjerg, 2019). Currently, most of the studies explored the application of various food additives to improve the quality of surimi or using antifreeze agents to improve the antifreeze performance of surimi during frozen storage. But consumers' demand for safe and natural food with no additives is enhanced due to their increased awareness of quality food and associated health benefits. Thus, it is of immense importance to explore a physical processing method focused to restrain the quality decline of surimi gel during the freezing process along with improving the original quality of surimi gel without adding any food or chemical additives except the addition of edible salt to form surimi gel.

Among the recently reported technologies, pressure shift freezing (PSF) technology has gained significant attention from food scientists as PSF technology reported to improve the quality of frozen food and the properties of ice crystals formed during the freezing process. Thus, researchers have also proposed PSF as an efficient alternative method to the existing freezing process (Su et al., 2014). The PSF treatment is based on the principle of cooling the sample to a desired temperature under high pressure (generally 100–200 MPa) and then quickly releasing the pressure to atmospheric pressure (about 101.3 KPa). Under high pressure, the freezing point of water present in a sample is much lower compared to the freezing point under atmospheric pressure, which increases the degree of supercooling of water present in a sample (Bridgman, 1912; Zhang et al., 2022). The pressure is released suddenly, and the sample with reduced temperature abruptly experiences atmospheric pressure. When the temperature difference between the sample temperature and that corresponding to the atmospheric pressure is too high, a sudden temperature rise is observed due to latent heat release by ice nucleation. At this stage, a large number of ice nuclei are produced and grow into many fine ice crystals (Su et al., 2014) which lead to the completion of approximately 25% of freezing. Thus, even if the freezing continues under subsequent atmospheric conditions, the ice crystal in the frozen products will remain small (Martino, Otero, Sanz, & Zaritzky, 1998). The fine ice crystals formed during PSF treatment can reduce mechanical damage to food materials and prevent juice loss compared to the large-size ice crystals formed by traditional air freezing. Previous studies have shown that PSF-treated muscle samples retain their natural microstructure better compared to the samples that undergone traditional air freezing as the ice crystals formed during PSF treatment are small and evenly distributed throughout the food system (Otero, Martino, Zaritzky, Solas, & Sanz, 2000; Zhu, Ramaswamy, & Le Bail, 2005).

Until now, the effect of PSF technology on the gel properties of surimi and the associated mechanism has not been fully studied (Zhang et al., 2023). The effects of PSF on water mobility, ice crystal structure, and rheological properties of grass carp surimi gel have not been reported. Thus, this study was conducted to explore the effects of PSF on breaking force, deformation, and gel strength of grass crap surimi gel, and compared these parameters with grass crap surimi gel samples undergone PSF pretreatment (Pr-PSF) and conventional air freezing (CAF) treatment. The variation and distribution of water in surimi gel were analyzed by low-field NMR (LF-NMR) and magnetic resonance imaging (MRI), respectively. The impact of different treatments on rheological changes during surimi gel formation was also analyzed. The mechanism of impact by different methods on various properties of surimi gels was analyzed based on the microstructure of samples, size and distribution of ice crystals of frozen samples, the intermolecular force of surimi gels, and the secondary structure of proteins. The output of this study revealed the significance of PSF technology in the surimi gel processing and meat processing industry.