## 3 Results and Discussion
### Transverse Relaxation Time (T\({}_{2}\)) of LF-NMR  
The distribution and mobility of water molecules are considered as relevant parameters for changes in fish products during freezing, freezing and thawing. Low-field nuclear magnetic resonance (LF-NMR) uses the principle of energy exchange between nuclei at a fixed magnetic distance (1H) to analyze sample composition. T\({}_{2}\) transverse relaxation times measured by low-frequency NMR have been shown to detect the presence of sev  
Figure 2: Change in pH value of cuttlefish during storage at \(-\)18 \({}^{\circ}\)C by different freezing methods. Error bars show standard deviation. eral water groups in the muscle tissue of aquatic products [21; 22]. As can be seen from the Figure 3A, two forms of water were detected from fresh cuttlefish meat: one is the fastest-emerging peak of the relaxation component T\({}_{21}\) representing bound water, which represents water tightly bound to macromolecules in the range of 1.80-2.01%, distributed in the sarcoplasmic, sarcolemma and myogenic fiber protein The other was the major relaxation component, T\({}_{22}\), which represents water that is not readily mobile, ranging from 98.00-98.19%, and represents water that was encapsulated or immobilized in highly organized structures, such as water adsorbed in protein tertiary and quaternary structures, or located within the space of high-density myogenic fibers, including actin and myosin filaments [23]. In cuttlefish meat stored after freezing, in addition to the two types of water mentioned above, another component with the slowest peak emergence, T\({}_{23}\), was detected, this relaxation component was the least abundant, between 0.02% and 0.4%, it is free water distributed outside the structure of the myogenic fibrin network, in this component was not detected in fresh samples, this may be due to the growth of ice crystals after freezing which destroyed some cells and increased the content of free water. The variation in composition and structure of different fish species led to the variation in relaxation time. As shown in Figure 3F, the content of free water in cuttlefish under the six groups of freezing methods showed a significant increasing trend throughout the storage cycle. Refrigerator direct-freezing had the slowest freezing rate, generated larger ice crystals, grew ice crystals in later storage, caused severe mechanical damage to cells, and had the highest free water content at 90 day. Although liquid nitrogen freezing had the fastest freezing rate, the lack of free water content was not the lowest, because the generated low temperature fracture phenomenon would cause some damage to the muscle tissue, which would convert some water to free water. Tunnel continuous freezing and air-blast freezing had the lowest content on day 90, which may be due to having a faster freezing rate and maintaining the morphological integrity of the cuttlefish during the freezing process. Magnetic resonance imaging (MRI) is a technique that can obtain MRI images of water and fat in muscle, which can visualize the distribution and migration of water during the processing of fish products. Usually, the higher the brightness value and the higher the proton density, the more it tends to be red, indicating a higher water content in that part of the fish, and the color of MRI of fresh cuttlefish meat is close to red. It can be seen from Figure 4 that the color of MRI maps of fish flesh in LF group was the reddest and the proton density was the largest at the 3 d, while the color of RF group had tended to be yellow, and some yellow-blue color had appeared in the edge part, and the difference between the brightness of proton density-weighted image of air-blast freezing treatment and spiral freezing was not obvious.