## 3 Results and Discussion
### Physicochemical Properties
#### 3.1.5 Determination of Water Distribution in Surimi Gel  
The T2 inversion spectrum of fermented hairtail surimi can be divided into three relaxation periods based on the distribution of peaks: segment a (0.5-4 ms), segment b (25-220 ms), and segment c (270-1011 ms) (Figure 3). The lateral relaxation time (T2) of protons varies, and they stem from the different chemical environment location. A shorter T2 will result in fewer degrees of freedom for the proton, and a longer T2 will result in more degrees of freedom for the proton. The size of T2 reflects the size of water freedom in the sample, where a longer T2 indicates that the water in the sample is less bound and is easier to be expelled [47; 48; 49]. Among the three peaks of T2, segment a has the shortest time; this part is bound water that most closely combined with other molecules and is considered to be the non-rotating \"bound water\" in the magnetic field. Segment b is semi-bound water, which is mainly composed of amide groups in protein molecules that combined with water at a small bond energy, and it is bound by the structure of the protein gel network; it is bound water that is next to free water and can easily transform, thus being considered as \"bound\" water that can rotate in the magnetic field. Segment c, which has the longest duration, consists of free water, which is mainly volume water and structural water; this part of the water has the molecular mobility of water in an aqueous solution. Figure 2: Scanning electron microscopic images of MFHS gel at different fermentation times. The T2 time of hairtail in segment a shifted to the left compared with the unfermented group, and the peak value and peak area did not change remarkably. This indicates that after the Monascus fermentation, the water molecules more closely combined with the polar groups on the surface of the protein. The possible factor that could explain this is that the enzymes and metabolites produced by fermentation hydrolyzed the myofibrillar protein and opened the helix, with the closer surimi binding with water occurring through the combination of hydrogen chains; this is also consistent with the results of the partial fermentation stage of mandarin fish [44]. The peak value and peak area of segment b were significantly larger. The T2 time shifted to the left, and the T2 value decreased compared with the unfermented group. This indicates that Monascus fermentation increased the peak and peak area of non-flowing water, which promotes the tight formation of a three-dimensional network gel structure of hairtail surimi with a more uniform pore size, which consists of bound water in the gel network and reduced water loss. By comparing the peak value and peak area of segment b of surimi at four fermentation times, it can be seen that with the increase in fermentation time, the peak value and peak area of hairtail surimi first increased and then decreased. The 1 h fermentation group was the highest, which revealed the 1 h fermentation group as having the highest semi-bound water content and best WHC. At the same time, it can be seen from the Figure 3c segment that the peak value and peak area of segment c in the Monascus fermentation group decreased and had the peak shift right, indicating that the free water content of the Monascus fermentation group was lower than the unfermented group but also that the binding was stronger than the unfermented group; the results reflect that fermentation reduces the water activity of surimi, which is consistent with the decrease in water activity of fermented mandarin fish [43]. The peak value and peak area of segment c were the smallest in the 1 h fermentation group. As the fermentation time went on, the free water content of segment c first decreased and then increased. There was no significant difference in the free water content between the 5 h fermentation group and the unfermented group, but it was slightly longer at T2.