*2.12. Statistical Analysis*

All experiments were performed at least three times, and the results were presented as the mean ± standard deviation (SD). Statistical Package for Social Science (SPSS 26, SPSS Inc., Chicago, IL, USA) was used to carry out all statistical analysis with the significance level set at 0.05 based on one-way analyses of variance (ANOVA). Significant differences were evaluated by Duncan's multiple range test (*p* < 0.05).

#### **3. Results and Discussion**

*3.1. Changes of Relaxation Time and Water Distribution of Starch–Surimi Gels with Non-Setting or Setting Effect*

The results of LF-NMR can effectively determine water mobility and distribution in gel matrix [27]. T2b (1–20 ms), T21 (20–300 ms), and T22 (400–2000 ms) represented the transverse relaxation time of bound water, immobilized water, and free water (Figure 1a, b), respectively. It was obvious the relaxation time of different water migrated. As shown in Figure 1c, d, the peak relaxation time of bound water (TP2b) in gels without starch was 2.39 ms (CG) and 1.94 ms (SCG), between which there was a significant difference (*p* < 0.05) that might be related to the gel structure heated by dissimilar processes. TP2b increased by starch addition and showed a positive correlation with starch content. The migration of TP2b was relevant to different water mobility bound by the protein and starch. The peak relaxation time of immobile water (TP21) dropped from 101.71 ms to 71.71 ms in CG, and from 94.84 ms to 65.36 ms in SCG. The shorter the relaxation time (T2), the stronger the binding force of water molecules to matrix [12]. Thus, the water mobility of bound water significantly increased with starch incorporation (*p* < 0.05), while the mobility of non-mobile and free water decreased in both matrices. This was consistent with the investigation by Li et al. [28], who found that starch showed a better restriction capacity on the free motion of water molecules. However, the relaxation time between the two types of gels had a significant difference (*p* < 0.05), indicating that starch-swelling differed in gel matrices with different heating treatments.

**Figure 1.** T2 relaxation time (**a**,**b**) and water mobility (**c**,**d**) of starch–surimi matrix in direct heating process and two-step heating process. CG: gel obtained by direct heating; SCG: gel obtained by two-step heating. 0: without starch; 3, 6, 9, and 12: incorporation with 3%, 6%, 9%, and 12% starch.

The relative moisture content was observed by peak area proportions (PT2b, PT21, and PT22) in Figure 2. It was discovered that immobile water made up the majority of the primary proportion in surimi gel, followed by bound water and free water. PT21 showed the relative content of immobile water that increased initially with the addition of starch, resulting in a reduction in free water in both CG and SCG. The PT21 of CG and SCG reached a maximum at 6% and 9% starch content, respectively. Subsequently, it decreased and was possibly associated with the dehydration of surimi gels suffered by the compression from starch swelling. Moreover, starch incorporation reduced the relative surimi content in the entire matrix, causing a decrease in immobile water content held by surimi gel network. Notably, in the non-starch containing gels, PT21 of CG and SCG was 97.20% and 96.27%, showing that the gel matrix had higher PT21 without preincubation at 40 ◦C. The changes might presumably contribute to overheated time and the squeeze by gel formation in SCG, resulting in a transition from immobile water to free water. Although direct heating formed a poor gel, the juiciness mouthfeel of surimi products improved [29]. Since the hydroxyl groups in starch bound more water, the matrix with starch incorporation significantly differed in PT22 (*p* < 0.05). However, PT2b significantly increased with the continuous addition of starch (*p* < 0.05), especially in CG. With 12% starch, PT2b of CG increased by 173.15% compared to 113.89% in SCG. Among the above changes, it indicated that water absorbed by starch existed in the form of immobile water and bound water. Moreover, it was inferred that the starch in direct cooking gels showed better swelling states, which resulted in the phenomena where the internal starch structure tended to easily combine with water.

**Figure 2.** Peak area proportion (PT2b, PT21, and PT22) of different water in starch–surimi matrix subjected to direct heating and two-step heating. Caption: see Figure 1.
