*3.3. MRI Analysis*

The hydrogen proton MRI has been used as a noninvasive method to evaluate the distribution of moisture content within food products [43]. The T2 weighted images taken at the transverse geometric center of each sample during the drying process revealed the distribution of the water within high-mobility protons (Figure 4). With increasing drying time, a continuous decrease is observed in the size of the brighter regions, suggesting the loss of a longer relaxation signal of water during drying. In addition, a decrease in the signal intensity from the external surface to the inner region is evident. Similar phenomenon was also observed by Ling et al. who found that the red region gradually changed to blue, and the color and size of the blue region remarkably decreased from the exterior to the interior part with increasing drying time of shrimp [39]. These results confirmed that the water relaxation signal gradually weakens and the water content continuously decreases during the drying process, in agreement with the above-described changes observed in moisture content in shrimp during drying.

**Figure 3.** (**a**) Distribution of T2 relaxation spectra and (**b**) change of T2 relaxation times obtained by multi-exponential fitting of the continuously distributed Carr–Purcell-Meiboom-Gill relaxation curve of different shrimp samples during drying.

**Figure 4.** T2 weighted MRI images of shrimp dried by hot air drying at different levels.

*3.4. Correlation between LF-NMR and Physicochemical Properties*

As a rapid, noninvasive method, LF-NMR relaxation is often applied to investigate water mobility in materials and foods [44]. However, the correlation between water distribution state and the physicochemical parameters of shrimp during the drying process needs deeper exploration. Therefore, the relationship between T2 relaxation times (T21, T22, and T23) and shrimp physicochemical properties was determined by Pearson correlation analysis (Figure 5). The results indicated good correlations between LF-NMR data and shrimp moisture content, hardness, adhesiveness, stickiness, and chewiness. Specifically, moisture content was significantly positively correlated with T21 (*R* = 0.943), T22 (*R* = 0.914), and T23 (*R* = 0.903), which may be explained by the substantial effect of moisture on the

proteins and myofibril in shrimps. This was similar to the findings of Cheng et al., who also reported a positive correlation between the decrease in moisture content and the change in relaxation times [42]. Regarding shrimp texture, its hardness, stickiness, and chewiness were negatively correlated with T21 (*R* = −0.877, *R* = −0.889, *R* = −0.852) and T23 (*R* = −0.846, *R* = −0.875, *R* = −0.844), whereas adhesiveness was positively correlated with T21 (*R* = 0.832) and T23 (*R* = 0.872). These results agree with those of Wang et al., who reported that T22 was highly correlated (*p* < 0.01) with hardness, elasticity, and chewiness, thereby consequently leading to moisture changes that will affect muscle fiber contraction and alter the texture of shrimp meat [44]. However, the color indicators exhibited a weaker correlation with the LF-NMR, which may be due to the fact that the color change is mainly caused by fat and pigmentation, and is weakly related with water splitting. In summary, the strong correlations between LF-NMR data and shrimp moisture content and texture properties indicate the potential of LF-NMR as a fast and nondestructive alternative method of detecting quality changes during shrimp drying.

**Figure 5.** Correlation analysis between LF-NMR relaxation parameters and moisture content, color, texture of shrimp during drying.
