## 3 Results and Discussion
### Low-Field NMR Spin-Spin Relaxation (T\({}_{2}\)) Measurement  
Low-field NMR is a non-destructive technique for testing the mobility and proportion of different fractions of water molecules. The T\({}_{\rm 2b}\) (T\({}_{\rm 2b1}\): 0.1-1 ms, T\({}_{\rm 2b2}\): 1-10 ms,) component corresponds to water closely associated with macromolecules (tightly bound water), the T\({}_{\rm 21}\) (10-100 ms) component reflects weakly bound water, and T\({}_{\rm 22}\) (100-1000 ms) represents the free water [33]. Figure 2 shows the distributions of T\({}_{\rm 2}\) relaxation times of black rice during the soaking and cooking stages. As shown in Figure 2a, the signal amplitude of the bound water (T\({}_{\rm 2b1}\)) in untreated samples occupied the major proportion since the black rice is a dried matter with little proton signal present. Soaking in excess water is an essential preliminary step to rice cooking. During the soaking process, the water diffused into black rice due to the moisture gradient between the surface and the center of the grain [34]. After soaking for 30 min, the bound water (T\({}_{\rm 2b2}\)) occupied the dominant portion of the grain and the immobilized water (T\({}_{\rm 21}\)) occupied a small proportion, indicating the water absorption process. A longer T\({}_{\rm 2}\) corresponds to more molecular degrees of freedom [35]. Milled black rice displayed longer T\({}_{\rm 2b2}\) values than whole black rice, suggesting that bran destruction facilitates water diffusion and absorption through crevices and chalky areas during soaking. During the cooking process, water is absorbed into the rice grains and hydrogen bonds between the starch molecules are gradually replaced by bonds between the starch and water molecules [36]. Water diffuses slowly from outside to inside the rice due to the crystalline organization of starch during cooking [36]. With the increase in cooking time, the T\({}_{\rm 2}\) relaxation times continuously increased, indicating water absorption into the black rice grain. After 40 min of cooking, weakly bound water (T\({}_{\rm 21}\)) prevailed, and its peak value increased as the milling degree increased (Figure 2f). This suggested that the presence of bran hindered water absorption in the black rice starch. The proton-weighted images of the water content of black rice with different cooking stages obtained by magnetic resonance imaging (MRI) are shown in Figure 3a. The proton-weighted images reveal the distribution of flow dynamic hydrogen protons, and the proton's density is interpreted as the number of hydrogen protons. The pseudo-color image's color scale from 0 to 300 corresponds to the color from blue to red, indicating that the fluidity of water increases gradually. The internal water distribution of black rice was obviously affected by both milling treatment and cooking time. The fluidity of water inside black rice increased with increasing cooking time. The fluidity of water inside C-F samples was greater than that of A and B after cooking for 20 min, which was due to differences in the integrity of black rice seeds. The distribution of red areas in the pseudo-color image of black rice gradually increased with the increase in milling degree after cooking for 40 min, indicating that the fluidity of water in various phases inside the rice gr \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Gelatinization Time (min)** & **Water Uptake Ratios (\%)** & **Expanded Volume (\%)** & **Starch–Iodine** \\  & & & **Blue Value/Solid** \\  & & & **Substance Weight** \\ \hline E & 23.54 \(\pm\) 0.51  & 194.05 \(\pm\) 0.59  & 190.91 \(\pm\) 0.00  & 0.81 \(\pm\) 0.21  \\ F & 23.82 \(\pm\) 0.74  & 194.05 \(\pm\) 1.86  & 193.94 \(\pm\) 5.25  & 0.82 \(\pm\) 0.03  \\ \hline \hline \end{tabular}  
A–F: Cooked black rice with different milling degrees (A: 0%, B: 0.57%, C: 1.27%, D: 4.16%, E: 6.97%, F: 10.19%). Different lowercase letters in the columns represented statistically significant differences (\(p<0.05\)).  
\end{table}