JIA-2019-11

2647 WANG Hao et al. Journal of Integrative Agriculture 2019, 18(11): 2644–2651 relaxation time and intensity of the transverse relaxation time were determined in every component of a corn kernel with CPMG; the mobility of water or grease molecules was reflected in the relaxation time T 2 , and A corresponded to the signal intensity (Ruan and Litchfield 1992). According to the principles of hydrogen proton NMR, the strength of hydrogen bonds and moisture fluidity were related to the length of the relaxation time (Garnczarska et al . 2007). Specifically, the longer the relaxation time was, the lower the strength of hydrogen bonds was, and the greater the degree of moisture fluidity was. As shown in Fig. 1, the relaxation time T 2 before 1 ms-transverse relaxation time of free water (T 21 ), after 1 ms-transverse relaxation time of bound water (T 22 ), and T 2 , corresponded to the signal intensity of proton intensity of free water (A 1 ), proton intensity of bound water (A 2 ), and proton intensity of total water (A), respectively. Free water moves relatively freely in plants cells, so it gets lost more rapidly. The T 22 peak generally reflects free water content in corn. The decrease in the T 22 peak and its shift to the left side reflected the decline in free moisture content during storage in this study. Bound water refers to the stable form of water which exists in a bound state with chemical substances in corn and generally cannot be dried out and removed. Thus, the T 21 peak represents the moisture bound with chemical substances. In Fig. 1, the T 21 peak increased and shifted to the right side with increase in storage time, which is in agreement with the conclusion drawn by Ruan and Litchfield (1992), suggesting that bound water relatively increases during the storage period. In the other words, there was an increase in the proportion of bound form of moisture, which is more stable than free moisture, out of the total moisture content during storage of fresh ear corn. Significant changes in T 2 , T 21 , T 22 , A, and A 1 were frequently observed between two adjacent measuring time during fresh ear corn storage (Fig. 2). In Fig. 2, the signal intensity of moisture (A) generally showed a decreasing trend in fresh ear corn during storage, which corresponded to the loss of water. Since it also involved concomitant moisture migration from the inner parts to the periphery of corn kernels during the entire storage process, the A 2 of T 22 peak showed an obvious inflexion point after 1-h storage. Water bound with nutrients and enzymes gradually converted into free water beyond 1-h storage. Our results suggest that water loss is greater than water migration within the first hour of storage; but water loss is weaker than water migration thereafter. 3.2. Moisture distribution analysis In this study, the status of the entire hydrogen protons in a sliced fresh ear corn sample was reflected in the proton density image. Each pixel point was depicted respectively in the direction to up, down, left, right, and center in ear corn. Statistically significant variations in gray values were frequently observed among different storage time of fresh ear corn in a drier at each of the up, down, left, right, and center points of sliced ear corn (Fig. 3). The size of the signal was proportional to the amount of gray values. Just as illustrated in Fig. 3, with the increase in storage time, the semaphore of each point showed a declining trend although the intensities of decline were different among the five direction points. The gray value of pixel point of center in sliced fresh ear corn was obviously higher than those of the other four direction points. Moreover, the rate of decrease in moisture content was significantly lower at the center (Fig. 3-E) than those in the peripheral parts (Fig. 3-A–D). These results are consistent with the fact that the peripheral areas dry faster than the inner parts of fresh ear corn during storage. 3.3. Daily analysis of relaxation time Fig. 4 exhibits changes in moisture relaxation time in fresh ear corn at different storage time during a total of one-week storage at room temperature. Dehydration in fresh ear corn happened at room temperature, so in general the peak area of T 2 decreased with storage time. The most significant differences in T 2 among the fresh ear corn samples under the different storage time occurred at the right region of 100 ms, as this part of corn grains had homologous bound moisture content. The 1–10 ms peaks corresponded to the most inactive bound form of water. With increase in storage time, the T 2 peak area was gradually increased on the third or fourth day, and decreased thereafter. The increase of the T 2 peak was explained by the increase in concentration of carbohydrates including glucose and fructose as the free water converted into bound water. Reduction in the T 2 peak might be owing to the fact that the molecules of nutrients Fig. 1 Change in moisture distribution in fresh ear corn at different storage time.