JIA-2019-11

2646 WANG Hao et al. Journal of Integrative Agriculture 2019, 18(11): 2644–2651 were placed at a room temperature of (25±5)°C and 50% humidity; water distribution and migration were assessed using NMR and MRI every day for a week. All the samples in both groups were repeatedly used during experimentation. 2.2. Low field (LF)-NMR measurements Transverse relaxation measurements of the samples in groups a and b were performed on a PQ001 Benchtop Pulsed NMR Analyzer (Niumag Electric Corporation, Shanghai, China) operating at a resonance frequency for protons of 22.6 MHz. Samples were placed in a 40-mm tube. Spin-spin relaxation amplitude (A) and transverse relaxation time (T 2 ) were measured using the Carr–Purcell– Meiboom–Gill (CPMG) sequence (Carr and Purcell 1954; Meiboom and Gill 1958). The NMR data were primarily analyzed with discrete exponential fitting and continuous inverse distribution. The measurements were collected repeatedly for ten times for each storage time treatment of all samples. The T 2 measurements were made with a value (time between 90° and 180° pulses) of 250 μs. Data on 10 000 echoes were acquired as 32-scan repetitions at 32°C. The repetition time between subsequent scans was 6.5 s. 2.3. MRI measurements MRI is the application of a soft pulse spin echo (SE) sequence, the graphics synthesizer (GS) accumulative acquisition button. The acquisition was performed for eight times with phase encoding of 128 steps. After the acquisition, the “process” tab in the parameter setting area was clicked, and then the two dimension (2D) was clicked to get the required 2D image. By changing time of repetition (TR) and echo time (TE), the corresponding weighted images (proton-weighted images, T 1 -weighted images, and T 2 -weighted images) were obtained. With the use of weighted image technique, the contribution of different hydrogen protons to the image was distinguished. In proton-weighted imaging, the higher the proton density per unit volume was, the stronger the signal was, and the brighter the image was; conversely, the lower the proton density was, the weaker the signal was, and the darker the image was. The samples in Group a were scanned, and images corresponding to the storage time of 0, 0.5, 1.0 1.5, 2.0, and 4.0 h were obtained, respectively. The samples in Group b stored at room temperature were scanned daily, and images were obtained for every day in a week. The imaging plane was perpendicular to the vertical axis and passed through the middle of the ear corn samples. The coil diameter was 156 mm. The spin-echo pulse sequence was set to obtain T 2 -weighted images of the samples. The sequence parameters used in this study were set as repetition time: 2 000 ms, echo time: 85 ms, shear layer thickness: 5 mm, and data matrix: 1024×128. If there was only one layer for imaging, then only the center slice was imaged. Asilicone oil reference sample was used to calibrate the signal intensity. A gray value which was the average brightness value in the target area of an ear corn sample was calculated with the ImageJ Software (National Institute of Health, Bethesda, Maryland, USA). 2.4. Corn grain moisture content determination An empty aluminum container was weighed (W0). A fresh corn grain was sampled, put into the aluminum container, and weighed (W1), oven-dried, cooled down in a drier, and then weighed again (W2). Drying was carried out at a constant temperature of 80°C. The mass moisture content of fresh corn grain was calculated with the following equation: Moisture content (%)=(W1–W2)/(W1–W0)×100 2.5. Statistical analyses Analysis of variance was conducted on moisture relaxation time and semaphore, gray value, signal intensities of bound water, free water, and total water, and bound water percentage over total water with the ANOVA procedure in SAS for Windows V9 (4) (SAS Institute, Cary, NC). The measuring time were used as treatments for the above measurements since the focus of this study was to evaluate the moisture distribution and migration of ear corn at different storage time. A randomized complete block design with three replicates was used for all the analyses. Treatment means were separated with the Fisher’s protected least significant difference test. All the analyses were designated as significant at P< 0.05. The standard deviation was calculated for each treatment of moisture relaxation time and semaphore, gray value, signal intensities of bound water, free water, and total water, and bound water percentage over total water, separately, with the Summary procedure in SAS. 3. Results and discussion The moisture content in corn grain was 56.7% at the beginning of storage time, but decreased to 54.8% after 4-h storage. These results indicated that moisture content in fresh corn grain showed a downward trend during storage. 3.1. Relaxation time and signal intensity analysis Ruan and Litchfield (1992) measured the differences in moisture content and mobility among various components of a corn kernel during steeping at room temperature. The