Analysis of moisture detection and migration law during paddy drying process based on LF -NMR

Hot air drying is one of the primary processing steps after paddy harvest. There are complex heat and moisture transfers inside the paddy during drying. The distribution of moisture can also dominate the quality of dried food in industrial production, such as the nutritional content, safety, and weight. The water transfer can greatly contribute to the product quality and the efficiency of drying. Low-field nuclear magnetic resonance (LF-NMR) technology can be expected to rapidly monitor and observe the internal moisture distribution of the sample in real time, due to its easy operation, wide applicability, high accuracy, and stability. Furthermore, the LF-NMR has been widely applied to monitor the moisture status and distribution during food drying, storage, and rehydration, whether the sample morphology, color, and size or not. The transverse relaxation time T₂ without damage or invasion can be used to analyze the migration of sample moisture from a microscopic perspective. This study aims to clarify the internal water migration during drying paddy using LF-NMR and imaging techniques. The drying characteristics were also determined using the transverse relaxation time T₂ inversion spectrum. A systematic analysis was implemented to explore the proportion of water in each state of paddy and magnetic resonance imaging (MRI) images under different drying conditions. The results showed that the higher the tempering temperature was, the shorter the drying time and the higher the drying rate of paddy was. The tempering temperature dominated the waist-bursting rate of paddy. Specifically, the lowest and highest waist bursting rates of paddy were achieved in the tempering temperatures of 40℃ and 20℃, respectively. Three peaks were detected to represent three types of water in paddy grains. Each peak precisely indicated the T₂ value for the respective type of water, namely: bound, semi-bound, and free water, representing successively from left to right as T₂₁, T₂₂, and T₂₃. There was the largest proportion (about 90.5%) of bound water in paddy. While the proportions of free water and semi-bound water were relatively small, only 9.5%. There was a significant linear relationship between the moisture content of the paddy and the amplitude of the NMR signal. The peak area of the T₂ inversion spectrum better reflected the moisture content of the paddy. The lateral relaxation time T₂ of the paddy gradually shifted to the left with increasing drying time, indicating the tighter binding of water and substrate in each state. Drying also changed the distribution of water and the moisture content in the paddy. There was a transformation between moisture in different states when the external environmental conditions changed. There was a variation among different states of moisture inside the paddy during drying, indicating the mutual transformation between bound and semi-bound water, as well as between semi-bound and free water. Tempering reduced the rate of moisture transformation. MRI images showed that the first thing to be lost was the free water attached to the surface of the paddy during drying. Among them, the moisture in the embryo spread outwards, as the moisture decreased on the surface of the paddy. The moisture in the embryo spread out again until the paddy reached a safe storage water content after the surface moisture was dried out again. Therefore, LF-NMR technology can serve as an effective way for the rapid detection of water migration during paddy drying. The finding can provide a strong reference to determine the optimal paddy drying.