Analysis of drying characteristics and moisture migration for paddy rice under low temperatures and low relative humidities

The thin-layer drying experiments of paddy rice with different initial moisture contents ( 18.9% ~ 29.6%) were conducted under low temperatures and low relative humidities. Mathematical models, drying duration and effective moisture diffusivity were used to investigate the effects of initial moisture content, drying temperature and air relative humidity on the external moisture diffusion characteristics of paddy rice. LF-NMR was adopted to analyze the internal moisture dynamic of kernels during the drying process of paddy rice. The results showed that the lower drying temperature, the more significantly humidification could shorten the durations of paddy rice.The Page model could fit the drying curves of paddy rice very well, and the mathematical expressions for parameters k and n could be better described by the experimental variables. Besides, the effective moisture diffusivity of paddy rice was high significantly ( p < 0.01) influenced by MC0, T and RH, and Deffvaried from3.0697 × 10-10m2/s to 5.0369 × 10-10m2/s.During the whole drying process of paddy rice, the data of LF-NMR indicated that both the A21 peak area representing for “bound water ” and A22 peak area representing for capillary water decreased significantly, while the A23 peak area representing for free water kept steady. When the moisture content of paddy rice was decreased to 14.5% ~16.5%, the LF-NMR signal peak of capillary water disappeared.We can combine the continuous weighing method and LF-NMR technology to effectively analyze the laws of moisture diffusion and moisture state transformation during the low-temperature and low-relative humidity drying process of paddy rice.