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1. In paddy grain, moisture is present at two places: at the surface of the grain, ‘surface moisture’, and inside the kernel, ‘internal moisture’.
2. Surface moisture will readily evaporate when grain is exposed to hot air. Internal moisture evaporates much slower because it first has to move from the kernel to the outside surface.
3. As a result, surface moisture and internal moisture evaporate at a different rate. This difference results in a different ‘drying rates’ for different period of drying.
4. The drying rate is defined as the rate at which grain moisture content declines during the drying process.
5. It is normally expressed in percent moisture removed per hour [%/hr]. Typical drying rates of rice dryers are in the 0.5%/hr to 1%/hr range.