Modeling of moisture profiles in paddy rice during drying mapped with magnetic resonance imaging

During the drying process, internal water migration is often the limiting factor for the overall water transfer. In order to simulate a drying process, an accurate determination of the water diffusivity (D) in the material is always necessary. For biological products D is known to be low and vary with moisture content (X) and temperature (T). D=f(X, T) can be estimated from the fitting of moisture profiles determined by non-intrusive NMR imaging. This work first presents the experimental results obtained on paddy rice. The moisture profiles were measured during the drying process using a Bruker AMX400 spectrometer, equipped with a micro-imaging device at the H⁺ frequency of 400MHz. Four drying experiments at two different air temperatures were performed. A constant time imaging (CTI) technique proved to be useful to obtain the moisture content map in a central slice section of the kernel with 3mm thickness, allowing a spatial resolution of 0.1mm. This method provided access to low moisture content and low water mobility data. Then a diffusive model was developed using a cylindrical geometry, taking into consideration the shrinkage during drying. Uni-dimensional water profiles from axes of the elliptic section of the kernel were selected in order to determine the diffusivity parameters of the drying model.