Assessment of water diffusivity in gelatine gel from moisture profiles. II. Data processing adapted to material shrinkage
2005
Ruiz-Cabrera, M.A. | Foucat, L. | Bonny, J.M. | Renou, J.P. | Daudin, J.D.
A method to assess the relationship between water diffusivity and water content was developed. Experiments designed to promote isothermal and unidirectional drying of samples and calculations of 1D water content profiles from 2D NMR images were presented in Part I (Ruiz-Cabrera et al., 2005). In theory water content profiles in rigid materials at different drying times can form a unique 'master curve' when Boltzmann's transformation is applied and the relationship D = f(X) can be directly derived from this curve. A procedure is proposed to account for local shrinking due to water removal: the measured water content profiles are first transposed into Lagrangian coordinates so that they correspond to the dry matter. This procedure was applied to the 1D profiles already measured on gelatine gel samples whose initial moisture content, pH and temperature were in the ranges 1.8-5 kg/kg dry basis, 4-8 and 10-24 degrees C, respectively. The assessed water diffusivity strongly decreased with the decrease in water content and slightly increased with temperature increase. The closer the pH is to the isoelectric point, the lower the water diffusivity. A unique relationship between water diffusivity and water content was not found. Regardless of the initial water content, the initial water diffusivity is always close to that of pure water. This suggests that structure changes induced by shrinking are a determining factor and that water content is just an indirect means of referring to that effect.
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