Approach to calculation time-dependent moisture diffusivity for thin layered biological materials

A method is presented to determine the effective diffusivity from experimental drying kinetics as a time-dependent parameter. The method combines an analytical solution of Fick's equation in which the Fourier number is approximated using the empirical coefficients a and b with a semi-theoretical equation derived for quasi-stationary conditions. The resulting equation has been applied to calculate the effective diffusivity from literature data on the drying of tobacco lamina and sliced celery. The applicability of the method was confirmed by good agreement of calculated and experimental data. It was found that the effective diffusivity for tobacco lamina rises sharply from practically zero at the beginning of drying to a maximum of 9.10(-10) m(2)/s at 89 s, and then gradually decays with time of drying. The same trend was found for sliced celery, but the maximum of the effective diffusivity (1.6.10(-7) m(2)/min) was attained at 56 s. The exact definition of the effective diffusivity vs. drying time identifies two phases of drying: the first phase was characterized by the rising intensity of drying with the maximum at the end of this phase, followed by the phase of slow decrease in the intensity of moisture removal. The rising intensity of drying observed during the initial phase of the process can be explained by warming up of the dried material during the initial phase of drying. (C) 2008 Elsevier Ltd. All rights reserved.