Sorption kinetics experiments were performed a t 37 C with microcrystalline cellulose held between dryness and 51% water activity in the absence of other gases. Effective diffusion coefficients (>lO-4 cm*/s) were greater than any previously reported for similar experiments. A model based on heat and mass transfer properties of the sample is proposed. The vapor space permeability calculated by application of this model is related to the structure of the microcrystalline cellulose as determined from water desorption isotherm analysis, mercury intrusion porosimetry, and steady state permeability techniques. Microcrystalline cellulose, like some freeze-dried foods, contains micropores and macropores. During sorption, the mocropores are important in determining the balance between internal and environmental control during the first half of sorption. The micropores, which account for less than 1% of the void volume of the porous matrix but approximately 40% of the surface area, control the mass transfer properties of the sample during the latter stages of sorption. Because of the great difference between their mass transfer properties, the macropores approach local equilibrium faster than the micropores; this phenomenon suggests that changes in the effective diffusion coefficient and permeability as sorption proceeds are related to the structure of the sample rother than to the moisture content itself.Drying of foods is a process of major industrial importance. The physical phenomena that occur during drying and the mechanisms of moisture transport are so varied that sorption experiments have been used to study mass transfer in porous and nonporous food materials. Generally, these investigations have been conducted without concern for the environmental resistances to moisture transport or for the conduction of the heat released during sorption from the sample food through the environment to a heat sink, although heat factors have been shown to affect the rate of moisture sorption considerably (1, 2).Many analyses of sorption have been used to calculate a diffusion coefficient based on an appropriate solution of Fick's second law of diffusion, Equation ( 1).Crank ( 3 ) has presented various solutions of Equation (1) with a constant diffusion coefficient based on appropriate boundary conditions and sample geometry, and he has outlined methods for determining the functional dependence of the diffusion coefficient on the moisture content. This implies, sometimes incorrectly, that the diffusion coefficient is mechanistically related to the moisture content. King ( 4 ) has shown that the effective diffusion coefficient ( D E F F ) is related to various physical properties of the sample (k, AH,, ps, etc.). This relationship occurs because sorption is a heat and mass transfer process. As such, any one rate parameter must be related to those properties important to both heat and mass transfer. The present study was undertaken to determine the importance of both heat and mass transfer in this process with attention given to the ...
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