Drying of temperature sensitive materials such as food and agricultural products, particularly at temperatures sensitive enough to cells, lead to probable cell damage and gelatinization. Such effects lead to surface hardened or case hardened layers leading to restricted moisture movement through resulting surface hardened layers and retarded drying rates. In this paper, equations are developed, based on the model of Dworecki et al. [Dworecki, K., Slezak, B., Ornal-Wasik, S., 2005. Evolution of concentration field in a membrane system. Journal of Biochemical and Biophysical Methods 62 (2), 153], for the moisture concentration distributions across the material and the surface hardened layers, and for the respective drying rates. Experimental data are compared with the solutions of the above equations. It is observed that the experimental data fits the solution very satisfactorily. Plots of model equations were compared with experimental data giving correlation coefficients closer to unity for data on slices of different thicknesses of papaya and garlic. It is shown that the drying rates under the case hardened conditions could be characterized by two physical parameters lD and r=KD/Df. The respective values of lD and r evaluated for different sizes of papaya and garlic slices showed consistency among different sizes of the same species indicating a reasonable compatibility with the model.
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