Transport properties of cellular food materials undergoing freeze-drying

The samples of sliced and mashed apples were freeze-dried by controlling th eir surface temperatures over the usual pressure range of commercial operat ions. The surface of sliced samples could not be maintained at above 10 deg reesC in order to prevent the frozen layer from melting, while that of mash ed samples was allowed to heat up to 70 degreesC. Thermal conductivities and permeabilities were determined by applying the u niformly-retreating-ice Gent model to the dried layer of the samples underg oing freeze-drying. The values of permeability for the mashed samples were found to depend on the ice-crystallization time during freezing. The result s indicated that the drying rate of sliced samples was limited by the trans fer rate of water vapor flowing through the dried layer. A cellular structural model is proposed for predicting the permeability of the dried layer, based on the resistance of the cell membrane to molecular transfer of water vapor.