Transport properties of cellular food materials undergoing freeze-drying

The samples of sliced and mashed apples were freeze-dried by controlling their surface temperatures over the usual pressure range of commercial operations. The surface of sliced samples could not be maintained at above 10 degreesC in order to prevent the frozen layer from melting, while that of mashed samples was allowed to heat up to 70 degreesC. Thermal conductivities and permeabilities were determined by applying the uniformly-retreating-ice Gent model to the dried layer of the samples undergoing freeze-drying. The values of permeability for the mashed samples were found to depend on the ice-crystallization time during freezing. The results indicated that the drying rate of sliced samples was limited by the transfer 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.