Structural models related to transport properties for the dried layer of food materials undergoing freeze-drying

The values of thermal conductivity and permeability have been presented for the dried layer of raw beef, coffee solutions and sliced as well as mashed apples. A structural model for beef, coffee solution and mashed apple was developed for predicting the permeability of water-vapor flowing through the dried layer. In modeling the porous dried layer was assumed to be a bundle of capillary tubes with the pore space having an equivalent pore radius, porosity, and tortuosity factor. These results indicated that the structure for the solution systems was controllable by both solute concentration and freezing manners. Another cellular structural model has been presented for predicting the permeability of a sliced apple. The resistance of a cell membrane to the molecular transfer of water vapor was determined from the value of permeability and the size as well as number of the cells existed in the dried layer. Since the drying rate of cellular materials was limited by the mass flux across the dried layer, the model was considered to play an important role in predicting the optimum heating program for the surface temperature of cellular food materials such as fresh fruits and vegetables.