MEASUREMENT OF TRANSPORT PROPERTIES FOR THE DRIED LAYER OF SEVERAL FOOD MATERIALS UNDERGOING FREEZE-DRYING

The transport properties of several food materials have been presented as fundamental information to determine the drying rate of freeze-drying process. As an example, the measuring method of thermal conductivity and permeability has been demonstrated for the samples of sliced and mashed apples undergoing freeze- drying. Both samples were freeze-dried at constant surface temperatures ranging from —10 to 70 under the usual pressure range of commercial operations. A mathematical model, based on a quasi-steady state analysis, was formulated and then applied to the drying data to determine these transport properties for the dried layer of the sample undergoing freeze-drying. Values of thermal conductivity were found to be almost the same between sliced and mashed samples. However, the permeability data of the mashed apples were more than 4 times greater than that of sliced apples. Both temperature and pressure dependences on these transport properties were not recognized apparently, and the effects of freezing rate on transport properties were found to be critical for the mashed samples. The results indicated that the drying rate of sliced samples was limited by the transfer rate of water vapor flowing through the dried layer. The transport properties for raw beef, minced beef, shrimp and coffee solutions were also presented indicating the effects of operating conditions as well as the structural parameters of the samples on these values.