8. Measurements of Dried Layer Transport Properties During Freeze-Drying of Coffee Solutions(Synopsis paper presented at the 28th Annual Meeting)

Abstract

Experiments were conducted to determine the transport properties of the dried layer using a radiant heating upon the sample surface. The samples of 6-30% coffee solution were freeze-dried in a dish of 70.5mm in diameter and 25 mm in height at constant surface temperatures ranging from 20 to 55℃ under about 10-95 Pa total pressure range, the usual operating condition of commercial freeze-dryer. Thermal conductivities and permeabilities of the dried layer were determined by applying the drying data to a model based on heat and mass transport in the sample. Thermal conductivities had a tendency to decrease as the sample concentration was increassed, and there was no definite effect of the dried layer temperature on thermal conductivity. Permeability decresced with increasing the sample concentration and it increased as the dried layer temperature increased. These transport properties were not affected by the pressure of water vapor in the dried layer under experimental conditions. From these results it may be concluded that transport properties depend mainly on concentration of sample tested and the concentration was one of the most important factors which affected the structure as well as transport properties of the dried layer. The relationships between transport properties and controlling factors were discussed in connection with the freezing and freeze-drying operations of the sample.