Abstract
To study the drying characteristics of beef samples during freeze-drying process which includes the 2nd drying period, experiments with different conditions of heating methods, modes and sample temperatures were made using the freeze-drying apparatus which had been reported in reference (14). The results obtained from these experiments and the drying conditions are summarised as follows.
1. Different heating methods as well as the temperature conditions of the sample were shown in Fig. 1 and Table 1, respectively.
2. Experimental results provided important data pertinent to the drying characteristics of the sample tested and the corresponding operating conditions. The former consists of the change in sample weight, calculated drying rate and temperature distribution in the samples while the latter includes the surface temperatures of the heater, platen and condenser as well as the total and partial pressures of the non-condensing gas in vacuum chamber. A result obtained for the case in which both sample surfaces were heated by radiation was shown in Fig. 3. The changes in moisture contents, dimensions and densities of raw and dry samples were presented in Table 2, while the moisture distribution in sample was shown in Fig. 10.
3. Three typical patterns of temperature distributions of beef samples during freeze-drying were shown in Fig. 5. During the 1st drying period where no water vapor transfered from the sample bottom (patterns 1 and 2 in Fig. 5), the temperature gradient in the dryed region indicated larger value than that in the frozen region with the sublimation front showing a minimum temperature value. As the sublimation fronts proceeded inward from both top and bottom surfaces (patterns 3 in Fig. 5), the temperature of the frozen region decreased gradually and finally attained the minimum value at the instant immediately before the 2nd drying period began. In the latter period, the temperature distribution was linear between the sample surface and bottom.
4. The drying time for the 1st drying period was defined as the time required to dry 80 percent of the sample water content. Measured value under the present experimental donditions were given in Table 3. For example, the difference of 17.30 hours was observed in the 1st drying time when the surface temperature differed 20°C from each other keeping the bottom temperatures the same.
5. When the total pressure was 0.02-0.09torr at the end point of the drying process, the moisture distribution in the sample was influenced significantly by the temperature distribution within the sample. The higher the heating temperature of the sample, the lower the value of the moisture content.
