Fig. 4 shows the change in the temperature of the carrot center
with time for the sample subjected to hot-air fluidized bed drying.
The bottom figure shows the temperature change for the earlier
drying period (0e30 min). For the untreated carrot, the carrot
center was initially at room temperature (about 298 K), while the
center temperature of the freezing pretreated carrots was 233 K. In
the early period of the drying process, the temperature of the carrot
center increased drastically in all cases. For the sample subjected to
freezing pretreatment, the degree of temperature increase at the
center of the carrot declined notably at about 273 K. During this
period, thawing occurs, and the time required is longer at 12 kPa
than at 101 kPa. From visual observation of the carrots, it was
confirmed that the carrot remained frozen when the temperature
of the carrot center was lower than 273 K, while there was no
frozen region when the temperature of the carrot was higher than
273 K. That is to say, the frozen region persisted for a longer period
at 12 kPa than at 101 kPa. At 12 kPa, the boiling point of water is
lower than that at 101 kPa. Therefore, during the drying process,
the temperature of the carrot is lower at 12 kPa than at 101 kPa.
Thus, at 12 kPa, the frozen part persists for a relatively long time.
After thawing, the temperature of the carrot center increased
drastically again. For the untreated carrots, after the initial drastic
increase, the temperature of the carrot center became almost
constant at a slightly lower temperature than that in the fluidized
bed (333 K). For the sample subjected to freezing pretreatment,
there is a period when the temperature of the carrot center decreases
with time. This phenomenon is often observed in the drying of porous materials immersed in fluidized beds, and an in-depth
explanation is provided in our previous paper (Tatemoto et al.,
2004).