In our previous study on drying disk-shaped carrot samples
(Tatemoto and Michikoshi, 2014), a high drying rate was achieved
at a low drying temperature (333 K) by using a fluidized bed of inert
particles under reduced pressure (12 kPa), regardless of the type of
drying gas (dry air or superheated steam) employed, although a
higher drying rate was achieved with hot-air fluidized-bed drying
than with superheated-steam fluidized-bed drying. Shrinkage of
the dried carrot sample by the developed drying method was
similar to that observed using conventional hot-air drying at low
temperature. On this background, we suggest a drying method that
combines freezing pretreatment and fluidized-bed drying under
reduced pressure to maintain the size (volume) of the carrot sample
during drying. Vacuum-freeze drying is currently utilized in the
food industry as a method to maintain the size of dried materials;
however, this technique has some disadvantages, as mentioned
previously, including long drying time and high cost. Arolde and
Fernanda (2007) evaluated the effect of freezing pretreatment on
the drying time. They froze (and then thawed) select materials
(carrot and pumpkin) and dried them in a conventional hot-air
dryer. They reported that the pretreated materials required a
shorter drying time than the untreated counterparts, although the
volume change in the pretreated material during dryingwas almost
equal to that in the untreated material. The present study adopts an
approach in which frozen materials are immersed in a fluidized bed
of inert particles under reduced pressure without thawing prior to
the drying process. In the present method, evaporation and thawing
may occur during drying, successively or simultaneously. There
is no research on the drying of frozen materials in a fluidized bed of
inert particles under reduced pressure.