166
FRBEZE—DRYING
_
the liquid phase, a possible sticking—together of particles to each other and to the tray, and
a short time peak load for the water removal system—limit its application even for solid"
_
raw material.
_
‘
‘
“
R. I. N. Greaves 2 has described a method by which evaporative freezing can also be
used for liquids or pulps if they can be pumped. The reduction in handling cost is added to
the economical advantages of evaporative freezing. But until more results are published
it is diîcult to know how far some major problems with any evaporative freezing method —
.
of liquids could be overcome : the at least partial loss of highly volatile components, a
predictable crystal structure,an intended density and colour of the product, a pre—determined grain Size of the‘frozen product or at least a limited grain size range to obtain well
dened drying times. In addition to that it should be known how far evaporative freezing
Wouldchange the structure of colloidal suspensions.
Since very little has been published recently about the results with other freezing methods
to prepare food for freeze-drying it might be interesting to give at least some examples for
techniques which have been tried. Flow—freezing as a known technique in frozen—food
preparation can be adopted to freeze pieces, chips or dices continuously and quickly imme—
diately after the preparation of the foodstu‘. Quality losses or product changes can be
,
minimized thereby, and the handling of non—liquid products is facilitated. Figure 1 shows
a photograph of a ow—freezer.”
‘
For some liquids and pulps a partial freezing in ‘a continuously scraped cooler, producing
a high viscose mixture of small ice crystals, solids and water can be useful if a second stage
for the nal freezing of the product is added, as shown for a pilot plant in Figure 2. As
known from ice—cream freezing such a System- can be used best if the product contains some
amounts of sugar or generally speaking has a prolonged temperature range in which it
gets hardened as the temperature goes down. Other liquids, especially when a time-tempe'
rature control is ,desirable, çanbe frozenin smaller orlarger plates, on a belt or in
trayS or
«(€XPôÿimentalece Figure 3)…Such systems can be operated ina fairly wide
temperature time range. Unless the
of the frozen product is already
by a corr68ponding shapezof:thc
the;frozen
by suitable
mechanical means
low ,ÎQŒPQÏ3ÊUËQ- ln any. o£ these « cases the product; has
the bel£ org out
systemsië
principle in & lay-out
.
of Figure 4 a and-b. TÏhediquid
in» a temperature controlled blast
room (Figure 4
4 :
meçhanicany taken but 9f tha
trav
screemagmightraqtalwavs be.
dust as possible in the
(the
particles can befed.back into the liquid
for freezing). The dust problem will be discussed later in the freeze-drying process.
«
P B. Mason
3 has desŒibed a freeze—drying method in which the liquid product is frozen
on tubes for plates which Can be cooled during the freezing and heated during the drying
period. __Theyplates
are sùbmergedat the beginning in the liquid and cooled for a
_
certain period of time. The product
plates or tubes up to a wanted thickness,
ï
,
“then
the excess liquid is removed, the chamber surrounding the tubes evaCuated, and the
drying period can”; start
is applied to the
similar principles
,
_
(gure5)havebeensuggestedbyU Hackenberg4ao,andalsoby H JEenberg5"
-
f
'
FRBEZE—DRYING
_
the liquid phase, a possible sticking—together of particles to each other and to the tray, and
a short time peak load for the water removal system—limit its application even for solid"
_
raw material.
_
‘
‘
“
R. I. N. Greaves 2 has described a method by which evaporative freezing can also be
used for liquids or pulps if they can be pumped. The reduction in handling cost is added to
the economical advantages of evaporative freezing. But until more results are published
it is diîcult to know how far some major problems with any evaporative freezing method —
.
of liquids could be overcome : the at least partial loss of highly volatile components, a
predictable crystal structure,an intended density and colour of the product, a pre—determined grain Size of the‘frozen product or at least a limited grain size range to obtain well
dened drying times. In addition to that it should be known how far evaporative freezing
Wouldchange the structure of colloidal suspensions.
Since very little has been published recently about the results with other freezing methods
to prepare food for freeze-drying it might be interesting to give at least some examples for
techniques which have been tried. Flow—freezing as a known technique in frozen—food
preparation can be adopted to freeze pieces, chips or dices continuously and quickly imme—
diately after the preparation of the foodstu‘. Quality losses or product changes can be
,
minimized thereby, and the handling of non—liquid products is facilitated. Figure 1 shows
a photograph of a ow—freezer.”
‘
For some liquids and pulps a partial freezing in ‘a continuously scraped cooler, producing
a high viscose mixture of small ice crystals, solids and water can be useful if a second stage
for the nal freezing of the product is added, as shown for a pilot plant in Figure 2. As
known from ice—cream freezing such a System- can be used best if the product contains some
amounts of sugar or generally speaking has a prolonged temperature range in which it
gets hardened as the temperature goes down. Other liquids, especially when a time-tempe'
rature control is ,desirable, çanbe frozenin smaller orlarger plates, on a belt or in
trayS or
«(€XPôÿimentalece Figure 3)…Such systems can be operated ina fairly wide
temperature time range. Unless the
of the frozen product is already
by a corr68ponding shapezof:thc
the;frozen
by suitable
mechanical means
low ,ÎQŒPQÏ3ÊUËQ- ln any. o£ these « cases the product; has
the bel£ org out
systemsië
principle in & lay-out
.
of Figure 4 a and-b. TÏhediquid
in» a temperature controlled blast
room (Figure 4
4 :
meçhanicany taken but 9f tha
trav
screemagmightraqtalwavs be.
dust as possible in the
(the
particles can befed.back into the liquid
for freezing). The dust problem will be discussed later in the freeze-drying process.
«
P B. Mason
3 has desŒibed a freeze—drying method in which the liquid product is frozen
on tubes for plates which Can be cooled during the freezing and heated during the drying
period. __Theyplates
are sùbmergedat the beginning in the liquid and cooled for a
_
certain period of time. The product
plates or tubes up to a wanted thickness,
ï
,
“then
the excess liquid is removed, the chamber surrounding the tubes evaCuated, and the
drying period can”; start
is applied to the
similar principles
,
_
(gure5)havebeensuggestedbyU Hackenberg4ao,andalsoby H JEenberg5"
-
f
'