Note: Descriptions are shown in the official language in which they were submitted.
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Nethod and apparatus for drying solid foodstuff~
Field of the invention
The invention relates to a method and apparatus
for drying solid foodstuffs, in particular vegetables which
have been washed. More specifically, the invention relates
5 to a method and apparatus for drying sliced vegetables
following industrial slicing of these vegetables in a
vegetable slicing plant.
Background of the invention
The European patent publication EP-A-0271376
10 describes a process and apparatus for drying plums by
supplying them to an oven on a conveyor belt, and by circu-
lating hot air through the oven. The air temperature is
uniform throughout the oven, and the temperature of the
plums at the discharge is higher than their temperature at
15 entering the oven.
The European patent publication EP-A-0079523
discloses a device and process for drying solid particles in
a suspension, like e.g. urban sludges or non-edible waste.
The drying installation disclosed in this document comprises
20 a first upstream section in which the product is dried by
circulating air which is heated in calorific heaters. The
air that is discharged from this first section is led
through a heat exchanger, mixed with discharged air from a
second, downstream section and recirculated into the first
25 section. In the second, downstream section, the product is
dried by air that is heated by condensers of a heat pump,
the evaporator of which is connected to the heat exchanger
of the first section. The temperatures at which this drying
installation operates are in the range of about 100C, and
30 therefore much higher than necessary for drying vegetables.
Nowadays, vegetables to be dried after washing are
supplied to a centrifuge, and dried therein in a discon-
tinuous process. The drawbacks of this drying mode are that
the product to be dried is damaged by the rotating drum and
35 by the forces which the products exert on each other, and
that the damaged and torn apart parts are left behind in the
centrifuge, which may lead to the centrifuge becoming
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unbalanced and less hygienic due to possible material
infection.
Summary of the invention
The present invention seeks to provide an improved
method of drying solid foodstuffs, in particular vegetables
which have been washed. Such vegetables may include carrots,
potatoes, cauliflower, lettuce and alike. The principle
underlying the method can, however, also be used for other
vegetables and foodstuffs in solid condition.
The present invention further has for its object
to provide a method of drying solid foodstuffs with which no
centrifugal forces are exerted on the foodstuffs.
The present invention also has for its object to
provide a method for drying solid foodstuffs with which the
temperature of the foodstuffs after drying is substantially
the same as before drying.
According to the present invention, the foodstuffs
are supplied in a substantially flat bed, and a relatively
cool drying gas is also supplied. At a first location the
drying gas is urged to flow through or along the foodstuffs
substantially perpendicularly to the direction of the bed,
after which the gas is heated and dried. At a further
location, spaced from said first location, the drying gas is
urged to flow through or along the foodstuffs substantially
perpendicularly to the direction of the bed. This further
location lies upstream of the first location as seen in the
direction of supply, and the temperature of the drying gas
at the further location is higher than that at the first
location. Subsequently, the gas is cooled and redirected to
the first location, and the dried foodstuffs are discharged.
Preferably, the steps of heating and drying the
gas and urging it to flow through or along the foodstuffs at
a location further upstream is repeated at least once before
the drying gas is cooled.
Preferably, also the steps of supplying the
relatively cool gas, urging it to flow through or along the
foodstuffs, heating and drying the gas, urging it to flow
through or along the foodstuffs again and cooling the gas
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for renewed supply are repeated at least once before the
dried foodstuffs are discharged.
The drying gas may be heated and dried before
being urged to flow through or along the bed for the first
time. The direction of flow through the bed may be upwards,
and the gas may be transported back under the bed along the
outside of the bed, thus resulting in a substantially
corkscrew-shaped path of the gas flow.
Preferably, the gas used as drying gas is air.
With foodstuffs that are sensitive to oxygen it is also
possible, however, to use nitrogen or another inert gas for
drying, also because the gas used in the drying process is
recirculated after the absorbed water vapor has been
removed.
The temperature of the foodstuffs during drying
preferably does not exceed 40C, and the foodstuffs in the
bed are preferably first heated and then cooled down again
during drying, such that their temperature at discharge is
substantially the same as the temperature at which the
foodstuffs are supplied.
The invention also seeks to provide an improved
apparatus for drying solid foodstuffs, specifically washed
vegetables. According to the invention, such apparatus
comprises supply means for supplying a flat bed of solid
foodstuffs, and gas supply means for generating a supply of
relatively cool drying gas. The apparatus also comprises
flow generating means at a first location for causing the
drying gas to flow through or along the foodstuffs in a
direction of flow that is substantially perpendicular to the
direction of the bed. The apparatus further comprises means
for heating and drying the gas that has flown through or
along the bed. Connected in series with the heating means
are further flow generating means for causing the drying gas
to flow through or along the bed at a second location
further upstream of the first location. The temperature of
the drying gas at this further location is higher than that
at the first location. Finally, the apparatus comprises
means for recirculating the drying gas from the further flow
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.
generating means to the cool gas supply means, and discharge
means for discharging the dried foodstuffs.
Further details of the method and apparatus of the
invention will become apparent from the following
description, wherein reference is made to the appended
drawing, in which the sole figure diagrammatically
illustrates the operation of the drying apparatus of the
invention.
Detailed descri~tion of a preferred embodiment
In the drawing the device is shown to comprise six
successive heaters 1-6, coolers 7, 8 and 9 and fans 11-16.
It will be apparent, however, that it is also possible to
use a larger or a smaller number of heaters, fans and
coolers, dependent on the foodstuffs or product to be dried,
the nature and humidity thereof, the temperature at the
supply point and the desired temperature at the discharge
point and the manner in which the foodstuffs are stacked on
the conveyor belt. The optimum number of heaters, coolers
and fans can be adjusted by a person skilled in this field
of the art, also in dependence on the capacity of the
coolers, heaters and fans.
The following explanation of the system is only by
way of example, therefore. The product to be dried, such as
sliced carrots or lettuce, is supplied at arrow 17 by means
of the perforated conveyor belt 10 or a perforated shaker
conveyor 10 and, after drying, discharged at the other side,
indicated by the arrow 18. Air is used as the cooling gas,
said air being supplied at 19 and having a humidity of 100 %
and a temperature of for example 5C. This air flow is
passed through the cooler 8 at 31, whereby the air is cooled
down to -5C. A large part of the water vapor contained
condenses thereby and freezes on to the cooler. That is why
the cooler 9 is arranged beside the cooler 8, so that when
the cooler 8 has built up too much resistance as a result of
icing, the cooler 9 can be used and the cooler 8 can be
defrosted. The air obtained at 32 has a temperature of -5C
and a relative humidity of 100 %. This cold air is supplied
to the heater 6 and the heater S by means of the line 20.
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Different temperatures are set for these two heaters, so
that, seen in the longitudinal direction of the conveyor
belt, drying of the product to be dried takes place from the
starting point 17 to the end point 18 with air having a
decreasing temperature and, along the first part, a
decreasing moisture content. Upon being supplied the air
flow 33 has a temperature of -5C and a relative humidity of
100 % said air being heated in the heater 6 to a temperature
of 3C and a relative humidity of 55 %. This air flow is
passed through section 22 of the conveyor belt 10, whereby
the quantity of foodstuffs present thereon, which has
already been largely dried, is in particular cooled down to
the temperature at which the product is supplied at 17. This
temperature is about 2C. The product which at 17 is
supplied to the conveyor belt 10 having openings 21, is sub-
stantially dried at a temperature of 12-15C in the first
five sections of the conveyor belt indicated at 27-23, and
cooled down again to the supply temperature, viz. about
2-3C, in the last part of the conveyor belt 22. Numeral 35
indicates the air leaving the conveyor belt at section 22,
which air is supplied, via line 61, to the heater 4 by means
of the fan 16. The product to be dried present on section 23
of the belt 10 is dried by means of air from the air flow
33a having a temperature of -5C, said air being supplied to
the heater 5 and having a temperature of 20C and a relative
humidity of 25 % upon exiting the heater 5. As a result of
the low relative humidity the last remainder of moisture is
removed from the product to be dried as well as possible,
and the air discharged from section 23 of the conveyor belt
10 at 44 is supplied, by means of fan 15, to the heater 3
via the line 62. In the central sections 24 and 25 of the
conveyor belt 10 an air flow 37 having a temperature of 25OC
and a relative humidity of 25 % is obtained via the line 36
and heater 4, and after drying of the product on section 24
of the belt an air flow 38 is obtained, which at 39 is
supplied to the heater 2 by means of fan 14, via line 63. In
the heater 2 the air is heated to 27C and a relative
humidity of 35 %, and then passed through the product to be
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dried on section 26 of the belt 10, after which the air is
discharged at 41, the air flow 42 being passed through
cooler 7 by means of fan 12, where it is cooled down to 5C
and a relative humidity of 100 %.
The air flow at 45 is passed through the heater 3,
whereby an air flow having a temperature of 25C and a
relative humidity of 25 % is obtained, which is passed along
and through the product to be dried present on section 25 of
the conveyor belt 10, and the cooled-down, more humid air is
supplied, via lines 64 and 48, to the heater 1 by means of
the fan 13, so that air having a temperature of 27C and a
relative humidity of 35 % is obtained at 49, which is passed
through section 27 of conveyor belt 10, resulting in the air
flow 50, which is passed through the cooler 7, by means of
fan 11, as air having a temperature of 20C and a relative
humidity of 65 %, and supplied to cooler 8 again as a gas
flow 19 having a temperature of 5C and a relative humidity
of 100~, in order for the cycle to be repeated.
Initially the air flows from the fans 15 and 16 do
not have the same temperature and humidity, but this air is
preferably mixed prior to being passed through the heaters 3
and 4, so that the air supplied to the heaters 3 and 4 has a
common temperature of 12C and a relative humidity of 50 %.
Instead of the six fans 11-16 used in the
embodiment described, it is also possible to use fewer fans,
for example three, by using six fans, however, it is
possible to achieve a more or less perpendicular
displacement of the air flow through the product to be
dried. This will prevent the occurrence of considerable
accelerations in horizontal direction in the air flow, which
may result in the product to be dried being blown off
locally. The air velocity through or along the product to be
dried should preferably be limited to about 2 m/sec., since
at a higher air velocity the product is blown off or
displaced in a disadvantageous manner, so that an irregular
distribution over the belt of the product to be dried
results. If the air supplied were to be distributed over six
passages at 32a, viz. over the six heaters 1-6, the maximum
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air circulation per hour would be too small to remove the
desired amount of moisture form the product. That is why the
air flows through the fans are combined in two streams, so
that the total amount of air circulating per hour is
doubled. By using six independent heaters the temperature
and the drying rate can readily be controlled at any point
along the belt.
Every product to be dried has its own warming-up
time and its own cooling-down time, dependent on the
diameter of the particles to be dried. Six different
temperatures can be set by the six independent heaters,
dependent on the requirements of the product to be dried.
Each heater can be controlled separately, therefore. The
fact that in the embodiment described the heating capacity
is respectively controlled by two streams does not mean that
it is not possible to adjust different temperatures for all
six heaters.
After the usual slicing and washing freshly sliced
vegetables contain 8-30 % adhering water. In order to obtain
an attractive appearance and a satisfactory storage life of
the packed, fresh product it is necessary to reduce the
amount of adhering water by 60-80 %.
The above-described system is capable of removing
about 140 kg of moisture per hour. After washing, sliced
lettuce contains 30 % adhering moisture. During drying the
amount of adhering moisture is reduced to 6 %. The time
duration of the lettuce in the drier of the invention is
about 15-20 minutes. The maximum layer thickness on the
conveyor belt is about 12-15 cm. All this can be controlled
by suitably selecting the width of the conveyor belt,
whereby for the present embodiment a length of about 6 m and
a width of 1,3 m have been selected.
The drawing does not show any control equipment
for the temperature and humidity measurement equipment,
because an expert in this field of the art will be familiar
therewith.
It is possible to combine the heaters 3 and 4, as
well as the heaters 1 and 2, since these heaters 1,2 and
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.
3, 4 respectively transmit an equal amount of heat to the
gas to be passed therethrough.
The advantages obtained by using this method and
this apparatus lie in the fact that with this apparatus
S there is no loss of product, as is the case with
centrifuges. The temperature of the product to be discharged
can readily be controlled, which is why the product obtained
from the drying plant has about the same temperature as the
product to be supplied, and will normally not be higher than
40C during the drying process of the invention.
Furthermore, the method can be carried out continuously,
whilst the apparatus is easy to clean, in particular in
those places where the product to be dried comes into
contact with the apparatus.
