Note: Descriptions are shown in the official language in which they were submitted.
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HEAT PROCESSING OF A PRODUCI
Back~round of the Invention
1. Field of the Invention
S The present invention relates to a method of heat processing
a product, ~,pecially but not exclusively foodstuff, and relates particularly but
not exclusively to the baking of bread.
The term "heat processing" is intended to cover, inter alia, the
cooking of foodstuffs, including for example, the delicate vacuum cooking of
meat, fish, vegetables or fruits, the pressure cooking of foodstuffs, and bakingof farinaceous products such as bread.
With regard to bread, the time required to bake a loaf can be
considerably shortened by baking a loaf's interior by rnicrowaves and rapidly
browning the exterior with conventional thermal heating, at temperatures
higher than those usual for baking.
2. Discussion of the Prirr Art
In industriali~ed countries it is common to cool bread to 21DC
in order to slice it. Conventional cooling, achieved by conduction of heat
from the interior of the loaf to its surface, demands a long period of time.
From there heat is conducted to the surrounding air, which heats and leaves
the loaf surface by c~ - ection currents. Forced air convection is frequently
used to accelerate heat removal. The heat transfer is an unsteady state
process, one major problem being the low heat conductivity of bread.
Increasing the velocity of the surrounding cooling air does not significantly
increase the cooling rate of the bread. Using such method a standard 800
gram loaf of white bread generally requires two and a half hours to cool.
Vacuum cooling is a technique which has been used for some
years and can be used to cool standard loaves of bread in around three and
a half minutes. Both microwave heating and vacuum cooling dehydrate the
product to a greater degree than using the normal techniques of conventional
thermal heating and cooling thereby increasing the weight loss normally
resulting from these normal techniques. This creates an economic disadvan-
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tage for low profit margin product sold by weight such, for example, as bread.
The advantages of microwave-assisted baking followed by vacuum cooling of
bread are extremely high speed, the option of using soft wheat flour only (that
cannot usually be used alone to ba~ce bread) and a product with a very great]y
reduced microbial contamination free of secondary contamination from
cooling air, due to the airless vacuum cooling.
Objects and Summary of the Invention
An alternative heat energy source which can be used for baking
is radio-frequency heat which, like microwave heating, permits acceleration
of the baking process compared with conventional electrical resistance
heating.
For convenience the term "dielectric heating" is used herein,
where applicable, to include both microwave heating and radio-frequency
heating.
The speed of baking by dielectric heating is still limited by the
danger of product explosion caused by the rapid creation of internal steam
pressure within the product, and it is an object of the present invention to
obviate or mitigate this danger.
According to the broadest aspect of the present invention there
is provided a method of heat processing a product, comprising the steps of:
placing said product in an enclosed zone; and
subjecting said product while in said zone to dielectric heating;
and during at least a portion of said heating processing vithin said enclosed
zone by adding gas and/or subtracting gas from the enclosed zone.
Preferably the product is subjected to microwave heating.
Alternatively the product is subjected to radio frequency
heating.
In the cases of roasting meat, coffee, cocoa or the baking of
farinaceous products, if there is only steam and no oxygen and/or other gases
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present in the oven, some of the customary dehydration characteristics and/or
thermal disintegration and oxidation reactions will not take place. These
reactions can be desirable and essential for the marketability of certain food
products, such as the brown crus~ and the fresh baked aroma of bread. In
addition, if a food product is heat treated without oxygen and cooled,
consequent oxidation may create undesirable flavors.
The selection of the constitution of supra-atmospheric pressure
environment vithin the oven can deterrnine such organoleptic characteristics
of the food product.
The dielectric heating hereinbefore referred to in the various
aspects of the present invention may be supplemented by conventional
thermal heating which may be used simultaneously with or separately of the
dielectric heating depending upon heat processin~ circumstances or require-
ments.
In the heat treatment of other products, particularly non-food,
it may be necessary to prevent oxidation, in this case the oven-contained
environment may be steam and/or nitrogen, and/or other nonoxidating gases
or mixtures of gases.
According to another aspect of the present invention there is
provided apparatus for heat processing of products, comprising:
structure defining an enclosed chamber having selectively openable door
means permitting placement of a product to be heat processed therein;
means associated with said chamber-defining structure for dielectric heating
of products placed therein; and
pressure-altering means operatively coupled vith said chamber-defining
structure for selective addition or subtraction of gas from the chamber in
order to selectively increase or decrease pressure conditions therein.
Preferably, the apparatus includes means within said chamber
for ~enerating radiant heat.
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Preferably the dielectric heating means comprises means for
generating microwaves.
Preferably the pressure altering means comprises a supply of
gas and conduit means operatively connecting said supply and said chamber
Preferably pressure altering means comprising a vacuum pump
operatively coupled with said chamber.
According to a further aspect of the present invention there is
provided a method of heat processing a gas-permeable product containing
vaporizable water, said method comprising the step of:
placing said product within an enclosed pressurizable zone:
subjecting said product while in said zone to dielectric heating sufficient to
cause vaporization of some of said water therein; and during at least a
portion of said heat processing, increasing the pressure within the enclosure
sufficiently to prevent significant disruption of the product by virtue of rapidmigration of vaporized water therefrom.
According to yet another aspect of the present invention,
therefore, there is provided a method of vacuum cooling a partially or
completely baked dough-based product within a gaseous atmosphere
comprising the steps of simultaneously subjecting the product to radiant
~0 heating sufficient to develop the crust of said product while reducing the
pressure of the gaseous atmosphere around the product for rapid cooling
thereof.
Brief Description of the Drawing Fi~ures
An embodiment of the present invention will now be described,
by way of example, with reference to the accompanying drawings in which:
Fig. 1 is a schematic diagram of baking apparatus; and -
Fig. 2 is a perspective view of an oven including a door
arrangement.
petai!ed Description of the Preferred Embodiment
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Referring to Fig. 1, there is shown an oven 1 cylindrical in cross-
section for example and provided with a steel pressure housing or casing 2
capahle of withstanding pressures up to S atmospheres for example and lined
with ceramic 3 for electrical insulation and heat resistance purposes.
The oven 1 is provided with, at its respective ends, a sealable
inlet door arrangement 4 and a sealable outlet door arrangement 5.
The oven 1 houses a product metal conveyor 6, or a non-metal i .
conveyor supported and movable across a metal platform or table.
The metal conveyor 6 (or metal supporting platform or table)
forms one electrode of a radio-frequency heating system which also includes
the discrete electrodes 7 (although these may be replaced by a plate electrode
extending the length of the oven 1) above the metal conveyor 6. he
electrodes 6 and 7 are connected to a radio-frequency generator 8.
In lieu of (or in addition to) th~. RF heating structure, a
schematically illustrated microwave generator may be used, trans. rring
microwaves via wave guide outlets 7a, which evenly distribute the microwave
field strengths throughout the chamber. The microwave generator may be,
for example, a cavity magnetron, a plurality of magnetrons or a reflux
klystron.
A preferred alternative to ceramic lining is to provide adequate
air space (clearance) between the discrete electrodes 7 and the steel pressure
casing 2 by appropriately dimensioning the diameter of the latter. This will
achieve the same desired result as ceramic lining.
Conventional electrical resistance hea~ers 9 connected to main
supply for example are also provided within the oven 1.
In accordance with this invention, the volumetric interior of the `
oven 1 is connected to a gas supply 10 by a pump 11 whereby said volume~ric
interior can be placed under supra-atmospheric pressure during the baking
process. ~-
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The gas supplied may be steam, air (oxygen), nitrogen or any
other gas required during the particular baking process being ~arried out
within the oven 1.
Also in accordance with the present invention, the volumetric
interior of the oven 1 is connected to a vacuum pump 12 whereby airless
vacuum cooling of the baked product can be effected within the oven 1
without the need to transfer same to a separate cooling chamber.
External to the oven 1 there are two separate conveyors 13, 14.
An inlet conveyor 13 is at the entrance and an outlet conveyor 1~ at the exit
of the oven 1. Inlet conveyor 13 supplies a product 15 to the oven and outlet
conveyor 14 carries the baked product 15 away from the oven 1. Between the
internal conveyor 6 and the inlet and outlet conveyors ~3, 14 there is a gap
16. Drawbridge conveyors 17, 18 are employed to bridge the gap 16, one at
either end of the oven 1, hen it is being loaded or unloaded. The drawbridge
conveyor 17, 18, are rotatable about the inlet and outlet conveyors 13, 14
between a horizontal position aligned with the external 13, 14 and internal
conveyor 6 and a position substantially vertical to the line of conveyors 6, 13,14.
With the inlet door arrangement 4 open, outlét door arrange-
ment 5 shut and the drawbridge conveyor 17 in its hoAzontal position a batch
of products 15 to be heat processed are loaded into the oven 1 from the inlet
conveyor 13 through inlet onto internal conveyor 6. The inlet door arrange-
ment 4, is then shut and sealed and heat processing can commence. The
door arrangements 4, 5 are described in more detail below.
Product 15 is heated by the dielectAc 7 and electrical resistance
heaters 9. T-he microwave or radio-frequency waves cause molecules of the
product to vibrate and heat is generated by the fAction between them, as is
well known. The waves penetrate deeply into the product so that heat is
generated throughout its bulk. lf the product is a food product then, duAng
heat processing, the pressure builds up within the food product due to rapid
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creation of heat and therefore steam. This allows for an external steam to be
created that will not only counter balance the internal steam pressure within
the product, but can furtherrnore, prevent a product from exploding. With
this external steam counter pressure the heat processing speed can be greatly
increased, no longer being limited due to the danger of product explosion.
While the external pressure creation must primarily utilize steam, there is alsoa built in option for filling the pressure oven 1 vith a controlled percentage
of additional steam, pressuri~ed air or oxygen via pump 11. Treating the
product 15 with oxygen allows oxidation under pressure to take place. This
can be desirable and essential for saleability of certain products, such as the
brown crust and the fresh baked aroma of bread.
The heat treatment is followed by vacuum cooling of the
product 15 in which the oven 1 is evacuated by a vacuum pump 12. Both
dielectric heating and vacuum cooling dehydrate the product, but if baked in
saturated pressurized s~eam there is no water vapor pressure gradient present
to allow the moisture to escape. The product stays more moist than it usually
would. The oven 1 is enclosed in the steel casing 2 which can withstand the
pressure gradients relative to the surroundings during cooling. The dielectric
7 and/or 7a and electrical resistance heaters 9 are switched off and the
pressure released prior to commencement of cooling.
~t the end of the heat processing and cooling, the outlet door
arrangement 5 is opened and the heat processed product 15 can exit through
the outlet onto drawbridge conveyor 18 in its horizontal position to outlet
conveyor 14.
ID the case of certain products, particularly non-food, it may be
necessary to prevent oxidation and therefore a gas such as nitrogen is pumped
into the oven 1 during baking by the gas pump 11.
It is to be appreciated that the vacuum cooling could take place
in a separate chamber with a conveyor system running between the oven 1
and the vacuum chamber.
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The oven 1 and the separate vacuum cooler (if such) are both
equipped with the capacity to monitor and record parameters such as
temperature, pressure, percentage oxygen in the gases, heating medium and
the percentage of other gases in the heating medium.
Figure 2 shows in detail the door arrangements 4, 5 mentioned
above. The arrangement 4, S is the same at the oven inlet and outlet and will
be described only once.
The inlet to oven 1 is closable by sliding door 19, slidable in a
vertical direction between two parallel guide brackets 20, fitted to either sideof the oven 1 inlet. The sliding door 19 is a substantially square plate. In thelower position it fully closes the oven inlet, when raised it provides for access
to the inlet. Each bracket 20 is L-shaped in cross section.
A clamping door 21 is used to clamp the sliding door 19 when
in the closed position. It comprises a frame work of an outer circle 22, two
horizontal straight frames 23 (one broken midway) and two vertical straight
frames 24 disposed inside the outer circle 22 and such that they provide for
a central window 25 aligned with the oven inlet. The clamping door 21 is
slidable horizontally on horizontal rods 26 (four shown) positioned around the
periphery of the inlet and fixed thereto such that their axes are parallel with
that of the oven 1. The rods 26 are received in holes in the outer frame 22.
On the side facing the oven the clamping door 21 has a rectangular guide
framework incorporating four protruding legs 27 positioned at the four points
where the horizontal frames 23 and vertical frames 24 intersect. The guide
legs 27 protrude at right angles to the rest of the frame.
To load the oven 1 the clamping door 21is slid away from the
oven 1 on the horizontal rods 26 and the sliding door 19 is raised vertically
to open the oven inlet. The inlet drawbridge conveyor 17 is then lowered to
communicate with the internal conveyor 6 and loading can commence. Note
that the window 25 in the frame is designed to allow the drawbridge conveyor
17 to be correctly positioned. When the product 15 batch has been loaded
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the drawbridge conveyor 17 is raised, the sliding door 19 is lowered to close
the inlet and the clamping door 21 is slid towards the oven 1 such that the
]egs 27 about the sliding door 19 and hold it tightly in position. A seal (not
shown) such as silver seal used in firearms or some other heat-resistant
S sealing element, that can stand up to 350C without deteriorating, is provided
between the oven 1 and the slidlng door 19 such that when clamped the
arrangement can vithstand an internal vacuum or an internal pressure in
excess of the surroundings, of about 5 atmospheres maximum. The clamping
door 21 is locked in position by means of a hydraulic or pneumatic actuator
28 (as shown in Fig. 2) or alternatively by virtue of a screw type device used
to move the door 21 along the rods 26 (not shown), for example.
Unl~ading is achieved at the exit end 11 (see ~i,. 4) by
releasing the clamp door 21 opening the sliding door 20 and lowering the
drawbridge conveyor 18.
It is possible to produce sterile food with the oven according to
this invention due to the supra-atmospheric pressure as a result of which the
temperature inside the food, for example a loaf of bread, can ~each, say,
121C (the boiling point of water at about 2 atmospheres of pressure), w h
sterili~es food. At this temperature the heat resistant bacterial spores are also
killed. Also, since the heat processing and vacuum cooling are done in the
same vessel there is no air contamination when the product passes fror~ -he
oven to separate cooler.
The significance of sterile products are that they have
considerably increased shelf-life and usually superior quality befor~ le given
shelf-life expires. Prevented spillage is twofold:
a.) Apparent spoilage in which the food can look and/or taste bad,
but is not necess~ y toxic.
b.) Toxic spoilage in which the spoilage is not necess.;rily apparer t.
and can therefore be highly dangerous.
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Bread is usually not sterile. Microbial deterioration of bread is
usually the growth of mold or going sticky on the inside due to mucus
producing bacteria ("ropy bread"). These are prevented in practice by adding
about 0.2%-0.3% of a salt of propionic acid - sodium or calcium propionate
S usually. A baking process as defined herein that produces sterile bread, and
obviates the use of chemical preservatives, would be highly advantageous.
In addition to food any other implements or substances could
also be sterili~ed using the oven according to this invention. Examples are
cotton wool for use in surgery or microbiology laboratories, bandages,
substrates for microorganisms, pipettes, bottles of sterile water of infusions,
surgical instruments, etc. By using radio waves the sterilization is accelerated,
in certain cases, and, in some instances, vacuum cooling could simultaneously
cool and dry unwanted, residual moisture.
The equipment, as described could be used in vacuum cook a
product. Foods requiring delicate cooking could be cooked by applying a
partial vacuum to the oven 1 via vacuum pump 12 in combination with the
dielectric heating process. This allows cooling to be carried out at lower
temperatures (for example boiling of water at 70 instead of 100C) and thus
desirable flavor compounds and other organoleptic qualities, destroyed at the
higher temperature and not at the lower, would be preserved. The dielectric
heat rays are able to travel in a vacuum or partial vacuum and they can bake
the outer crust of, for example, a loaf of bread while the center of the loaf iscooled by the escape of water vapor. The escaping water vapor collects heat
while traveling to the surface and thus removes the heat on escape thereby
cooling the surface of the loaf also.
The above-described oven with vacuum environment facility can
be used to ensure very rapid drying with additional advantage that water will
boil away at temperatures lower than the usual 100C of atmospheric pressure
drying thereby creating less damage to the product being dried.
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The oven with vacuum environment facility consequently is
suitable, inter alia, for rapid drving of fruit and vegetables.
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