Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF :lNVENTION
CONTROL OF HEATING BY MICROWAVE ENERGY
FIELD OF INVENTION
The present invention relates to the conversion of
microwave energy to thermal energy for the purpose of
heating foodstuffs and the control of heating of such
foodstuffs thereby.
BACKGROUND TO THE INVENTION
It is known that thin films of certain
electroconductive metals are capable of converting
incident microwave energy into thermal energy and
certain thicknesses such as is described in U.S. Patent
No. 4,641,005, and that such thermal energy can be used
during microwave cooking of foodstuffs to achieve
desired heating effects, such as browning the crust of a
frozen pizza.
It is sometimes difficult to control the heating of
the foodstuff to achieve a satisfactory uniform heating
of the product for consumption. For example, with a pot
pie or pizza pie, often the pie contents are fully
cooked by the application of microwave energy before the
crust is suitably crisp.
SUMMARY OF INVENTION
It has now surprisingly been ~ound that it is
possible to control the heating of the foodstuff by
microwave radiation by controlling the level of
microwave energy entering the foodstuff and by
converting a portion of the incident microwave energy
into thermal energy to heat external portions of the
food product.
In this regard, it has been found that, above the
threshold thickness of a layer of electroconductive
material below which no significant thermal energy
results from exposure of the electroconductive material
to microwave energy, the level of thermal energy which
is generated lncreases with increasing thickness of
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electroconductive material until a maximum level of
thermal energy generation is achieved.
Further increases in the thickness of the
electroconductive metal layer produces the same degree
of heat generation but also results in reflection in
microwave energy, preventing penetration oE a portion of
the incident microwave energy through the
electroconductive ma~erial layer and its subs~rate. The
degree of reflection of the incident microwave energy
increases with increasing thickness until the thickness
is beyond susceptor thickness and no thermal energy is
generated from the incident microwave energy. The
electroconductive material now has reached a thickness
which generally induces arcing and destruction o~ the
integrity of the electroconductive material layer.
These observations enable greater control over
microwave cooking of foodstuffs to be achieved. Thus,
by permitting a greater or lesser proportion of the
incident microwave energy to pass through the
z0 electroconductive material layer to the foodstuff being
cooked, the rate of cooking of the foodstuff by the
microwave energy passing through the electroconductive
material layer may be speeded up or slowed down, while
the rate of cooking at the surface of the foodstuff by
the thermal energy generated from the electroconductive
metal layer.
GENER~L DESCRIPTION OF INVENTION
~he present invention employs a layer of
electroconductive material of thickness controlled to
provide the desired degree of transparency to microwave
radiation so as to provide a desired combination of
thermal energy generation and microwave transmission.
The electroconductive material may be any
convenient material which may be provided as a layer of
sufficient thinness that the electroconductive material,
in effect, becomes semi-conductive and hence converts a
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portion of microwave energy incident thereon to thermal
energy.
The electroconductive material usually is a metal,
although other electroconductive materials, such as
carbon and some metal oxides, may be used. The
electroconductive metal most commonly used is aluminum,
but other electroconductive metals, such as stainless
steel and copper, may be employed.
The thickness of the layer of electroconductive
lo material at which the various effects noted above are
observed depend on the material chosen.
For aluminum, the threshold thickness for the
generation of thermal energy for heating is at a
thickness corresponding to an optical density of about
0.08. As the thickness of the aluminum layer increases,
the thermal energy generated increases to a maximum
value at a thickness coxresponding to an optical density
of about 0.2 to 0.3. The same level of thermal energy
generation is achieved as the thickness of the aluminum
layer is increased, but an increasing degree of
reflection of microwave energy occurs, thereby
decreasing the proportion of incident microwave energy
passing through the aluminum layer. The effect
continues to a thickness o~ aluminum layer corresponding
to an optical density o~ about 0.8. With further
increases in thickness of the aluminum layer, a further
proportion of the incident microwave energy is reflected
and the level of production of thermal energy declines,
at a thickness of aluminum corresponding to an optical
density of about 2.0, the aluminum metal layer is
essentially microwave energy opaque.
The film of electroconductive material may be
supported on a polymeric film or may be provided on a
paper substrate. The polymeric film may comprise any
convenient heat-resistant flexible polymeric material,
such as polyester or a polyolefin. The polymeric film
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substrate often is laminated to a single sheet of paper
or paperboard or sandwiched between two sheets of paper
or paperboard for utilization in a packaging structure.
This lamination also prevents the polymeric film from
distorting and enables the laminate to retain its
effectiveness in converting microwave energy into
thermal energy.
The novel laminates of this invention may be
employed to control microwave hea~ing of the foodstuff
to obtain less rapid heating than otherwise would be the
case, as a result of reflection of a portion of the
microwave energy, while the thermal energy generated by
conversion of a portion of the microwave energy is able
to produce the desired heating to adjacent means.
The manipulation o~ the thermal energy generation
and degree of microwave energy transmission in
accordance with the present invention may be combined
with other effects within the packaging structure.
For example, the layer of electroconductive metal
may be transferred from a polymeric material substrate
onto a paper substrate and achieve a higher level of
heat generation from the incident microwave energy than
is achieved when the electroconductive metal is
supported on the polymeric film, as described in my
copending United States patent application Serial No.
354,217 filed May 19, 1~89 ("Crisp Met"), the disclosure
of which is incorporated herein by reference.
In addition, the shielding effects may be combined
with an enhanced heating effect for multiple layers of ~!
electroconductive metal, as described in my copending
United States patent application Serial No. 374,~55
filed June 30, 1989 ("Multi-Met"), the disclosure of
which is incorporated hereby by reference.
DESCRIPTION OF PRI~FE:RRED h'MBODIMENTS
In one embodiment of the invention, the laminate is
incorporated into a pot ple d-sh. To obtain even
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heating during microwave cooking, the pot pie dish is
formed of rigid card or any other convenient material
and has a layer of high heat release coating on the
inside next to the pie to assist in release following
cooking. The side walls of the container have
incorporated therein a layer of metal of a thickness
corresponding to an optical d~nsity of from about 0.15
to about 1.1 and typically an optical density of about
0.8, to achieve reflectance to slow down cooking of the
pie contents but still get surface browning.
At the bottom wall, however, more rapid thermal
heating is required and two or more metal layers are
provided of a thickness corresponding to an optical
density of from about 0.10 to about 0.50. The use and
effect of multiple metal layers is described in my
copending United States patent application Serial No.
374,655 mentioned above.
By combining multiple metal layers with partially
reflective metal layers, the desired degree of heating
from various regions of the dish is controlled to the
desired degree to provide an even degree of cooking
throughout the product with the desired degree of
crispening.
Another product where a similar result is desired
is in pizzas. Oftentimes when a pizza is exposed to
microwave energy, the filling becomes unevenly cooked
throughout its depth, with the upper portion cooking
faster than the lower portion.
This problem may be overcome by providing a
laminate according to the invention, having a partially-
reflective metal film, in the direct path of the
microwave energy, such as by incorporating the same into
a container. This structure has the effect of
deflecting a portion of the microwave energy to the
bottom of the tray. Alternatively, the laminate of the
invention may be located below the pizza in contact with
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the crust, to achieve less microwave heating and more
thermal heating.
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SUMM~RY OF DISCLOSURE
In summary of this disclosure, the present
invention provides a novel manner of controlling heating
by microwave energy by using a heat susceptor thickness
of metal layer, which also is of a thickness suitable to
reflect a portion of the microwave energy and thereby
slow down the rate of heating by microwave radiation.
Modifications are possible within the scope of this
invention.
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