CA2003985A1 - Control of heating by microwave energy - Google Patents
Control of heating by microwave energyInfo
- Publication number
- CA2003985A1 CA2003985A1 CA 2003985 CA2003985A CA2003985A1 CA 2003985 A1 CA2003985 A1 CA 2003985A1 CA 2003985 CA2003985 CA 2003985 CA 2003985 A CA2003985 A CA 2003985A CA 2003985 A1 CA2003985 A1 CA 2003985A1
- Authority
- CA
- Canada
- Prior art keywords
- layer
- microwave energy
- microwave
- heating
- electroconductive
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 238000010438 heat treatment Methods 0.000 title description 19
- 239000000463 material Substances 0.000 claims abstract description 28
- 238000010411 cooking Methods 0.000 claims abstract description 11
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 13
- 229910052782 aluminium Inorganic materials 0.000 claims description 12
- 230000003287 optical effect Effects 0.000 claims description 9
- 239000000123 paper Substances 0.000 claims description 6
- 239000011087 paperboard Substances 0.000 claims description 4
- 230000005855 radiation Effects 0.000 claims description 4
- 239000010410 layer Substances 0.000 claims 10
- 238000000034 method Methods 0.000 claims 8
- 239000002356 single layer Substances 0.000 claims 1
- 235000013305 food Nutrition 0.000 abstract description 2
- 238000012216 screening Methods 0.000 abstract 1
- 229910052751 metal Inorganic materials 0.000 description 16
- 239000002184 metal Substances 0.000 description 16
- 230000000694 effects Effects 0.000 description 8
- 239000010408 film Substances 0.000 description 7
- 235000013550 pizza Nutrition 0.000 description 4
- 239000000758 substrate Substances 0.000 description 4
- 230000005540 biological transmission Effects 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 230000020169 heat generation Effects 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 238000004806 packaging method and process Methods 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- XEGGRYVFLWGFHI-UHFFFAOYSA-N bendiocarb Chemical compound CNC(=O)OC1=CC=CC2=C1OC(C)(C)O2 XEGGRYVFLWGFHI-UHFFFAOYSA-N 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 239000010949 copper Substances 0.000 description 1
- 230000006378 damage Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 235000012396 frozen pizza Nutrition 0.000 description 1
- 238000003475 lamination Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 229910044991 metal oxide Inorganic materials 0.000 description 1
- 150000004706 metal oxides Chemical class 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000001151 other effect Effects 0.000 description 1
- 230000035515 penetration Effects 0.000 description 1
- 229920000728 polyester Polymers 0.000 description 1
- 229920000098 polyolefin Polymers 0.000 description 1
- 229910001220 stainless steel Inorganic materials 0.000 description 1
- 239000010935 stainless steel Substances 0.000 description 1
- 239000010409 thin film Substances 0.000 description 1
Landscapes
- Cookers (AREA)
- Package Specialized In Special Use (AREA)
- Electric Ovens (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Microwave cooking of food products is controlled to a desired degree by providing a desired combination of thermal energy generation to achieve surface browning and microwave energy screening to limit the proportion of incident microwave energy passing to the foodstuff.
The control is achieved by controlling the thickness of a layer of electroconductive material.
Microwave cooking of food products is controlled to a desired degree by providing a desired combination of thermal energy generation to achieve surface browning and microwave energy screening to limit the proportion of incident microwave energy passing to the foodstuff.
The control is achieved by controlling the thickness of a layer of electroconductive material.
Description
~oa~3~
" SHIELD MET "
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 : : . . . . .
" ' -'. ` '; : : .
, 2~
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 : . - , . . .
- .: . .
-, . ., . ;
, ,. .. , . . ~ , ;20039~ r~
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 . ~
, ' ~ , ', , :
' ~0~3~
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 ' : ,' . ~ ' .~: ' - .-' :. ' `,', :' , :
: -' ~
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 :
~3~3~5 the crust, to achieve less microwave heating and more thermal heating.
: ` :
.
, :
:: , ~ :
:: :
, ~ . , :,: - , ~ .
" SHIELD MET "
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 : : . . . . .
" ' -'. ` '; : : .
, 2~
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 : . - , . . .
- .: . .
-, . ., . ;
, ,. .. , . . ~ , ;20039~ r~
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 . ~
, ' ~ , ', , :
' ~0~3~
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 ' : ,' . ~ ' .~: ' - .-' :. ' `,', :' , :
: -' ~
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 :
~3~3~5 the crust, to achieve less microwave heating and more thermal heating.
: ` :
.
, :
:: , ~ :
:: :
, ~ . , :,: - , ~ .
3~3$~5 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.
:`
.
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.
:`
.
Claims (8)
1. A method of controlling the microwave cooking of prepared foodstuffs for consumption to a desired degree, which comprises:
positioning a layer of electroconductive material between a source of microwave radiation and said foodstuff so that said microwave energy is incident on said electroconductive material layer, and providing said electroconductive material layer with a thickness effective to convert a portion of the incident microwave energy to thermal energy for crispening and browning an outer surface of the foodstuff coincident with said electroconductive material layer and also effective to reflect a portion of the incident microwave energy to a desired degree.
positioning a layer of electroconductive material between a source of microwave radiation and said foodstuff so that said microwave energy is incident on said electroconductive material layer, and providing said electroconductive material layer with a thickness effective to convert a portion of the incident microwave energy to thermal energy for crispening and browning an outer surface of the foodstuff coincident with said electroconductive material layer and also effective to reflect a portion of the incident microwave energy to a desired degree.
2. The method of claim 1 wherein said electroconductive material is aluminum.
3. The method of claim 2 wherein said aluminum has an optical density of at least about 0.8.
4. The method of claim 2 wherein said aluminum has an optical density of about 0.8 to about 2Ø
5. The method of claim 2 wherein said aluminum layer is supported on a polymeric film layer.
6. The method of claim 5 wherein said polymeric film layer is laminated to a layer of electrically non-conductive support material.
7. The method of claim 6 wherein said support material comprises a single layer of paper or paperboard.
8. The method of claim 6 wherein said support material comprises two outer layers of paper or paperboard between which said polymeric film layer is sandwiched.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB888827709A GB8827709D0 (en) | 1988-11-28 | 1988-11-28 | Control of heating by microwave energy |
| GB8827709.0 | 1988-11-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2003985A1 true CA2003985A1 (en) | 1990-05-28 |
Family
ID=10647565
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002003974A Expired - Lifetime CA2003974C (en) | 1988-11-28 | 1989-11-27 | Differential thermal heating in microwave oven packages |
| CA 2003985 Abandoned CA2003985A1 (en) | 1988-11-28 | 1989-11-27 | Control of heating by microwave energy |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002003974A Expired - Lifetime CA2003974C (en) | 1988-11-28 | 1989-11-27 | Differential thermal heating in microwave oven packages |
Country Status (2)
| Country | Link |
|---|---|
| CA (2) | CA2003974C (en) |
| GB (1) | GB8827709D0 (en) |
-
1988
- 1988-11-28 GB GB888827709A patent/GB8827709D0/en active Pending
-
1989
- 1989-11-27 CA CA002003974A patent/CA2003974C/en not_active Expired - Lifetime
- 1989-11-27 CA CA 2003985 patent/CA2003985A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| CA2003974C (en) | 2002-01-08 |
| CA2003974A1 (en) | 1990-05-28 |
| GB8827709D0 (en) | 1988-12-29 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Dead |