Claims
- 1. An apparatus for adjusting the temperature profile of a food product to be heated by microwave radiation having a frequency of 2.45 GHz comprising:
- a food product to be heated by microwave radiation;
- a conductive grid, the grid comprising a conductive surface surrounding a plurality of tuned openings, the grid having a plurality of tuned openings of a size between 0.75 inch and 1.75 inches, the openings being spaced apart a distance between 0.6 inch and 1.8 inches, the grid being spaced a distance from a first opposing surface of the food product between 0.25 inch and 2.4 inches;
- a conductive ring, the conductive ring being in close proximity to a second surface of the food product; and,
- the grid and ring being mutually cooperable to provide a predetermined temperature profile representative of heating of the food product when the food product is exposed to microwave radiation.
- 2. An apparatus for adjusting the temperature profile of a food product to be heated by microwave radiation, comprising:
- a food product to be heated by microwave radiation having a dielectric constant E';
- a conductive sheet having a tuned opening located in close proximity to an opposing surface of the food product, the opening having a minimum size D.sub.min of: ##EQU14## where D.sub.min is the minimum size of the opening expressed in inches, D is equal to 0.75 inch, E' is the dielectric constant of the food product, and F is the frequency of the microwave radiation expressed in gigahertz;
- the opening having a maximum size D.sub.max of: ##EQU15## where D.sub.max is the maximum size of the opening expressed in inches, D is equal to 1.75 inches, E' is the dielectric constant of the food product, and F is the frequency of the microwave radiation expressed in gigahertz;
- the conductive sheet surrounding the opening defining a conductive border having a minimum width of 0.25 inch; and,
- the conductive sheet and tuned opening being mutually cooperative to provide a predetermined temperature profile for the food product during microwave heating.
- 3. An apparatus for adjusting the temperature profile of a food product to be heated by microwave radiation at a frequency of 2.45 GHz comprising:
- a food product to be heated by microwave radiation;
- a conductive grid, the grid comprising a conductive surface surrounding a plurality of tuned openings, the grid having a plurality of tuned openings of a size between 0.75 inch and 1.75 inches, the openings being spaced apart a distance between 0.6 inch and 1.8 inches, the grid being spaced a distance from a first opposing surface of the food product between 0.25 inch and 2.4 inches; and,
- the grid being operable to provide a predetermined temperature profile representative of heating of the food product when the food product is exposed to microwave radiation.
- 4. The apparatus according to claim 3, wherein:
- the size of the openings in the grid are between 1 inch and 1.5 inches.
- 5. The apparatus according to claim 4, wherein:
- the grid is spaced a distance from the food product between 0.6 inch to 1.8 inches.
- 6. The apparatus according to claim 5, further comprising:
- a conductive side wall encircling a second surface of the food product, the conductive side wall being in close proximity to the second surface of the food product.
- 7. The apparatus according to claim 5, wherein:
- the grid has a conductive border width greater than or equal to 0.5 inch.
- 8. The apparatus according to claim 4, wherein:
- the grid is spaced a distance from the food product between 0.9 inch and 1.5 inches.
- 9. The apparatus according to claim 8, wherein:
- the openings are spaced apart a distance between 0.125 inch and 2.4 inches.
- 10. The apparatus according to claim 9, further comprising:
- a conductive side wall encircling a second surface of the food product, the conductive side wall being in close proximity to the second surface of the food product.
- 11. The apparatus according to claim 8, wherein:
- the food product has a dielectric loss factor E", measured prior to microwave heating, which is less than 0.8.
- 12. The apparatus according to claim 8, wherein:
- the food product has a dielectric loss factor E", measured prior to microwave heating, which is less than 0.2.
- 13. The apparatus according to claim 8, further comprising:
- a conductive side wall encircling a second surface of the food product, the conductive side wall being in close proximity to the second surface of the food product.
- 14. The apparatus according to claim 8, wherein:
- the grid has a conductive border width greater than or equal to 0.5 inch.
- 15. The apparatus according to claim 4, wherein:
- the grid is spaced a distance from the food product of 1.2 inches.
- 16. The apparatus according to claim 15, wherein:
- the openings are spaced apart a distance between 0.125 inch and 2.4 inches.
- 17. The apparatus according to claim 4, further comprising:
- a conductive side wall encircling a second surface of the food product, the conductive side wall being in close proximity to the second surface of the food product.
- 18. The apparatus according to claim 4, wherein:
- the grid has a conductive border width greater than or equal to 0.5 inch.
- 19. The apparatus according to claim 3, wherein:
- the openings in the grid have a size of 1.25 inches, the size of the openings being substantially uniform.
- 20. The apparatus according to claim 3, wherein:
- the grid has a region where the openings in said region have a size of 1.25 inches.
- 21. The apparatus according to claim 20, wherein:
- the grid is spaced a distance from the food product of 1.2 inches.
- 22. The apparatus according to claim 21, further comprising:
- a conductive side wall encircling a second surface of the food product, the conductive side wall being in close proximity to the second surface of the food product.
- 23. The apparatus according to claim 22, wherein:
- the food product has a dielectric loss factor E" greater than 4.
- 24. The apparatus according to claim 3, wherein:
- the grid is spaced a distance from the food product between 0.6 inch to 1.8 inches.
- 25. The apparatus according to claim 24, further comprising:
- a conductive side wall encircling a second surface of the food product, the conductive side wall being in close proximity to the second surface of the food product.
- 26. The apparatus according to claim 25, wherein:
- the food product has a dielectric loss factor E" greater than 4.
- 27. The apparatus according to claim 3, wherein:
- the grid is spaced a distance from the food product between 0.9 inch and 1.5 inches.
- 28. The apparatus according to claim 27, wherein:
- the openings are spaced apart a distance between 0.125 inch and 2.4 inches.
- 29. The apparatus according to claim 28, further comprising:
- a conductive side wall encircling a second surface of the food product, the conductive side wall being in close proximity to the second surface of the food product.
- 30. The apparatus according to claim 29, wherein:
- the food product has a dielectric loss factor E" greater than 4.
- 31. The apparatus according to claim 27, wherein:
- the food product has a dielectric loss factor E", measured prior to microwave heating, which is less than 0.8.
- 32. The apparatus according to claim 27, wherein:
- the food product has a dielectric loss factor E", measured prior to microwave heating, which is less than 0.2.
- 33. The apparatus according to claim 27, further comprising:
- a conductive side wall encircling a second surface of the food product, the conductive side wall being in close proximity to the second surface of the food product.
- 34. The apparatus according to claim 33, wherein:
- the food product has a dielectric loss factor E" greater than 4.
- 35. The apparatus according to claim 3, wherein:
- the grid is spaced a distance from the food product of 1.2 inches.
- 36. The apparatus according to claim 35, wherein:
- the openings are spaced apart a distance between 0.125 inch and 2.4 inches.
- 37. The apparatus according to claim 3, wherein:
- the grid is generally parallel to the first opposing surface of the food product, the grid covering an area which is at least coextensive with said opposing surface of the food product.
- 38. The apparatus according to claim 3, wherein:
- the grid has a conductive border width greater than or equal to 0.5 inch.
- 39. An apparatus for heating a food product in a microwave oven to provide for generally uniform heating, comprising:
- a package including a food product for microwave heating, the package including a grid, the grid having an impedance at the microwave frequency to be used for microwave heating, the grid being spaced a distance from the position occupied by the food product, such that a combination of impedance of the grid and spacing between the grid and the food product is selected to fall within the region defined as region "C" of FIG. 38.
- 40. An apparatus for adjusting the temperature profile of a food product to be heated by microwave radiation at a frequency of 2.45 GHz, comprising:
- a food product to be heated by microwave radiation;
- a first means for adjusting the temperature profile of the food product comprising a dimensionally stable electrically continuous conductive surface which surrounds a plurality of transmissive areas, the conductive surface being formed from a material having a bulk resistivity less than 100 microohms per centimeter, the conductive surface being substantially parallel to an opposing surface of the food product, the transmissive areas having a size between 0.75 inch to 1.75 inches, the transmissive areas being spaced apart from each other by a distance between 0.6 inch to 1.8 inches, the conductive surface being spaced from the opposing surface of the food product by a distance between 0.25 inch to 2.4 inches.
- 41. The apparatus according to claim 40, wherein:
- the conductive surface is planar.
- 42. The apparatus according to claim 41, wherein:
- the transmissive areas are essentially microwave transparent.
- 43. The apparatus according to claim 42, wherein:
- the transmissive areas comprise circular openings in the conductive surface.
- 44. The apparatus according to claim 43, wherein:
- the circular openings are arranged in a staggered lattice configuration.
- 45. The apparatus according to claim 43, wherein:
- the circular openings are arranged in a square lattice configuration.
- 46. The apparatus according to claim 43, wherein:
- the circular openings are arranged in a circular array.
- 47. The apparatus according to claim 40, further comprising:
- a dielectric material on one side of the conductive surface and in close proximity to the conductive surface, the dielectric material having a dielectric constant E', the transmissive areas having a size in inches between ##EQU16## where F is the frequency of microwave radiation that is to be used to heat the food product.
- 48. An apparatus for heating meat, comprising:
- a meat patty initially in a frozen condition, to be heated to an edible temperature in a short period of time;
- means for preheating the meat patty using microwave radiation to establish a predetermined temperature profile in the meat patty, such means including a generally cylindrical dielectric bowl adapted to receive the meat patty, a circular conductive ring which encircles the sides of the meat patty, the conductive ring having a conductive bottom, the conductive bottom having an opening therein which is substantially transparent to microwave radiation, the conductive ring being supported by the bowl; and,
- infrared means for heating the meat patty using infrared heat after microwave preheating.
- 49. The apparatus according to claim 48, wherein:
- the meat patty has a diameter of 4.6 inches; and,
- the opening in the conductive bottom is circular and has a diameter of 3 inches.
- 50. The apparatus according to claim 49, wherein:
- the conductive ring has a height of 13/8 inches.
- 51. The apparatus according to claim 50, further comprising:
- a planar conductive top sheet placed over the top of the dielectric bowl and spaced from the conductive ring thereby defining an essentially microwave transparent gap between the conductive top sheet and the conductive ring.
- 52. The apparatus according to claim 51, wherein:
- the conductive top sheet is substantially parallel to the meat patty and spaced from the meat patty by a distance of 1 inch.
- 53. The apparatus according to claim 52, wherein:
- the meat patty has a thickness of 3/4 inch.
- 54. The apparatus according to claim 52, wherein:
- the conductive bottom has a diameter of 51/4 inches.
- 55. The apparatus according to claim 54, wherein:
- the conductive top sheet has an opening therein having a diameter of 31/8 inches, the opening in the conductive top sheet being generally aligned concentrically with the opening in the conductive bottom.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application discloses subject matter related to application Ser. No. 119,381, filed Nov. 10, 1987, entitled "Susceptor in Combination with Grid for Microwave Oven Package", by Dan J. Wendt et al., the entire disclosure of which is incorporated herein by reference.
Microwave heating of food products has a number of disadvantages and problems which have eluded practical and satisfactory solution for many years. Food products typically heat differently in a microwave oven, than would be the case if the same food product were placed in a conventional oven or grill, etc. In many cases where a food product has been cooked in a microwave oven, the result was an inferior tasting food product as compared to that which can be produced by conventional cooking means.
In the past, significant problems have been encountered involving the nonuniform heating of food in a microwave oven. When certain foods are cooked in a microwave oven, it is not uncommon to have areas which are overcooked. Such overcooking can result in undesirable attributes such as hardness, toughness, discoloration, off taste, as well as other undesirable attributes. Nonuniform heating of food sometimes results in raw spots in the food product. In some instances, some areas of a food product may be overcooked, while other areas of the same food product remain undercooked or raw. Thus, extreme examples of nonuniform heating exist where a food product may have both overcooked areas and undercooked areas in the same food product.
A basic challenge in consumer microwave design is to ensure a uniform field strength within the oven. Manufacturers have used different cavity designs, methods of feeding the microwave cavity, mode stirrers and product turntables in attempts to provide more uniform heating of food. If an oven design could be optimized to a single food product, in a controlled geometry, good uniformity results might be obtained. Instead, consumer microwave ovens are used on a multitude of food products, some of which will have good uniformity results in an oven and some which will have local hot and cold spots.
Designing a product for microwave preparation is difficult because of the range of oven designs that are in use. A product that works well in one microwave oven may not work well in another. Microwave brownies are particularly sensitive to overheating in microwave hot spots. The dehydration that results from a hot spot creates a hard texture after the brownies are cool. In particularly nonuniform ovens, raw spots and overcooked spots may exist in the same pan of brownies.
In the past, significant problems have been encountered with food products whose dielectric properties change significantly during microwave heating. It has been discovered that the amount of microwave energy which is absorbed by a food product during heating is related to the dielectric properties of the food product. More specifically, the relative dielectric loss factor (E") will significantly affect the extent to which microwave energy is absorbed by the food product. Some foods, when heated in a microwave oven, will undergo dramatic changes in the dielectric properties of the food, and specifically the relative dielectric loss factor. For example, in frozen meat the relative dielectric loss factor E" may initially be 0.34 when the meat is frozen at -24.degree. C., and change during microwave heating to a value over 10 at 50.degree. C.
During exposure to microwave radiation, a large piece of frozen meat may have one or more localized spots which thaw first. When these spots thaw, the dielectric properties of that portion of the meat change dramatically so that those spots then begin to absorb disproportionately large share of the microwave energy. The areas of the meat which are still frozen continue to absorb significantly less microwave radiation because the dielectric properties of the frozen areas of the meat have not changed significantly. This can result in a phenomenon which may be referred to as "thermal runaway." In this phenomenon, the significant change in dielectric properties which result as the food is heated by microwave radiation, such as when frozen meat is thawed, may result in spots which become very hot and/or cooked, separated by only a few centimeters from spots which are still frozen solid. Some mechanism has long been needed for dealing with significant changes in dielectric properties, which result in consequent differences in absorption of microwave radiation, during microwave heating.
In the past, many foods cooked in a microwave oven have had a problem with the criticality of the heating time. Microwave heating tends to heat foods more rapidly than may be the case with conventional cooking. The difference between an undercooked food product, and an overcooked food product, may be only a few seconds of heating time. Although most microwave ovens have timing mechanisms for precisely setting the microwave heating time, the criticality of heating time causes significant problems when efforts are made to mass produce food products for use in a large variety of microwave ovens. Microwave ovens commonly available to consumers vary significantly in cooking characteristics. Differences in power, oven cavity dimensions, stirrers, standing wave patterns, etc. from one microwave oven model to another result in significant differences in microwave heating characteristics among various microwave ovens. Food products which are mass produced for consumption by consumers having different models of microwave ovens need a reasonable margin for error in microwave heating time required to produce a satisfactorily cooked food product. Thus, with many foods the need has existed to reduce the criticality of heating time for a food product intended to be heated in a microwave oven. In other words, the "window" of time between the point in time where the food product is undercooked, and the point in time when the food product is overcooked, needs to be as large as possible without compromising the other advantages of microwave heating, such as unduly lengthening overall cook time.
The differences in various models of microwave ovens used by consumers makes it difficult to mass produce a food product for general consumption. The same food product will cook differently in different models of microwave ovens, and even in different examples of the same model oven. A need has long existed for some means to handle differences in microwave ovens so that a food product may be cooked in a variety of models of microwave ovens without significantly varying the cooking parameters.
Many commercial fast food establishments have equipment for the conventional cooking of hamburgers which has been optimized for hamburger patties of a particular size and mass. For example, a moving belt conveys hamburger patties through a flame broiler. The length of travel through the flame broiler, the temperature, and time of exposure have all been adjusted so that when the hamburger patties emerge at the end of the moving belt, hamburger patties of a standard size and weight are properly cooked. One difficulty with such an arrangement is that once it has been optimized for a particular size hamburger patty, it cannot be practically used alone to cook significantly larger hamburger patties. Such a mechanism may be difficult to adjust, and once it is properly set up to correctly cook hamburger patties of a standard size, it cannot conveniently be changed to cook hamburger patties which are significantly larger or smaller. It is desirable in commercial fast food establishments to provide a choice of larger hamburgers to consumers. Many commercial fast food establishments have attempted to meet consumer demand for larger hamburgers by offering "double hamburgers", where two standard size hamburger patties are provided in a single hamburger. Another complicating factor in such commercial fast food establishments is the need to provide food on demand. Hamburger meat must be conveniently maintained in a frozen condition. Thus, it is desirable to transform the hamburger patty from a frozen state to a properly cooked state as quickly as possible, and without advance notice of a customer order.
A need has existed to provide a means for producing a predetermined heating profile in a food product. For example, in the above example of a commercial fast food establishment having a standard infrared cooking mechanism which is optimized for a standard size food product, a need exists to provide a precooking means which can produce a predetermined heating profile in the food product, such that when the food product is subsequently heated by the standard cooking apparatus, the food product will be properly cooked. For example, a means has been needed to provide a heating profile for a larger hamburger patty which preheated the hamburger patty in a manner where the temperature distribution throughout the meat, prior to introducing the meat onto the standard moving belt for flame broiling hamburger patties, was set so that the large hamburger patty would be properly cooked when it emerged from the conventional cooking apparatus. The flexibility which would be provided by an invention which gave the ability to design a particular heating profile in a food product, using microwave technology, would open up remarkable opportunities for improving the quality of microwave cooking. It would also open up significant opportunities for combination heating utilizing microwave technology in combination with conventional heating to produce highly satisfactory and pleasant tasting food products. Also, significant reductions in total cook time could be achieved.
The present invention provides a method and apparatus for adjusting the temperature profile of a food product during exposure to microwave radiation. In a particularly advantageous application of the present invention, the temperature profile may be adjusted for uniformity. Thus, the present invention may include a means for providing a substantially uniform heating profile for a food product in a microwave oven.
The means for adjusting the temperature profile of the food product during microwave heating includes a conductive sheet containing one or more tuned apertures or perforations whose size and geometry may be adjusted in accordance with the teachings of this invention to provide a predetermined or desired temperature profile for the food product. The conductive sheet may take the form of a grid, particularly where uniformly heated food is desired.
Conductive side walls defining a conductive ring may be used in combination with a grid, particularly where it is desirable to alter an electric field boundary condition with the result of a reduction in edge heating of a food product.
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Foreign Referenced Citations (5)
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Patent Grant
4972059
