Methods and devices used in the microwave heating of foods and other materials

• US 5,519,195 A
• Filed: 02/09/1993
• Issued: 05/21/1996
• Est. Priority Date: 02/09/1989
• Status: Expired due to Fees

• Current Assignee: Graphic Packaging International Incorporated
• Original Assignee: Beckett Technologies Corp.
• Inventors: Keefer, Richard M., Lacroix, Cindy M.
• Primary Examiner(s): HOANG, TU BA
• Application Number:
  US08/015,311
• Time in Patent Office: 1,197 Days
• Field of Search: 219/727, 219/728, 219/730, 219/745, 219/736, 219/734, 219/725, 219/759, 426/107, 426/113, 426/234, 426/243, 99/DIG. 14
• US Class Current: 219/728
• CPC Class Codes: B65D 2581/3441 3-D geometry or shape factors, e.g. depth-wise
• CPC Class Codes: B65D 2581/3487 Reflection, Absorption and Transmission [RAT] properties of the microwave reactive package
• CPC Class Codes: B65D 2581/3489 Microwave reflector, i.e. microwave shield B65D2581/3487 takes precedence
• CPC Class Codes: B65D 81/3453 Rigid containers, e.g. trays, bottles, boxes, cups
• CPC Class Codes: Y10S 99/14 Induction heating

Abstract

A method and device for enhancing the heating of a surface layer of an article being heated by microwave energy is characterized by directing the energy through the surface layer into a main portion of the article in such a manner that the modes of the energy are in cut-off in the surface layer.

Claims

1. A method of enhancing the heating of a surface of an article to be heated by microwave energy, comprising the step of directing said energy through a plurality of microwave-transparent apertures defined by annuli of microwave-reflective, substantially non-absorptive material and into the surface layer and thence into a main portion of the article wherein said microwave energy enters into said surface layer in the form of cut-off propagation to directly heat said surface layer without first converting such microwave energy into heat.

2. A method according to claim 1, wherein the surface layer has a lower dielectric constant than the main portion and said microwave energy is not in the form of cut-off propagation in the main portion, whereby to cause absorption of energy in the surface layer per unit distance into the article to be greater than absorption of energy per unit distance into the main portion and hence raise the surface layer to a temperature higher than that of the main portion.

3. A method according to claim 1 wherein each of said apertures has a width dimension from 5 to 25 mm.

4. A method according to claim 1, further comprising the step of generating higher order modes of microwave energy in the article to improve the uniformity of heating of the article in lateral directions transverse to a direction of propagation of the energy through the surface layer.

5. A mode-filtering device for enhancing the heating of a surface of an article to be heated by microwave energy and having a surface layer having a dielectric constant, comprising a sheet of microwave-transparent material including a plurality of microwave-transparent apertures defined by annuli of microwave-reflective, substantially non-absorptive material and for transmission of microwave energy at a frequency into the article to be heated by the microwave energy, wherein said sheet is located adjacent to the surface layer and said apertures have dimensions so that microwave energy enters through the apertures in the form of cut-off propagation into said surface layer to directly heat said surface layer without first converting such microwave energy into heat.

6. A device according to claim 5, the apertures have dimensions so that the microwave energy in the form of cut-off propagation propagated through the apertures is not absorbed in a main portion of the article located beneath the surface layer and having a higher dielectric constant than the surface layer.

7. A device according to claim 5, wherein said annuli are arranged in an array.

8. A device according to claim 7, wherein microwave-transparent material between said annuli traces out tortuous paths.

9. A device according to claim 7, wherein the annuli in said array are interconnected with each other by microwave-reflective, substantially non-absorptive material.

10. A device according to claim 5, wherein said annuli each has a shape and said shape is selected from the group consisting of rectangular, substantially square, substantially circular, substantially triangular, substantially hexagonal and combinations of at least two of such shapes.

11. A device according to claim 5, for use with a food article to be heated by microwave energy at a frequency of 2.45 GHz, wherein said apertures each have a transverse dimension in the range of 5 to 25 mm.

12. A device according to claim 5, wherein said annuli have an outer width of 10 mm to 16 mm.

13. A device according to claim 5, for use with an article the surface layer of which has a relatively low dielectric constant, wherein said annuli have an outer width of 20 mm to 25 mm.

14. A device according to claim 7, wherein said array provides a spacing between annuli of from 3 mm to 6 mm.

15. A device according to claim 5, further including an electrically conductive plate located on said sheet of microwave-transparent material, said plate defining at least one said aperture having a closed outer periphery, and at least one electrically conductive island disposed substantially in register with said aperture to define a microwave energy transmissive gap between the outer periphery of the island and the outer periphery of the aperture for generating in the article to be heated at least one microwave energy mode of a higher order than a fundamental mode in said article.

16. A device according to claim 15, wherein said gap is continuously open.

17. A device according to claim 15, wherein said gap is bridged at spaced intervals by electrically conductive material spanning the gap between said plate and said island.

18. A device according to claim 15, wherein said island has an aperture.

19. A device according to claim 15, wherein said plate and said island are disposed in coplanar relation to each other.

20. A device according to claim 15, wherein said plate and said island are respectively disposed in parallel planes spaced apart in a direction transverse to said planes.

21. A device according to claim 20, wherein said island is smaller in area than said aperture.

22. A device according to claim 20, wherein said island is at least equal in area to said aperture.

23. A device according to claim 15, wherein said plate defines a plurality of said apertures distributed over its area in spaced relation to each other, with a plurality of said islands disposed in register with respective apertures to provide an array of annular gaps distributed over the area of said plate.

24. A device according to claim 15, wherein the device forms a first wall portion of a container for said article.

25. A device according to claim 24, further comprising a second said device forming a further wall portion of the container opposed to the first wall portion.

26. A device according to claim 24, wherein said container comprises an upwardly opening tray for holding said article and a lid for covering the upward opening of the tray, said device being disposed on said lid with the conductive plate of the device extending over substantially the entire area of said lid.

27. A device according to claim 5, wherein said plurality of apertures are positioned on said sheet of microwave-transparent material and have an electromagnetic property different from that of the sheet.

28. A device according to claim 27, wherein said electromagnetic property is selected from conductivity, lossiness, dielectric constant, spatial thickness, a stepwise discontinuity and a magnetic property.

29. A device according to claim 5, wherein at least one of said annuli has at least one interruption therein.

30. A combination comprising:

an article to be heated by microwave energy, said article having a surface layer and a dielectric constant,

a container in which said article is mounted, and

a mode-filtering device incorporated in at least one wall of the container for enhancing the microwave energy heating of the article, said at least one wall being at least one of a bottom and a lid of said container,

said device comprising a sheet of microwave-transparent material including a plurality of microwave-transparent apertures defined by annuli of microwave-reflective, substantially non-absorptive material and for transmission of microwave energy at a frequency into said article,

wherein said sheet is located adjacent to the surface layer and said apertures have dimensions so that microwave energy enters through the apertures in the form of cut-off propagation into said surface layer to directly heat said surface layer without first converting such microwave energy into heat.