Cooking utensil for uniform heating in microwave oven

A continuous conductor pattern applied to a dielectric cooking utensil improves the distribution of microwave energy to the bottom surface of food being cooked. The energy is coupled from the electromagnetic field within the oven cavity by metal strip pickup probes on the hand grips or side walls which are part of the embedded metal layer. The conductor pattern on the bottom wall couples energy from the pickup probes to the central region of the utensil.

BACKGROUND OF THE INVENTION 
This invention relates to microwave oven cookware and more particularly to 
utensils which aid in distributing the microwave energy more uniformly 
throughout the food being cooked. 
Microwave ovens are designed to heat a large variety of loads having a 
range of material properties, sizes, and shapes. In order to achieve 
relatively uniform heating, the various microwave coupling elements are 
proportioned for best average heating. In addition, a rotary vane stirrer 
assembly is employed to vary the intensity, spatial distribution, and 
frequency of the microwave energy. Uneven heating persists in spite of 
these features and the recipe instructions generally require the user to 
manipulate the food by turning or inversion at specified times during the 
cooking cycle. Finally, the instructions may specify a holding period 
after the microwave energy source is shut off. During this time the heat 
within the food will diffuse to produce a more nearly isothermal product. 
SUMMARY OF THE INVENTION 
Dielectric cooking utensils have metal layer conductor patterns applied to 
distribute the microwave energy over the surface of the vessel or dish, 
and to couple the energy to the central region of the utensil and thus 
reduce variations in heat input to the food load being cooked. The 
patterned metal layer is continuous and includes one or more pickup probes 
and a bottom wall conductor pattern. The energy is coupled from the 
electromagnetic fields within the oven cavity by means of the pickup 
probes which are embedded in or on the handle grips, side walls, or other 
outer region of the utensil. The bottom wall conductor pattern couples the 
energy to the central region of the utensil to realize more uniform 
heating of the food. 
One embodiment is a meat dish having metal strip pickup probes embedded in 
the hand grips on either side and a meandering conductor pattern to couple 
energy to the central region of the dish. A second embodiment is a cake 
pan having T-end metal strip pickup probes within the side wall and a 
bottom wall conductor pattern with a disk at the center of the pan. The 
conductor pattern is different for different general classes of foods and 
is shaped to optimize heat uniformity for that utensil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The rectangular dielectric dish in FIGS. 1 and 2 is used to cook roasts and 
other meats in a microwave oven. It is common for these high loss 
materials to overcook near the corners of such a dish (without a conductor 
pattern) while the center remains cooler. It is proposed to provide 
dielectric microwave cookware which have patterned metal layers applied to 
distribute the microwave energy over the surface of the utensil. The metal 
layer is fashioned to pick up energy from the electromagnetic fields 
within the oven cavity and bring it into the central region of the 
cookware underneath the food. 
Utensil 10 is made of a dielectric material with low microwave energy 
absorption and has a bottom wall 11, a raised edge 12, and two laterally 
projecting hand grips 13 and 14. The patterned metal layer indicated 
generally at 15 is continuous and is embedded within the utensil. The 
monopole metal strip pickup probes 16 and 17 in the hand grips serve to 
couple microwave energy from the electromagnetic fields within the oven 
cavity to a meandering conductor pattern 18 in the bottom wall of the 
utensil. The patch of the metal pattern may be varied to concentrate the 
energy as desired. Energy is thus coupled to the central region of the 
cookware, underneath the food load, and thus reduces the variation in heat 
input to the food being cooked. 
Referring to FIG. 3, a conventional microwave oven 19 with top feed of the 
microwave energy to the oven cavity is assumed. A magnetron source 20, a 
feed box 21, the oven cavity 22, and a shelf 23 are shown schematically. 
Wave energy is incident on the top and sides of roast 24 and the meat 
cooks from the outside toward the inside, and metal layer conductor 
pattern 15 extracts energy from the electromagnetic fields within the oven 
cavity and feeds it to the central part of even heating utensil 10 so as 
to cook the roast from underneath as well as from the sides and top. A 
uniformly cooked product is obtained without rotating or inverting the 
food itself. 
Metallic pattern 15 may be formed in a variety of ways. In one case, this 
may consist of a metal overlay on the lower or outer surface of the glass, 
plastic, or ceramic utensil body. In another case, a thin metal sheet or 
foil is sandwiched between layers of dielectric material. Embedding the 
patterned metal within the dielectric utensil is preferred in that 
mechanical wear of the conductor is avoided and potential sparking at the 
conductor due to casual contact with metal shelves or such is avoided. The 
metal pattern is aluminum or copper for good conduction and the dielectric 
utensil material is typically Pyrex.RTM. glass or polysulfone plastic. 
Meander conductor 18 is narrow relative to the dimensions of the utensil 
in order to enable a desired heat pattern resolution to be realized. There 
is a region of high electric field intensity at every half wavelength and 
thus increasing the total length of the meander conductor and decreasing 
its pitch, particularly in the central region of the utensil, gives a 
higher heat density. Other patterns are possible such as a slanting 
straight conductor which crosses the center of the utensil. Pickup probes 
16 and 17 are relatively short and are not resonant structures. 
The metal layer conductor pattern in or on the dielectric utensil is shaped 
to optimize the heat uniformity for that utensil and the general class of 
foods for which it is suitable. There are different conductor patterns for 
different general classes of foods, three of these being cakes and breads, 
meat, and casseroles or other semi-liquid foods. A set of microwave 
cookware is provided and the user selects the cookware to fit the class of 
foods being heated. It is desirable to cook more than one item at the same 
time and this goal is furthered by delivering energy to food in that 
vessel or dish at a rate that uniform cooking of the food occurs. 
FIGS. 4 and 5 show a dielectric cake pan 25 having an appropriately 
patterned metal layer 26 to aid in distributing the microwave energy 
uniformly throughout the material to be cooked. The three T-end metal 
strip of pickup probes 27-29 are embedded within the side wall 30 of the 
pan and are connected by metal stripes in the bottom wall 31 to a 
centrally located conductor pattern which includes a disk 32 and three 
stubs 33. The entire conductor pattern is symmetrical. The pickup probes 
couple energy from the fields within the oven cavity to the bottom wall 
conductor pattern to attain more uniform heating of the cake mix. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that various changes in form and details may be made 
therein without departing from the spirit and scope of the invention.