DE68916798T2 - SUSCEPTOR FOR HEATING A SINGLE FOOD. - Google Patents

Abstract

A packaging system is disclosed which includes a susceptor heating means having selective responsiveness to microwave radiation. The susceptor surface has a plurality of regions, where at least one region has an altered responsiveness to microwave radiation which is achieved by disruptions in the susceptor surface. A method for making regions of a susceptor selectively responsive to microwave heating by disrupting the continuity of the metallized film of the susceptor is also disclosed.

Description

In the past, there have been difficulties with various attempts to brown or crisp foods in a microwave oven. A microwave oven heats foods differently than a conventional oven. Generally speaking, foods are heated in proportion to their tendency to absorb microwave radiation, which can result in heating patterns that are considerably different from those that exist in a conventional oven. In addition, microwave radiation penetrates most foods in a manner that results in heating patterns that are considerably different from those that otherwise exist in a conventional oven. In most cases, microwave energy will heat the foods more quickly than in a conventional oven. For example, a food that would require 30 minutes to properly "cook" in a conventional oven may require only three or four minutes to "cook" in a microwave oven. In a conventional oven, the oven atmosphere is heated to relatively high temperatures to transfer heat to the food surface, resulting in the surface always being the hottest area of the food. In a microwave oven, the oven atmosphere is generally not heated; the food itself heats the surrounding air and transfers heat to it, resulting in the food surface being cooler than the interior. These differences significantly affect the ability to brown or crisp a surface of a food in a microwave oven.

Many attempts have been made to brown or crisp the surface of a food product in a microwave oven. One such attempt has involved the use of packaging devices called susceptors. Suitable susceptors may include microwave-absorbing coatings deposited on a microwave-transparent backing layer. These susceptors heat up when exposed to microwave radiation. A susceptor can reach temperatures high enough to brown or crisp the surface of a number of food products. The susceptor can be placed in close proximity to or in direct contact with the surface of the food product. A typical commercially available susceptor includes a thin film of vacuum-deposited aluminum on polyester, which is then adhesively laminated to paper or cardboard.

The use of susceptors, however, has led to additional problems. Available susceptors typically do not heat evenly. As a result, such susceptors may not brown or crisp the food evenly. For example, the outer region of a susceptor may become much hotter during microwave irradiation than the central region of the susceptor. As a result, the outer region of the food may tend to brown or crisp, but the central region may not, without overcooking the outer region. This is a particular problem with foods that have large surface areas, e.g., the baked crust of a large frozen pizza. When a susceptor pad is used, e.g., to crisp several fish sticks placed side by side on the susceptor, microwave heating can typically cause the fish sticks to stick to the ends of the susceptor are crispy, but the fish fingers in the middle of the susceptor base cannot be crispy enough.

An additional problem with heating foods using susceptors is the lack of control of the heating profile across the susceptor surface. It is often desirable to adjust the amount of heat delivered in sections of a susceptor to accommodate different food characteristics. This is a particular problem when two or more foods with varying browning or crisping requirements are placed in conjunction with a common susceptor. During heating, one contact surface of the food may be overcooked while a contact surface of an adjacent food may remain wet or mushy.

When cooking a food by microwave radiation, consideration must be given to the overall energy balance achieved during the heating process. If an attempt is made to simply increase the magnitude of microwave radiation or the cooking time, in an attempt to brown or crisp a particular area of the surface of the food, this may result in overheating or overcooking of other surfaces and/or the interior of the food itself. In other words, if one attempts to achieve browning or crisping of the center region of a pizza crust by simply increasing the heating time or by increasing the intensity of microwave radiation, the likely result would be overcooking of the outer surface of the pizza and/or overcooking of the pizza toppings.

Heating food in a microwave oven, especially where susceptors are used, usually involves a complex balancing of the energy absorbed throughout the food. Although the use of susceptors has resulted in some improvement in browning or crisping food in a microwave oven, it has been necessary to solve the problem of susceptors that do not heat uniformly. Some means of uniformly browning or crisping food with a susceptor have been needed. It has also been necessary for some means to achieve uniform browning and crisping without disturbing the complex energy balance necessary to properly heat all areas of the food. In addition, it has been necessary to brown or crisp different types of food products in different ways.

Examples of attempts to achieve crisp baking of food products are shown in U.S. Patent 4,267,420 issued to Brastad and U.S. Patent 4,230,924 issued to Brastad et al. Brastad attempted to make a flexible wrapping material that was completely wrapped around a fish stick to brown the surface of the fish stick. However, Brastad did not work out the problem of uneven crisp baking of the food product. Brastad does not disclose how to compensate for uneven heating caused by the flexible wrapping material.

Another example is US Patent 4,641,005, granted to Oscar E. Seiferth. A thin film susceptor is disclosed for heating food. However, Seiferth did not address the problem of uneven heating of the susceptor surface. Seiferth does not disclose how food loads are to be compensated or how susceptor preferential edge heating is to be compensated.

It is clear from the above discussion that prior art attempts to achieve crisping of the surface of a food in a microwave oven have been overall unsatisfactory. The use of susceptors has often resulted in uneven crisping of the food surface and undesirable uneven heating patterns.

In US Patent 3,302,632 a relatively expensive microwave cooking appliance is shown which has a plurality of compartments used to keep different food items separate. Grids of different mesh sizes are incorporated into the bottom portion of the appliance, being sandwiched between or embedded within two nested and adhered plastic containers.

The grids are made of a metal with a high conductivity that attenuates microwave radiation that comes from a suitable source and is directed through the appliance. The grids reduce the transmission of microwaves through the container walls and thus retard the cooking process.

Only a single food product is contained in any given compartment. For example, the small mesh size grid is under a compartment containing peas, the open spaced or large mesh size grid is under a compartment containing potatoes, and a compartment that has no grid or screening under it contains meat. Thus, the US Patent 3,302,632 cooking three different foods to a given state in the same time interval.

The previously mentioned US patent 4,641,005 shows a susceptor for heating a food product in response to microwave radiation. These types of susceptors have problems in that they do not heat evenly. A susceptor can have hot spots and cold spots during microwave heating. As a result, the food product being heated can have burnt areas and at the same time have areas that are still moist or mushy or nearly frozen.

Thus, the object to be achieved by the invention is to provide a convenient and inexpensive way of adjusting the heating of a susceptor of the type shown in U.S. Patent 4,641,005 so that the heating can be controlled.

According to the invention, the electrically continuous thin conductive film is separated with thin line conductivity breaks formed in the conductive film to reduce heating of selected areas of the susceptor in response to microwave radiation. The breaks in the electrical continuity of the conductive film "tune" the susceptor in the broken area. This technique can be used to adjust the heating of the susceptor.

The susceptor according to claim 1 is further developed by the features according to claims 2 to 8; claims 9 and 10 show a method for producing a susceptor, and claim 11 shows a packaging system comprising a susceptor according to one of claims 1 to 8, which can be produced by a process according to claims 9 or 10.

According to the present invention, a system is provided for heating a food in a microwave oven that can be used to achieve more uniform heating of the surface of the food. The system includes a susceptor device having variable sized conductive regions. The size of the conductive regions is adjusted to compensate for undesirable uneven heating patterns that would otherwise exist.

The susceptor means is in close proximity to or in direct contact with the surface of the food to be crisped or browned. The susceptor means generally comprises a layer having a conductive coating, typically a metallized film, which absorbs microwave energy while exposed to microwave fields. The susceptor means therefore heats in response to microwave radiation. According to the present invention, the conductive coating is divided into a plurality of regions having susceptor regions which may have a different size in each region. The susceptor regions may be formed, for example, by scoring, cutting, etching, stamping, printing or other methods to interrupt the conductive coating of the susceptor means. At least one region has its responsiveness to heating effects of microwave radiation altered by the interruptions in the conductive coating.

Portions of the susceptor which would otherwise tend to overheat can be provided with small susceptor areas which are comparatively less responsive to microwave radiation. Portions of the susceptor assembly which would otherwise tend to be underheated are provided with larger susceptor areas which are comparatively more responsive to microwave radiation. By adjusting the size of the susceptor areas within the limits of this invention, it is possible to compensate for non-uniform heating patterns which would otherwise be present in a susceptor. Uniform crisping and browning of a food product can thereby be achieved. Alternatively, if a non-uniform crisping or browning pattern is desired, the susceptor assembly can be designed in accordance with the present invention to provide a specific desired heating pattern.

For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the drawings. In the drawing:

Fig. 1 is a plan view of a susceptor pad having two sections, each section being provided with different sizes of susceptor regions formed thereon;

Figure 2 is a graph showing the heating profile of a susceptor pad not constructed in accordance with the present invention;

Fig. 3 is a diagram showing the temperature profile of a similar susceptor pad, but with regions with different dimensioned susceptor areas according to the present invention;

Fig. 4 is a bar chart illustrating the influence of the different sized susceptor regions formed according to the present invention on crisping;

Fig. 5 is an image of a temperature pattern obtained during microwave irradiation of a susceptor that did not have variable sized susceptor regions according to the present invention. The image was created by an infrared camera;

Fig. 6 is a similar picture of the heating pattern of the susceptor having variably sized susceptor regions according to the present invention;

Figure 7 is a plan view of a susceptor having variable susceptor areas according to the present invention for use in crisping the surface of a food product, such as a pizza;

Figure 8 is a graph depicting the temperature profile of a round pizza susceptor that did not have stepped sized areas in accordance with the present invention;

Fig. 9 is a diagram showing a temperature profile of a similar susceptor but without the stepped dimensioned susceptor regions of Fig. 7;

Fig. 10 is a bar graph illustrating the influence of variable sized susceptor areas using a susceptor pad constructed for tanning according to Fig. 7;

Fig. 11 is an image of the heating pattern of a susceptor constructed as shown in Fig. 7. The image was created with an infrared camera;

Fig. 12 is a graph showing the temperature reached during microwave heating as a function of the size of the susceptor regions;

Fig. 13 is a graph showing the percentage power absorbed, transmitted and reflected as a function of the size of the susceptor regions;

Fig. 14 a diagram of capacitance/reactance as a function of the size of the susceptor regions;

Figure 15 is a plan view of an alternative embodiment of a susceptor using a labyrinth pattern to reduce the microwave heating effect on a discontinuous region of the susceptor;

Fig. 16 is a plan view of a susceptor demonstrating the effect of interrupting the conductive layer without cutting during microwave heating;

Fig. 17 is an infrared image of the microwave heating effects on a susceptor without any interruptions;

Fig. 18 is an infrared image of the microwave heating effects on the susceptor illustrated in Fig. 16;

Figure 19 is a plan view of a susceptor having four sections of different responsiveness formed using the present invention;

Fig. 20 is a graph showing the temperature profile during microwave heating of the susceptor constructed according to Fig. 19;

Fig. 21 is a top view of a susceptor using the principle of "directed flow";

Fig. 22 is an infrared image of the microwave heating effects on a susceptor without modification, which was used as a control example for comparison;

Fig. 23 is an infrared image illustrating microwave heating of a susceptor constructed as shown in Fig.

Fig. 24 is a plan view of a susceptor constructed according to Fig. 21, subsequently modified with additional cuts to interrupt the electrical conductivity between the central region and the strips of the susceptor;

Fig. 25 is an infrared image illustrating microwave heating of the susceptor constructed as shown in Fig. 24;

Fig. 26 is a plan view of an example of a susceptor using the principle of "directed flow";

Figure 27 is a plan view of a susceptor constructed in accordance with the present invention with a spiral cut therein to achieve "directed flow";

Figure 28 is a top view of the susceptor constructed in accordance with the present invention with a rectangular spiral cut to achieve "directed flow";

Fig. 29 is a top view of an alternative embodiment of a circular susceptor using "directed flow".

To crisp the surface of a food, a susceptor pad 10 may be used. The food to be crisped may be placed in close proximity to or in direct contact with the susceptor pad 10. The susceptor pad 10 may include a layer of metallized polyester comprised of a layer of polyester having a thin film of a metal, such as aluminum, deposited thereon. The polyester layer serves as a support for the thin metal film. The conductive metal layer may be deposited on the polyester substrate by a vacuum deposition process. The metallized polyester layer is preferably adhesively bonded to a support surface, such as paper.

For disclosure of the details of a suitable composition for the susceptor pad 10, reference is made to application number 070,293 filed July 6, 1987 by Michael R. Perry et al, entitled "Package for Crisping the Surface of Food Products in a Microwave Oven," the entire disclosure of which is incorporated herein by reference. Another disclosure of a susceptor pad is contained in U.S. Patent 4,641,005 issued to Oscar E. Seiferth and entitled "Food Receptacle for Microwave Cooking," the entire disclosure of which is incorporated herein by reference.

It has been found that when a susceptor pad with a continuous metallized layer is exposed to microwave radiation, an uneven heating pattern typically results, as shown in Figure 5. The heating effects are often most pronounced at the edges of the susceptor pad, while the center may not be adequately heated. If such a susceptor pad is used to, for example, crisp up a plurality of fish fingers placed side by side on the susceptor pad, typically microwave radiation may result in the fish fingers being crispy at the ends, but the fish fingers in the center of the susceptor pad may not be adequately crispy. At a minimum, the fish fingers will not be evenly crispy.

It is desirable to have some means to compensate for the uneven heating that can result when a susceptor pad is exposed to microwave radiation. In accordance with the present invention, variable sized susceptor regions are provided that compensate for the otherwise undesirable uneven heating characteristics of a susceptor pad. As shown in Fig. 1, a susceptor pad 10 is provided with larger sized susceptor areas in the central region 11 and relatively smaller sized areas in end regions 12. The relatively more microwave responsive central region 11 contains larger susceptor areas 13. The less microwave responsive end regions 12 contain relatively small susceptor areas 14.

In this illustrated example, the conductivity of the metallized film is interrupted by cuts or notches 15. The notches 15 may be cuts in the metallized layer created by a sharp tool such as a razor blade. Or the notches 15 may be formed by stamping a sharp die on the susceptor substrate 10. Any means for forming discontinuities or conductivity breaks between the metallized layer of one susceptor region 13 and an adjacent susceptor region 13 should provide satisfactory results. For example, conductivity breaks may be formed by etching, notching, cutting, punching or embossing, or photoresistive techniques. It has surprisingly been found that it is only necessary to interrupt the metallized layer, which can sometimes be done by drawing a line with a ballpoint pen across the surface of the susceptor pad 10. In general, any method that interrupts the electrical continuity of the thin metal film has been found to be effective. The grooves 15 similarly form conductivity interruptions in the metallized film between one small susceptor region 14 and an adjacent small susceptor region 14.

The smaller susceptor regions 14 are made sufficiently small so that the susceptor regions 14 are less responsive to microwave radiation than the larger susceptor regions 13. Thus, when the susceptor base 10 is exposed to microwave radiation, the smaller susceptor regions 14 will be less responsive to the heating effects of the microwave radiation than would be the case if the notched grooves 15 were not provided on the susceptor base 10. The smaller susceptor regions 14 act to "detune" the responsiveness of the end region 12 to microwave radiation.

The larger susceptor regions 13 are comparatively more responsive to heating effects of microwave radiation. It is believed that the larger susceptor regions 13 have less of a "detuning" effect on the central region 11. The larger susceptor regions 13 may also improve the uniformity of heating of the central region 11. If the susceptor regions 13 were not cut in the central region 11, some peripheral heating of the central region 11 could occur in this example. The susceptor regions 13 may be designed so that no "detuning" effect is achieved. In some applications it is only important that the relative heating of one region 12 be less than that of another region 11.

The configuration of the small susceptor regions 14 and the larger susceptor regions 13 illustrated in Figure 1 tend to balance the tendency of the end regions 12 to overheat compared to the central region 11.

Fig. 2 illustrates the heating profile of a susceptor pad used to heat fish fingers in a microwave oven. Fig. 2 includes a susceptor pad that does not have variable sized susceptor regions 13 and 14. The temperature of various positions on the horizontal centerline of the susceptor pad were measured using an infrared camera. Line 16 represents the temperature profile of a susceptor pad after being exposed to microwave radiation for 30 seconds. Line 17 represents the temperature profile of the same susceptor pad after being exposed to microwave radiation for 60 seconds. Line 18 represents the temperature profile of the same susceptor pad after being exposed to microwave radiation for 210 seconds.

As shown in Figure 2, the temperature of the center of the susceptor pad increased rapidly to a relatively high temperature within 30 seconds, and then decreased around the time the 60-second temperatures were measured. The temperature of the center of the susceptor pad remained low during 210 seconds of microwave radiation, as shown by line 18. The edges of the susceptor pad, however, remained at relatively high temperatures. The result was that the fish fingers at the end regions of the susceptor pad were crispier than the fish fingers located in the center region of the susceptor pad. Non-uniform crisping of the fish fingers was observed. The total cooking time for the fish fingers was about 31/2 to 4 minutes.

The temperatures were measured with an infrared camera. The temperatures are "uncorrected" because the infrared camera was directed through a wire mesh shield. The wire mesh shield was used to prevent microwave energy from escaping from the microwave oven. The wire mesh probably led to lower average temperature readings on the infrared camera. Here, however, the relative temperature differences are of primary interest. It is therefore assumed that, although the actual temperatures measured cannot be precise, the relative temperatures are accurately represented.

Fig. 3 illustrates temperature profiles of a susceptor pad 10 constructed according to Fig. 1. As shown in Fig. 3, the relative heating of the central region 11 compared to the end regions 12 was accomplished by using smaller susceptor areas 14 at the end regions 12.

Line 19 represents the temperature profile of the susceptor pad 10 after being exposed to microwave radiation for 30 seconds. Line 20 represents the temperature profile of the susceptor pad 10 after being exposed to microwave radiation for 60 seconds. Line 21 represents the temperature profile after being exposed to microwave radiation for 210 seconds. The temperature of the end regions 12 remained relatively low for the 30 second measurement represented by line 19 and for the 60 second measurement represented by line 20. The temperature of the end regions 20 increased toward the end of the heating period as represented by line 21.

Fig. 4 is a graph illustrating the effect of the smaller susceptor areas 14 at the end regions 12 and the larger susceptor areas 13 in the middle region 11 on the crispiness of fish fingers. A plurality of fish fingers were placed on the susceptor base 10 as shown in Fig. 1. The fish fingers were placed parallel to each other in a side-by-side relationship. The fish fingers were aligned with their longitudinal extent vertically in Fig. 1. In other words, the fish fingers were aligned with their longitudinal extent in the same direction as the width of the susceptor pad 10. Thus, some fish fingers were completely in contact with the end regions 12, while other fish fingers were completely in contact with the middle region 11.

In a test using a matched sensor array, fish fingers in contact with the central region 11 showed, on average, about a 10% change (increase) in producing crispness, as shown by bar graph 22 in Figure 4. Fish fingers cooked on standard susceptors and on grooved susceptors were compared by scale. Fish fingers in contact with the end regions 12 of the susceptor pad 10 experienced a -11% change (decrease) in producing crispness, as shown by bar graph 23 in Figure 4. The percent changes were based on a comparison with fish fingers cooked on a susceptor pad that did not contain variable susceptor regions 13 and 14. Thus, the use of variable susceptor areas 13 and 14 resulted in an increase in the crispiness of fish fingers on the central region 11 and a decrease in the crispiness of fish fingers on the end regions 12. The variable sized susceptor areas 13 and 14 thus compensated for uneven heating that would otherwise have occurred during microwave irradiation with the combination of susceptor pad and fish fingers.

The effect of the variable dimensioned susceptor areas 13 and 14 is further illustrated by a comparison of Fig. 5 with Fig. 6. Fig. 5 shows an image taken with an infrared camera during of microwave irradiation of a susceptor pad that did not include variable sized susceptor regions 13 and 14. Fig. 6 illustrates the temperature profile of a susceptor constructed according to Fig. 1. The infrared image of Fig. 6 was taken after 30 seconds of microwave irradiation. Fig. 6 corresponds to the 30 second temperature profile shown in Fig. 3 (represented by line 19). Fig. 6 shows that microwave heating of the end regions 12 was greatly reduced compared to the middle region 11.

In the susceptor pad 10 illustrated in Figure 1, the small susceptor regions 14 are formed in the shape of small squares that are approximately 1/16 inch on each side. In other words, the small susceptor regions 14 are formed in the shape of squares having a height and a width of 0.0625 inches.

The large susceptor areas 13 in the central region 11 of the susceptor pad 10 illustrated in Fig. 1 are formed in the shape of rectangles having a length of 11/2 inches and a width of 7/8 inches. In other words, the large susceptor areas 13 have a length of 1.25 inches and a width of 0.875 inches.

The overall length of the illustrated susceptor pad 10 was 61/2 inches. The overall width was 33/4 inches. Each end region 12 was about 2 inches x 33/4 inches. The center region was about 21/2 inches x 33/4 inches. The grooves 15 used to separate the small and large susceptor regions 14 from each other and from adjacent large susceptor regions 13 were razor blade width cut into the metallized polyester layer.

Fig. 7 illustrates a round susceptor pad 24 used for browning the crust of a pizza or the like. In the case of a round susceptor for use in heating pizza, it has been found that the outer periphery of the susceptor pad tends to heat more than the central region of the susceptor pad. This often results in browning of the outer surface of the pizza crust while only 50 to 60% of the central region of the pizza crust is browned. This is demonstrated by the information presented in Figs. 8 and 10, which are explained in more detail below.

It is desirable to have some means for reducing the heating of the outer region 26 of the susceptor pad 24 while increasing the relative heating of the central region 25 of the susceptor pad 24. In the present invention, this is provided by the provision of conductivity breakers or grooves 27 in the outer region 26 of the susceptor pad 24. The grooves 27 may be in the form of cuts made with a razor blade or the like. It is sufficient if the grooves 27 are made in any manner that interrupts or breaks the electrical conductivity of the metallized layer of the susceptor pads 24.

The grooves 27 define small susceptor areas 28 in the outer region 26 of the susceptor base 24. The central region 25 defines a larger susceptor area 29. The small susceptor areas 28 are less responsive to the heating effects of microwave radiation as compared to the large susceptor area 29. This has the effect of reducing the level of heating in the outer region 26 of the susceptor base 24 where the susceptor 24 would otherwise tend to become overheated. Providing variable susceptor areas 26 and 29 has the effect of increasing the temperature of the central region 25 relative to the outer region 26.

Figure 8 is a graph illustrating temperature profiles of a round susceptor pad used to brown the crust of a pizza. Line 30 represents temperature measurements at various horizontal positions of the susceptor pad after exposure to microwave radiation for 30 seconds. Line 31 represents temperature measurements at the same locations after exposure to microwave radiation for 120 seconds. Line 32 represents temperature measurements after exposure to microwave radiation for 300 seconds. Line 33 represents temperature measurements after exposure to microwave radiation for 390 seconds. Figure 8 shows that the outer portion of the susceptor pad became much hotter than the center portion of the susceptor pad.

Figure 9 illustrates the temperature profile of a susceptor pad 24 constructed in accordance with the embodiment illustrated in Figure 7. Line 34 shows temperature measurements at various horizontal positions on the susceptor pad 24 after exposure to microwave radiation for 30 seconds. Line 35 shows temperature measurements after exposure to microwave radiation for 120 seconds. Line 36 shows temperature measurements after exposure to microwave radiation for 300 seconds. Line 37 shows temperature measurements taken after exposure to microwave radiation for 390 seconds.

A comparison of Fig. 9 with Fig. 8 shows that the use of variable susceptor regions 28 and 29 dramatically changes the temperature profile of the pizza susceptor 24. The central region 25 became much hotter after 390 seconds of irradiation than the central region of the susceptor pad not constructed in accordance with the present invention. The temperature of the outer region 26 was reduced while the temperature of the central region 25 was increased.

Figure 10 is a bar graph illustrating the effect on pizza crust browning as a result of using different sized susceptor areas 28 and 29 on susceptor pad 24. The bar graph represents the percent crust area that was browned after microwave heating.

Column 38 in Figure 10 represents the percent crust area that was browned using a susceptor pad that did not have differently sized susceptor regions. Slightly less than 80% of the pizza crust area was browned in this example. More than about 85% of the area of the exterior of the pizza crust was browned, as represented by column 40 in the bar graph of Figure 10. However, less than 60% of the center area of the pizza crust was browned, as represented by column 39 in Figure 10.

By using a susceptor pad 24 with different sized susceptor areas 28 and 29 as shown in Figure 7, the amount of browning that occurred in the central region 25 was greatly increased while the amount of browning that occurred in the outer region 26 was greatly reduced. Column 42 represents the amount of browning that occurred in the central region 25. About 95% of the area of the crust in the central region 25 was browned in this example. Only about 5% of the area of the crust in the outer region 26 was browned as represented by column 43 in Figure 10. The total percentage of the area of the crust that was browned was less than 30%, as shown by column 41 in Fig. 10.

Fig. 11 is an image taken with an infrared camera showing the heating pattern of a susceptor pad 24 constructed in accordance with the embodiment shown in Fig. 7. The infrared image was taken at a time during the heating period corresponding to 390 seconds of exposure to microwave radiation. The infrared image of Fig. 11 corresponds to line 37 shown in the temperature profile diagram of Fig. 9. The areas corresponding to the central region 25 and the outer region 26 are labeled in Fig. 11.

In the particular susceptor pad 24 illustrated in Figure 7, the diameter of the susceptor 24 was 9 inches. The diameter of the central region 25 was about 4.5 inches. The small susceptor areas 28 were generally formed as squares having a height and a width of 1/16 inch or 0.0625 inches. The grooves 27 were formed by razor blade cuts in the metallized layer of the susceptor pad 24.

If a region 26 of a susceptor 24 is made less responsive to microwave heating, the amount of heating of an uninterrupted region 25 can be increased. This phenomenon is referred to as "load sharing." It is believed that if a region 26 is made less responsive to microwave heating, more energy is available to heat other regions 25.

Fig. 12 is a graph showing the heating effect of the small susceptor regions 14 as a function of the size of the region. In In this case, the susceptor areas were formed as squares. The dimensions shown are the height and width of the squares.

Fig. 12 shows that the responsiveness of the small susceptor areas 14 to the heating effects of microwave radiation drops sharply when the squares 14 are made smaller than 0.625 inches on either side where the metallized susceptor base 10 has a relatively high resistivity of 1,650 ohms (reference area) or ohms per unit square area. For lower resistivities on the order of 18 ohms (reference area), the responsiveness of the small squares 14 to the heating effects of microwave radiation decreases when the squares are made smaller than 0.3125 inches on either side.

In Fig. 12, line 44 represents temperature as a function of size for small squares 14 where the resistivity of the metallized layer of the susceptor pad was 10 18 ohms (reference area). Line 45 represents temperature as a function of size of squares 14 where the resistivity of the metallized layer of the susceptor pad was 10 60 ohms (reference area). Line 46 represents temperature as a function of size of susceptor areas 14 where the resistivity of the metallized layer was 1,650 ohms (reference area). These temperatures have not been corrected for differences in emissivity of the susceptor surface. The relative temperatures along each line (44, 45, 46) are correct. The comparative heating between susceptors of different resistivities is caused by emissivity differences of the susceptor surfaces and has not been corrected in Fig. 12.

Figure 13 represents data taken with a network analyzer for the susceptor pad 10 which was 60 ohms (reference area) and which was designed based on the measurements shown by line 45 in Figure 12. A 5 inch square uncut susceptor pad 10 provided reflectance, transmission and absorption measurements shown on the rightmost portion of the graph of Figure 13. For the uncut pad, absorption was measured to be about 30%. Reflectance was measured to be about 68%. Transmission was measured to be about 2%.

Fig. 13 shows that the reflection, transmission and absorption of a susceptor pad are affected by discontinuities or conductivity breaks in the susceptor surface. The curves begin to change significantly when the size of the squares 14 created by the discontinuities or conductivity breaks are made 0.625 inches on a side or smaller. The percent power absorbed decreased significantly for squares that were 0.625 inches on a side or smaller. An absorption of about 33% was measured for squares 14 with a width of 0.625 inches. An absorption of about 27% was measured for squares 14 with a width of about 0.3125 inches. An absorption of about 20% was measured for squares 14 with a width of about 0.1563 inches. An absorption of about 11% was measured for squares 14 with a width of about 0.0781 inches.

All measurements were taken by the network analyzer before heating the susceptor pad 10 in a microwave oven. This technique, ie using network analyzer data, can be used to explain the reduced responsiveness of the susceptor pad regions having discontinuities or conductivity breaks forming complex patterns other than those defining simple squares 14 as illustrated in the above examples. Therefore, it should be appreciated that reduced responsiveness to microwave heating can be achieved by using discontinuity patterns or conductivity breaks of various configurations in addition to the illustrated example of squares 14.

The influence of discontinuities or conductivity breaks in the susceptor surface can be better understood with reference to Fig. 14. Fig. 14 is a graph showing the influence on the reactive component of the impedance of a susceptor pad when small squares 14 are formed in the susceptor surface. The data plotted in Fig. 14 were measured with a network analyzer using the same susceptor pad which had an initial resistivity of 60 ohms (reference surface). More specifically, the susceptor pad's impedance was essentially completely resistive prior to cutting, as shown by the point at the upper right corner of the graph, measured for the uncut susceptor pad with a side length of 5 inches square.

Conductivity discontinuities in the surface of the susceptor pad created a negative reactance, i.e. a capacitive reactance. The total impedance Zs of the susceptor pad can be expressed as:

Zs = Rs - jXs,

where Rs is the resistance component of the impedance and Xs is the reactance component of the impedance. If Xs is positive, then the reactance is inductive. If Xs is negative, then the reactive component is capacitive. If the surface of the susceptor is discontinuous as a result of breaks or disconnections in the conductivity of the susceptor support surface, the susceptor typically demonstrates capacitive reactance.

Measuring the reactance of the susceptor surface provides an indication of the magnitude of the discontinuity or discontinuity of a region of the susceptor surface. This is proportional to the extent to which the responsiveness of that region to heating during microwave irradiation is affected by the discontinuity or discontinuity in the susceptor support surface.

The relative difference in capacitive reactance of different regions of the susceptor pad 10 resulting from discontinuities in the susceptor pad can be used as a means of determining whether one region is less responsive to heating effects of microwave radiation than another region of the susceptor pad 10. Thus, complex patterns can be used to create discontinuities in the susceptor pad surface. Measurements with the network analyzer can be used to determine the altered responsiveness of a region of the susceptor pad to heating effects of microwave radiation as a result of any complex patterns or discontinuities.

Fig. 15 illustrates an embodiment of a susceptor pad surface with a complex "maze" pattern that forms discontinuities in the susceptor pad surface. For complex patterns such as those shown in Fig. 15, network analyzer measurements can be used are used to determine the relative responsiveness of different regions to microwave radiation.

Fig. 15 shows a first region 47 of the susceptor pad having discontinuities or breaks in the form of a labyrinth pattern. The discontinuities in the first region 47 make it less responsive to heating effects from microwave radiation than would be the case if the discontinuities in the susceptor surface were not present in the first region 47. A second region 48 is also shown, in this example as a central rectangle of susceptor material.

Disruptions in the susceptor surface do not necessarily have to take the form of cuts in the surface. The susceptor surface can be disrupted, e.g., by drawing lines using a ballpoint pen. An example of the ability to achieve lower responsiveness through disruptions created, e.g., with a ballpoint pen is shown in the experiment illustrated in Figure 16. A square susceptor pad 49 was used in this experiment. A grid pattern covering a first region 50 was drawn on the susceptor pad 49 using a ballpoint pen. Three circular regions 51 were randomly selected and were left uninterrupted. The relative heating of the two susceptor pads is shown in Figures 17 and 18, without the grid pattern and with the grid pattern illustrated in Figure 16, respectively.

Fig. 17 shows an image taken with an infrared camera showing the heating effects on a susceptor pad without any interruptions. This susceptor pad was used as a control for the experiment.

Figure 18 is an infrared image of the heating effects on a susceptor pad 49 with a grid pattern drawn thereon using a ballpoint pen. The relative difference in heating of the three circular regions 51 whose susceptor pad surface was uninterrupted can be clearly seen from the infrared image of Figure 18. This experiment demonstrated the effectiveness of interruptions in causing a region of the susceptor pad to respond to heating. Thus, actual cuts in the susceptor pad surface are not needed. Interruptions can be created by pressing or embossing the susceptor pad surface. Interruptions can be created that are essentially invisible. However, the effect of the interruptions can be detected by measurements using a network analyzer.

Figure 19 shows a susceptor pad 52 having a first region 53, a second region 54, a third region 55 and a fourth region 56, each having different patterns of conductivity separations in the surface of the susceptor pad 52. In this example, squares 57 were formed in the fourth region 56. The squares 57 had a side length of 1/2 inch. The squares 57 were formed by providing cuts 61 in the surface of the susceptor pad 52 using a razor blade.

The third region 55 had smaller squares 58 formed by incisions 61 having a side length of about 1/4 inch. The second region 54 had even smaller squares formed therein. which had a side length of about 1/8 inch. The first region 53 had the smallest squares 60 which were formed by incisions 61 which had a side length of about 1/16 inch.

Fig. 20 illustrates the temperature profile of the susceptor pad 52 constructed as shown in Fig. 19. The heating effects of the microwave radiation on the fourth region 56 were much greater than the heating effects on the other regions 53, 54 and 55. The smaller the size of the squares in the region, the less heating was observed. The temperatures were measured using an infrared camera.

Cuts or interruptions in the surface of the susceptor can be applied to create an effect that can be called "directed flow". This can be illustrated by reference to the experiment shown in Figures 21 to 25.

Fig. 21 illustrates a susceptor pad 62. Parallel cuts 63 were machined into the surface of the susceptor pad 62. A central non-cut region 64 was left in the middle of the susceptor pad 62. The parallel cuts 63 defined stripes 65 on the surface of the susceptor pad 62. There were no conductivity breaks or discontinuities between the ends of each stripe 65 and the central region 64 of the susceptor 62.

Fig. 22 is an image taken with an infrared camera showing the heating pattern of a non-cut susceptor. This was used as a control for the experiment. Fig. 23 is an image taken with an infrared camera showing the Heating pattern of the susceptor 62 constructed according to Fig. 21. An intense heating of the central region 64 is evident. The strips 65, which are connected without interruption to the end region 64, appear to increase heating of the central region 64.

Fig. 24 shows a susceptor pad 66 constructed as shown in Fig. 21, except that additional cuts 67 have been made to interrupt or separate the continuity between the strips 65 and the central region 64. Fig. 25 is an image taken with an infrared camera showing the heating pattern of the susceptor pad 66 constructed as shown in Fig. 24. The heating of the central region 64 is not as extensive as that in the example shown in Fig. 21.

Fig. 26 illustrates an alternative embodiment of a susceptor pad 68 in which the principle of "directed flow" was applied. In this example, the susceptor pad 68 was a circular susceptor suitable for use with pizza and the like, for example. The susceptor pad 68 illustrated in Fig. 26 has radial cuts or discontinuities 69. The cuts 69 define stripes 70 that extend radially inward toward the central region or target area 71. The stripes 70 are connected to the central region 71 without interruption. It will be appreciated that the target area 71 may be located at a position other than the center of the susceptor 68.

Secondary cuts 72 may be provided to extend only partially toward the central region 71. A secondary region 73 is defined by the region extending radially outward from the center of the base 68 to the ends of the secondary cuts 72. This results in a relatively hot central region 71. The secondary region 73 will generally be warmer than the outermost region 74 of the susceptor pad 68.

In general, the more notches 69 provided in the susceptor pad 68, the hotter the central region 71 will be. In practice, it has also been observed that the uniformity of heating of the outermost region 74 of the susceptor pad 68 is improved by providing an increased number of notches 69 in the susceptor pad 68.

A circular incision could be made around the central region 71 to interrupt the electrical conductivity between the central region 71 and the strips 70. In such an example, it has been observed that the central region 71 becomes preferentially hot during microwave heating, but not as hot as compared to an example in which the central region 71 is connected to the strips 70 without interruption, as shown in Figure 26.

An alternative embodiment of a round susceptor 75 is shown in Fig. 29. The example illustrated has a plurality of cuts 76 extending radially inward from the outer periphery towards the central region 77. The cuts 76 define a plurality of strips 78 extending radially from the central region 77. In this case, all of the cuts 76 extend from the periphery of the susceptor 75 to the edge of the central region 77. All other things being equal, the central region 77 of the example illustrated in Fig. 29 would get hotter than the central region 71 of the example illustrated in Fig. 26.

Various geometric arrangements have been used to demonstrate the principle of "directed flow". For example, round spirals as shown in Fig. 27, square spirals as shown in Fig. 28, pinwheel-like incisions, cross-shaped regions, etc. have been tried. All of these various geometric arrangements demonstrate the ability to create a relatively hot central region that is connected to variously shaped strips without interruption. It has been observed that the central region generally has a maximum size at which the principle of "directed flow" works most effectively. If the area of the central region is made too large, the central region will not get as hot. It is believed that the maximum size of the central region is a function of the resistivity of the susceptor pad material. The lower the resistivity, the larger the central region can be and still effectively provide significant heating of the central region. Generally speaking, the smaller the central region is, the hotter or the more intense the warming influence on the central region will be.

A susceptor may be constructed by initially providing the susceptor surface with discontinuities or separations in the conductive layer. Additional disclosure is contained in the application entitled "Microwave Heater and Method of Manufacture" by Turpin et al., filed concurrently herewith, the entire disclosure of which is incorporated herein by reference.

In the above description, measurements of resistivity, reflectivity, transmittance, absorbance, etc. were carried out at room temperature (21ºC) unless otherwise specified.

In the descriptions given above, measurements were made with a network analyzer, all of which included the procedure described below. A Hewlett Packard Model NL 8753A network analyzer in combination with a Hewlett Packard Model No. 85046A S-parameter test set was used. All measurements were made on a microwave oven operating at 2.45 GHz. All measurements were made at room temperature unless otherwise specified. All measurements were made using a WR-282 waveguide. The reflectivity, transmittance and absorption measurements were made without the presence of any food.

The measurements were preferably made by placing a sample to be measured between two adjacent pieces of waveguide. A conductive silver paint is preferably placed around the outer edges of a sample layer cut slightly larger than the cross-sectional opening of the waveguide. The colloidal silver paint from Ted Pella, Inc., has given satisfactory results in practice. The sample is preferably cut to have an overlap of about 50/1000 inch (0.127 cm) around the edge. The waveguide is calibrated according to procedures specified and published by Hewlett Packard, the manufacturer of the network analyzer.

Scattering parameters S₁₁, S₁₂, S₂₁ and S₂₂ are directly measured by the network analyzer. These measured parameters are then used to calculate the microwave reflection power, transmission power and absorption power.

The reflection directed into aperture 1 is of magnitude S₁₁ squared. The reflection directed into aperture 2 is of magnitude S₂₂ squared. The transmittance directed into aperture 1 is of magnitude S₂₁ squared. The transmittance directed into aperture 2 is of magnitude S₁₂ squared. The absorption power directed into either aperture 1 or aperture 2 is equal to one minus the sum of the reflection power and the transmission power in the aperture.

The complex surface impedance of an electrically thin layer is obtained from the measured scattering parameters using the formula presented in "Properties of Thin Metal Films at Microwave Frequencies" by R.L. Ramey and T.S. Lewis, published in the Journal of Applied Physics, Vol. 39, No. 1, pages 3883-84 (July 1968) together with the information in J. Altman, Microwave Circuits, pages 370-71 (1964), both of which are incorporated herein by reference. For uninterrupted susceptor material, the impedance is essentially completely resistive. Interruptions or conductivity breaks introduce a capacitive reactance component into the impedance.

The infrared images and temperature measurements taken with an infrared camera were taken using a Thermovision 870 scanner (infrared camera). The infrared camera was used in conjunction with a TIC-8000 thermal image computer. Image analysis was performed using CATS software (version 1.04). The infrared camera, computer and software are commercially available from Agema Infrared System A.B., located in Danderyd, Sweden.

The original infrared images of Figs. 5, 6, 11, 17, 18, 22, 23 and 25 were made in color. For convenience, black and white copies have been used here. It is not believed that the color originals are necessary. However, the color originals are incorporated herein by reference.

The above disclosure has been directed to a preferred embodiment of the present invention. The invention may be embodied in a number of alternative embodiments other than those illustrated and described above. Those of ordinary skill in the art will be able to recognize a number of modifications to the above-described embodiments after having the benefit of the above disclosure and the teachings herein. The full scope of the invention is to be determined by appropriate interpretation of the claims and is not to be unnecessarily limited to the specific embodiments described hereinabove.

Claims (11)

1. A susceptor for heating a single food item in response to microwave radiation, the susceptor (10) having a selective heating responsiveness to microwave radiation, the susceptor being adapted to brown or crisp the surface of a food substance placed in close proximity thereto, and the susceptor having an initially electrically conductive continuous layer formed on a support, characterized in that 2the conductive layer has a first electrically continuous region (12) which acts to heat in response to microwave irradiation, and a second region (11), the second region having thin-line conductivity interruptions (15) formed on the conductive layer which interrupt the electrical continuity of the conductive layer to reduce heating of the second region in response to microwave irradiation, the second region being formed by interrupting the electrical continuity of the conductive layer in a predetermined pattern with the thin-line conductivity interruptions, the thin-line conductivity interruptions being used to adjust the selective responsiveness of the susceptor Warming device works when the individual food item is heated. 2. A susceptor according to claim 1, characterized in that the second region of the conductive layer comprises a plurality of small discrete regions (13) of the conductive layer defined by the conductivity discontinuities in the conductive layer, the small discrete regions of the conductive layer having dimensions small enough to reduce the responsiveness of the second region to heating by microwave radiation relative to the first region. 3. Susceptor according to claim 2, characterized in that the regions of the conductive layer are substantially square in shape. 4. Susceptor according to claim 3, characterized in that the square-shaped areas of the conductive layer have a length of - less than about 0.6250 inches (1.5875 x 10⊃min;² m), or - less than about 0.3125 inches (0.7938 x 10⊃min;² m), or - less than about 0.1563 inches (0.3970 x 10⊃min;² m), or - less than about 0.0781 inches (0.1984 x 10⊃min;² m) exhibit. 5. Susceptor according to one of the preceding claims, characterized in that the conductive layer is a metallized layer of aluminum with a specific resistance of - between about 10 Ohm (reference surface) and about 1700 Ohm (reference surface) or - between approximately 60 ohms (reference surface) and approximately 1650 ohms (reference surface), or - between approximately 0.1 Ohm (reference surface) and approximately 2000 Ohm (reference surface), or - greater than about 60 ohms (reference surface) is. 6. Susceptor according to one of the preceding claims, characterized in that the first region of the susceptor has small discrete areas (14) defined by discontinuities in the susceptor surface, the discontinuities causing a different responsiveness of the areas to microwave irradiation, the areas having dimensions small enough to reduce the responsiveness to heating by microwave irradiation relative to the second region, and/or the areas having a substantially rectangular shape. 7. A susceptor according to any preceding claim, wherein the second region has an additional significant capacitive component in the impedance of the susceptor. 8. Susceptor according to one of the preceding claims, characterized in that the susceptor (52) includes a third region (54) and/or a fourth region (53) each having a plurality of regions (59) and/or fourth regions (60) which are formed by conductivity interruptions which are formed in the conductive layer in front of microwave radiation, so that the third and/or fourth region respond less strongly to microwave radiation than the second region. 9. A method for producing a susceptor according to any one of the preceding claims, marked by reducing the responsiveness of a first region of a metallized thin film susceptor to the heating effects of microwave irradiation as compared to a second region of the metallized thin film susceptor by disrupting the electrical continuity of the thin metallized layer of the susceptor over the first region prior to microwave cooking, wherein disrupting includes: Cutting the thin metallized layer of the susceptor in the first region, or separating the thin metallized layer of the susceptor in the first region into a plurality of distinct subregions of the continuous metallized layer. 10. The method of claim 9, characterized by notching the second region of the thin metallized layer of the susceptor, where the dimensions of the continuous metallized layer areas in the second region are longer than the dimensions of the continuous metallized layer in the first region. 11. A packaging system comprising a susceptor according to any one of claims 1 to 8, which can be manufactured by a method according to claim 9 or 10.