Abstract:
A food thermalization device is provided, which permits the food to be thermalized and held for extended periods of time without causing the food to deteriorate. The device includes an electrically-resistance-heated plate, which is controlled to equilibrate at a set temperature in the range of 160° F. to 185° F., with a fluctuation not exceeding plus or minus 5° F. The plate draws much less power than other food cooking devices and occupies much less space for the amount of food it can prepare. A browning oven is also provided for exposing unwrapped food to very high temperature radiation for a short period of time to brown the food.

Description:
This application is a CIP of U.S. patent application Ser. No. 09/083,777, filed Aug. 13, 1998, now U.S. Pat. No. 5,948,301, which is a CIP of U.S. patent application Ser. No. 08/794,271, filed Jan. 31, 1997, abandoned, which is a continuation of Provision Appl. No. 60/063,430, filed Oct. 28, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device and method for heating and staging food, and, in particular, to a device that can pasteurize and hold food at a ready-to-eat, heated temperature for an extended period of time without causing the food to dry out. 
     People have been heating and cooking food for a long time and, over the centuries, have created a variety of devices for heating food, beginning with an open fire, and moving on to a stove, an oven, a microwave oven, a griddle, a fryer, and so forth. Almost all of the heating devices heat to a high temperature, well above the boiling point of water. These devices can be very useful for cooking the food if the cook is attending closely to be sure the food does not burn or dry out during the cooking process, and if the food is to be eaten promptly after it is cooked. However, these devices are not good for thermalizing and holding frozen foods, raw foods, or pre-cooked foods at a ready state at optimal quality for any extended period of time without causing deterioration of the food. Also, because these devices must be able to heat to a high temperature, they use large amounts of energy, so that, for example, an electric stove must be connected to 220 volt power rather than the usual 110 volt household current. 
     There are several devices that have been constructed for holding food, but they all involve the use of temperatures at or above the boiling point of water. Food is mostly made up of water, so, at those temperatures, the vapor pressure inside the food increases, driving water out of the food, and thereby causing the food to dry out and deteriorate as it is held. 
     For example, it is possible to purchase pre-cooked, frozen chicken breasts, but there is currently no good, practical way, with the existing technology, to rethermalize those chicken breasts and hold them at a safe temperature, in which pathogens would be killed, without also drying out the meat. If the chicken breasts are put into an oven, they dry out. If they are put into a microwave, they dry out, and a microwave is not designed to hold food, only to heat it quickly. If they are heated on top of the stove or in an electric griddle or frying pan, again the high temperatures cause them to dry out. 
     U.S. Pat. No. 5,701,804 “Liebermann”, which is hereby incorporated by reference, describes a clamshell grill with upper and lower plates that are heated by a circulating fluid to a set temperature, which is below the boiling point of water but high enough to kill pathogens. This device solves the problem of how to thermalize and hold food without causing the food to deteriorate, but it is a very expensive device to manufacture and maintain, and it occupies a large volume of space for a relatively small food contact surface area. The Liebermann patent teaches that the way to thermalize the food and maintain the food at the desired temperature is to provide a reservoir of fluid at a fixed temperature between 130° F. and 185° F., prepare special upper and lower hinged food contacting plates with a fluid pathway in each plate, press the food between the plates, and continuously pump fixed temperature fluid through the plates to maintain the plates at a fixed temperature. 
     As one might imagine, this fluid-heated grill is a large, heavy, bulky device. The surface on which the food rests is at counter-top height, and the large, hinged lid opens upwardly. The fluid reservoir, heater, and pump are housed below the surface. There are three cooking surfaces per device, so the device occupies a floor or counter top area approximately equal to one-third of the total cooking surface area and occupies a height of at least three feet, or a volume of at least one cubic foot per square foot of cooking surface area. The special plates with fluid pathways are expensive to manufacture and are heavy. Since the food is pressed between upper and lower plates, only one thickness of food can be heated between the plates at any given time. 
     Mr. Liebermann selected a heat transfer fluid rather than electrical resistance heating in the design taught in that patent, because he knew there were problems with hot spots in electrical heating devices, and hot spots would not be acceptable in a design that required a fixed temperature to prevent the deterioration or drying out of the food. 
     In a typical electric griddle or frying pan, a metal plate is heated by electrical resistance heating. There is a control that allows the cook to set a temperature from a relatively low warm or simmer temperature to a high temperature on the order of 400° F. In order to achieve the high temperature, the griddle or frying pan must be designed to draw a large amount of power when it is on. That means that, when it is set at a low temperature, it is actually cycling on and off, making large temperature swings on the order of ±20° F. Also, because of the high power draw of the resistance heater, the heater becomes hot very quickly, creating hot spots on the griddle or frying pan. Thus, the typical electric griddle or frying pan would not be suitable for heating and holding food in a narrow temperature range below the boiling point. 
     U.S. Pat. No. 5,403,997 “Wimpee” recognizes the problem of hot spots with electrical heating systems and suggests the use of two different sets of resistance heaters at different power levels, so that the control system would cycle between the two sets of resistance heaters rather than cycling on and off. However, even if the Wimpee system eliminates the problem of hot spots, it still teaches that a food warmer should operate above the boiling point, in the range of 215° F.-240° F., which causes deterioration of the food, and its proposed system for eliminating hot spots would be complicated and expensive. 
     SUMMARY OF THE INVENTION 
     The present invention takes advantage of the teachings of the cited Liebermann reference but goes much further, to provide a practical device which is much less complicated, less expensive to manufacture and maintain, and occupies far less volume per square foot of food contact surface area than that design. Also, the device of the present invention does not require the food to be pressed between two plates but heats the food on a single plate, which means that food of different thicknesses can be heated on the same plate at the same time and that the space and expense of a second plate are eliminated. 
     The present invention provides a simple, electric resistance heating system which maintains the food-contacting plate at a fixed temperature below the boiling point, so that there is minimal fluctuation of temperature. The temperature preferably equilibrates to a fixed point between 160° F. and 185° F., with a fluctuation of plus or minus 5° F. or less (in a preferred embodiment the fluctuation is ±2° F.). This temperature is high enough to ensure the safety of the food (killing any pathogens) and is low enough to prevent the food from drying out. 
     By limiting the temperature to a preset temperature below the boiling point, the present invention reduces the power requirements from prior art systems, requiring less than 200 watts per square foot of food contact surface area, as compared with greater than 300 watts per square foot of food contact surface area in prior art electric resistance heated plates. This means that the present invention can heat at least seven to ten square feet of food using normal household wiring, not exceeding 1.5 KW, and, in a preferred embodiment, in which each square foot of food contact surface area draws approximately 150 watts, the device can heat ten square feet of food contact surface area with normal 110 volt service, not exceeding 1.5 KW. Also, by designing the plate to operate only at low temperatures and by keeping the area of each plate relatively small, there are no problems with hot spots on the plate; the entire plate remains at one temperature. When cold food is placed on the plate, the temperature of the plate will drop temporarily, but then the plate and the food on the plate gradually rise in temperature, with the plate and the food equilibrating at the set temperature. 
     In a preferred embodiment, a controller is used to cycle the power on and off as needed to maintain the desired temperature below 212° F. and preferably at a set temperature in the range of 160° F. to 185° F., ±5° F. With this arrangement, food can be thermalized to a set temperature, such as 175° F., ±5° F., and can be held at that set temperature for extended time periods, on the order of eight hours, while maintaining the quality of the food. 
     In a preferred embodiment of the invention, several electrical-resistance-heated food contact plates are spaced vertically, each plate maintained at a fixed temperature in the preferred range, with little temperature variation. The space above each plate is small enough to prevent excessively large food products from being inserted into the device, which ensures that the food that is put into the device is properly thermalized. This device is not intended to be used to prepare the family&#39;s Christmas turkey but rather to thermalize and hold relatively thin foods such as hamburgers, hot dogs, chicken breasts, pizzas, burritos, calzones, rice dishes, and so forth. 
     By maintaining a relatively small spacing between plates, one can easily obtain ten square feet of food contact surface with a footprint of only one square foot, and the device can sit on the counter top, so it does not occupy the height of other devices but rather only about two cubic feet for ten one-foot square plates spaced above one another, or a volume of about 0.2 cubic feet per square foot of food contact surface area. This compares favorably with the earlier Liebermann design, which used fluid heat transfer plates and occupied approximately one cubic foot per square foot of food contact surface area, or approximately five times the volume of the present invention. In other words, the present invention requires approximately ⅕th the amount of space to prepare the same amount of food as the earlier Liebermann design. 
     With this preferred embodiment, a family could put its dinner choices into the home appliance version of the device in the morning and have dinner waiting in good condition in the evening, or the family could put frozen dinner choices into the device when returning home and have dinner ready in approximately half an hour to one hour. It is expected that a device for home use might have 4-6 one-square-foot plates, while a commercial unit might have ten one-square-foot plates. A small, one-square-foot counter top device with ten plates could prepare and hold 90 hamburgers or ten pizzas, making it ideal for catering parties, for fast food restaurants, and so forth, using only normal 110 volt service and without the need for special hoods or vents. A household device could be an even smaller unit, comparable in size to a toaster oven but with far greater capacity. 
     Since the plates are always at a low temperature, which one can bump into without getting burned, no fumes are driven off that would require hoods and vents, and the device could be used anywhere—in the home, store front, kiosk, and so forth. 
     The preferred embodiments of the present invention include electronic controls that make the device very easy to use and require no special training and little attention, while still ensuring that the food is heated to a pathogen-free and aesthetically pleasing temperature. It is not necessary to remove the product promptly—just leave it in the device for hours without harming the food. 
     One preferred embodiment includes an additional browning oven which exposes the food to very high temperatures in the range of 1500° F. to 1600° F. for approximately 45-60 seconds after thermalizing the food and before serving the food. This is especially desirable for a food product such as pizza. Using this device, several pizzas could be rethermalized and held, and then each pizza could be finished in the browning oven as needed. Even with the high temperature browning oven, it is not necessary to remove the food at exactly the time it is finished, because the browning oven automatically turns off at the end of a fixed time, and, as soon as it is turned off, the food is no longer exposed to the high temperatures, due to the cooling fan. 
     Another preferred embodiment includes a bun and pastry warmer compartment on the top of the device. 
     Another preferred embodiment includes a spring-biased electrically-heated plate, which can be used instead of Sterno to heat a chafing dish, eliminating the fire hazard and fumes of Sterno, while protecting the food against burning and drying out. Also, the electrically-heated plate may be curved to serve as the support for a chafing dish as well as the heat source for the chafing dish. 
     Another preferred embodiment adds a heat-conducting insert on top of the food. 
     Another preferred embodiment adds heat-conducting fins on the food-supporting tray. 
     Because the power draw of each plate is low, less than 200 watts per square foot of food contact surface area, it is possible to power at least seven square feet of food contact surface area with normal household 110-volt service, making the device useful anywhere—both for home and commercial use. 
     Thus, the present invention provides a simple, economical system for thermalizing food and for maintaining the food at a pathogen-free, ready-to-eat temperature without causing deterioration of the food. The plates of the present invention may be stacked to form a rack; they may be placed side-by-side; they may be made larger or smaller; and they may be used in commercial establishments or in the home. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of a food thermalization rack made in accordance with the present invention; 
     FIG. 2 is a rear perspective view of the food thermalization rack of FIG. 1; 
     FIG. 3 is a front view of the food thermalization rack of FIG. 1; 
     FIG. 4 is a right side view of the food thermalization rack of FIG. 1; 
     FIG. 5 is an exploded perspective view of the food thermalization rack of FIG. 1; 
     FIG. 6 is an exploded rear perspective view of one of the plates of the rack of FIG. 1; 
     FIG. 6A is a sectional view of an alternative plate; 
     FIG. 7 is a broken-away bottom view of the plate of FIG. 6; 
     FIG. 8 is a rear view of one of the plate supports in the rack of FIG. 1; 
     FIG. 9 is a perspective view of a pizza on one of the food trays of the rack of FIG. 1; 
     FIG. 10 is a perspective view of several hamburgers on one of the food trays of the rack of FIG. 1; 
     FIG. 11 is a broken-away side sectional view of the top portion of the food thermalization rack of FIG. 1; 
     FIG. 12 is a front perspective view of a browning oven made in accordance with the present invention; 
     FIG. 13 is a front perspective view of the browning oven of FIG. 12 with the shell, controls, rack, crumb catching tray, and cutting board removed; 
     FIG. 14 is a block diagram showing the control system for one plate of the food thermalization rack of FIG.  1  and for the single-plate arrangements of FIGS. 22,  22 A,  23 , and  24 ; 
     FIG. 15 is a perspective view of a combination of two of the food thermalization racks of FIG.  1  and one of the browning ovens of FIG. 12; 
     FIG. 16 is a left side sectional view of the combination device of FIG. 15; 
     FIG. 17 is an exploded perspective view of the browning oven of FIG. 12; 
     FIG. 18 is a block diagram of the control system for the browning oven of FIG. 12; 
     FIG. 19 is a perspective view of a curved chafing dish heater made in accordance with the present invention; 
     FIG. 20 is a perspective view of the chafing dish heater of FIG. 19, showing some of the internal elements in phantom; 
     FIG. 21 is a side sectional view of the chafing dish heater of FIG. 20; 
     FIG. 22 is an alternative embodiment of a chafing dish heater made in accordance with the present invention; 
     FIG. 22A is a second alternative embodiment of a chafing dish heater made in accordance with the present invention; 
     FIG. 23 is an alternative embodiment of a food thermalization rack made in accordance with the present invention; 
     FIG. 24 is another alternative embodiment of a food thermalization rack made in accordance with the present invention; 
     FIG. 25 is a perspective view of a food thermalization rack similar to that shown in FIG. 1 but with a few modifications; 
     FIG. 26 is a perspective view of the same rack as that shown in FIG. 25 but with a different insert on the food support tray; 
     FIG. 27 is an exploded perspective view of one of the plates of the racks of FIGS. 25 and 26; 
     FIG. 28 is a perspective view of the plate of FIG. 27 after it has been assembled; and 
     FIG. 29 is a broken-away perspective view showing how the plate of FIG. 28 is mounted in the racks of FIGS.  25  and  26 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-11 show an example of a food thermalization rack  10  made in accordance with the present invention. The rack  10  includes an outer shell  12 , which is preferably made of sheet metal but may also be made of plastic or other suitable materials. The shell  12  includes a left side  14 , a rear  16 , a right side  18 , and a bottom  20 . Left and right rack support sheets  22 ,  24  are mounted to the inside of the left and right shell portions  14 ,  18 , respectively. A rear view of the left rack support sheet  22  is shown in FIG. 8, where it is clear that each rack support sheet  22 ,  24  includes a plurality of elongated rack support projections  25 . The rack support sheets  22 ,  24  are preferably made of molded plastic, although other materials could be used. A control block  26  is mounted on the left side of the shell  12  by means of a hinge  37  and includes buttons and indicator lights, which will be described later. A plurality of resistance-heated plates  28  is inserted into the rack  10 , with each plate  28  resting on aligned left and right rack support projections  25 . In the preferred embodiment, the vertical spacing between rack support projections  25  is less than two-and-one-half inches and preferably less than 1-¾ inches. This ensures that the food placed in the rack  10  is thin enough to be properly thermalized by the plates  28  and permits a large quantity of food to be heated in a small volume. Food-receiving trays  36 , sized to fill the plates  28 , are slid onto each plate from the open front of the rack  10 , with the trays  36  resting directly on the respective plates  28 . The trays  36  are preferably made of metal or other rigid, heat-conducting material. As is described below, the plates may also be made of a variety of materials. 
     On the back portion of the rack  10  are electrical edge connector receptacles  30 , which receive respective edge connectors  32  on the plates  28  (shown in FIGS. 6,  7 , and  11 ). The receptacles  30  are electrically connected to control boards  66 , as shown in FIGS. 11 and 14, and the control boards  66  are in the control block  26 . For now, at the prototype stage, there is a separate control board  66  for each plate  28 , but, in later stages of development, it is thought that it may be more desirable to put all the controls on a single, multi-channel board. 
     On the top of the rack  10  is a bread and pastry warmer cover  34 , which is hinged to the back  16  of the shell  12  by hinges  35 . The bread and pastry warmer cover  34  is preferably made of a clear plastic. If the bread and pastry warmer is not desired, the top of the shell  12  may be closed off by a flat top (not shown), which would be identical to the bottom  20 . The food to be thermalized is placed on the conductive trays  36 , which simply rest on the plates  28  and can be slid in and out to put food into and out of the rack  10 . 
     FIGS. 6 and 7 show a preferred embodiment of a plate  28 . The plate  28  is made of three main pieces. First is a top plate portion  38 , which, in this preferred embodiment, is made of 0.093-inch-thick aluminum plate. The top plate portion  38  could alternatively be made of other metals or other conductive materials. Next is a silicone glass portion  40 , which includes a top sheet  42  of 0.025-inch-thick silicone glass, a resistance wire element  44  under the top sheet  42 , and a similarly-configured bottom sheet  46  of silicone glass. The resistance wire element  44  follows an undulating path, covering the entire area of the plate  28 , with the spacing between adjacent portions of the wire element  44  being approximately 0.25-inches, which helps ensure a uniform temperature in the plate  28 . Finally, there is a bottom sheet  47 , preferably made of 0.020-inch-thick aluminum. 
     In addition to these main elements, there is a sensor  48 , which preferably is a thermocouple or thermister, which indicates to the controller  66  the temperature of the plate  28 . As shown in FIG. 6, the sensor  48  is located at the center top surface of the bottom plate  47  and includes leads  50  which are soldered to the edge connector  32 . The bottom plate  47  defines a notch  51  in the area of the edge connector  32 . A silicone patch  52  encloses the connection between the edge connector  32  and the leads  50 . The edge connector  32  is also soldered to the two ends of the resistance heater wire  44  and to feedback lead  45  (shown in the schematic in FIG.  14 ). 
     The top and bottom metal sheets  38 ,  47  and the silicone glass portion  40 , with the wire  44  and edge connector  32 , are vulcanized together to form a single plate  28 . An unvulcanized rubber coating is applied to the silicone  42 ,  46 , and, when vulcanized under pressure, it holds the entire metal-silicone-wire sandwich plate  28  together. The plate  28  is then sealed around its perimeter with silicone to prevent anything from entering between the plates and to provide an aesthetically pleasing plate. 
     The plate  28  described above is one preferred embodiment. However, other materials and other assembly methods could be used to achieve plates that would function in the same way. For example, as shown in FIG. 6A, the plate  28 ′ could be made of the same serpentine resistance wire  44 ′, with its wire  44 ′ and its sensor  48 ′ embedded in a cast conductive ceramic material  49 , rather than sandwiched between silicone and metal sheets. 
     Also, instead of using resistance wire  44 , the electrical resistance could be provided by a thick film electrically resistive track, as taught in U.S. Pat. No. 5,177,341, which is hereby incorporated by reference. FIGS. 27-29 show another alternative embodiment of a plate  428 , which includes a top plate  138 , a silicone glass portion  140 , and a bottom plate  147  as in the plate  28 . However, instead of using a patch as in the earlier embodiment, this embodiment forms an indentation  152  in the bottom plate  147  to receive the bulging connector portion of the silicone glass layer. Instead of an edge connector, this plate  428  has a cord with a grommet that extends through an opening in the left side of the device. Also, instead of sealing around the edge of the plate with a sealant as in the previous embodiment, this embodiment includes edge flaps  149  on the upper plate  138 , which fold over the bottom plate  147  to close off the edge of the plate  428 . Also, on the sides are extensions  151 , which define U-shaped indentations  153 , that are used to mount the plate  428  into the shell of the device. As shown in FIG. 29, screws  154  are inserted through the U-shaped indentations  153  to fasten each plate  428  to the inside of the shell of the device. 
     To control the temperature of the plates  28  on the rack  10 , the control arrangement shown in the schematic diagram of FIG. 14 is used. There is a source of electrical power  54 , such as regular household, 110-volt, alternating current service. There is an on-off switch  56  on the rack  10 , which is used to turn the rack on and off. For each plate  28 , a button  58 , a red light  60 , and a green light  62  are mounted on the control block  26  at the front of the rack  10 . There is a transformer  64 , which supplies 5 v D.C. to each control board  66 , and there is a control board  66  connected to each receptacle  30 . 
     Each edge connector  32  connects to two leads for the resistance wire  44 , two leads to the sensor  48 , and a feedback lead, which indicates whether the high temperature switch  68  has opened the heater wire circuit due to overheating. There may also be an additional sensor (not shown), which would indicate whether or not a tray is present on a plate  28 , and this would also communicate with the control board  66  to cause an alarm signal if the tray is removed before the food on the tray is ready. 
     The operation of the preferred thermalization rack control system is as follows: 
     When the on-off switch  56  is turned on, power flows from the power supply  54 , through the transformer  64  to the controller  66 , and through the triac and drive to the plate  28 . So, whenever the rack is turned on, all the plates  28  are being heated. When a person slides out a tray  36 , puts food on the tray  36 , and slides the tray back in, the person also pushes the button  58  for the plate  28  on which the tray  36  rests. This tells the controller  66  that a cook cycle should begin, and the controller  66  turns on the red light  60 , indicating that the food is not yet ready to eat. The controller  66  monitors the temperature of the plate  28 , and, when the plate  28  reaches a set temperature, the controller  66  begins a timer. When a preset time is reached, the controller  66  turns off the red light  60  and turns on the green light  62 , indicating that the food is ready. The preset time is approximately thirty minutes, or whatever time test results show is sufficient to ensure that the food has reached the desired end temperature, which is the temperature of the plate. 
     The controller  66  turns the triac and drive on and off, turning the power to the resistance heater wire  44  on and off, as necessary, to maintain the preset temperature, whenever the main switch  56  is turned on. If the controller  66  senses that the temperature of the plate  28  has dropped more than a certain amount, such as 5° F. below the preset temperature, the controller  66  will restart the timer. An alarm (not shown) may be provided to sound if a tray is removed after the button for that tray has been pushed and before the green light  62  comes on. A digital indicator (not shown) may be used instead of the green light. 
     FIGS. 9 and 10 show examples of the types of foods that may be thermalized on the plates  28 . FIG. 9 shows a pizza  70  encased in a sealed, clear plastic wrapper  72 , resting on a tray  36 . FIG. 10 shows nine hamburger patties  74 , individually wrapped in sealed, clear plastic wrappers  72 , resting on a tray  36 . Each tray  36  may be similarly filled with wrapped or unwrapped food. When the respective trays  36  are slid into the rack  10  onto their respective plates  28 , the warm plates  28  begin conducting heat through the trays  36  into the food  70 ,  74 . The temperatures of the plates  28  will drop below the set temperature when cold food is initially inserted, and, as heat is conducted into the food, the plates  28  will come back up to their set temperatures. Once a plate  28  returns to its set temperature, the timer begins, and, once the plate has been at its set temperature for a set time, the green light will come on, indicating that the food is ready. The food may remain at that ready state for several hours, without deterioration of the food. Then, when it is desired to serve some of the food, the tray  36  is slid out, and the food is removed from the tray and served. 
     For foods such as pizza, which are tastier and more appealing to the eye after browning, a browning oven  80 , made in accordance with the present invention, may be used. FIGS. 12,  13 , and  17  show a preferred embodiment of the browning oven  80 . The browning oven  80  includes a shell  82 , defining a plurality of openings  84  in its front face, through which air can pass. At the front of the browning oven  80  are three handles for sliding drawer-like elements into and out of the browning oven  80 . The uppermost handle is attached to a food support grate  86 . The second handle connects to a drawer  88  to catch crumbs. The lowest handle connects to a slide-out cutting to board  90 . 
     FIGS. 13 and 17 show the eight radiant heating elements  92  (four upper elements  92 A and four lower elements  92 B), which, in the preferred embodiment, are sixteen-inch long, 1100-watt short wave infrared quartz tubes, gold-plated on the upper half of their circumference for high heat transfer efficiency, which achieve a radiation temperature of 1500-1600° F. in four seconds upon activation. The four upper heating elements  92 A lie above the food grate  86 , and the four lower heating elements  92 B lie below the food support grate  86 . Upper and lower guides  94 ,  96 , on both sides of the frame  98  guide the food support grate  86  as it is slid into and out of the oven  80 . A blower  100  is mounted on the back of the oven  80  and continuously pulls air into the openings  84  in the front of the oven  80 , through the oven  80 , and out the back of the oven  80  whenever the oven  80  is turned on. This keeps the exterior skin of the oven at a temperature that can safely be touched (ranging from 130° F. to 200° F.), even when the radiant heating elements  92  are turned on. 
     The electrical controls for the oven  80  are isolated in a compartment on the right side of the oven  80  by a vertical plate  102  and are shown in the block diagram of FIG.  18 . On the front face of the oven  80  are buttons for controlling the oven  80 . An on-off button  104  is used to turn the oven power on and off, and another button  106  turns the lower tubes off. There is also another button  108 , which is used to set the time (from zero to 99 seconds). There are three displays on the front of the oven  80 . There are two lights  110 ,  112 , which indicate when the upper and lower banks of heating elements, respectively, are on. There is also an LED display  114 , which indicates the amount of time the oven will be on. 
     Looking now at FIG. 18, it can be seen that the electrical system also includes a transformer  116 , which provides DC power to the controller. There is a micro-controller  118 . There is a high limit switch  119 , which will turn off the heating elements  92  if the temperature at the switch exceeds a certain set value. 
     To operate the browning oven  80 , first the power switch  104  is turned on. Then, the button  108  is used to set the time, and then the button  108  is hit again to turn on the heating elements  92 A,  92 B. If the switch  106  is opened, only the top bank of heating elements  92 A will turn on; if the switch  106  is closed, both the top and bottom banks  92 A,  92 B will turn on. The micro-controller  118  measures the time until the time set by the user is over, and then it turns off the heating elements  92 . Since this unit requires large amounts of energy, it does not operate on typical household electrical service. Instead, it is expected to use 220 volt service. The heating elements come up to temperature within four seconds, and, once the elements are turned off, the temperature drops quickly. Although this browning oven draws large amounts of energy when it is on, it is only on for short periods of time, thereby using far less energy overall than would be used by a conventional oven. 
     FIGS. 15 and 16 show a combined unit, with two of the food thermalization racks  10  of FIG. 1 mounted on top of a browning oven  80  of FIG.  12 . This combination is intended for commercial use. For example, it would allow twenty pizzas to be thermalized and held in the racks  10 , and, when someone wanted to eat a pizza, a pizza would be removed from its tray, unwrapped, and placed in the browning oven  80  for approximately thirty to ninety seconds to brown the pizza. This arrangement would permit the continuous dispensing of pizzas at a rate of approximately one per minute. 
     The combination device of FIGS. 15 and 16 is particularly well-suited to the preparation of pizzas, because the thermalization rack  10  provides ideal conditions for the air pockets in the pizza dough to enlarge as the dough is warmed and the air inside the pockets expands. 
     In the case of pizza, the pizza that is put on the thermalization rack  10  may have a precooked crust, a par-baked crust, or may be made with raw dough. With pre- or par-baked crusts, the cell structure is already set. With raw dough, the rack  10  provides an opportunity for the dough to rise and the air cells to expand, and for the dough to partially bake with the expanded air cells. The high temperature browning oven  80  then finishes the baking and browns the pizza, bringing out the flavor of the dough and the cheese. 
     Of course, the browning oven  80  is not intended for use solely with pizza and could be used with a wide variety of foods for which a short exposure to high temperatures is desirable. 
     Variations on the basic flat plate  28  are shown in FIGS. 19-22. FIGS. 19-21 show a first embodiment of a chafing dish heater, and FIG. 22 shows a second embodiment of a chafing dish heater. Chafing dishes are often used in catering, and the chafing dishes are usually heated by burning Sterno under them. This creates pollutants as well as being a fire hazard, and the Sterno is at temperatures that will cause the food to dry out over time. FIG. 19 shows a stand  120 , which supports the chafing dish heater  122 . The chafing dish heater  122  is essentially the same as the plates  28 ,  28 ′, except that it has been formed in a shape corresponding to the shape of a chafing dish, so that a chafing dish filled with water can be placed directly into the heater  122 . It would also be possible to put the water directly into the chafing dish heater  122 . 
     As shown in FIGS. 20 and 21, the chafing dish heater  122  includes top and bottom plates  38 ″,  47 ″, silicone sheets  42 ″,  46 ″, an embedded electrical resistance heater  44 ″ and sensor  48 ″, as well as an edge connector  32 ″ as found in the plates  28 . The chafing dish heater  122  has the same type of controls as are found at each individual plate  28  on the rack  10 . 
     FIG. 22 is an alternative type of chafing dish heater  124 . In this case, the chafing dish  126  is placed on a stand  128 , and the chafing dish heater  124  contacts the bottom surface of the chafing dish  126  to heat the food in the chafing dish  126  by conduction. The plate  124  heats the chafing dish, which heats the water inside the chafing dish, which heats the pan holding the food, which heats the food. The power of the plate  124  must be substantially greater than the power of a normal plate  28 , because it must maintain the temperature of the water in the chafing dish at the desired set temperature, which requires much greater energy input than when food is placed directly on the plate. Thus, this type of plate will draw substantially more than the 200 watts per square foot of other plates. The control for the plate  124  is set to maintain the equilibrium temperature of the plate  124  at the desired set temperature, which again is in the range of 160° F. to 185° F., ±5° F., with a maximum temperature of approximately 190° F. The plate  28 A of the chafing dish heater  124  is made the same way as the plate  28 , except that its dimensions are made to fit the size of a chafing dish, and it has a higher power rating. Also, springs  130  support the plate  28 A at all four corners to bias the plate  28 A upwardly into contact with the bottom of the chafing dish  126 . 
     FIG. 22A is another alternative embodiment similar to FIG. 22, but showing a control module  126  below the springs  130 . The control module  126  includes an on-off switch and additional outlets so that the chafing dish heaters can be connected together. 
     FIGS. 23 and 24 show alternative embodiments of a food thermalization rack. Instead of having several individual plates  28 , as shown in the embodiment of FIG. 5, these embodiments have a single plate  328 ,  228 , which has been formed into an undulated shape to create several parallel shelves  129 ,  229  for supporting food. These plates  328 ,  228  may be made as a flat metal sandwich, as was described above with respect to the plates  28 , and the flat metal sandwich may then be bent into the desired undulated shape to form the parallel, horizontal food support shelves  129 ,  229 , or the sheets of the metal sandwich may be preformed in the undulated shape and then the parts assembled as described above, or the undulated plate may be molded of a ceramic, plastic, or other conductive material with embedded electrical resistance heating elements in the desired shape. Other manufacturing methods may also be used. In these embodiments, the control blocks  326 ,  226  preferably are located at the bottom of the unit. A housing  327 , having a front door and an open bottom, may be placed over the plate  328  or  228  and rests on the control block  326 ,  226 . The control in these embodiments is essentially the same as the control in FIG. 14 for a single plate of the first embodiment. However, for these embodiments, which are intended to be made inexpensively for home use, there probably will not be red and green lights and a timer to show whether the food has spent a sufficient amount of time being heated. Instead, there may be a timer that can be set to have the heater come on at a certain preset time, so that, for example, frozen food may be placed on the shelves of the unit in the morning, as people are leaving home for school or work, and the timer may be set to come on for 4:00 in the afternoon, so the food will be ready to eat for supper. The undulations may be made as smooth curves, as shown in FIG. 23 or as sharp curves, as shown in FIG.  24 . The spacing between shelves preferably is the same as in the first embodiment, and these embodiments preferably would use food-supporting trays as in the first embodiment. 
     FIG. 25 shows a food thermalization rack  400  that is very similar to the rack  10  of FIGS. 1-11, except that the plates are spaced farther apart than in the first embodiment—approximately four inches apart. The plates  428  are those shown in FIGS. 27-29, which have already been described. Each food support tray  436  is the same as the trays  36  in the first embodiment, except that each tray  436  includes upwardly-extending projections  438  at its rear edge, and the shell of the rack  400  includes inwardly-projecting arms  440 , which contact the projections  438  when the tray  436  is pulled out, to prevent the tray from falling out of the rack  400 . 
     In this embodiment shown in FIG. 25, an optional heat-conducting insert  450  is placed on top of the sealed food to help heat the top surface of the food. The insert  450  preferably is just a piece of metal or other heat-conducting material with a slot  452  near its front edge. It does not have any embedded electrical heaters, but it does absorb heat from the plates  428  and conducts that heat to the top surface of the food. In order to remove the heat-conducting insert  450 , a simple tool  454  is used. The tool  454  is inserted into the slot and then functions as a handle for the heat-conducting insert  450 . 
     FIG. 26 shows the same embodiment of the rack  400  as in FIG. 25, but a different type of insert has been added to the tray  436  in this embodiment. Here, four U-shaped channels  460 , preferably made of the same type of heat-conducting material as the tray  436 , are inserted onto the tray  436 , and the sealed packages of food  462  are placed on edge in the channels  460 . In this view, the packages of food  462  preferably are hamburgers or chicken sandwiches, including the bun. By inserting the sandwiches vertically in the channels  460 , the buns do not become soggy or hard at the outer crust as they might if they were placed in a normal horizontal position. The channels  460  serve as heat transfer fins, improving the heat transfer to the food. Each channel  460  preferably is approximately 2-⅞ inches wide and 2-⅝ inches high in order to accommodate normal hamburgers and sandwiches, but the dimensions could be changed depending upon on the type of food to be inserted. Also, while this preferred embodiment uses individual channels, it would also be possible to simply make the trays  436  with vertically-projecting fins, which would use less material. An advantage of the channels  460  is that it would be possible to use just one or two channels on a tray and leave the rest of the tray for use with flat, horizontally-oriented food products, as needed, thereby making the availability of the trays more flexible. 
     It will be obvious to those skilled in the art that modifications may be made to the embodiments described above without departing from the scope of the present invention.