WO2005094646A1

WO2005094646A1 - Conduction oven for food thermalization

Info

Publication number: WO2005094646A1
Application number: PCT/US2004/006293
Authority: WO
Inventors: Benno E. Liebermann
Original assignee: B E Aerospace, Inc.
Priority date: 2004-03-02
Filing date: 2004-03-02
Publication date: 2005-10-13

Abstract

An oven for thermalizing and staging pre-cooked foods in containers, including a plurality of shelves (35A-F), each shel having an electrical resistance heating element, and a temperature controller having a first, relatively high power setting to elevate the food to a first temperature level, and a second, relatively low power setting for attaining and maitaining a second equilibrium temperature level

Description

CONDUCTION OVEN FOR FOOD THERMALIZATION

Technical Field and Background of the Invention [0001] This invention relates to a food preparation apparatus, and more specifically to a conduction oven for re-thermalizing or maintaining the temperature of food items, such as portion-controlled food packaged in sealed containers, such as served on aircraft. The particular disclosure of this application relates to an oven for preparing pre-cooked food for aircraft passengers as a part of regular meal service. The oven is characterized by being capable of thermalizing individual food items and holding the food items at a pathogenically safe temperature for extended times, while maintaining the food in a succulent condition for service within a wide time range. [0002] The oven permits service of hot meals on demand by maintaining an inventory of food items in a safe and palatable condition that can be served anytime a passenger wants a meal, or to take into account events, such as ground delays or en route weather, that may require unanticipated rescheduling of meal service. [0003] The oven permits the food items to be prepared and optionally refrigerated and/or re-heated on the ground, thus avoiding use of far more expensive energy generated by the aircraft. As disclosed, the oven operates principally on a conduction principle, with additional convective heat supplied by radiation of heat from contact surfaces and the surrounding heated air.

[0004] The oven also has application in any environment where large quantities of meals must be served, such as in cafeterias, hotels, hospitals, schools and prisons. [0005] Flight kitchen food preparation is a conventional form of "mass catering", but with distinct differences from food preparation techniques and practices in, for example, restaurants and hotels. The time difference between food preparation in the prior art flight kitchen and serving on board an aircraft with limited kitchen facilities makes flight catering a high-risk food preparation operation. The complexity of the production procedures in the flight kitchen also increases the microbiological hazards associated with this type of food preparation. Major factors affecting the hygienic quality of the food are the size of the operation, the complexity of the in-flight service, the number of airlines catered for, the number of flights serviced during a given period, and the duration of the flights serviced.

[0006] Each airline has its own food service specification, further increasing the complexity of planning and meal preparation control. Production planning for flight caterers is in many instances similar to "just in time" production techniques, meaning producing the necessary quantity of meals at just the necessary time, taking into account whether meals will be carried for more than one flight leg, and whether foreign flight kitchens will be used, or only local suppliers. In many cases, frozen meals may be transported for an extended period of time, and thawed over time as needed. This also adds risk to the operation.

[0007] A flow chart of a typical prior art flight kitchen is shown in Figure 1. Prior art flight kitchens typically use a cook-chill system, whereby cooked items are rapidly chilled in blast chillers within 4 hours of preparation from a temperature of 60-65 C. to 5-1 O C A cold kitchen is used for preparation of snacks, appetizers, salads and some desserts. Until portioning and packaging, all prepared items are kept chilled. After making up the meal trays— typically thin plastic or aluminum foil trays-the trays are loaded into trolleys for the flight. In some cases the trolleys are loaded with dry ice in order to minimize the temperature rise in the aircraft galley before service. [0008] Food storage and preparation for serving takes place in the aircraft galley, which, given the space and weight constraints of the aircraft, provides cold storage areas, regeneration ovens, water boilers and beverage storage and machines, and waste product storage. On narrow-bodied aircraft meals are often kept chilled by dry ice located within the trolley. Wide-body aircraft used for long-haul flights more typically utilize refrigerators or chiller units for the trolleys.

[0009] Chilled and frozen meals served hot must be re-heated so that a core temperature of at least 72 C. is reached in order to destroy any surviving pathogenic micro-organisms. In the 1970's, hot meal trays were transported to aircraft in hot ovens for short-haul flights and kept there at a temperature of over 63 C. until serving. More current prior art practices include the above-described "cook-chill" system, although hot served foods are sometimes transported to small aircraft if they are not equipped with ovens. See, Hatakka, Maija, Hygienic Quality of Foods Served On Aircraft. Academic Dissertation, October 2000, University of Helsinki, Dept. of Food and Environmental Hygiene.

[0010] As can be seen, these practices are wasteful of energy and product, and are inefficient from the standpoint of flexibility to the airline company and caterer, while guaranteeing safety.

[0011] Research and analytical comparisons into consumer perception with means to enable the consumer to verbally express food quality indicate that most consumers follow a relatively uniform, subconscious food evaluation sequence; namely: 1. eye appeal; 2. inviting aroma; 3. taste and tenderness; 4. safe food temperature at consumption; 5. prompt, courteous service; 6. price (value); 7. ambience of the establishment; 8. restroom hygiene; and 9. brand appeal from prior experiences.

[0012] This sequence culminates in the generation of a memory value in the consumer's mind that to a large extent influences subsequent food consumption decisions. This fact can have a critical impact on whether a consumer patronizes a specific establishment in the future.

[0013] Research in the targeted area of pathogenic food safety has demonstrated that factually addressing the food safety issue is a principal priority in situations where large numbers of people must be served within a relatively short period of time.

[0014] Food safety issues are regulated and supported by specific guidelines and

Federal and State regulations regarding time and temperature conditions to limit production of life-threatening bacteria. Universally practiced cooking techniques base the function of converting raw or fresh protein food items to a safely ingestible state as a function of time related to temperature at the center of any particular protein food article. Research has demonstrated that the time factor associated with any cooking technique is irrelevant. The important factor in the cooking process is the achievement of a precise internal end temperature at the center of the food article, thereby guaranteeing thermal lethality of pathogens present in raw protein food articles. [0015] As noted above, succulent food is preferred to overcooked, dry food.

Maximum retention of moisture within the food article during any cooking process is therefore a function of precisely thermalizing the food to a specific end internal temperature of approximately 82 to 85 C. Within this range, the liquid component of the food does not reach its gaseous state and therefore does not dry the food by extraction and evaporation of the water from the food.

[0016] Protein food products are approximately 70-76 percent water. By preventing the overall elevation of the internal food temperature beyond a maximum of 85 C, satisfactory moisture retention within the food is assured. In this condition, it has been established that food can be staged, i^, held at this temperature, for up to ten hours and yet maintain a high quality, succulent condition.

[0017] United States Department of Agriculture data shows that meat and other bacteria-containing foods are safe for human consumption if cooked to a very low internal temperature of 55 O, if held at that temperature for 124 minutes. The time the food must be held at an internal temperature of 71 C. is reduced to only 1 second. [0018] Current food preparation practices in large-volume facilities require a variety of specifically designed equipment types to cook the food, transfer the food to serving containers, for example, steam tables, and display the previously-cooked food for visual display and service to the consumer.

[0019] In an aircraft food service environment, many of the conditions that exist in other food service environments do not exist. For example, in present aircraft food service systems, food is served strictly when determined by the flight food service personnel, consistent with overriding concerns such as weather. The food is not displayed for selection by the passenger, but rather unwrapped immediately prior to service and handed to the passenger. Particularly in this environment, delivery of tasteless, unattractive, dried-out food on a "take it or leave it basis" generates ill will, diminishes the travel experience of the passenger, and ultimately involves substantial wastage of food items that are either not consumed, or refused outright. [0020] Accordingly, it has been determined that food items can be safely cooked and staged for extended times if maintained at a precise, equilibrated internal temperature of from 71 to 85 C, thereby complying with accepted standards for bacteriological food safety.

[0021] By never exceeding a predetermined end temperature, such as 82 C. for hamburger patties, during the cooking and staging process, moisture in the food is retained, and the food remains tender, moist and delicious without cellulardeterioration. Many products, such as chicken, beef or pizza, can then be cosmetically and organoleptically "finished" with high-temperature infrared or other ancillary equipment in 60-90 seconds just before serving.

[0022] This slow-cooking and staging process provides significantly higher product yields than conventionally-practiced techniques, and is extremely flexible in terms of shape and form to accommodate kitchen and serving area design requirements. This is a particularly important consideration in serving aircraft passengers, since the galley areas are typically small, compact and oddly-shaped. Electrical requirements will generally be 110V (US). [0023] Conduction allows high efficiency energy transfer and relatively low temperatures, thus minimizing energy costs and heat gain in surrounding areas. This type of heating is particularly advantageous when using plastic or aluminum containers as often found in airline meal service. Direct contact between the heating elements and the containers effectively makes the container an "oven" in which the food item is cooked and staged until ready for serving. The term "equilibrate" is used herein to describe the process of using a first, relatively high heat to begin the process of bringing the food to a suitable temperature, and then a second, lower heat setting for completing the process, bringing the food to an equilibrium temperature where the food is approximately the same temperature throughout. The relatively high heat setting may raise the food product to a temperature below, at or above the optimal serving temperature, with different areas of the food at different temperatures. The second, lower temperature setting "equilibrates" the temperature of the food by bringing all of the food, whether below, at or above the desired temperature, to the same temperature, where it can be held for an extended period of time, as discussed herein.

Summary of the Invention [0024] Therefore, it is an object of the invention to provide an electric, dual temperature conduction oven that permits food items to be safely re-thermal ized and/or maintained at temperature for extended periods of time.

[0025] It is another object of the invention to provide a conduction oven that permits food items to be re-thermalized and held at temperature before being loaded onto an aircraft for service at a later time.

[0026] It is another object of the invention to provide a conduction oven that uses power efficiently to re-thermalize and hold food at a predetermined temperature for an extended period of time before service, without degradation in the quality of the food. [0027] These and other objects of the present invention are achieved in the preferred embodiments disclosed below by providing an oven for thermalizing and staging pre-cooked food products in suitable containers, comprising an oven enclosure having a top wall, a bottom wall and opposed, spaced-apart side walls. A plurality of vertically spaced-apart shelves are provided for being positioned in the enclosure for carrying containers of food on a horizontal, major surface thereof. Each of the shelves have an electrical resistance heating element for conductively heating the food containers. A temperature controller is operatively connected to an electrical source and to each electrical resistance element. The temperature controller has a first, relatively high heat output setting for elevating the food to a first temperature level and a second, relatively low heat output setting for equilibrating the food temperature and maintaining the food at the second temperature level for a predetermined period of time.

[0028] According to one preferred embodiment of the invention, the shelves each comprise a metal sheet and a membrane carrying the resistance elements, the metal sheet and membrane positioned in intimate, surface-to-surface with each other. [0029] According to another preferred embodiment of the invention, the shelves each comprise an upper metal sheet, a lower metal sheet and a membrane carrying the resistance elements, the membrane sandwiched between and positioned in intimate, surface-to-surface with the upper and lower metal sheets.

[0030] According to yet another preferred embodiment of the invention, the membrane comprises a sheet of silicone-bonded and cured rubber with an array of electrical resistance wires applied thereto.

[0031] According to yet another preferred embodiment of the invention, the enclosure includes shelf retainer elements for cooperating with complementary retainer elements on the shelves for supporting the shelves in the oven. [0032] According to yet another preferred embodiment of the invention, the shelf retainer elements comprise a plurality of studs mounted on the oven and projecting into the enclosure, and the complementary retainer elements comprise notches in sidewalls formed on opposing side edges of the shelves for being positioned over the studs and supporting the shelves on the studs for easy removal and cleaning.

[0033] According to yet another preferred embodiment of the invention, the upper metal sheet is selected from the group consisting of aluminum and corrosion-resistant stainless steel.

[0034] According to yet another preferred embodiment of the invention, the upper sheet includes a raised lip extending laterally between opposed side edges of a rearward end of the shelf for retaining the containers of food thereon during removal of the shelves from the oven.

[0035] According to yet another preferred embodiment of the invention, first heat output setting is 177 C. and the second heat output setting is 82 C.

[0036] According to yet another preferred embodiment of the invention, the temperature controller includes a timer for maintaining the first and second heat output settings for a predetermined period of time.

[0037] According to yet another preferred embodiment of the invention, an aircraft food service unit is provided, and comprises a galley for insertion into an aircraft interior, including an electrical connector for connection of the food service unit to an electrical power supply of the aircraft, a plurality of oven bays for receiving a respective plurality of portable ovens. Each of the ovens comprises an enclosure having a top wall, a bottom wall and opposed, spaced-apart side walls. A plurality of vertically spaced-apart shelves are provided for being positioned in the enclosure for carrying containers of food on a horizontal, major surface thereof, each of the shelves having an electrical resistance heating element for conductively heating the food containers.

A temperature controller is operatively connected to an electrical source and to each electrical resistance element, the temperature controller having a first, relatively high heat output setting for elevating the food to a first temperature level and a second, relatively low heat output setting for equilibrating the food temperature and maintaining the food at the lower temperature level for a predetermined period of time. [0038] According to yet another preferred embodiment of the invention, the shelves each comprise a metal sheet and a membrane carrying the resistance elements, the metal sheet and membrane positioned in intimate, surface-to-surface with each other.

[0039] According to yet another preferred embodiment of the invention, an oven is provided for thermalizing and staging pre-cooked food products, and comprises an enclosure having a top wall, a bottom wall and opposed, spaced-apart side walls. An enclosure heating element is positioned around the top wall, bottom wall and side walls to define an encircling heat source for the enclosure. A plurality of vertically spaced- apart shelves are provided for being positioned in the enclosure for carrying containers of food on a horizontal, major surface thereof. Each of the shelves have an electrical resistance heating element for conductively heating the food containers. A temperature controller is operatively connected to an electrical source, to the enclosure heating element, and to each electrical resistance element, the temperature controller having a first, relatively high heat output setting for elevating the food to a first temperature level and a second, relatively low heat output setting for equilibrating the food temperature and maintaining the food at the second, equilibration temperature level for a predetermined period of time.

[0040] According to yet another preferred embodiment of the invention, the enclosure heating element comprises a resistance wire embedded in a pad. [0041] According to yet another preferred embodiment of the invention, the enclosure heating element comprises a resistance wire embedded in a pad, and the enclosure heating element is covered with an insulation layer. [0042] According to yet another preferred embodiment of the invention, the enclosure heating element comprises a resistance wire embedded in a pad. The enclosure heating element is covered with an insulation layer. The enclosure is positioned in an exterior oven cabinet, the cabinet including a door through which food products are placed into and removed from the enclosure.

Brief Description of the Drawings [0043] Some of the objects of the invention have been set forth above. Other objects and advantages of the invention will appear as the invention proceeds when taken in conjunction with the following drawings, in which: [0044] Figure 1 is a flow diagram of a prior art flight kitchen;

[0045] Figure 2 is a flow diagram of a flight kitchen according to an embodiment of the invention;

[0046] Figure 3 is a flow diagram of a flight kitchen according to a further embodiment of the invention;

[0047] Figure 4 is a fragmentary view of an aircraft galley incorporating conduction ovens according to an embodiment of the invention; [0048] Figure 5 is a perspective view of a conduction oven according to an embodiment of the invention;

[0049] Figures 6 and 7 are perspective views of a conduction oven according to an embodiment of the invention, with the door removed for clarity; [0050] Figure 8 is an enlarged, fragmentary view of the conduction oven shown in Figures 5-7;

[0051] Figure 9 is an enlarged, fragmentary view of the conduction oven shown in Figures 5-8, indicating the manner of suspending and removing the shelves; [0052] Figure 10 is a simplified, exploded view of the oven according to an embodiment of the invention;

[0053] Figure 11 is a perspective view of an oven shelf with individual serving meal containers thereon;

[0054] Figure 12 is a simplified perspective view of an oven showing insertion of shelves with individual serving meal containers thereon; [0055] Figure 13 is a perspective view of a shelf used in an oven according to an embodiment of the invention;

[0056] Figure 14 is another perspective view of a shelf used in an oven according to an embodiment of the invention;

[0057] Figure 15 is an exploded, perspective view of the shelf shown in Figures

11-14;

[0058] Figure 16 is a further exploded, perspective view of the shelf shown in

Figures 11-14;

[0059] Figure 17 is a perspective schematic view of the resistance wiring pattern of the shelf according to an embodiment of the invention; and

[0060] Figure 18 is a circuit diagram of a conduction oven according to an embodiment of the invention.

Description of the Preferred Embodiment and Best Mode [0061] Referring now specifically to the drawings, a flow diagram of a flight kitchen according to one embodiment of the invention illustrated in Figure 2. Food to be cooked is received from a delivery location 1 to cold storage facilities 2A, 2B, where it is stored, either chilled or frozen, until ready for use. The food is delivered to a pre- prep facility 3, pre-prepared for cooking, including thawing, if necessary, and then prepared in the hot kitchen 4. The prepared, cooked food is then assembled into individual serving portions at an assembly location 5 by placing, for example, a cooked meat item, vegetable and starch into a plastic or aluminum foil tray and sealing the contents in the tray with a plastic or foil cover. See, for example, Figures 11 and 12 and discussion below.

[0062] As shown, the food can then be dispatched to the aircraft ovens 10 according to the invention in a frozen, chilled or warm condition. As explained below, the flexibility of the oven 10 permits the caterer and airline to utilize the oven 10 without regard to whether the food is placed on the aircraft in a frozen, chilled or tempered condition. After placement into the aircraft galley, the food is then prepared for service at the appropriate time as necessary, dependent on the temperature state of the food. [0063] Referring now to Figure 3, another embodiment of the invention is illustrated. Food to be cooked is received from a delivery location 11 to a cold storage facilities 12A, 12B, where it is stored, either chilled or frozen, until ready for use. The food is delivered to a pre-prep facility 13, pre-prepared for cooking, including thawing, if necessary, and then prepared in the hot kitchen 14. The cooked food is frozen or chilled using blast chillers 15 and delivered to chilled storage 16. The frozen or chilled items are assembled in an assembly location 17 and returned to chilled storage 16. [0064] Meanwhile, uncooked items are delivered to chilled storage 16 and held until ready for assembly, and then delivered to an assembly and tray assembly location 18. The chilled hot kitchen and cold kitchen items are dispatched to the aircraft 19, and stored in a chilled condition until ready to heat for service in the ovens 10. [0065] As shown in Figure 4, the ovens 10 are fitted into the aircraft galley 20 in a conventional manner.

[0066] Referring now to Figure 5, the oven 10 includes a control module 25 having an on/off switch 26 and "power on", "heating up" and "ready" lamps 27, 29, 30, respectively. The front opening door 32 includes a door latch 33 for locking the door 32 in a closed position and for opening the door 32 when desired. [0067] Figures 6-9 show the oven 10 with the door 32 removed for clarity. The oven 10 includes a plurality, for example, 6, shelves 35A-F positioned in vertically spaced-apart relation in the oven 10. Each of the shelves 35A-F are retained on retainer elements, for example, four inwardly projecting studs 37, one being shown in Figures 6 and 7.

[0068] Oven 10 is explained in further detail in reference to Figure 10. Oven 10 includes an outer enclosure 40 formed of top, left and right side, bottom and rear walls 41-45, respectively, formed of corrosion-resistant steel plates. An inner oven liner 47, also formed of corrosion-resistant steel plates, is wrapped on all four major sides with a heater pad 48. The heater pad 48 is glued onto the oven liner 47 and covered with standard DF700 insulation (not shown). The heater pad 48 is comprised of a rubber membrane sheet in which are embedded an array of resistance wires, not shown. The resistance wires alternate back-and-forth in a known manner across the length and width of the pad 48 in a pattern and spacing that provide uniform heating to the pad. The wires are connected to a power supply through the control module 25. [0069] As is shown in Figures 11 and 12, each shelf 35A-F is adapted to receive an array of food trays "T", which are placed directly on the top surface for being heated principally by conduction through direct heat transfer, as described further below. [0070] Referring now to Figures 13-17, an exemplary shelf 35A is shown and further described. Shelf 35A includes a top surface 50, left and right downwardly- extending supports 51 , 52, an upwardly-extending retainer wall 53 and a bottom surface 55. The supports 51 , 52 each have two notches 54 that are captured and held by the studs 37, as shown in Figures 6-9.

[0071] As is best shown in Figures 15 and 16, shelf 35A is formed of a sandwich of top and bottom aluminum or corrosion-resistant stainless steel sheets 56, 58, between which is sandwiched a shelf heating pad 59. As illustrated in Figure 17, the heating pad 59 comprises a rubber membrane sheet in which is embedded an array of resistance wires 60. The wires 60 are arrayed according to a pattern whereby uniform heating is provided to the surface of the shelf 35A. Leads 62 connect to the control module 25.

[0072] Referring to Figure 18, a block wiring diagram of the oven 10 is shown.

Power to the control module 25 is through a power terminal 70, preferably 115V, 60 Hz, fused with a mains fuse 71. The on/off switch 26 and "power on", "heating up" and "ready" lamps 27, 29, 30, provide power control and a visual indication of oven status. See also Figure 5. An auto timer/power off switch 73 is also provided. Controls are connected to the heating units through a control connector 74. A one shot "heating time" control circuit 75 having a range of from 0-30 minutes, "begin temp" control circuit 76 (approx. 175 C. and a "hold temp" control circuit 77 (approx. 70 C.) provide temperature control settings for the oven 10.

[0073] Power is supplied through a power connector 80 from a power supply 81.

The oven heater pad 48 and the shelf heater pads 59, one for each of the shelves 35A- F are connected to the power supply 81 in parallel through heater connectors 83A-F. Thermistors 85A-G permit temperature measurement and control. [0074] Further details regarding the design, construction and operation of the oven 10 are now provided for purposes of illustration. These details may vary based on the conditions applicable to particular circumstances.

[0075] The heater pads 59 of the shelves 35A-F have both upper and lower heating capacity. The upper heating capacity is by conduction, le , direct contact between the food trays "T" and the top surface 50 of each of the shelves 35A-F. The bottom surface 55, see Figure 13, radiates heat energy downwardly, and provides convective heat to the top of the trays "T" on the shelf directly beneath, thus providing greater uniformity of heating. Each heater pad 59 according to the preferred embodiment disclosed herein has a dimension of 395 x 215 mm, with 115 V, 200 W, 66 Ohms nominal, including wiring and thermistor. As observed in Figure 18, each heater pad 59 switches on and off independently of the other heater pads 59 according to the adjusted temperature setting. The various dimensions and power supply criteria are tailored to meet particular size and power criteria as required by the end use. [0076] Significant supplemental convective energy is provided by the heater pad

48 surrounding the oven liner 47. The heater pad 48 provides additional uniformity to the heating process and a heat ballast when the oven door 32 is open. The heater pad 48 also maintains the air in the oven at a constant temperature, thus substantially reducing the amount of water condensation in the oven 10. The heater pad 48 according to the preferred embodiment disclosed in this application has dimensions of 1330 x 310 mm, 115V, 500W, 16 Ohms nominal, including wiring and thermistor. Total power of the oven 10 is therefore 1700 W. [0077] Thickness of the shelves 35A-F is variable, with the thickness of an aluminum sheet material of between 1 mm and 3 mm providing adequate heat transfer, with the thicker sheet retaining heat more efficiently when the oven door 32 is open. In circumstances where corrosion is a particular concern, such as in aircraft, corrosion- resistant stainless steel is preferred, while aluminum may be suitable in non-aerospace applications such as in cafeterias.

[0078] A typical thermalization cycle comprises placing aircraft meals in conventional individual serving trays in the oven 10 at a nominal temperature of 7 C. and heating the meals for approximately 15 to 30 minutes at a full power setting wherein the shelf temperature is limited to a maximum of 177 C. The maximum shelf temperature is then dropped to a hold temperature of 70 C. The object is to heat the food up to a temperature of 70 C. and then hold the food at that temperature. [0079] The amount of time required at full power depends on a variety of factors, including the starting temperature of the food, the type of food to be heated; i.e. dense foods heat faster than foods with entrapped air, such as whipped potatoes; and the number of trays "T" in the oven 10.

[0080] It is anticipated that an increase in power to the shelf heater pads 59 to

250 W, and lowering the power of the oven heater pad 48 to 200 W will provide faster heating times while maintaining total power consumption at about 1700 W, thus permitting a normal 15 amp U.S. wall outlet to be used.

[0081] Preferably, the "heating up" light 28 remains on for approximately five minutes after the temperature reaches the "hold" temperature. This permits a "soak time" that results in a better equilibrated temperature and greater food safety. [0082] It has been determined that typical airline food and similar frozen and refrigerated foods can be re-thermalized and held at 70 C. for 4-5 hours with no degradation in taste, texture or appearance. The oven 10 thus permits service of hot meals on demand by maintaining an inventory of food items in a safe and palatable condition that can be served anytime a passenger wants a meal, or to take into account events, such as ground delays or en route weather, that may require unanticipated rescheduling of meal service.

[0083] The oven 10 also permits the food items to be prepared and optionally refrigerated and/or re-heated on the ground, thus avoiding use of far more expensive energy generated by the aircraft.

[0084] A conduction oven for re-thermalizing or maintaining the temperature of food items, such as portion-controlled food items such as served on aircraft is described above. Various details of the invention may be changed without departing from its scope. Furthermore, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation-the invention being defined by the claims.

Claims

I claim:

1. An oven for thermalizing and staging pre-cooked food products, comprising: (a) an oven enclosure having a top wall, a bottom wall and opposed, spaced-apart side walls; (b) a plurality of vertically spaced-apart shelves for being positioned in the enclosure for carrying containers of food on a horizontal, major surface thereof, each of the shelves having an electrical resistance heating element for conductively heating the food containers; and (c) a temperature controller operatively connected to an electrical source and to each electrical resistance element, the temperature controller having a first, relatively high heat output setting for elevating the food to a first temperature level and a second, relatively low heat output setting for equilibrating the food from the first temperature level to a second equilibrium temperature level and maintaining the food at the second temperature level for a predetermined period of time.

2. An oven according to claim 1 , wherein the shelves each comprise a metal sheet and a membrane carrying the resistance elements, the metal sheet and membrane positioned in intimate, surface-to-surface with each other.

3. An oven according to claim 1 , wherein the shelves each comprise an upper metal sheet, a lower metal sheet and a membrane carrying the resistance elements, the membrane sandwiched between and positioned in intimate, surface-to- surface with the upper and lower metal sheets.

4. An oven according to claim 2 or 3, wherein the membrane comprises a sheet of rubber with an array of electrical resistance wires applied thereto.

5. An oven according to claim 4, wherein the enclosure includes shelf retainer elements for cooperating with complementary retainer elements on the shelves for supporting the shelves in the oven.

6. An oven according to claim 5, wherein the shelf retainer elements comprise a plurality of studs mounted on the oven and projecting into the enclosure, and the complementary retainer elements comprise notches in sidewalls formed on opposing side edges of the shelves for being positioned over the studs and supporting the shelves on the studs.

7. An oven according to claim 3, wherein the upper metal sheet is selected from the group consisting of aluminum and stainless steel.

8. An oven according to claim 3, wherein the upper sheet includes a raised lip extending laterally between opposed side edges of a rearward end of the shelf for retaining the containers of food thereon during removal of the shelves from the oven.

9. An oven according to claim 1 , wherein the first heat output setting is 250

F. and the second heat output setting is 180 F.

10. An oven according to claim 1 , wherein the temperature controller includes a timer for maintaining the first and second heat output settings for a predetermined period of time.

11. An aircraft food service unit, comprising: (a) a galley for insertion into an aircraft interior, including electrical connector means for connection of the food service unit to an electrical power supply of the aircraft; (b) a plurality of oven bays for receiving a respective plurality of portable ovens, each of the ovens comprising: (i) an enclosure having a top wall, a bottom wall and opposed, spaced-apart side walls; (ii) a plurality of vertically spaced-apart shelves for being positioned in the enclosure for carrying containers of food on a horizontal, major surface thereof, each of the shelves having an electrical resistance heating element for conductively heating the food containers; and (iii) a temperature controller operatively connected to an electrical source and to each electrical resistance element, the temperature controller having a first, relatively high heat output setting for elevating the food to a first temperature level and a second, relatively low heat output setting for equilibrating the first food temperature to a second, equilibrium temperature level and maintaining the food at the second temperature level for a predetermined period of time.

12. An aircraft food service unit according to claim 11 , wherein the shelves each comprise a metal sheet and a membrane carrying the resistance elements, the metal sheet and membrane positioned in intimate, surface-to-surface with each other.

13. An aircraft food service unit according to claim 11 , wherein the shelves each comprise an upper metal sheet, a lower metal sheet and a membrane carrying the resistance elements, the membrane sandwiched between and positioned in intimate, surface-to-surface with the upper and lower metal sheets.

14. An aircraft food service unit according to claim 12 or 13, wherein the membrane comprises a sheet of silicone rubber with an array of electrical resistance wires applied thereto.

15. An aircraft food service unit according to claim 14, wherein the enclosure includes shelf retainer elements for cooperating with complementary retainer elements on the shelves for supporting the shelves in the oven.

16. An aircraft food service unit according to claim 15, wherein the shelf retainer elements comprise a plurality of studs mounted on the oven and projecting into the enclosure, and the complementary retainer elements comprise notches in sidewalls formed on opposing side edges of the shelves for being positioned over the studs and supporting the shelves on the studs.

17. An aircraft food service unit according to claim 13, wherein the upper metal sheet is selected from the group consisting of aluminum and stainless steel.

18. An aircraft food service unit according to claim 13, wherein the upper sheet includes a raised lip extending laterally between opposed side edges of a rearward end of the shelf for retaining the containers of food thereon during removal of the shelves from the oven.

19. An aircraft food service unit according to claim 11 , wherein the first heat output setting is 250 F. and the second heat output setting is 180 F.

20. An aircraft food service unit according to claim 11 , wherein the temperature controller includes a timer for maintaining the first and second heat output settings for a predetermined period of time.

21. An oven for thermalizing and staging pre-cooked food products, comprising: (a) an enclosure having a top wall, a bottom wall and opposed, spaced-apart side walls; (b) an enclosure heating element positioned around the top wall, bottom wall and side walls to define an encircling heat source for the enclosure; (c) a plurality of vertically spaced-apart shelves for being positioned in the enclosure for carrying containers of food on a horizontal, major surface thereof, each of the shelves having an electrical resistance heating element for conductively heating the food containers; and (d) a temperature controller operatively connected to an electrical source, to the enclosure heating element, and to each electrical resistance element, the temperature controller having a first, relatively high heat output setting for elevating the food to a first temperature level and a second, relatively low heat output setting for equilibrating the food from the first temperature level to a second, equilibrium temperature level and maintaining the food at the second temperature level for a predetermined period of time.

22. An oven according to claim 21 , wherein the enclosure heating element comprises a resistance wire embedded in a pad.

23. An oven according to claim 21 , wherein the enclosure heating element comprises a resistance wire embedded in a pad, and the enclosure heating element is covered with an insulation layer.

24. An oven according to claim 21 , wherein: (a) the enclosure heating element comprises a resistance wire embedding in a pad; (b) the enclosure heating element is covered with an insulation layer; and (c) the enclosure is positioned in an exterior oven cabinet, the cabinet including a door through which food products are placed into and removed from the enclosure.