US 20030213718 A1
A method for conveniently and hygienically providing warmed food items in a catering location, while reducing handling of such items. The method includes providing a packaged tray assembly of a plurality of individual trays that are adapted to accommodate a series of food items, the trays being ovenable and being arranged in at least one stack or a stacked orientation which has a predetermined compact size and shape that fits at least in at least part of a cavity of an oven at a catering location, wherein the stacked orientation also provides spaces to promote hot air circulation inside the trays for uniform heating of the food items. The method allow frozen cooked or pre-cooked food products to be packaged and transported from a manufacturing location to a catering location where they are reheated in stackable trays for packaging, transport and reheating, which can be nested inside each other after use. The trays are preferably designed with sloped portions and openings allowing the products or food items to be uniformly reheated.
1. A method for conveniently and hygienically providing warmed food items in a catering location, while reducing handling of such items, which method comprises providing a packaged tray assembly of a plurality of individual trays that are adapted to accommodate a series of food items, the trays being ovenable and being arranged in at least one stack or a stacked orientation which has a predetermined compact size and shape that fits at least in at least part of a cavity of an oven at a catering location, wherein the stacked orientation also provides spaces to promote hot air circulation inside the trays for uniform heating of the food items.
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a bottom surface for supporting the series of food items thereon,
complementary opposite shaped support means and are configured and dimensioned such that in the stacked orientation, the trays can be stacked upon each other to provide sufficient room for the food items and spaces to promote hot air circulation inside the trays for uniform heating of the food items, while in a nesting orientation, empty trays can be nested together for compact storage, and
venting means to facilitate hot air circulation within the tray.
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11. A packaged food tray assembly for convenient transporting, reheating and serving of warmed food items in a catering location comprising
a series of food items, and
a plurality of individual trays having a base adapted to accommodate the series of food items, the trays being ovenable and arranged in at least one stack of a stacked configuration which has a predetermined compact size and shape that fits at least in at least part of a cavity of an oven at a catering location, wherein the stacked configuration also provides spaces to promote hot air circulation inside the trays for uniform heating of the food items, and, optionally,
a removable outer protective package that maintains the integrity of the trays in the stacked orientation for transport to the catering location.
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 Owing to the economic advantages which it provides, this method is suitable for any type of catering location, such as a company restaurant or school canteen. Because of the advantages it provides also as regards handling and reduction in space requirement, it is still better suited to the service of hot food items to airline passengers for breakfast, lunch or dinner.
 In order to implement the method, wherein the trays are stacked on top of each other when they contain food products and nested inside each other when they are empty, the trays have opposite complementary support means which fit inside each other when the trays are all in the same direction and which are superimposed to each other when the trays are oriented at 180° with respect to each other, leaving wide air inlets between the sides which do not form the support. Therefore, one important aspect of the invention is to take advantage of this support means to create a space between the trays in this stacking configuration that promotes hot air circulation from the oven. In order to reheat the food products arranged in the trays as evenly as possible, the trays can also include openings through the support means or sides of the trays and/or through the bottom of the trays and/or indentations made on the upper edge of at least one side of the trays. Therefore, air circulation is promoted both in horizontal and vertical directions through the trays.
 Even reheating is further improved by forming a succession of sloped portions in the bottom such that the edges of each product to be reheated are also placed in the hot air flow of the oven. This variant also has the advantage of improving the display, arrangement and grasping of hand-held food items. It also gives the advantage of being able to fill the trays with products, which by nature or due to cooking, will have substantially different dimensions.
 In another possible embodiment, the trays are stacked one on top of the other by means of removable intermediate spacing means arranged to leave air circulation paths between each tray. The intermediate spacing means may preferably be made of a low-cost, disposable and, preferably pliable, material such as cardboard and the like. The spacing means are shaped to let hot air circulate between the trays in the oven so that an homogeneous and quick heating of the food items can successfully be achieved.
 It will be observed that these trays may be made of a material which enables them to be reused, for example, aluminum. However, according to a preferred embodiment, said trays are made of a disposable or recyclable material, such as a plastic material or ovenable paperboard able to withstand a temperature of up to 230° C. This offers the further advantage of providing a light product open to numerous design variations, by the choice of colors of the material used, or by adding marks or logos to the sides of the trays, such marks being representative of the products contained in the tray, the supplier or suchlike. The trays can be made of injection molded or thermoformed plastics. Suitable food grade plastics are those that can withstand temperatures up to 230° C. during at least 30 minutes, preferably 1 hour, while substantially maintaining good mechanical characteristics. Preferred plastics may be chosen among the group of heat resistant food grade polymers or copolymers consisting of a syndiotactic polystyrene (SPS), crystalline terephthalate polyethylene (CPET), polycarbonate, polyetherimide, polyimide, polyetherketone (PEEK), fluoropolymers, polyphthalamide, polyphenylene sulfides, polyether sulfone, polyaryl sulfones, polypropylene and mixtures and co-polymers of these thermoplastics. The material could also be made of a thermosetting polymers such as a polyester or epoxide resin.
 Preferred plastic materials for the tray are those with a glass transition temperature preferably higher than 80° C., even more preferably higher than 95° C. and a melting point higher than 200° C., even more preferably higher than 225° C. At oven temperatures higher than the glass transition, the tray may be slightly pliable but still retains its shape. In the preferred example, the trays comprise a matrix of syndiotactic polystyrene (SPS) and a mechanical filler, preferably of reinforcing material such as glass and the like. SPS is a crystalline polymer that has a glass transition temperature of about 100° C. and a melting point of about 270° C. The tray can be molded with such a material at a thickness lower than 3 mm, preferably less than 2 mm, even more preferably at about 1.5 mm while still keeping high mechanical properties, and in particular, excellent flexural strength. Within this range of thickness, trays of this material achieve a sufficient strength, are stackable in a reduced volume after use, can be economically dedicated for a single use and be destroyed after use by incineration.
 The mechanical filler has the advantage to further increase the flexural strength of the tray under heated conditions, in particular, at temperatures greater than the glass transition temperature. The matrix of SPS and the mechanical filler are preferably used together in a ratio of from 99:1 in % wt. to 60:40, even more preferably 90:10 to 70:30.
 The tray may preferably be made of a material that withstands high temperatures and even preferably is a disposable molded plastic material with a high melting point and glass transition temperature as aforementioned.
 The diagram of FIG. 1 schematically describes the steps of the method according to the invention. Products 1 for manufacturing food products are delivered to a preparation unit 2, outside catering location 7, where they are cooked, or pre-cooked. They may be, for example, products for breakfast (Viennoiserie, Danish pastries, croissants, etc.), lunch or dinner (calzone, pizzas, quiches, etc..), or any other pastry or bread products, or baked or fried products which freeze and regenerate well in an air convection oven or an oven using both air convection and steam. After preparation, in a following step 3, the products may be individually wrapped in a flow wrap or ovenable paper sachet then arranged in trays and are then deep frozen in step 4; equally the products may be deep frozen first and then placed in the trays. In an alternative, the food products may be directly placed in the trays without individual packaging.
 Deep freezing may be carried out by any suitable means such as by rapidly cooling to the required temperatures, i.e., minus 18 to 40° C. in a spiral freezer or in a freezing tunnel under liquid nitrogen jets or any other suitable freezing technology. In all cases, the cold chain should be maintained all the way through the preparation to the delivery to the catering location by storing the packages in freezers in order to ensure safe and hygienic conditions.
 In the following step 5, the trays are stacked head to tail on top of each other and packaged in a packaging, for example in a carton able to contain 5 to 10 trays, the packaging then being closed to firmly maintain the stack of trays in position. The cartons are then brought to the foodservice location 6 of a catering location 7, where the stacked trays are removed from their packaging and reheated in a convection oven, then separated from each other to be distributed in catering location 7, for example to the passengers of an aircraft. The tray may be either directly handled with a heat protecting glove for being directly used as a serving tray to the passengers or, alternatively, be fitted in a larger insulation basket that holds the tray. When the trays are empty, they are nested inside each other to occupy a minimum of space to then be either thrown away 8, or reused 9 after preparation unit 2.
 The method thus allows hot or warm food products to be served conveniently and hygienically, in particular in locations where the preparation space and storage space before and after use are limited.
 In order to make this method even more efficient, the shape of the trays has been optimized, as explained in the following description.
FIG. 2 shows a carton 10 in which, for instance, seven trays 20 are stacked with the food items properly placed therein, the carton including a front panel 10 a able to be closed, for example by means of adhesive strips or glue which are not shown. The dimensions of carton 10 depend on those of the trays, and of those of the reheating oven. The dimensions and shape of the carton are determined to provide a sufficiently tight enclosure around the stack of trays that maintains the stack in place even in turbulent conditions of transport. The carton may preferably be sufficiently closed with as few holes or openings as possible, although this is not an absolute requirement, but to ensure a greater degree of hygiene and play an effective thermal insulation barrier. For instance, a carton 10 may measure for example 40×40×25 cm (i.e., height of 40 cm, depth of 40 cm and width of 25 cm), with a thickness of between 1 to 20 mm, which allows it to be easily handled, although other types of packaging can be envisaged with a smaller or greater number of trays.
 The carton may advantageously comprise means for facilitating the arrangement of the stack of trays in the oven while requiring as little handling as possible. Preferably, both the front and rear panels 10 a, 10 d have a tear-off portion with tear or perforated line 10 b that upon tearing off is capable of removing a sufficiently wide portion of the carton to let the stack of trays easily slip through one opening when pushing is exerted on the stack. The tear line may be advantageously combined with a precept zone 10 c preferably provided in the vicinity of an edge of the carton at or close to the periphery of the tear line.
 As shown in FIG. 10, when the operator needs to load the stack of trays in the oven, her (or she) tears off the tear portions in the front and rear panels. The carton can be placed in front of the cavity 51 of oven 50, such as on a planar support surface 52, with the rear of the carton facing the cavity of the oven. The user can then push on the whole stack of trays, in direction A, to slip it right into the oven. The carton is then thrown away.
 The trays and their complementary support means are preferably designed, dimensioned and shaped so that upon arranging a desired number of trays in a stack, the resulting stack of trays fits, at best, an available room in the cavity of the oven. Of course, depending on the oven, its cavity may accommodate a single stack of trays or, alternatively two or more stacks of trays placed in parallel.
 Preferably, the stack of tray is sized to fit in the oven's cavity in such a way that it leaves a predetermined gap between the trays and the walls of the oven that promotes a sufficient air velocity along the walls in convection heating mode. If the gap between the trays and the oven's cavity is too small, air cannot easily circulate from one tray to another once the trays are all full of food. On the contrary, if the gap is too large, the velocity drops and the heating performance decreases accordingly. It has been found that air velocity measured along the walls of the oven should preferably be higher than 1 meter/second in convection mode, even more preferably of about 2 to 3 meters per seconds to achieve a rapid heating of the food in the trays. For conventional convection ovens used, the air velocity required can be obtained with a total clearance with values of less than 15 mm, preferably of about 4-8 mm (total clearance as measured at the “lip” of the side of each tray when the tray is pushed against one sidewall for measurement).
 It is also for example possible to keep the trays stacked with a shrink-wrapped plastic film or belts, either used alone, or in combination with an outer carton especially if the food items in the trays are not individually packed in a flow wrap.
 The trays may in fact be designed with shapes allowing them to be manufactured economically, for example by heat molding a plastic material, such as syndiotactic polystyrene (SPS), crystalline terephthalate polyethylene (CPET), polyamide, polyimide, PEEK or other suitable heat resistant polymers or copolymers or by shaping an ovenable paperboard withstanding temperatures of up to 230° C. Preferably, the plastic is reinforced by a mechanical filler such as glass, ceramic, carbon and the like. Depending upon the choice of the polymer(s) or copolymer(s) for the trays, the molding process may be carried out by injection molding, compression molding, thermoforming or injection-compression molding.
 With reference to FIGS. 3 and 4, showing the preferred embodiment, it can be seen that a tray 20 has the shape of a rectangular case including two small sides 21, 22, two large sides 23, 24 and a bottom 25.
 Also with reference to FIGS. 5 to 7, it can be seen that the two small sides 21, 22 of tray 20 have complementary support means or shapes allowing them to be stacked on top of each other, as shown in FIG. 5, when they are oriented head to tail with respect to each other, or to nest them inside each other as shown in FIG. 7, when they are all oriented in the same direction.
 On its inner face close to the edges joining large sides 23, 24, the small side 21 has two small shelves 11 separated by a space 13 to form, on the outer part of small side 21, two recesses 15 of complementary shape to that of small shelves 11, these recesses 15 delimiting on bottom 25 a support zone 17. The opposite small side 22 has, in a complementary manner, in its inner face close to its edges joining the large sides 23 and 24, two recesses 14 of substantially complementary shape to that of small shelves 11, these recesses 14 being joined by a support strip 18 of substantially the same shape as support zone 17 at the base of small side 21. The support strip 18 delimits in the outer surface of small side 22, a recess 16 of substantially identical shape to that of support zone 17 of the other small side 21. The recesses 14 of small side 22 are closed on the bottom side to form legs 12 which will rest on small shelves 11 of small side 21 when the trays are stacked head to tail, support zone 17 then resting on support zone 18, as shown in FIGS. 5 and 6 for two trays 20 a, 20 b. Conversely, as shown in FIGS. 7 and 8, when trays 20 are all oriented in the same direction, they nest inside each other. For small side 22, legs 12 engage in recesses 14 and the inner shape delimited by support strip 18 engages in recess 16. The same is true of small side 21 with complementary shapes 11/15 and 13/17. The shape, size and dimensions of the complementary support means may greatly vary without departing from the spirit of the invention. In particular, those attributes may be determined so as to optimize the volume available for the food inside the tray while not being detrimental to the stability of the stacked assembly.
 Returning to FIGS. 3 and 4, it can be seen that bottom 25 is formed by a succession of slightly sloping portions 26 connected to each other by almost vertical strips 27, these sloping portions 26 and strips 27 being respectively provided with venting means 36 and 37. Likewise openings 31 and 32 are provided in small sides 21, 22. It can also be seen that large sides 23, 24 have upper and lower edges form series of indentations 28 a, 28 b with a substantially identical or similar profile to that of the bottom, with sloping portions 29, 19, connected by substantially vertical portions 28.
 As can be seen in FIGS. 5 and 6, when two trays 20 a, 20 b are stacked, edges of large sides 23, 24 delimit the openings 38 between portions 19, 28 and 29.
 The indentations 28 a, 28 b of the upper and lower edges take position so that when the trays are properly stacked in a head-to-tail configuration, the indentations are substantially vertically aligned so as to leave side openings 38 in number and section sufficient for the air to circulate at the upper surface of each array of food items. Furthermore, the section of the side openings 38, as obtained from the stacked arrangement of trays, progressively widens from the bottom of the sloping portions to the top of the sloping portions, therefore providing an homogeneous air circulation all along the upper surface of the food items.
 Furthermore, the support means are arranged such that when two trays are stacked together, the large openings 31 of small sides 21, 22 are substantially uncovered so as to allow air to easily circulate through these small sides.
 All the openings 31, 32, 36, 37 and 38 provided in these trays thus allow hot air to circulate very easily between small food portions 30 a, 30 b, etc. arranged on bottom 25, at least around four faces if the small portions have the shape of rectangular rods, as shown in FIG. 9. It can also be seen that this configuration allows food items such as preferably hand-held snacks 30 a, 30 b of substantially different lengths to be arranged, which allows a reduced assortment of trays to be obtained for a wide range of food products to be reheated.
 The number of vent means and their distribution through the tray surfaces may be determined as a function of the size of food items to be placed in the trays so as to achieve a quick and uniform reheating of the food items while avoiding cold or, conversely, too hot spots. In particular, the number of vents should be sufficient so that each individual food item placed in the tray should be correctly vented. Preferably, the food supporting surface area to vent surface area for each individual tray should preferably be within a range of 3:1 to 1:10, even more preferably 2:1 to 1:5.
 The venting of the trays is also determined to form both vertical and horizontal air paths provided in a sort of matrix configuration that enables to uniformly raise the temperature of a plurality of vertical layers of arrays of discrete food items as arranged in the stacked trays. This effective air circulation enables to raise the food temperature from negative or chilled temperatures to a minimum of about 60° C., more preferably at least 70° C., at core of the food items, in less than 30 minutes, even preferably less than 20 minutes.
FIGS. 11 and 12 show a second embodiment of the tray assembly according to the invention which differs from the first embodiment described above in that the stacking/nesting is obtained by means of a two-part arrangement. As illustrated in FIG. 11, trays 40 are still stacked for example in a “A” box type carton 20, but they are spaced one from each other by reversed U-shaped spacing means 45 made for example of carton. These reversed U-shaped spacing means have sidewalls that engage in the bottom of the large sides 43, 44 of a lower tray 40 in the stack which extend upwardly beyond the level of the tray's circumferential edges to form a tunnel 49 for circulating hot air in the direction of the small sides 41, 42. The spacing means 45 forms an upper flat support surface that provides support to an upper tray in the stack. Optionally the top and sides of the U-shaped spacing means may be provided with openings 46, 48. Thus, when the trays 40 are removed from the oven, the U-shaped spacing means are folded and thrown away, and when the trays 40 are empty, they may be nested whatever are their orientation. Like in the first embodiment the bottom of the trays 40 may be provided with sloped portions in array and/or openings. The spacing means may also be configured with four to six sides such as an elongated box comprising evenly distributed vents. The spacing means are preferably made of low cost, disposable material such as preferably cardboard or paperboard material that can withstand heat in the oven.
 A more preferred material for the tray is syndiotactic polymer reinforced with 15 to 40% wt. glass, preferably 20 to 30% wt. glass. Suitable syndiotactic polymers are polymers of polystyrenes (SPS). Among the important properties of SPS, as compared to food grade polymers used traditionally, SPS experience a lower coefficient of linear thermal expansion that make them easy to mold, more stable and less subject to deformation in a stacked configuration of trays when heated in the oven. These materials exhibit low melting viscosity, good heat properties but cool quickly after removal of the heat, have relatively high impact toughness, absorb little water, have a relatively high glass transition temperature, and a relatively long elongation to break. These properties result in a tray which can be manufactured at relatively thin thicknesses, preferably at less than 3 mm, at a much lower cost and which still exhibits excellent performance characteristics which do not vary significantly over the range of temperature and relative humidity encountered during the heating and serving. Furthermore the tray can be incinerated without causing environmental problems.
 Syndiotactic polystyrene, which is sold under the trade name QUESTRA® by Dow Chemical Company, is a semi-crystalline thermoplastic polymer. However, as opposed to the unordered atactic configuration of amorphous general purpose polystyrene, syndiotactic polystyrene is comprised of styrene monomer units arranged in a highly ordered alternating configuration along the polymer chain. This ordered structure allows the polymer chain to crystallize. The crystallinity improves the strength and heat resistance of the material near and above the glass transition temperature. Since the material absorbs very little moisture (less than 0.1% after 24 hrs immersion according to ISO 62), the glass transition does not depend upon moisture. The glass transition temperature of plastic material is the temperature at which the amorphous phase of the material undergoes a transition from a glassy state to a flexible state involving motion of long segments in the polymer chain. The glass transition for various materials is shown in the following table:
 Accordingly, on account of the improved heat resistance, strength properties and cost, syndiotactic polystyrene is preferred for the tray.
 The invention is not limited to hand-held snacks but encompasses the possibility to provide warmed food meals in individual plates or containers. For that, the trays have preferably an enclosure that is shaped to receive a plurality of frozen food meals in plates or dishes. The sloped portion of the bottom of each tray that is appropriate for hand-held snacks may preferably be replaced by a flat bottom that can accommodate the open plates or dishes such as “plats a sabot” and the like. Large vents or inlets as aforementioned are still necessary to provide a proper convection heating of the meals in their dishes. The container for the meal has preferably no lid thereon that would reduce heat transfer to the top of the meal although a lid could be used to prevent liquid food such as soup or sauce from spilling over their recipient.
 The invention provides numerous advantages such as:
 a) A better and more hygienic food delivering method to foodservice locations with no direct handling of the food itself until the consumer serves him- or herself,
 b) A more efficient, less cumbersome, easy-to-serve and preferably disposable, food delivery system that provides more convenience to the in-flight operators when boarding, passing around the food to the passengers and cleaning the food service cabin before landing,
 c) A tray arrangement that enables optimization of the spacing and venting for hot air circulation depending upon the nature and size of the food items to be regenerated in the oven,
 d) A more secured package that can go through the security check points of airports, in particular metal detectors, more easily as compared to the usual metal tray inserts as the package may be made of disposable, X-ray transparent materials and is intended for first opening only once on board in the aircraft when the food is ready to be reheated in the oven.
 e) An improved fitting with airline trolleys and overhead galley containers when packed in outer carton and better use of allocated space within a standard aircraft oven considering product size/dimensions and heating performance.
 The following examples illustrate the most preferred embodiments of the invention.
 Seven trays of the type as described and illustrated in FIGS. 3 to 9 were assembled with frozen prebaked Calzone type snacks. Each tray contained an average of 9 snacks with about 3 snacks per array. The snacks individually weighed from 70 to 80 grams. The trays were then placed as a stack of trays in an airline convection oven of the type commercialized by B/E Aerospace. The average temperature in a prewarmed oven as controlled by a thermostat was set at respectively 19020 C., 180° C. and 170° C. It took respectively 18, 20 and 22 minutes to regenerate the snacks to a serving temperature in core of about 70° C. minimum. All snacks were evenly heated within the trays within an average temperature range of +/−8 to +/−10° C. The snacks did not experience any cold spots.
 A stack of Atlas-sized trays of 240 mm in width was placed in an Atlas-sized oven interior width of 245 mm resulting in a total clearance of 5 mm as measured at the “lip” of the trays. With the profile of the cross-section of the trays, the actual air space ranged from 15 mm to 21 mm in width when pushed against one sidewall of the oven. Air flow of 2-3 meters per seconds were measured at the front edge of the oven with these trays loaded and in place, when operating in convection mode, and heating times to 70° C. product center temperature were 20 +/−2 minutes.
 When these Atlas-sized trays were placed in a wider KSSU-sized oven (interior of 261 mm), the increased air space between the trays and oven side wall resulted in reduced air speed with all points less than 1 meter/sec. in convection mode, and heating times increased to more than 33 minutes to reach a temperature at core of 70° C.
 Other features and advantages of the present invention will appear more clearly upon reading the following description of a preferred embodiment for implementing the method according to the present invention, this description being made with reference to the annexed drawings, in which:
FIG. 1 is a diagram schematically showing the different steps of the method according to the invention;
FIG. 2 shows a first embodiment of a stack of trays properly filled in with food items packaged in a carton;
FIG. 3 in perspective shows an isolated tray of the stack of FIG. 2;
FIG. 4 is a top view of the tray shown in FIG. 3;
FIGS. 5 and 6 show respectively, in perspective and in side view, two trays stacked on top of each other,
FIGS. 7 and 8 show respectively, in perspective and in side view, two trays nested inside each other;
FIG. 9 shows a tray containing two small portions of food products;
FIG. 10 shows the easy loading of the trays in the oven;
FIG. 11 shows a second embodiment of a stack of trays according to the invention, and
FIG. 12 shows in perspective an isolated tray assembly of FIG. 11.
 The present invention concerns a method intended for industrial catering for conveniently and hygienically packaging, transporting to a catering location and serving food products using sets of stacked trays where each set of trays will be preheated in a oven before the food products are distributed, the empty trays then being able to be nested inside each other to occupy less space.
 A certain number of methods are already known in the catering industry for facilitating the packaging and transportation of food products from a total or partial manufacturing location to a catering location where said products can undergo final preparation.
 In airline catering, the food products to be reheated are traditionally supplied on board on metal trays that are slidably inserted in securely closed metal tray inserts for transport and boarding. A caterer has to manipulate the food to put it on trays. Those inserts usually fit within the galleys of the in-flight cabin. Once it is time for consumer service, the tray inserts are opened, then, the trays are individually removed and slipped into the cavity of an oven which is provided with receiving support means to receive each individual tray. The support means in the cavity are placed at sufficient vertical distance to allow hot air to circulate between the trays. When heating has been achieved, the food items are individually transferred in baskets or any suitable serving container for serving them to passengers. The trays are then re-installed empty in the tray inserts that are replaced in the galleys before the landing.
 This traditional method requires too much handling and time both for the caterer and the in-flight service staff. Furthermore, too much space must be allocated in the catering cabin for those tray inserts even after they have been emptied of the food. These inserts are also heavy and can usually only be inserted at the floor level of the galley. Finally, the air circulation around the trays might not be optimal as the support means might not be optimally placed in the oven depending on the nature and size of the food product to be reheated.
 U.S. Pat. No. 4,269,169 discloses a method wherein the trays, which are sealed by a cover, can be stacked on top of each other in a container which is open on one side and has orifices allowing hot air to circulate in the spaces between the trays, these spaces being obtained by legs located at the base and resting on the cover of the subjacent tray. The trays and the container have a generally pentagonal shape so that the volume of a conventional oven with a rectangular base plate cannot be used in an optimum manner, which constitutes a drawback when reheating prior to consumption has to be carried out in a confined space, as is the case in the foodservice location of an aircraft. These containers are in fact intended to be introduced into a special rotary circular oven, disclosed in U.S. Pat. No. 4,625,867, such ovens also having the drawback of requiring more space than a conventional oven. It will also be observed that the individual sealing of each tray, which is required in order to stack said trays and because the container is not hermetic at all, contributes to the increased cost of the final product.
 In U.S. Pat. No. 4,384,191, filed by the same author subsequently to the two previously cited patents, the container, which is still provided with orifices, has a conventional parallelepiped shape having, on either side of its opening, in a conventional manner, rails or shelves enabling ordinary sealed trays to be positioned without touching each other, one or more warm air generators being arranged between the containers after they are introduced into a heating chamber.
 This method has substantially the same drawbacks as before, in that it requires individual sealing of the trays and a special heating installation.
 It will be observed that the aforementioned methods enable the trays to be nested after use but does not allow them to be easily positioned in an oven for reheating without having to use additional means.
 Within the field of palletization, for moving finished or semi-finished products from a manufacturing location to a delivery or sales location, without the products having to undergo any transformation on the actual pallets, there exist numerous devices, all relying on the same principle and allowing the pallets to be stacked when they contain products and to be nested when they are empty. In accordance with this principle, all the pallets, or equally all the trays, have the same shape with flared walls allowing them to be stacked but having two opposite sides with complementary shapes allowing them to be stacked head to tail which leaves a space free for arranging products. For example, one may review U.S. Pat. Nos. 4,522,301 and 5,896,992.
 U.S. Pat. No. 4,522,301 discloses a process consisting of forming raw croissant dough in open cells of plastic trays where they are frozen, the plates being then stacked head to tail and packaged in a carton to be delivered for example to a baker's where the frozen raw croissants will be emptied onto the shelves of an oven to be cooked, all the trays then being turned the same way to be nested.
 For transporting pastries or bread products, U.S. Pat. No. 5,896,992 discloses flat-bottomed rectangular cases the small sides of which have complex complementary shapes allowing them to be stacked on top of each other when they are arranged head to tail and to be nested inside each other when they are all placed in the same direction.
 There still remains a need, however, for improvements in devices intended for industrial catering, and the present invention now provides such improved devices.
 The present invention thus concerns a method which, with minimum handling, allows food products, arranged in a stacked trays assembly to be both packaged and transported conveniently and hygienically from a manufacturing location, where the products are cooked or pre-cooked and possibly frozen, to a catering location, where the trays are placed in a conventional oven to reheat the products prior to distribution.
 The invention also concerns the trays themselves, the shape of which has been designed to allow them to be easily stacked to enable food products which may have the same or slightly different dimensions to be reheated uniformly, and to allow the products in the trays to be easily removed after reheating for distribution. Thereafter, the trays can be nested for compact storage.
 The method according to the invention comprises providing a packaged tray assembly of a plurality of individual trays that are adapted to accommodate a series of food items, the trays being ovenable and being arranged in at least one stack or a stacked orientation which has a predetermined compact size and shape that fits at least in at least part of a cavity of an oven at a catering location, wherein the stacked orientation also provides spaces to promote hot air circulation inside the trays for uniform heating of the food items.
 The method includes packaging the stack of trays in an outer protective package that maintains the integrity of the trays in the stack for transport to the catering location. The protective package, which may preferably be a carton or a shrink-wrapped plastic film, is removed before introducing the stack of trays into the oven for heating of the food items. For airline catering applications, the outer carton is preferred as it allows an easy palletization from the manufacturing point and eliminates handling at the caterers except the pure logistic of receiving, storing and boarding the cartons and brings the on-board handling to an absolute bare minimum.
 When the food products, which are for example small frozen snacks or pre-cooked meals, have reached the right temperature for consumption in the oven, the trays are dismantled or separated from each other to allow the products to be served in the catering location. The trays are configured such that the empty trays are capable of being nested together in a nesting orientation of reduced space after the food items have been served from the trays, so that the trays can be compactly stored after serving the food items from the tray.
 The food items are generally individual frozen hand-held snacks or meals, and the trays include apertures to assist in promoting air circulation inside the trays. Preferably, the stack of trays provides a predetermined clearance which, in association with the walls of the oven is effective to maintain an air velocity that is greater than 1 meter per second, in a convection mode, when the stack of trays is placed in the oven for heating of the food items.
 The invention also relates to the trays that are used in the method. These trays include a bottom surface for supporting the series of food items thereon and venting means to facilitate hot air circulation within the tray. The venting means may be openings provided in at least one side of the trays or by indentations formed on an upper edge of at least one side of the trays. Also, the venting means may be openings arranged in the bottom of the tray to facilitate vertical hot air circulation within the tray.
 The invention also relates to a packaged food tray assembly for convenient transporting, reheating and serving of warmed food items in a catering location. As above, the tray includes a series of food items, and a plurality of individual trays as described herein. These trays preferably include a removable outer protective package that maintains the integrity of the trays in the stacked orientation for transport to the catering location.
 The packaged food trays are stackable upon each other by means of a removable intermediate spacing member arranged to allow air circulation paths between each tray. When in the stacked configuration, the trays include air inlets adjacent the sides of the trays to assist in promoting hot air circulation inside the trays. Preferably, the base of each tray has a sloped surface that promotes display and convenient handling of the food items in the tray.