GB2112257A

GB2112257A - Shield for improved cooking of frozen foods in a microwave oven

Info

Publication number: GB2112257A
Application number: GB08232806A
Authority: GB
Inventors: Stanley H Kwis; Roland C Fulde
Original assignee: Campbell Soup Co
Current assignee: Campbell Soup Co
Priority date: 1981-11-19
Filing date: 1982-11-17
Publication date: 1983-07-13
Also published as: AU557574B2; NL8204466A; CA1202088A; AU9067382A; DE3242402C2; BE895052A; DE3242402A1; GB2112257B

Abstract

This invention provides a microwave shield which facilitates uniform heating of frozen food products. The microwave shield is formed from a material which is non- transmissive of microwaves. The shield comprises a base member 31 having an aperture, and a continuous side wall 26 which extends upwardly away from the base member. The invention also provides a container for frozen foods which includes the microwave shield with an interior liner 22 and/or an exterior layer 23 the interior liner and exterior layer substantially conforming to the shape of the shield. The invention further provides frozen food products integrated with the microwave shield of the invention, and methods of uniformly heating frozen food products by use of the microwave shield. <IMAGE>

Description

SPECIFICATION Shield for improved cooking of frozen foods in a microwave oven TECHNICAL FIELD This invention relates to microwave cooking of frozen foods. More specifically, this invention relates to a device which facilitates uniform microwave cooking of frozen foods. This device and its method of use find particular utility in the microwave oven cooking of frozen foods marketed in trays having outwardly sloped sides, such as meat pies and pastries.

BACKGROUND ART Illustrative of the problems that have occurred in heating frozen foods in a microwave oven are the problems normally encountered when cooking frozen meat pies. it has been found that heating an eight ounce frozen pie in a microwave oven normally results in boil-over of the liquids contained in the pie at the sides after about six minutes heating time, although the center of the pie still remains unacceptably cold. If the cooking time is extended to eight minutes in an effort to heat the cold center, extensive burning of the pie, especially the pie gravy, occurs at the sides while the center of the pie is still below an acceptable serving temperature. After ten minutes cooking time, weight loss of the pie due to evaporation of water can approach 48%, rendering the pie unpalatable.

One solution to this problem of non-uniform heating has been to pre-thaw the meat pot pie prior to cooking it in the microwave oven, which promotes more uniform absorption of the microwaves and uniform heating. However, the primary advantage of frozen foods to the consumer lies in the convenience in their preparation, and the need to pre-thaw a frozen food product eliminates this desirable convenience by requiring foresight or extra preparation time.

U.S. Patent No. 2,600,566 (Moffett) discloses a process for selectively dielectrically heating one segregated portion of a composite food product while preventing any heat from reaching other portions of the composite. For example, a frozen product containing separate areas of ice cream and syrup is heated in such a manner as to provide hot syrup and cold ice cream. This process is accomplished by using a container having an electrically conductive shield portion with a circular aperture centrally located in the bottom, and a conductive cover with a similar aperture located in the center. Moffett also mentions the possibility of using such a container to uniformly soften excessively frozen ice cream. The problems associated with cooking frozen foods in a microwave oven are markedly different from those encountered in softening ice cream.

British Patent No. 1 ,368,947 discloses that more uniform heating of a solid portion of frozen food may be obtained by placing a microwave-transmitting plate of specified size under a frozen food portion being heated.

The prior art generally teaches heating separate segregated portions of a composite frozen food product at different rates in a microwave oven by using a device which controls the exposure received by each portion. See, e.g., U.S. Patent No.3,219,460 (Brown), U.S. Patent No. 3,302,632 (Fichtner), U.S. Patent No.3,547,661 (Stephenson), U.S. Patent No. 3,615,713 (Stevenson), and U.S. Patent No.

3,865,301 (Pothier). While these patents disclose variable heating of segregated portions of a composite frozen food product in a microwave oven, none of these patents disclose cooking a single portion of a frozen food product by subjecting certain areas of this single portion to differential microwave heating.

The prior art demonstrates that the need continues for improved devices and methods to facilitate the uniform cooking of frozen food products in microwave ovens.

DISCLOSURE OF INVENTION Accordingly, it is the primary objective of the present invention to obviate the problems presented by prior art methods and devices for the microwave cooking of frozen foods. More specifically, it is an object of the present invention to remove the need for pre-thawing of frozen food products prior to their cooking in a microwave oven.

It is another object of the present invention to provide a microwave shielding device for use in heating frozen foods in a microwave oven, which eliminates excessively rapid heating of the peripheral portions of the product which may result in boil-over, excessive dehydration or burning.

It is yet another object of the present invention to provide packaged frozen food products which can be conveniently heated or cooked in a microwave oven at a high power setting without danger of boil-over, excessive dehydration or burning.

It is still another object of the present invention to provide convenient and time-saving methods of uniformly heating or cooking frozen food products in a microwave oven.

It is another object of the present invention to provide a packaged frozen food product capable of achieving enhanced top peripheral browning effects upon cooking in a microwave oven.

In accordance with the present invention, these and other objects which will be apparent are achieved by providing a microwave shield which facilitates uniform heating of frozen food products. The microwave shield of this invention prevents the passage of microwaves through the container into shielded portions of the food product. The shield may either absorb or reflect incident microwave energy. In the case of microwave absorbing shield materials, desirable, slow and uniform heating by conduction may be achieved. In a preferred form this microwave shield comprises a base member having a central aperture and a continuous side wall which extends upwardly away from the base member. The top of the shield is open to penetration of microwaves. A frozen food product may be inserted in the microwave shield and heated by exposure to microwaves.The portions of the frozen food product which are adjacent to the open top and to the central aperture are exposed to microwave energy while the shielded portions are not. This selectively-increased exposure of the central portion of the frozen food product to microwave energy results in more uniform heating of the frozen food product.

In one embodiment, the continuous side wall extends away from the base member at an obtuse angle, forming a pie-dish shape, in which the side wall prevents premature overheating of the peripheral portion of the frozen food product adjacent to the side wall.

The invention additionally provides a container for frozen foods which includes the above described microwave shield and an interior liner generally conforming to the ultimate package shape, which is transparent to microwaves. The liner normally does not have a central aperture in the base member. The side wall of the liner may optionally extend further away from the base member of the liner than does the side wall of the microwave shield. In this manner, the liner supports the peripheral edge portions of the frozen food product, and microwaves can be transmitted through the portion of the side wall which is not coextensive with the microwave shield. Microwave heating of only a portion of the peripheral edge of the food product in such a configuration may in some cases result in more rapid overall heating of the product, while still avoiding boil-over or burning.

According to yet another embodiment, the frozen food container may include an exterior layer of microwave transmissive material into which the microwave shield is inserted to form a decorative assembly. The interior liner and exterior layer aspects of the invention may be employed either alone or together, and these elements may be formed of a durable material such as plastic or glass so that a reusable microwave shield is produced.

According to a further embodiment, the invention provides frozen food products integrated with the microwave shields of the invention, which may be uniformly heated by microwaves in a microwave oven.

In still another embodiment, the invention provides methods for uniformly heating frozen food products comprising the steps of inserting a frozen food product in a container comprising the microwave shield of the invention, followed by subjecting the shielded frozen food product to microwave energy in a microwave oven.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side view of a frozen pot pie body supported by a conventional microwavetransparent pie tray.

Figure 2 is a side view of a preferred microwave shield of the invention having a central aperture.

Figure 3 is a side view of a preferred microwave shield of the invention having a central aperture, together with a liner and exterior layer, in which the side walls of the interior liner and exterior layer extend farther from the bottom of the assembled microwave shield than does the side wail of the microwave shield itself.

BEST MODE OF CARRYING OUT THE INVENTION According to this invention, uniform heating of a frozen food product in a microwave oven may readily be obtained. This invention broadly may be used in connection with the heating or cooking of any frozen food product provided as a solid, frozen portion and is particularly applicable to heating and cooking frozen meat pies, fruit pies and pastry shells with an edible filling. The invention will be described particularly in connection with its use for heating or cooking frozen food products provided in pie-shaped portions. However, it is to be understood that the invention has broad applicability to heating and cooking of any frozen food product provided in a container having a bottom and a continuous side wall.

When a frozen pie supported by a microwave-transmissive container (such as a paper tray) is heated in a microwave oven, two interdependent problems result in uneven heating and cooking of the pie. The first problem can be seen from an examination of Figure 1. Figure 1 is a schematic side view of a frozen pie (1) contained in a conventional microwave-transparent pie tray (2). Microwaves from a microwave oven (not shown) impinging on the pie surface (3) encounter a decreased path length through the pie in the peripheral portion (4) of the pie. Microwaves impinging on the side wall of the pie tray (5) also penetrate into the peripheral portion (4) of the pie, while penetrating into the main portion (6) of the pie to a lesser degree. The result is that more microwaves per unit volume penetrate into the peripheral portion (4) of the pie than in the main portion (6) of the pie and the peripheral portion begins to thaw before the main portion.

The second problem relates to the absorption of microwaves which induces the desired heating of the pie. Water which is present in the pie is polar and thus a good absorber of microwaves. Ice, however, does not absorb microwaves but merely transmits and reflects them. When in a frozen state, the pie initially contains ice, but no water. Areas of the pie which are first to thaw, yielding water, will thus be the first areas to be cooked. Unthawed areas, on the other hand, remain uncooked until water in them develops to absorb microwaves.

Since the peripheral portions of the pie initially receive the greatest amount of microwaves per unit volume (see Figure 1) these portions are the first to be thawed. As the microwaves thaw the peripheral portions, water is produced and the amount of microwaves absorbed by the peripheral portion progressively increases. The rapid heating of the peripheral portion of the pie due to the microwave absorbing capabilities of the water, taken in combination with the persistence of ice in the main portion of the pie, results in a progressively increasing imbalance in temperature from the peripheral portion to the main portion of the pie. In the case of an eight ounce frozen pie, the temperature of the peripheral portion of the pie may be as much as 500F greater than the temperature of the center of the pie after six minutes of heating in a microwave oven.The weight loss in the pie due to water evaporation after six minutes may be about 21 percent. After eight minutes of heating in a microwave oven, the weight loss may be as great as about 36 percent, accompanied by charring of the pie gravy in the peripheral area of the pie.

According to the present invention, a special microwave shield is provided which obviates these problems. The microwave shield is made of a material which is non-transmissive of microwaves and which further can be either reflective or absorbtive of microwaves.

Figure 2 illustrates a microwave shield of the invention. The shield comprises a base member (11) having a continuous side wall (12) which extends upwardly away from the base member (11) for a distance (A). The base member (11) of the shield is provided with a centrally-located aperture (13).

Normally, the shield is provided with a lip (14). The top of the shield, into which a frozen pie may be inserted, is completely open, allowing unrestricted exposure to microwave energy. The microwave shield facilitates uniform microwave heating of a frozen pie, and prevents boil-over, burning and dehydration of the pie during the heating or cooking process.

In a microwave oven, reflection of microwaves from the walls of the microwave oven chamber results in a stabilized field of microwaves which propagate in all directions. Hence, if a pie is placed in a microwave oven, suitably spaced away from the bottom surface of the oven to facilitate reflection of microwaves from the bottom, the pie will be exposed to microwaves from all directions. By employing the microwave shield of the invention to heat or cook a pie, this exposure is substantially modified.

Microwave impinging on the top surface of the pie in the shield may penetrate into the pie without any attenuation. However, penetration of microwaves into the pie from the side wall and bottom is substantially reduced. The side wall entirely prevents passage of microwaves impinging on the exterior of the side wall into the pie. Absorption of microwaves into the pie through the bottom of the microwave shield is eliminated except for microwaves passing through the central aperture.

Elimination of absorption of microwaves into the pie from the exterior of the side wall by the shield reduces the amount of microwaves absorbed by the peripheral portion of the pie and thus reduces the rate of heating of the peripheral portion. The controlled absorption of microwaves into the pie through the bottom of the shield, as limited by the aperture in the bottom of the shield, maximizes absorption of microwaves in the center of the main portion of the pie, while reducing absorption of microwaves into the remainder of the main portion of the pie.

In operation, microwaves penetrate the center of the main portion of the pie from both the top and bottom, causing the center portion to thaw and become heated. Heat from the center of the main portion of the pie accumulates and begins to be conducted from the center towards the periphery of the pie. The controlled absorption of microwaves into the pie, as induced by the microwave shield, results in uniform heating of the pie without adverse effects such as boil-over, burning or excessive dehydration.

The microwave shield is generally made of any material which is both reflective and nontransmissive of microwaves. Excessive absorbance of microwaves by the shield is desirably avoided to prevent heating of the shield to the extent which may result in burning of the exterior surfaces of a frozen food product. Limited microwave absorbance, however, is not harmful and may provide beneficial conductive heating of the shielded food product. Materials which may be used to produce the shield include metals such as aluminum, copper, iron, magnesium, manganese, steel, stainless steel, tin, and alloys. The preferred metal is aluminum in view of its widespread use in making containers for food products, and its economical availability.

The microwave shield may have a base member and side wall of any desired thickness. The shield base member and side wall are preferably of an equal thickness, and should generally be thick enough to provide adequate mechanical strength to support a frozen food product without distortion or breaking of the shield, while not being so thick as to become excessively heavy or uneconomical to produce.

Where the shield is fabricated from one or more of the above-mentioned metals, the bottom and side wall should generally have a thickness of at least about 9 microns. Preferably, the thickness may range from about 9 to 75 microns, and most preferably from 10 to 25 microns. For technical considerations there is no upper limit to the thickness of the shield. Economic considerations, however, will dictate the thickness of the shield.

The shape of the microwave shield can vary widely depending on the nature of the frozen product being heated. Where the frozen food product is a pie, the bottom of the shield generally is circular.

However, microwave shields having oval, square, rectangular, triangular, or other geometrically shaped bottoms may also be employed.

The overall size of the microwave shield, which generally varies in direct proportion with the size of the bottom of the shield, may be determined generally according to the desired size of the frozen food product to be contained by the microwave shield or to be sold as an integral part of a product.

The aperture in the bottom of the microwave shield is normally placed near or at the center of the bottom of the shield, so that conduction of heat toward the peripheral regions of the frozen food product will occur in a substantially uniform fashion. The invention is generally described in connection with central apertures, but other aperture placements may be desirable in particular cases. The shape of the aperture is not critical, but circular or oval apertures will result in the most uniform conduction of heat toward the periphery of the frozen food product and thus are preferred.

The size of the central aperture should generally be correlated with the shape and volume of the frozen food product to be inserted into the shield. The size of the aperture should be determined with a view towards balancing the need for efficient heating time (larger aperture) with the need to avoid overheating peripheral areas of the frozen food product (smaller aperture). In the case of an eight ounce round pie such as a meat pie, having a bottom with a diameter of 80 mm, the central aperture preferably has a diameter of about 20 mm. In the case of a ten ounce round pie, having a bottom with a diameter of 95 mm, the central aperture preferably has a diameter of 60 mm. Frozen food products will of course be matched with shields substantially conforming to their shape. As the size of the bottom of a frozen food portion increases, the size of the central aperture should be increased.Similarly, as the thickness of a frozen food portion (in a direction perpendicular to the plane of the bottom) increases, the size of the central aperture should be increased. These factors may be generally correlated by those skilled in the art, and appropriate aperture sizes may further be determined by experimentation.

The distance A (see Fig. 2) to which the side wall extends upward from the base member of the microwave shield, as well as the angle which the side wall makes with the base member are both determined generally in accordance with the desired shape of the frozen food product to be contained or shielded. In the case of a meat pie, the side wall normally makes an obtuse angle of about 1 200 with the base member of the microwave shield. It has been found, however, that where the side wall makes approximately a 900 angle with the base member of the shield, boil-over, burning and overheating of the peripheral portions of a frozen food product may not be significant problems.The microwave shield of the invention nevertheless is useful in heating and cooking such products, because the center portion still tends to be heated more slowly than the periphery. Also, a side wall making a 900 angle with the bottom of a microwave shield for heating and cooking a frozen food product may in some cases be undesirable. For example, there is an accepted aesthetic value to having a side wall making an obtuse angle with the bottom of the frozen food product, particularly where the frozen food product is a pie such as a meat pot pie. Additionally, due to the sharp corners created by a 90angle, the consumer may experience difficulty in removing the peripheral portions of such a product.

In conformance with the usual shape of trays used for heating and serving frozen food products, the microwave shield may optionally incorporate a lip extending from the opening formed by the side wall, the lip normally constituting a continuous ring which is substantially parallel to the base member of the microwave shield.

Optionally, the central aperture in the microwave shield may be supplemented by adding a plurality of peripheral holes or openings in the base member of the shield. Such peripheral holes provide added but limited access of microwaves to the bottom of the frozen food products inserted in the shield, which microwaves proceed upward through the shield and aid in conduction of heat toward the periphery of the frozen food product. The size, positioning and number of such peripheral holes should be calculated to avoid overheating or burning of the periphery of the frozen food product. These peripheral holes are normally evenly spaced around the periphery of the central aperture so as to equally distribute the resultant heat towards the periphery of the frozen food product.

The microwave shield of the invention may generally be produced using conventional technology for the production of frozen food trays. Where the shield is made of aluminum, for example, the known means for producing disposable frozen pie tins may be used. The microwave shield is preferably stamped out of a single sheet of metal so that the base member and side wall together are part of a continuous piece of metal. The central aperture may be produced in the metal prior to shaping the side walls or may be stamped out of a pre-formed pie tin, as will be apparent to those skilled in the art.

According to a preferred embodiment of the invention, the microwave shield will include a microwave transparent interior liner. The liner is shaped so that it closely fits the interior of the microwave shield. The liner normally does not have a central aperture. The liner may generally be made of any microwave-transparent material. Commonly, the liner is made of a pulp product, such as paper or cardboard. The liner material should be selected so that no undesirable taste or odor is imparted to frozen food products which come in contact with it. Normally, the liner will be adhesively bonded to the microwave shield; however, such permanent attachment is optional and other attachment means, as will occur to those skilled in the art, may alternatively be used.

Optionally, when the liner and microwve shield are joined together, the liner side wall may extend farther away from the base member of the shield than does the side wall of the shield itself. If the frozen food product which is inserted in the liner substantially fills the liner, then microwaves can penetrate into a portion of the periphery of the frozen food product through the part of the liner which extends beyond the side wall of the shield itself. In this manner, partial heating of the periphery of a frozen food product by direct impingement of microwaves may be effected. This may be desirable where browning effects are desired at the top of the periphery of the frozen food product, such as in a frozen pie having a crust.

In yet another embodiment, an exterior layer is provided into which the microwave shield can be inserted. Normally, the exterior layer has substantially the same shape and size as the interior liner. The exterior layer may be made from any material which is transparent to microwaves, and is conveniently made of a pulp product such as paper or cardboard. In this manner, decoration of the visible exterior of the microwave shielded container may readily be achieved. In addition to the decorative function, an exterior layer is also useful to protect the microwave shield itself from mechanical damage, e.g., in the case of a microwave shield which is a thin sheet of aluminum foil. An exterior layer may be used in combination with a liner, as discussed above, or may be employed separately.If the liner extends upward beyond the side wall of the microwave shield, the exterior layer preferably should extend upward for the same distance.

Such a microwave shield is illustrated in Figure 3. The shield (21) is sandwiched between an interior liner (22) and an exterior layer (23). The side wall (24) of the liner, and the side wall (25) of the exterior layer, both extend farther away from the base member of the interior liner in the assembled laminate microwave shield than does the side wall (26) of the microwave shield itself. In one particular embodiment, the side wall (24) and the side wall (25) extend about twice as far away from the base member of the interior liner as does the side wall (26) of the microwave shield.

In still another embodiment, the microwave shield comprises a liner and an exterior layer which are both made of a durable material which is transparent to microwaves, such as glass or plastic. In this event, the microwave shield can serve as a reusable device constituting a permanent microwave oven accessory. The construction of such a durable laminate may be carried out using any of the well known fabrication methods employed in this art.

Durable laminates may also be produced by forming a plastic, glass or similar container having a layer of microwave shielding material integrally formed therein, for example, by fusing a glass or plastic material around a shield layer or by forming a suspension of shielding particles in a desired pattern in a fluid precursor and setting this suspension-containing precursor to form a unitary article containing a layer of embedded shielding particles.

In another embodiment of this invention, the microwave shield-containing container may be fabricated by providing a microwave non-transmissive coating on either the interior liner or exterior layer. This coating layer is continuous in the desired shielding areas and absent in areas where transmission is desired, e.g., the aperture or top of the side walls. Any of the well-known coating methods for providing discontinuous coatings can be employed.

A detachable seal or top cover which is appropriately sized to fit over the opening formed by the side wall of the microwave shield is optionally provided. This top cover is made of a material which is transparent to microwaves, so that the pattern of impingement of microwaves on a frozen food product inserted in the shield is not altered by the cover. Such a top cover may be used to substantially isolate the interior of the container formed by the microwave shield. This results in accelerated heating inside the microwave shielded container. Additionally, where a product having a crust is to be heated and/or cooked, the use of a top cover may improve browning of the crust.

In the case of a thin, disposable microwave shield optionally also comprising a liner and, or an exterior layer made of inexpensive materials such as paper or cardboard, the top layer is preferably also made of an inexpensive material, such as paper or cardboard. In the case of a durable microwave shield comprising, e.g., a liner and exterior layer made of glass or plastic, the top cover is preferably also made of a durable material.

In use, a frozen food product is placed within a microwave shield substantially conforming in shape and size to the frozen food product, and the frozen food product is heated and/or cooked in a microwave oven for a desired period of time at a desired microwave power setting. Frozen food products may be supplied in microwave shields of a suitable size and shape by producers of frozen food products, or frozen food products may be inserted in microwave shields of suitable size and shape immediately prior to use, by the consumer.

Heating and cooking times and microwave power settings are primarily a matter of choice, depending on the article being heated or cooked, and can be determined readily by the microwave oven user. In the case of eight ounce frozen pies, it has been found that a microwave cooking time of about six minutes using a high energy setting on a microwave oven will generally be adequate.

EXAMPLE 1 In this example, ten ounce pies were cooked using microwave shields in accordance with the present invention. Ten ounce chicken pot pies were produced by combining 48 grams of cooked chicken meat with 182 grams of cooked pie garnish (vegetables in a gravy base) and mixing until the meat was uniformly distributed. The pie fillings were placed in microwave shields and top dough layers were added. The chicken pot pies were then frozen to a uniform temperature of about --100 F.

Three types of microwave shields were tested. All three types of microwave shields were shaped generally as pie tins, i.e., they were round, and comprised a base member, a continuous side wall extending upward from the base member, and a lip extending from the terminus of the side wall at the top. The side wall/base member angle was 1310. In one series of tests, the microwave shield had a circular central aperture in the base member of 40 mm in diameter. In another series of tests, the shield had a circular central aperture in the base member of 60 mm in diameter. In a third series of tests, the shield had no aperture in the bottom. A series of control tests employed conventional microwavetransmissive pie plates (i.e., no shielding).

The ten ounce chicken pies were baked for eight minutes in a Litton microwave oven (model D1250, rated at 700 watts). The percent weight loss of each pie was then measured by comparing the pre-cooked weight to the weight after cooking. Temperature variations in the pies were then measured by probing each pie simultaneously in four locations, including the center bottom, center top, side bottom and side top. A thermocouple (type T) needle probe was used to make the temperature measurements, which were recorded 20 seconds after penetration of the thermocouple.

The tests were conducted individually with each pie being positioned in the direct center of the microwave oven chamber. The above procedures were then repeated except that the cooking time was extended to a total of ten minutes.

In all test cases, microwave cooked pies were organoleptically evaluated and physically tested against identical pies that were cooked in a conventional oven at 4250F for 45 minutes. The pie evaluations included the following: average serving temperature, weight loss, dough crusting and browning, boilout during cooking, burnt areas and temperature uniformity.

The preferred characteristics for a microwave cooked pie are: 1. Average serving temperature between 1600--1750F. Most preferred serving temperature 1650--1700F.

2. Difference between any temperature location should not exceed 250F.

3. Minimal weight loss.

4. No boilout during cooking.

5. No burning.

6. Uniform cooked gravy appearance.

7. Flaky, golden brown crust.

The results from the weight loss tests for ten ounce pies are shown in Table 1. The results for the temperature variation tests for ten ounce pies are shown in Table 2.

TABLE 1 Weight Loss in Ten Ounce Pies Percent Weight Loss Shielding 8 Minute Bake 10 Minute Bake None 20 31 Full 6 11 60 mm 6 15 40 mm 6 14 TABLE 2 Temperature Gradient in Ten Ounce Pies Temperature Baking Probe Time Temperature OF Location (Minutes) No Shield Full Shield 40 mm Aperture 60 mm Aperture Center Bottom 8 111 54 160 163 Center Top 8 108 140 190 180 Side Bottom 8 A 81 120 147 Side Top 8 B 152 159 168 Center Bottom 10 188 56 188 190 Center top 10 182 158 197 200 Side Bottom 10 B 100 160 180 Side top 10 B 157 167 195 A - Boil out B - Burnt Table 1 shows that weight loss after baking ten ounce pies for eight or ten minutes was substantially reduced where a microwave shield was used.

The results in Table 2 show that much more uniform heating was obtained using the microwave shield with a 40 mm or 60 mm aperture than was obtained using the full shield or no shield.

Overall test results demonstrated that a 60 mm aperture-shield gave the most preferred characteristics.

EXAMPLE 2 In this example, eight ounce pies were cooked using microwave shields in accordance with the present invention. Eight ounce chicken pies were produced by adding 38 grams of cooked chicken to 143 grams of cooked pie garnish (vegetables in a gravy base) and the meat and garnish were mixed until the meat portion was uniformly distributed. The pie fillings were placed in microwave shields and top dough layers were added. The chicken pies were then frozen to a uniform temperature of about -100F.

Eleven types of microwave shields were tested. All eleven types of microwave shields were shaped generally as pie tins, i.e., they were round, and comprised a base member, a continuous side wall extending upward from the base member, and a lip extending from the terminus of the side wall at the top. The side wall/base member angle was 124 . In one series of tests, the microwave shield had a circular central aperture in the base member of 12 mm in diameter. In additional series of tests, the shield had circular central apertures in the base member of 14 mum, 16 mm, 18 mm, 20 mm, 22 mm, 26 mm, 30 mm, 40 mm, and 50 mm, respectively. In yet another series of tests, the shield had no aperture in the bottom. A series of control tests employed conventional microwave-transmissive pie plates (i.e., no shield).

The eight ounce chicken pies were baked for six minutes in a Litton microwave oven (model Do 250). The percent weight loss of each pie was then measured by comparing the pre-cooked weight of the weight after cooking. Temperature variations in the pies were then measured by probing each pie simultaneously in four locations, including the center bottom, center top, side bottom and side top. A thermocouple needle probe was used to make the temperature measurements, which were recorded 20 seconds after penetration of the thermocouple.

The tests were conducted individually with each pie being positioned in the direct center of the microwave oven chamber. The above procedures were then repeated in two additional sets of tests, with the total cooking time extended to eight minutes and ten minutes, respectively.

As in Example 1 the pies were organoleptically evaluated and physically tested against identical pies that were cooked in a conventional oven at 4250F for 45 minutes.

The results for the weight loss tests for eight ounce pies are shown in Table 3. The results for the temperature variation tests for eight ounce pies are shown in Table 4.

TABLE 3 Weight Loss in Eight Ounce Pies Percent weight Loss Shielding 8 Minute Bake 10 Minute Bake None 35 47 Full 6 15 12 mm 8 15 14mm 9 15 16 mm 10 15 18 mm 9 18 20 mm 9 15 22 mm 9 18 26 mm 9 15 30mm 9 15 40 mm 10 18 50 mm 13 20 TABLE 4 Temperature Gradient in Eight Ounce Pies Temperature Baking Temperature OF Probe Time No Full Shield Aperture Diameter in Millimeters Location (Minutes) Shield Shield 12 14 16 18 20 22 26 30 40 50 Center Bottom 6 111 55 52 52 84 97 170 140 107 142 183 192 Center Top 6 103 100 115 89 110 131 157 148 158 153 177 180 Side Bottom 6 A 60 60 60 60 87 97 62 55 86 148 170 SideTop 6 A 120 116 91 123 128 144 135 88 116 162 130 Center Bottom 8 181 76 90 76 153 102 179 180 188 188 185 194 CenterTop 8 150 148 158 144 168 144 175 170 181 185 180 182 Side Bottom 8 A 100 97 81 128 92 130 130 142 130 174 175 Side Top 8 B 146 141 140 148 130 158 148 148 147 172 174 Center Bottom 10 184 90 100 100 170 143 181 182 189 B B B CenterTop 10 165 151 160 145 171 168 176 171 183 186 185 177 Side Bottom 10 B 143 143 140 163 144 162 153 172 B B B SideTop 10 B 160 160 154 168 163 170 165 176 B B B A - Boil out B - Burnt Table 3 shows that the percent weight loss in eight ounce pies after baking for eight or ten minutes using either a full shield or a shield according to the invention, was significantly lower than the weight loss encountered in the trials where no microwave shield was employed.

Table 4 shows that shielding pies according to the present invention significantly improves the uniformity of the microwave cooking process.

The test results generally indicated that a shield of at least 16 mm and not exceeding 26 mm in diameter was necessary to result in a 10 minute microwave cooked 8 oz. pie with the preferred characteristics. A 20 mm shield resulted in the most preferred characteristics. However, if the cooking time was shortened to 8 minutes with the same shield, the pie would have marginally acceptable characteristics.

While the preferred embodiments of this invention have been described above and illustrated by the examples, modifications in the above may be readily made by those skilled in the art, and the invention is limited only by the scope of the appended claims.

Claims

1. A microwave shield for promoting uniform heating of a frozen food product, the shield comprising a material which is both non-transmissive of microwaves, said shield comprising a first base member having at least one aperture therein and a first continuous side wall which extends upwardly away from said first base member, said shield having an open top, and said shield constituting a container in which a frozen food product may be exposed to microwave energy.

2. The microwave shield of claim 1 additionally comprising a microwave-transparent interior liner having the same general configuration as said shield, said liner comprising a second base member and a second continuous side wall which extends upwardly away from said second base member.

3. The microwave shield of claim 1 additionally comprising a microwave-transparent exterior layer having the same general configuration as said shield, said exterior layer comprising a third base member and a third continuous side wall which extends upwardly away from said third base member.

4. The microwave shield of claim 2 additionally comprising a microwave-transparent exterior layer having the same general configuration as said shield, said exterior layer comprising a third base member and a third continuous side wall which extends upwardly away from said third base member,

5. The microwave shield of claim 4 wherein said interior liner and said exterior layer do not have an aperture corresponding to the aperture in said first base member of said shield.

6. The microwave shield of claim 2 in which the second continuous side wall extends away from the second base member for a distance which is greater than the distance to which the first continuous side wall extends away from the first base member.

7. The microwave shield of claim 3 or 4 in which the third continuous side wall extends away from the third base member for a distance which is greater than the distance to which the first continuous side wall extends away from the first base member.

8. The microwave shield of claim 1 in which the first continuous side wall extends upwardly away from the first base member at an obtuse angle.

9. The microwave shield of claim 1 in which the first base member and first continuous side wall are formed from a metal selected from the group consisting of aluminum, copper, magnesium, manganese, stainless steel, steel, tin and alloys.

10. The microwave shield of claim 1 in which the first base member and first continuous side wall are formed from a single sheet of metal.

11. The microwave shield of claim 1 in which the first base member has a shape selected from the group consisting of oval, circular, square and rectangular.

1 2. The microwave shield of claim 1 in which the aperture is located at the center of the first base member.

1 3. The microwave shield of claim 11 wherein said first base member is oval in shape and said aperture is oval in shape.

1 4. The microwave shield of claim 1 or 12 in which the aperture is circular.

1 5. The microwave shield of claim 14 in which the first base member is circular and has a diameter of about 80 mm, and the aperture has a diameter of about 20 mm.

1 6. The microwave shield of claim 14 in which the first base member is circular and has a diameter of about 95 mm, and the aperture has a diameter of about 60 mm.

1 7. The microwave shield of claim 12 in which the first base member additionally comprises a plurality of peripheral holes spaced symmetrically around the aperture.

1 8. The microwave shield of claim 2 in which the liner is made of a pulp product selected from the group consisting of paper and cardboard.

1 9. The microwave shield of claim 2 in which the liner is made of a durable material selected from the group consisting of plastic and glass.

20. The microwave shield of claim 3 or 4 in which the exterior layer is made of a pulp product selected from the group consisting of paper and cardboard.

21. The microwave shield of claim 3 or 4 in which the exterior layer is made of a durable material selected from the group consisting of plastic and glass.

22. The microwave shield of claim 2 wherein said first base member and said first continuous side wall comprise a coating on said interior liner.

23. The microwave shield of claim 3 wherein said first base member and said first continuous said wall comprise a coating on said exterior layer.

24. The microwave shield of claim 1 integrally formed within a container having the same general configuration as said shield.

25. The microwave shield of claim 1 containing a frozen food product.

26. The microwave shield of claim 25 in which the frozen food product is a meat pie.

27. The microwave shield of claim 26 wherein said meat pie is a chicken pie.

28. The microwave shield of claim 25 wherein said frozen food product is a fruit pie.

29. A method for uniformly heating a frozen food product comprising the steps of providing a frozen food product in the microwave shield of claim 2, 3 or 4 and subjecting the partially microwaveshielded frozen food product to microwave energy to uniformly heat the product.

30. A food product package in which frozen foods may be conveniently heated or cooked in a microwave oven at a high power setting without boil-over, excessive dehydration or burning, said package comprising the microwave shield of claims 2, 3 or 4.

31. A microwave shield, substantially as hereinbefore described with reference to and as shown in Fig. 2 or 3 of the accompanying drawing.

32. A method of uniformly heating a frozen food product, substantially as hereinbefore described with reference to the Examples.

33. A food product package comprising a microwave shield in accordance with Claim 1 and substantially as hereinbefore described with reference to Fig. 2 or 3 of the accompanying drawings.