CA2185179C

CA2185179C - Pastry construction for pie casing

Info

Publication number: CA2185179C
Application number: CA002185179A
Authority: CA
Inventors: Neilson Zeng; Richard Mackay Keefer; Emily Bright
Original assignee: Graphic Packaging Corp
Current assignee: Graphic Packaging International LLC
Priority date: 1995-11-28
Filing date: 1996-09-10
Publication date: 2007-04-24
Anticipated expiration: 2016-09-10
Also published as: GB9524318D0; CA2185179A1

Abstract

A microwavable container includes a base, at least one upstanding side wall about the periphery of the base and a microwave energy heating element within the container. A food product including a pastry casing and a filling within the casing conforms to the base and at least a portion of the at least one side wall.
Selected portions of the casing have a thickness to provide a reflection co-efficient at the interface of the container and casing to maintain the electric field strength at the microwave energy heating element above that necessary to effect crisping and browning of the casing.

Description

The present invention relates to packages for food products and in particular to food products and microwavable containers and to a method of distributing incident microwave energy into a food product having a pastry casing and a filling.
The microwave oven is well recognized as a convenient manner of preparing foods. In the preparation of food products with a microwave oven it is preferable for the cooked food product to have an appearance similar to that found with a conventional oven. A further requirement is to ensure that the food product is uniformly cooked once the food product has the appearance of a conventional food product which includes the crisping and browning of the surface of the food product normally associated with a conventional oven.
Accordingly various proposals have been made to ensure uniform distribution of energy and to achieve a browning and crisping effect. It is however still found that uniform cooking of a food product is difficult to obtain with particular food products.
It is often found that portions of the food product are cooked but others are relatively uncooked which naturally leads to dissatisfaction on the part of the consumer. If the cooking time is increased, then not only is the food product less convenient for the consumer but parts of the food product may then become overcooked leading to a deterioration in the perceived quality of the food product.
A particular problem arises with pastry which may be used to encase a food product, such as for example, a pot pie. The pastry is normally stored in a frozen state within a microwavable container and is then introduced into the microwave oven for thawing and cooking. The pastry will conventionally be contained within a dish having a base and upstanding side walls and it is frequently found that portions of the pastry overlying the base remain uncooked whilst other portions are fully cooked. Intuitively, this indicates the need for a reduction in t'1 L~ ~ ~' r;~ J !

-2-thickness of the pastry but it is found that this does not lead to the desired results.
It is therefore an object of the present invention to obviate or mitigate the above disadvantages.
According to one aspect of the present invention there is provided a food product and a microwavable container, said microwavable container having a base, at least one upstanding side wall about the periphery of said base and a microwave energy heating element within said microwavable container, said food product including a pastry casing overlying and conforming to said base and at least a portion of said side walls, and a filling within said casing, selected portions of said casing having a thickness to provide a reflection coefficient at the interface of said container and casing to maintain the electric field strength at said microwave energy heating element above that necessary to effect crisping and browning of said casing.
Typically, the thickness should be of at least 2.5 mm and preferably not more than 7 mm although improved performance may be observed with a thickness up to 15 mm. Preferably the thickness of the pastry casing is not less than

3 mm and not more than 5 mm.
According to another aspect of the present invention there is provided a method of distributing incident microwave energy into a food product having a pastry casing and a filling to effect heating thereof, said method comprising the steps of:
locating said food product in a microwavable container having a base, at least one upstanding side wall about the periphery of said base, and a microwave energy heating element within said container, said food product conforming to said container; and selecting a thickness of said casing at predetermined locations to provide a reflection coefficient at an interface between said casing and container to maintain the electric field strength at said microwave energy heating element above that necessary to effect crisping and browning of said casing.

It has been found surprisingly that by increasing the thickness of the casing, the power absorption is increased and reflected power ratio is decreased.
The effect of this is to increase the electric field strength at the susceptor and thereby promote crisping and browning. In practice, by maintaining the thickness of the casing in excess of 2.5 mm a substantially uniform cooking is achieved and crisping and browning is enhanced.
An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which:
Figure la is a section through a pot pie within a microwavable dish;
Figure lb is a side elevational view of the microwavable dish within an outer carton;
Figure 2 is an enlarged section of a portion of Figure l;
Figure 3 is a section similar to Figure 2 showing an analysis of wave propagation in the pot pie and dish of Figure 2;
Figure 4 is a curve showing the relationship between pastry casing thickness and dielectric constant of the filling;
Figure 5 is a schematic representation of a test apparatus; and Figure 6 is a schematic representation of a further test apparatus.
Refernng therefore to Figures la, lb and 2, a food product in the form of a pie 12 is shown and is located within a microwavable container 10 including a dish 14. The dish 14 has a base 16 and upstanding side walls 18 about the periphery of the base that terminate in a peripheral flange 20. The dish includes a substrate 22 formed from a microwave transparent material such as for example, polymeric film, paper or paperboard. A microwave energy heating element 23 is within the dish 14. The microwave energy heating element includes a pattern 24 of microwave energy interactive material overlying the substrate 22. The microwave energy interactive material may be electroconductive or semiconductive

-4-material such as metal foil, vacuum deposited metal or metallic ink. The electnxonductive material is preferably aluminum although other metals such as copper may be employed, In addition, the eleCtroconductive material may be replaced with a suitable efectroconductive, semiconductive or non-conductive artificial dielectric or ferroelectri~ Artificial dielectrics comprise conductive subdivided material in a polymeric ar other suitable matrix or binder and may include flakes of electroconductive metal such as aluminum. in the present embodiment, the layer 24 is formed of a thin metaliic film.
The pattern 24 is covered by a plastic film 28.
The dish 14 is positioned within a generally rectangular outer carton 112.
The outer carton 192 is foided from a paperboard blank and has top and bottom major panels 120 interconnected by side panels 124. Sine flaps 126 extend about the edges of the major panels 120. The side claps 126 can be folded to seat the carton 112. The exact details of the carton and paperboard blank wilt vary according to food product dimensions and characteristics of the carton are provided for illustrative purposes only.
IS The tap major panel 120 of.outer Qrton 112 supports am active microwave energy heating element 128. The actnre microwave energy heating element is bonded or adhered to the inwardly directed face of the top panel 120 so that the active microwave energy heating element 128 overlies the dish 14 when the dish is inserted into the outer carkon 112. The microwave energy heating element 128 also includes a pattern of 2o microwave energy interactive material disposQd on a su~strate._ AlkernatiuRty, thP.mirSrQwave. .... .. ..._.._....__-_ _ energy interactive material may be in the farm of a patterned susceptor including one or mare layers of suscepting material.
The pie 12 has a pastry casing 28 that conforms to the base 16 and side walls 18 of the dish 14. The casing 28 contains a filling 30 and has a iid 32 to enclose the filling 2s 30. A small air gap 33 typically between 0.5 mm and 1 mm may exist between the filling 30 and the lid 32. The casing 28 includes walls 34 that abut the aide walls 18 of tPle dish 14 and merge smoothly with a base 3f3_ The base 3fi has a thickness d as shown in Figure 2 which is at least 2.5 mm thick. The wall 34 may have a thickness less than 2.5 mm.
It has been found surprisingly that by McCarthy 1"~~rault LLP TDp-RED it83t79889 v. 1 ~i85~7R

-5-having a base thickness of at least 2.5 mm, a uniform cooking of the base 36 is obtained with effective browning and crisping of the pastry casing 28 by the microwave energy heating element 23.
An analysis of the interface between the pie 12 and microwave container 10, as illustrated in Figure 3, shows that the voltage reflection coefficient r (which when squared is the ratio of power transmitted to the total incident power) is given by:
rrorpt - ri + a Wd 'r~1 _(r~ )Z~ . 1 1 + r.r~-.e -i~
where o- _ ZZ - Zi _ JE i - JEZ .
Z2 + Zl JE 1 + JE2 _ Z3 - Z2 - JE2 - JE3 Z3 + ZZ JE2 + JE3 and izn~E . a a Wa -where:
each of the superscripts and subscripts refers to the location identified on Figure 3;
7lo is the wavelength of the incident microwave radiation in air; and en is the dielectric constant of medium n.
Thus, the reflective coefficient for the field incident at the microwave energy heating element pastry interface is a function of the relative dielectric constants E1, E2, E3 and the thickness of the pastry d.
In order to maintain the field strength at the microwave energy heating element 23 sufficient to achieve browning and crisping, a power absorption

-6-in the order of 50 % or -3db is required. A lower power absorption (i.e. a higher reflection at the pastry surface) means a reduced electric field intensity at the microwave energy heating element. Similarly, in order to achieve maximum power absorption (and hence surface field strength), a pastry thickness of ~,/4 is required, typically ' 15 mm for most pastries at normal domestic microwave oven operation of 2450 MHz.
In tests conducted on commercial food products, it is found that:
E1 = 5-j0.1 for a glass tray;
EZ = 6-j0.5 for pastry; and E3 = 70-j7 for a chicken arid vegetable filling.
In order to adjust the reflection coefficient for a 50% power absorption, the pastry thickness, d, may be adjusted and for the parameters listed above is found to be about 3.5 mm.
Accordingly, a pastry thickness of about 3.5 mm provides browning and crisping of the base of the casing together with complete cooking of the food product.
A similar analysis for the lid 32 of the casing provides:
E~ = 1 JO (~r)~
E2 = 4.8 j0.35 (pastry + air); and E3 = 70 j7 (filling).
Accordingly, the optimum pastry thickness is between about 3 mm and 6 mm. It should be noted that the microwave energy heating element 128 at the lid will typically be spaced away from the pastry lid. The phase differences between these components also influences the net field strength at the microwave energy heating element. In general however, the gap is reasonably small and the effect is secondary.
Similarly, the side wall of the pie may be analysed using values of:
E1 = 1 j0 (air);
EZ = 6-j0.5 (pastry); and E3 = 70 j7 (filling).
An optimum thickness of between about 3 mm and 6 mm was found.

2~~5119 _7_ The above analysis is based on a filling having a dielectric constant of 70 j7. As shown in the graph of Figure 4, as the dielectric constant of the filling changes, so should the thickness of the pastry change with a range of thickness of between about 2.5 mm and 5.5 mm for filling dielectric constants of between 48 and 82. For example, if a pie filling has a higher dielectric constant an accordingly thicker pastry is required.
The curve shown in Figure 4 is representative of pastry having a dielectric constant of E2 = 6 j0.5. Different pastries will have different dielectric constants and accordingly a similar curve will exist for each constant.
Accordingly, for a given filling and pastry, the thickness of the pastry may be selected to provide a reflection coefficient that maintains the electric field strength at the microwave energy heating element above that necessary for effective crisping and browning. Moreover, the thickness of the pastry may be adjusted where dielectric constants may change, for example with the introduction of an air gap between the lid and filling, and thereby achieve uniform cooking and crisping.
In practical tests, a minimum thickness of pastry of about 2.5 mm has been found necessary to achieve acceptable commercial results. Sometimes, commercial considerations suggest an upper limit of about 7 mm for pastry thickness and typically pastry thicknesses of between about 3 mm and 7 mm have been satisfactory. Where unsatisfactory browning and crisping is obtained, an increase in pastry thickness will raise the power absorption and therefore the field strength at the microwave energy heating element and improve the browning and crisping performance.
In one test conducted with a Marie Callender Pot Pie, a total reflected power ratio of -2.44 db (5790 was observed with a pastry thickness of about 2 mm.
An additional 1.5 mm of pastry was added to provide a nominal thickness of about 3.5 mm and the reflected power was then observed to reach the desired - 3db reflected power ratio.
The two configurations were then cooked under similar conditions w 2~85E79 _g_ and original pie with the 2 mm pastry was observed to remain raw with no surface crisping or browning. By contrast, the modified pie with 3.5 mm pastry was observed to be fully cooked with uniform crisping and browning.
Confirmation of the effect of pastry thickness was simulated using the apparatus of Figure 5. An oil-based plasticine was used in place of pastry as it has a dielectric constant E of 5.0, similar to typical pastries.
Plasticine sheet 40 was set under a plastic container 42 with diameter of 88 mm and height 80 mm. 1000.1 gram of water was filled in the plastic container 44. Both plasticine 40 and the plastic container 42 were shielded by a microwave energy reflective shield 46 of diameter 100 mm and length 90 mm. The power reflection at a frequency of 2450 MHz was measured under a Wiltron Network Analyzer 48. Measurement results for three different thicknesses of plasticine sheets are listed below. A "thin" plastic beaker held the water forming an extra layer.
Plasticine RaW n Loss VSWR Power TransferPower Transfer Thicl~ess (dB) (~) (~) (mm) 0.00 1.80 9.67 34.0 0.00 2 2.30 7.67 40.8 0.79 3 2.82 6.26 47.5 1.45 5 4.50 3.99 64.1 2.75 6 4.74 3.76 66.4 2.91 9 18.8 1.26 98.7 4.63 As can be seen from Table 1 above, an increased thickness from 3 mm to 9 mm resulted in progressively increased power transfer into the plasticine.
A similar test apparatus was used to measure the effect on a water load as shown in Figure 6. The plasticine sheet 40 was set on a styrofoam support layer 50 (thickness of 8 cm), which was placed on the glass tray of the microwave oven. A plastic container 42 with 1000.1 gram water, was set on top of the plasticine sheet 40. Both plastic container and the plasticine sheet were shielded with the same cylindrical shield 46 as mentioned above. The initial temperature and 2~85~19 the temperature after a 60 second cook in the microwave oven was measured by a digital thermometer. The incident power P transmitted through the plasticine sheet was calculated via the formula:
P(watt) = M*S*8T/t where:
M is the net weight of the water (gram);
S is the average value of the specific heat of water which is 4.186 J/g/ ° C at room temperature;
8T is the temperature rise of the water after cooking (°C); and t is the time of heating in seconds.
Temperature rise measurement data with different thickness of the plasticine sheet are listed below:
Plaeticine Temperature Power Transfer Power Transfer Thickness (mm) Rise (v~ (dB) (C) 0.00 15.2 106.1 0.00 2 16.6 115.9 0.3 3 18.5 129.1 0.8 5 21.0 146.6 1.40 6 24.5 171.0 2.07 9 27.1 189.2 2.51 Accordingly, these tests confirmed that as the thickness of the intermediate layer increased, the power absorption increased with a corresponding increase in the surface electric field strength.
In summary, therefore, by maintaining the pastry thickness d above 2.5 mm and adjusting the pastry thickness to provide sufficient power absorption to maintain an effective field strength at the microwave energy heating element, an improved cooking and browning performance will be obtained. It is preferred that a pastry thickness of between 2.5 mm and 7 mm be used, more preferably between 3 mm and 6 mm.
Although the microwave energy heating element has been described ~~ ~~~ ~9 as including a layer of microwave energy interactive material covered by a plastic film, those of skill in the art will appreciate that the microwave energy interactive material may be patterned to achieve the desired heating effect. Also, although the microwave energy interactive material has been described as being in the form of a thin metallic film, the microwave energy interactive material may be in the form of a susceptor layer.
As those of skill in the art will appreciate variations and modifications may be made to the present invention without departing from the spirit and scope thereof as defined by the appended claims.

Claims

We Claim:

1. A food product and a microwavable container, said microwavable container having a base, at least one upstanding side wall about the periphery of said base, and a microwave energy heating element within said microwavable container, said food product including a pastry casing overlying and conforming to said base and at least a portion of said side walls, and a filling within said casing, selected portions of said casing having a thickness to provide a reflection coefficient at the interface of said container and casing to maintain the electric field strength at said microwave energy heating element to effect crisping and browning of said casing.

2. A food product and microwavable container according to claim 1 wherein said casing has a thickness to provide a reflected power ratio of less than 50%.

3. A food product and microwavable container according to claim 1 wherein the thickness of said casing is selected to provide a power absorption of greater than 50%.

4. A food product and microwavable container according to claim 1 or 2 wherein said casing has a thickness of not less than 2.5 mm.

5. A food product and microwavable container according to claim 4 wherein said casing has a thickness of not more than 7 mm.

6. A food product and microwavable container according to claim 5 wherein said casing has a thickness between about 3 mm and 6 mm.

7. A food product and microwavable container according to claim 2 or 3 wherein said microwave energy heating element includes a layer of microwave energy interactive material on said container and a plastic film overlying said microwave energy interactive material.

8. A food product and microwavable container as defined in claim 7 wherein said microwave energy interactive material covers said base and said side walls.

9. A method of browning and crisping a food product having a pastry casing and a filling by heating thereof in a microwave oven, the method comprising:
providing a microwavable container including a base and at least one upstanding wall at least partially defining an inner surface and a cavity, and a microwave energy heating element at least partially overlying the inner surface;
selecting a thickness of said casing at a predetermined location to provide a reflection coefficient at an interface between the casing and the microwavable container;
positioning the food product within the cavity of the microwavable container, the step of positioning including locating at least a portion of the casing proximate to the microwave energy heating element; and exposing the microwavable container and food product to microwave heating, the step of exposing including the step of generating an electric field strength between the casing and the microwave energy heating element in response to that exposure to microwave energy;
maintaining the electric field strength at the microwave energy heating element to effect crisping and browning of the casing.

10. A method according to claim 9 wherein said thickness of casing is selected to provide a reflected power ratio of less than 50%.

11. A method according to claim 9 wherein the thickness of casing is selected to provide a power absorption of greater then 50%.

12. A method according to claim 10 or 11 wherein the thickness is selected to be not less than 2.5 mm.

13. A method according to claim 9, wherein the step of selecting the thickness of the casing includes the steps of:
defining a reflected power ratio at a predetermined location between the casing and the microwave heating element;
determining the dielectric constant of the filling;
determining the dielectric constant of the casing; and determining a thickness of the casing as a function of the dielectric constant of the filling and of the dielectric constant of the casing to provide the reflected power ratio in the predetermined location.

14. A method according to claim 11 wherein said the thickness of casing is selected to be not more than 7 mm.

15. A method according to claim 13 wherein the thickness of the casing is selected to be between about 3 mm and 6 mm.

16. A food product system for heating a food product in a microwave oven, the system comprising:
a container including a base and at least one upstanding wall at least partially defining an inner surface; and a microwave energy heating element overlying the inner surface;
and a food product including a casing at least partially surrounding a filling, the casing being at least partially adjacent and at least partially conforming to the inner surface of the container at an interface therebetween, wherein the casing has a reflected power ratio for browning and crisping associated therewith in response to exposure to microwave energy, an electric field is generated at the interface between the casing and the microwave energy heating element in response to exposure to microwave energy, the electric field having an electric field strength based on the reflected power ratio of the casing, and at least a portion of the casing has a thickness selected to provide the reflected power ratio of the casing, such that the electric field strength at the interface between the casing and the microwave energy heating element is sufficient to brown and crisp the casing.

17. The system of claim 16, wherein the microwave energy heating element comprises a susceptor.

18. The system of claim 16, wherein the thickness of the casing is sufficient to provide a reflected power ratio of less than 50%.

19. The system of claim 16, wherein the thickness of the casing is sufficient to provide a reflected power ratio of less than 30%.

20. The system of claim 16, wherein the thickness of the casing is from about 2.5 to about 7 mm.

29. The system of claim 16, wherein the thickness of the casing is from about 3 to about 6 mm.

22. The system of claim 16, wherein the thickness of the casing is about 3.5 mm.

23. A food product system for heating a food product in a microwave oven, the system comprising:
a container including a base end at least one upstanding wall at least partially defining an inner surface, and a susceptor overlying the inner surface; and a food product including a casing substantially surrounding a filling, the casing at least partially lying adjacent to and at least partially conforming to the inner surface of the container at an interface therebetween, wherein at least a portion of the casing has a thickness selected to provide a reflected power ratio of less than about 50% in response to exposure to microwave energy, such that an electric field generated of the interface between the casing and the susceptor has a strength sufficient to brown and crisp strength.

24. The system of claim 23, wherein the thickness of the casing is from about 2.5 to about 7 mm.

25. The system of claim 23, wherein the thickness of the casing is from about 3 to about 6 mm.

26. The system of claim 23, wherein the thickness of the casing is about 3.5 mm.

27. A method of making a food product system for heating a food product in a microwave oven, the method comprising:
providing a microwavable container including a base and at least one upstanding wall at least partially defining an inner surface and a cavity, and a microwave energy heating element overlying the inner surface of the container;
providing a food product including a casing at least partially surrounding a filling; and positioning the flood product within the cavity of the microwavable container such that the casing is proximate the microwave energy heating element at an interface therebetween, wherein the casing has a reflected power ratio associated therewith in response to exposure to microwave energy, an electric field is generated at the interface between the casing and the microwave energy heating element in response to exposure to microwave energy, the electric field having an electric field strength based on the reflected power ratio of the casing, and at least a portion of the casing has a thickness selected to provide the reflected power ratio of the casing, such that the electric field strength at the interface between the casing and the microwave energy heating element is sufficient to brown and crisp the casing.

28. The method of claim 27, wherein the thickness of the casing is selected to provide a reflected power ratio of less than about 50%.

29. The method of claim 27, wherein the thickness of the casing is selected to provide a reflected power ratio of less than about 30%.

30. The method of claim 27, wherein the thickness of the casing is selected to be from about 2.5 to about 7 mm.

31. A method of enhancing the browning and crisping of a first food component while heating a second food component, the first and second food components being components of a multi-component food product, the first food component having a first dielectric constant, and the second food component having a second dielectric constant, the method comprising the steps of:
providing a microwavable container including a base and at least upstanding wall at least partially defining an inner surface and a cavity, and a microwave energy heating element overlying the inner surface; and positioning the food product within the cavity of the microwavable container, at least a portion of the first food component being proximate the microwave energy heating element at an interface therebetween, wherein the first food component has a reflected power ratio associated therewith in response to exposure to microwave energy, an electric field is generated at the interface between the first food component and the microwave energy heating element in response to exposure to microwave energy, the electric field having an electric field strength based an the reflected power ratio of the first food component, and the thickness of at least a portion of the first food component is selected using at least the dielectric constant of each the first food component and the second food component, such that the electric field strength at the interface between the first food component and the microwave energy heating element is sufficient to brown and crisp the first food component while allowing the second food component to be heated.