WO2008122069A1

WO2008122069A1 - Food packaging with automatic steam safety release vent

Info

Publication number: WO2008122069A1
Application number: PCT/AU2007/001551
Authority: WO
Inventors: Peter Grech
Original assignee: Peter Grech
Priority date: 2007-04-09
Filing date: 2007-10-11
Publication date: 2008-10-16

Abstract

A tray (10) for a self venting, sealable, food container is disclosed that includes a receptacle suitable for holding food, a rim of the receptacle (100) for receiving a sealing film and an indentation formed in the rim of the tray that defines a substantially hemi-conically shaped vent (30). The seal (110) formed between the rim of the receptacle and the sealing film is equally strong at all points of the seal and venting occurs when the tension in the film breaks the seal (110) between the film and the rim of the receptacle (100).

Description

FOOD PACKAGING WITH AUTOMATIC STEAM SAFETY RELEASE VENT

TECHNICAL FIELD

This invention relates to pre-packaged meal containers and methods for sealing and delivering food in a ready to heat form. In particular, the invention relates to self venting pre-packed meal containers for use in microwave and/or conventional ovens. BACKGROUND ART

Sealed plastic containers are commonplace in packaging applications, such as food packaging, and in particular for packaging pre-packaged ready to heat meals which are heated either in a microwave oven or in a conventional oven. Such containers are sealed so as to protect the contents of the container and also to fix the volume of the container such that, when heated, results in an increase in the internal atmospheric pressure and an increase in temperature. In such conditions, food contained within the container can be cooked by steam generated by the heating, and therefore, be supplied to the customer, in a partially uncooked, and usually frozen state.

Traditionally, such containers include a tray with a thin polymeric film comprising a flexible substrate and heat-sealable layer. The manufacture of sealed containers using polymeric films involves the formation of a seal between the film and the tray. This seal is usually formed by placing the film on top of the rim of a tray and applying heat and pressure in order to soften or melt the sealable coating layer so that it adheres to the surface of the tray and forms an effective seal between the film and the rim of the tray. A common problem with ready-prepared convenience meals is that pressure generated during the cooking cycle by the steam can cause the packaging, for instance the film, to burst, causing fragments of the packaging to contaminate the contents of the tray, or alternatively, hot food to be ejected from the container, thereby posing a burn risk. The film may also fail locally at a position which results in the uneven cooking of the food or alternatively, poses as a danger to the user when handling the tray after the cooking cycle, as steam is likely to escape along the edge of the tray and burn the hand of the user when handled.

Previous packaging for ready to heat pre-packaged food generally required that the user pierce the packaging, and in particular, the thin film, to prevent these things from happening. However, the need for piercing prior to heating the food in its container is often forgotten or not understood by the user. Or, if it is done, it is either done too much, or possibly even, not enough.

It would be desirable to provide packaging which did not require the user to pierce it before cooking.

It is therefore an object of this invention to provide a film suitable for use in packaging ready-prepared ovenable meals, which exhibits self venting. DISCLOSURE OF INVENTION

According to one aspect of the invention there is provided a tray for a self venting, sealable food container comprising: a tray suitable for holding food, a rim of the tray for receiving a sealing film, and wherein the seal formed between the rim of the receptacle and the sealing film is equally strong at all points of the seal; venting means for opening a channel of air communication between the receptacle, and the area outside the receptacle, at a predetermined point, and at a given pressure, when heated, to allow for the venting of hot gasses from within the container. Preferably the venting means is actuated by tension provided by the film under the force of increased internal atmospheric pressure generated by the heating of food contained in the receptacle.

Preferably the venting means of the tray is comprised of an indentation formed in the rim of the tray that defines a vent and wherein the venting occurs when the tension in the film breaks the seal between the film and the rim of the tray across the vent.

Preferably the indentation in the rim of the tray defines a vent that is substantially hemi-conically shaped and wherein the venting occurs when the tension in the film breaks the seal between the film and the rim of the tray at the apex of the substantially hemi conical shaped protrusion.

Preferably the shape of the tray is a rectangular and the indentation is located in the middle of a side of the tray.

Preferably the rim of the tray is adapted to be sealed to the film through the application of heat and pressure.

Preferably the tray is made from materials suitable for use in both microwave and conventional ovens.

In an alternate embodiment of the invention there is provided a method of heating food wherein said food is contained within a closed and sealed food container with a fixed internal volume, said food being capable of generating increased internal atmospheric pressure therein when the temperature of said food is raised, the method comprising the steps of: depositing the food into a tray of a tray wherein the tray has around the tray, a rim for receiving a film, at least a portion of the film being in direct contact with the hot gasses generated through heating the food, and a vent being defined between the film and rim of the tray; sealing the tray with film around the rim to form a seal; heating the sealed container by placing it in a heating apparatus so as to cause an increase in the internal pressure of the container which causes the film to be urged upwards, whereby sufficient tension generated in the film by the increase in pressure preferentially is directed to one area of the seal across the vent of the tray whereupon the vent is opened to as to allow the hot gasses to escape from the container through the vent.

Preferably the transfer of sufficient tension preferentially to one area of the seal between the film and the rim of the tray is achieved by providing a tray that has at least one indentation in the rim of the container that defines a vent and that extends into an area of the film which is urged upwards by the internal pressure of the container.

Preferably the seal created between the film and the rim of the tray is equally strong at all points around the rim.

Preferably the indentation defines a substantially hemi-conical shaped vent between the film and the container.

Preferably the vent is opened to direct air communication with the contents of the container once the seal between the rim of the container and the film is broken at the apex of the hemi-conically shaped vent defined by the indentation of the rim of the tray. BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings in which:-

Fig. 1 is a plan view of a food container according to a preferred embodiment of the invention;

Fig. 2 is a side view of the food container of Figure 1 ; Fig. 3 is a cross sectional view of the food container of Figs. 1 and 2 taken along axis 50; Fig. 4 is a plan view of the food container of Figs. 1 and 2 showing pressures generated during heating; Fig 5. is a side view of the food container of Figs. 1 and 2 which contains food; Fig 6. is a side view of the food container shown in Fig. 5 when the food has been heated;

Fig 7. is a side view of the food container shown in Fig. 6 that is venting steam through its vent.

MODES FOR CARRYING OUT THE INVENTION

Referring to Figures 1 and 2, the food container of the present invention is comprised of tray 10 and film 20 which is heat sealed to tray 10. The tray can be made out of various different materials depending on where it is to be used. If the tray is only to be used in conventional ovens, then it can be made out of various materials including heat resistant plastics and resins, and metals such as aluminium. If the tray is only to be used in a microwave over, then a range of microwave safe plastics and other materials may be used to form the tray.

More recently dual ovenable trays have been developed from materials that can withstand temperatures of -40 degrees Celsius to 200 degrees

Celsius. These trays can be filled with food and delivered to the consumer with instructions prepare the meal by either heating in a conventional oven, or heating in a microwave. In most cases these trays are made from plastic and other types of coated paperboard substrate. The person skilled in the art will appreciate that the present invention can be worked by utilising any commonly used material for forming the tray 10 and film 20. For the tray 10 materials such as polypropylene, CPET, high impact polystyrene, PVC and APET lined paperboard can be used. Further conventional techniques can be used for form the tray including injection moulding and thermo forming. Where the invention lies, in the present case, is the shape and configuration of the tray 10. The film 20 is of a conventional sort and applied to the tray 10 using conventional techniques familiar to the person skilled in the art.

In particular tray 10 features rim 100 which extends all around the tray 10 and which provides the surface for the film 20 to be bonded to. Film 20 may be comprised of various layers that provide the sealed container with the properties suited for the storage of food including layers which are impervious to odours and oxygen. Conventional films 20, usually also feature a thermoactive bonding layer. Such a layer is often found on the underside of the film 20 such that when the film is laid over a container in which food has been added and pressure and heat is applied, the film 20 forms a bond with the tray 10, at and around the rim 100.

Turning to Figures 1 and 2, a vent 30 is depicted which allows for steam and other hot gasses generated in the heating of the food container to be vented from the contents of the container at a designated area. Vent 30 is in direct communication with the outside of the sealed food container, and is sealed off from direct communication with the contents of the sealed food container by the seal 110, formed between the indented side wall of the tray 35 and film 20. Vent 30 can be located at any position on the rim 100 of tray 10.

Vent 30 can be better appreciated as depicted in Figure 3 which is a cross sectional view of the food container of Figures 1 and 2 at axis 50. When the food container is viewed by cross section, the vent 30 comprises a substantially hemi-conical air cavity formed between the indented and depressed side wall of the tray 35 and film 20. The air cavity is in direct air communication with the area outside of the food container and is bounded by the film 20 on its upper surface, and indented and depressed side wall of tray 35 at its lower surface. The tray can be easily formed in this shape by injection moulding. The person skilled in the art will appreciate that the invention can be carried out by indenting and depressing a portion of the rim of the tray 10 such that a multitude of differently shaped air cavities open to air communication with the outside of the container are formed under lid 20.

Figures 4, 5, 6 and 7 show the food container of the present invention in use. As the tray 10, and film 20, are comprised of relatively inextensible material, the volume of the sealed container will be fixed. Therefore, according to Boyle's law, as the temperature of the gasses, including water vapour or steam, is increased, the pressure exerted by the gasses also increases. The arrows in Figure 4 depict the relative pressures exerted on the sealed areas of film 20 and rim 100. As depicted in Figure 4, the pressure 60, exerted on the seal 110 where the vent 30 is found in the tray 10, are greater than the pressure applied to other areas of the sealed areas of film 20 and rim 100.

Figure 5 depicts a food container of the present invention with food 40 in it and sealed. This container has yet to be heated to any extent. Figure 6 depicts a food container according to the present invention that has been heated significantly. In such a situation, the gasses in the sealed container expand, and also, steam is generated from liquid water contained in the food 40. This generates pressure 60 which acts upon film 20 such that the centre of the film 20 is urged upwardly thereby creating tension at all points of the seal formed between film 20 and rim 100. In Figure 7 the vent 30 is shown in operation with steam 70 escaping from the food container. The indented portion of the rim 120, defining vent 30 in tray 10 is offset from the rim 110. By indenting a portion of the rim into the region of the food container where the film 20 rises during heating, the portion of the seal 110 can be placed under more tension than at all other areas of the seal between film 20 and rim 110. By utilising a indented portion of the rim 120 as depicted in Figure 4, the food can be sealed using a strong seal. This is because in the present invention there is no need to pre-weaken any part of the seal that is formed between film 20 and rim 100. Indeed, the strength of the seal is constant all around the rim 100, including at seal 110, which results in a more robust food container. The sealed package of the present invention can therefore better withstand movement and impact during transit that would otherwise cause food packages with pre- weakened seals and other self venting mechanisms to open prematurely.

After the portion of the seal 110 is broken, and hot gasses and steam escapes, a constant internal pressure can be maintained for the remainder of the heating process. Various methods can be employed to vary the final temperature and pressure of the food container during heating. One way in which to vary the final temperature and pressure is to vary the strength of the bonding agent used to seal the film 20 to the rim 100. By using a weaker bonding agent, the seal 110 will be broken under less tension, and consequently, at a lower temperature than had a stronger bonding agent been used. Alternatively, and preferably, the size and shape of the indented portion of the rim 120 can be varied to provide for differing opening pressures and temperatures. For example, in order to raise the pressure at which the steam vent 30 opens, the degree of indentation into the centre of the container can be made smaller, thereby reducing the tension experienced at the portion of the seal 110.

The food container of the present invention may possess multiple sealed areas formed around the rim of the container, wherein each of the sealed areas has formed in the rim of the container, an indentation that defines a vent 30 (not shown). The degree of indentation of the multiple indentations formed in the rim of such a food container can be varied such that each sealed area can have different final internal pressures and temperatures to ensure that different food types are ready to be eaten at the same time.

The food container may be made in different shapes. Rectangular food containers have been shown in the present examples, but the present invention can work equally as well in substantially circular, elliptical, square and triangular shaped containers. The present invention will be applicable to any food container in which the sealing film is urged upwards across an area into which an indentation in the rim of the container defining a vent can be offset from the otherwise regular rim of the container.

The person skilled in the art will appreciate that modifications may be made in details of design and construction without departing from the scope and ambit of the invention.

Claims

CLAIMS:

1. A tray for a self venting, sealabie, food container comprising: a receptacle suitable for holding food, a rim of the receptacle for receiving a sealing film, and wherein the seal formed between the rim of the receptacle and the sealing film is equally strong at all points of the rim; and venting means for opening a channel of air communication between the receptacle and the area outside the receptacle, at a predetermined point and at a given pressure, when heated, to allow venting of hot gasses from within the container.

2. The tray of claim 1 wherein the venting means of the tray comprises an indentation formed in the rim of the tray that defines a vent and wherein the venting occurs when the tension in the film breaks the seal between the film and the rim of the tray across the vent.

3. The tray of claim 2 wherein the vent is opened by tension provided by upward movement of the sealing film under the force of increased internal atmospheric pressure of the container generated by the heating of food contained in the receptacle.

4. The tray of claim 3 wherein the vent is substantially hemi-conical and wherein the venting occurs when the tension in the film breaks the seal between the film and the rim of the tray, across the apex of the substantially hemi-conical vent.

5. The tray of any one claim 3 where the tray is made from materials suitable for use in both microwave and conventional ovens.

6. A method of heating a food container wherein food to be heated is contained within a closed and sealed food container with a fixed internal volume, said food being capable of generating increased internal atmospheric pressure and hot gasses when the temperature of said food is raised, the method comprising the steps of: depositing the food into a tray of a container wherein the tray has a rim for receiving a film, at least a portion of the film being in direct contact with the hot gasses generated through heating the food, a vent being defined between the film and rim of the tray; sealing the container with film around the rim to form a seal; heating the sealed container by placing it in a heating apparatus so as to cause an increase in internal atmospheric pressure inside the container which causes the film to urge upwards, whereby sufficient tension generated by the upwards urging of the film is directed to a portion of the seal across the vent, whereupon the vent is opened so as to allow the other hot gasses to escape at from the container through the vent.

7. The method of claim 6 whereby the seal created between the film and the rim of the container is equally strong at all points around the rim.

8. The method of claim 14 whereby the vent is defined by an indentation in the rim of the tray that extends into an area of the film which is urged upwards upon the increase in internal atmospheric pressure of the container.

9. The method of claim 8 wherein the indentation in the rim defines a substantially hemi-conical shaped vent.