[0001] This invention relates to packaging and is concerned with a method of packaging a
heat transformation product, that is to say a product which changes in shape, size,
texture or other physical attribute on heating. Preferred heat transformation products
are those food products which change in shape, size, texture and colour or release
flavour on heating. A prime example of a food transformation product is grains of
corn (maize) which upon heating, produce "popcorn" while another is poppadoms. The
present invention will be described mainly with reference to grains of corn although
it is applicable to other heat transformation products. For ease of reference, such
grains of corn, whether in the "popped" or "unpopped" condition will hereinafter be
referred to as "popcorn". The invention also relates to packages produced by the method.
[0002] There has been increasing interest in providing popcorn in packages which can be
placed in an oven or microwave oven and heated to pop the corn while still in the
package, so that on heating, the resulting popcorn is retained in the package and
does not fly all around the oven and so that the resulting atmosphere generated in
the package does not escape or rupture the package explosively or other than in a
controlled manner. A problem with such packages is that in the unpopped state the
popcorn occupies relatively little room but expands greatly in volume on heating.
The package therefore has to be big enough to contain the popped popcorn which means
that when the corn is in the unpopped state there is a large amount of empty package
which can flop about and become damaged or which has to be carefully held in position
before heating.
[0003] It is an object of the invention to provide a package for a food transformation product
which takes up relatively little volume before heating but can expand on heating to
contain the product.
[0004] According to one aspect of the present invention there is provided a method of packaging
a food transformation product, comprising the steps of forming a bag portion from
a film of heat-extensible plastics material bringing the mouth of said bag portion
into contact with the mouth of a rigid container having untransformed food product
therein, collapsing said bag portion by drawing a vacuum between the interiors of
said container and said bag portion or by external pressure thereon to cause the bag
portion substantially to follow the interior contour of the container and food product
therein and sealing the mouth of said bag portion to the mouth of said container.
The method is preferably carried out continuously and use may be made of a conventional
thermoforming vacuum packaging machine.
[0005] In one embodiment of the present invention, the method is carried out by forming
the bag portion, as by deep-drawing the film of heat extensible plastics material,
and then placing over the mouth of the bag portion an inverted rigid container which
has the untransformed food product adhered, as by an edible adhesive, such as sugary
or toffee-like mixture, to what would normally be the base of the container. After
the bag portion has been invaginated into the rigid container the container can be
turned up the normal way.
[0006] In another embodiment of the present invention, the bag portion is formed and then
allowed to collapse or forced by pressure or vacuum into a collapsed state in which
it is placed over the mouth of the rigid container in which the untransformed food
product can be held in a loose condition.
[0007] In either case, when the resulting package is stood on its base and heated in an
oven/hot plate or microwave oven, the bag portion will stretch and form a dome under
the influence of the heat and will contain the food product, e.g. popcorn, as it increases
in volume during heating the lined fibre tray and the bag portion being capable of
extensive expansion to relieve any excess pressure. However, the seal between the
bag portion and the container may be a broken or intermittent seal to allow the escape
of air and/or moisture during heating. A preferred safety release is the inclusion
of a strip of paper at one or more points in the seal.
[0008] The plastics material of which the bag portion of the package is made is a heat-extensible
material which is heat-resistant in that it can withstand the temperatures in an oven
or microwave oven at which the food product undergoes transformation. For use in a
microwave oven the plastics material should have low dielectric loss properties. Suitable
plastics materials for the bag portion include cast polyesters such as polyethylene
terephthalate or polybutylene terephthalate, polycarbonates, and polypropylene, but
other suitable materials may be employed. The plastics material may have a thickness
of 25 to 200 microns, but preferably has a thickness of 25 to 100 microns.
[0009] The container is preferably in the form of a flanged tray or dish which may be round
or substantially rectangular in plan view. It is most desirable that the tray is substantially
square in plan view although other constraints may favour a somewhat oblong plan view.
It is desirable for the container to have a depth at least one third of the diagonal/width
or diameter in order to achieve optimum results. The container is preferably made
of fibrous material, such as moulded fibre pulp, paper, cardboard or fibre board made
in conventional manner by moulding fibres deposited by paper-making techniques. Alternatively,
the container may be made from bonded wood chips or bonded fibre material provided
that the bonding material can withstand subsequent heating. All these containers are
lined with a lining of heat-resistant plastics material which is compatible with the
material of which the bag portion is made and which can be sealed to the bag portion
by processes such as heat-sealing, thermal bonding or even by adhesive means.
[0010] The container may also be made of plastics material provided that the material can
withstand the subsequent heating and can be sealed to the bag portion.
[0011] If the package is to be heated in a microwave oven to effect the transformation of
the food product, it is desirable that the container should be provided with susceptor
material to enhance the heating effect of the microwave radiation. The susceptor material,
of which ferrites, metal particles. e.g. aluminium particles, and various argillaceous
materials, e.g. bentonite, are examples, may be incorporated in the container in various
ways. Thus, for example, a pad of susceptor material may be incorporated between the
interior of the base of a container made of fibrous material and the heat-resistant
plastics lining of the container or a pad of susceptor material may be present externally.
Alternatively, the container may be sprayed or otherwise treated with susceptor material,
either internally before lining, or externally. For example, the container may have
a metalised film thereon of e.g. aluminium particles at an optical density of 0.18
to 0.3. However, with some containers, such as those made of moulded fibre pulp, the
pulp from which the container is made will normally contain clays, such as bentonite,
or other fillers which may then become incorporated in the pulp from which the container
is moulded in sufficient quantities to act as susceptor materials in conjunction with
the inherent moisture content of the normal moulded fibre containers available in
commerce, which moisture content is usually 10% but may range from 5 to 15%, so that
there is no need for the addition of further susceptor material, as the package will
perform satisfactorily as a result of its self-suscepting properties.
[0012] If the container is made of plastics, then the plastics may contain susceptor material
as a filler.
[0013] It will thus be appreciated that the susceptibility of the package or the container
can be selected both in degree and position in accordance with the requirements of
the transformation product. To this end, it may in some cases be desirable to provide
the base of the container with ribs or feet so as to position, for example, a pad
of susceptor material at an appropriate distance from the turntable of a microwave
oven.
[0014] The first embodiment of the present method is particularly useful for packaging popcorn
which is present in a sweet form, since in this case the individual grains are covered
with a sugary or toffee-like mixture.
[0015] Such a sugary or toffee-like mixture provides a convenient adhesive for holding the
popcorn to the base of the container when the latter is inverted over the deep drawn
bag portion. However, other suitable edible materials may be used in an adhesive capacity
if appropriate.
[0016] In accordance with another aspect of the present invention there is provided a package
of a food transformation product, comprising a container having untransformed product
therein and a bag portion of heat extensible plastics material lying within the container
and substantially following the interior contour of the container and food product
therein, the bag portion being sealed to the mouth of the container, preferably by
a broken or intermittent seal. When the package of food transformation product is
placed in an oven or microwave oven, the product expands and extends the bag portion
above the container to form a bag full of the product which is retained in the bag.
[0017] In order to enable the package to be opened readily, especially when the package
is hot after the transformation of the food product, a tear-strip or opening strip
may be incorporated in the side of the package. The tear strip may comprise a strip
of material which is different from the material of which the lining of the container
or of the container itself is made and which is also different from the material of
which the bag portion is made, but which can be adhered as by heat-sealing, adhesively
or otherwise both to the container or its linining and the bag. A suitable material
for such a tear-strip is that sold under the Registered Trade Mark "Melinex" 850,
which is a co-extruded plastics film comprising a layer of polyester and a layer of
a heat-seal material which is present at a rate of 4 to 5g/m²; the tear-strip is conveniently
about 30 microns thick.
[0018] If the package of food transformation product is to be stored then it is desirable
to place the package in an outer or secondary package as it is possible to provide
a desired and controlled atmosphere within such a secondary package thereby prolonging
the shelf life of the food transformation product as the secondary package can be
made with a very low gas and moisture vapour transmission rate. The primary package
containing the food product can be placed in a thermo-formed outer container and covered
with a lidding material which is sealed to the outer container in a controlled atmosphere
packaging machine by means of so-called easy peel seal, that is to say a seal in which
two compatible polymer materials are united by heat, but one of the polymers contains
a differentially soluble polymer additive which settles out on cooling to form a line
of weakness along which the polymers can be separated. Alternatively, the primary
package of food transformation product can be flow wrapped in conventional manner.
[0019] In order to enable the invention to be more readily understood reference will now
be made to the accompanying drawings which illustrate diagrammatically and by way
of example some embodiments thereof, and in which:-
Figure 1 is a schematic view of plant for packaging popcorn,
Figure 2 is a schematic view of another plant for packaging popcorn,
Figure 3 is a schematic view of a plant for packaging the package produced by the
plant shown in Figure 1 or Figure 2,
Figure 4 is a view of a package prepared by the plant shown in Figures 2 and 3,
Figure 5 is a perspective view of a flow wrapped package, and,
Figure 6 is a view of the package after it has been heated in an oven.
[0020] Referring now to Figure 1 of the drawings, a film 1 of polybutylene terephthalate
100 microns thick is unwound from a reel 2 and fed to a deep-drawing station 3 in
which the film is deep-drawn in conventional manner make a forming 4 which for convenience
will be referred to as a bag.
[0021] Moulded fibre trays 5, lined with a film of polyethylene terephthalate 36 microns
thick and containing unpopped popcorn 6 adhered to the bases thereof by a toffee-like
mixture, are fed along a conveyor 7 to a loading wheel 8. The loading wheel has a
number of arms 9 each fitted with holding means 10 to hold a tray 5 firmly in position,
either by means of grippers (not shown) or by suction. The loading wheel 8 is mounted
above the film 1 and positioned so as to place the mouth of a tray 5 over the mouth
of a bag 4, the peripheries of the mouths of the tray and bag being of the same shape.
As the popcorn is adhered to the tray 5 by the toffee-like mixture, the popcorn does
not fall into the bag. The bag 4 and inverted tray 5 are then are passed to a conventional
vacuum/gas packaging machine 11 connected via a pipe 12 to a source of vacuum, where
a vacuum is drawn between the bag 4 and the inverted tray 5 so that, as shown, the
bag 4 is drawn into the tray 5 so as to lie closely adjacent the popcorn and substantially
follow the interior contour of the container. Alternatively, the bag could merely
occupy the head space of the tray in a collapsed state. The resulting package is then
passed to a heat-sealing and cutting station 14 where the mouth of the bag 4 is heat-sealed
to the lining at the mouth of the tray 5 and the package is severed from the surrounding
film 1. The seal between the bag and the tray lining may be a broken or intermittent
seal for a reason to be explained hereinafter. The sealed packages 15 are then removed
by a conveyor 16.
[0022] When a package 15 of popcorn prepared in the manner described above, is placed in
an oven or microwave oven and heated, the grains of corn expand many-fold and generate
steam pressure and this expansion causes the grains to push out the bag 4 back to
its original shape so that the bag 4 and the tray 5 are filled with a mass of expanded
popcorn 17. Should it be desired for reasons of safety or function, during the heating
of the package in the oven heated air and moisture generated by the heat can escape
at the intermittent seal between the mouth of the bag and the mouth of the tray, if
such a seal is provided.
[0023] Referring now to Figure 2, there is shown another plant for packaging a food transformation
product which is not adhered to the base of a container. In this plant preformed containers
in the form of deep flanged trays or tubs 20 of moulded fibre material are taken from
a stock 21 of trays by destacking apparatus indicated by arrow 22 and placed in rows
across a conveyor 23 so as to form a closely spaced array of trays on the conveyor.
The trays 20 are fed beneath a loading device 24 which deposits a disc 25 of susceptor
material at the bottom of each tray. The trays 20 are then fed to a thermoforming
station 26. A reel 27 of polyethylene terephthalate film 28 which has a thickness
of 36 microns is mounted above the conveyor 23 and the film is united with the trays
20 at the thermoforming station and serves to hold the discs 25 of susceptor material
in the trays. The film is coherently bonded to the trays and discs by being heated
by a heater 29 and by being drawn downwardly by a vacuum applied at a vacuum outlet
30.
[0024] After leaving the thermoforming station 26, the trays pass to a station 31 at which
a tear strip 64 (c.f. Figure 4) is added. At this station 31 which is not shown in
detail a strip of "Melinex" 850 is fed to each tray in a direction transverse to the
conveyor so as to lie over the flange of the tray and is heat-sealed to the lining
of the tray inside the tray and at the flange by means of a heated wheel or block
(not shown).
[0025] After leaving the tear-strip station 31, the trays pass to a loading station 32 where
the food transformation product 33, e.g. grains of corn, is placed in the tray. The
trays then pass to an invaginating station 34.
[0026] A reel 35 of a film 36 of polybutylene terephthalate 100 microns thick is mounted
above the conveyor 23 and above another thermoforming station 37 at which the film
36 is thermoformed into a forming or bag portion 38 using a heater 39 and a vacuum
applied at a vacuum outlet 40. The bag portion 38 is shown as being of substantially
the same shape as a tray 20 but it may be of any convenient shape bearing in mind
its eventual purpose to hold the transformed food product.
[0027] After thermoforming and cooling of the resulting bag 38 at the thermoforming station
37, the bag 38 in a collapsed condition is then brought into contact with a tray with
the mouths of the bag 38 and tray 20 in register with one another and the tray and
bag are passed to the invaginating station 34 where by means of compressed air introduced
through an inlet 41 the bag 38 is invaginated so that the bag substantially follows
the interior contour of the tray and the grains of corn 33 in the tray. The tray 20
with the invaginated bag then passes to a vacuum sealing station 42 where a vacuum
is applied via a vacuum outlet 43 to the space between the interior of the tray and
the bag so that the bag is now forced closely to follow the interior contour of the
tray and the grains of corn. At the same time the mouth of the bag is heat-sealed
to the lining of the tray at the flange thereof. The seal is preferably an intermittent
seal as indicated above. After leaving the station 42, the filled and sealed packages
are separated by a knife or cutting means 44 into individual packages 45 which are
either passed for sale or are passed for packaging in the plant shown in Figure 3
in which the packages 45 are packaged in secondary or outer containers. The necessity
for packaging the packages 45 in outer containers may arise not merely for presentation
but in order to extend the shelf-life of the food transformation product by packing
it in a desired or controlled atmosphere.
[0028] Referring now to Figure 3, a film 50 of thermoplastic material comprising a layer
of polyvinylidene chloride sandwiched between two layers of ionomeric polymer such
as that sold under the Registered Trade Mark "SURLYN" and having a total thickness
of 125 microns is unwound from a reel 51 and passed to a thermoforming station 52
where the film is thermoformed to produce an outer container 53 shaped to receive
a package 45 leaving the plant shown in Figure 2 or a package 15 leaving the plant
shown in Figure 1. The outer container 53 then passes to a loading station in which
a package 45 (or 15) is placed in the container 53.
[0029] A lidding material 54 comprising a film of thermoplastic material comprising a layer
of polyvinylidene chloride sandwiched between a layer of ionomeric polymer and a layer
of polyethylene terephthalate and having a total thickness of 60 microns is unwound
from a reel 55 and is brought to cover the container 53 with the ionomer layer of
the lidding material in contact with a flange of the outer container. The lidding
material is then united with the flange of the container 53 in a vacuum/gas flush
controlled atmosphere packaging machine 56 of conventional kind in which the space
between the lidding material 54 and the interior of the container 53 is filled with
carbon dioxide or nitrogen and the lidding material is heat-sealed to the flange of
the container. Finally, the containers are separated by cutting means 57 to provide
the finished package 58.
[0030] Referring now to Figure 4, there is shown a cross-section through the package 58
showing the outer container 53 sealed by a lid 60 around a flange 61 of the container
53. The container 53 is formed with an internal shoulder 62 on which rests the outer
flange of the tray 20 with its lining 63 secured to the wall of the tray and holding
the disc 25 of susceptor material in place. The grains of corn 33 are closely covered
by the invaginated bag 38 which is sealed to the mouth of the tray. In addition, a
tear strip 64 is shown lying between the bag 38 and the tray lining 63.
[0031] Figure 5 shows an alternative wrapping in which the package 45 or 15 has been flow-wrapped
in an envelope of material similar to the lidding material 54 using a convential flow-wrapping
machine.
[0032] When the package shown in Figure 4 is to be used, the lid 60 is removed and to this
end a tear strip similar to the tear strip 64 may be present at the edge or across
the lid. The package 45 is then removed from the container 53 and placed in a microwave
oven and heated in accordance with instructions present on the lid, on the package
45 or on the container 53. Heat generated causes the grains of corn to "pop" to produce
popcorn and causes the bag 38 to be pushed out of the tray firstly to the position
indicated at 65 in Figure 6 but due to the heat generated, the expansion of the atmosphere
within the tray and the fact that the bag is made of heat-extensible plastics, the
bag eventually takes up the position shown at 66. It will be appreciated that as the
bag expands from the position shown at 65 to the position shown at 66, there is a
thinning of the bag material. Accordingly, the thickness of the bag material, the
shape to which the bag is thermoformed and the amount of food transformation product
are chosen so that when the bag takes up the position shown at 66, the transformed
product is contained wholly within the bag without any risk that the bag might burst.
Thus it is possible to influence the final volume of the bag by controlling the thickness
of the bag material.
[0033] Instead of providing the lid 60 of the package 58 with a tear strip similar to the
tear strip 64 it could be provided with so-called "easy peel" seal but in this case
the outer package 53 and the lidding material 54 would be made of different polymers
such as a nylon, polyvinylidene chloride, polythene composite or a polypropylene,
polyvinylidene chloride, polythene composite for the outer package and a polyester,
polyvinylidene chloride, polythene composite for the lidding material. In an easy
peel seal a differentially soluble polymer additive, such as polybutylene is incorporated
in the lidding material. After heat sealing the lidding material to the flange 61
of the outer package 53, the polymer additive, which has become incorporated in the
lidding material during the melting of the latter, settles out on cooling as a line
of weakness permitting easy opening of the outer package. This can be assisted by
the provision of a tab of the lidding material, if desired.
[0034] It will be appreciated that many modifications of the present packages are possible
depending upon the materials used, the food transformation product and the temperature
at which the transformation is to take place.
1. A method of packaging a heat transformation product, in which the product, held
within a rigid container, is covered by a film of plastics material which is sealed
to the mouth of the container, characterised in that the mouth of a bag portion formed
from a film of heat-extensible plastics material is brought into contact with the
mouth of the container which contains therein a heat transformation product in an
untransformed state, and in that the bag portion is collapsed to cause the bag portion
substantially to follow the interior contour of the container and product therein
by the application of a pressure differential between the outside of the bag portion
and the inside of the bag portion and container, the interior volume of the container
and the bag portion being at least sufficient to contain said product when in its
transformed state.
2. A method as claimed in Claim 1, wherein the method is carried out continuously
by forming the bag portion by forming a film of heat-extensible plastics material
unwound from a reel thereof, invaginating the bag portion into the container by vacuum
or external pressure and sealing the mouth of the bag portion to the mouth of the
container, preferably by an intermittent seal.
3. A method as claimed in Claim l or 2, wherein an inverted rigid container having
the untransformed product adhered therein by means of an adhesive, is placed over
the mouth of a deep-drawn bag portion which is then invaginated into the container.
4. A method as claimed in any one of Claims 1 to 3, wherein the rigid container is
formed of a heat-resistant plastics material or is formed of moulded or bonded fibrous
material provided with a lining of heat-resistant plastics material.
5. A method as claimed in any one of Claims 1 to 4, wherein the rigid container incorporates
susceptor material or a pad of susceptor material is incorporated in the container
between the lining and the fibrous material.
6. A method as claimed in any one of Claims 1 to 5, wherein the heat transformation
product is a food transformation product, such as grains of corn (maize) transformable
into "popcorn".
7. A method as claimed in 6, wherein the resulting package of food transformation
product is packed within a secondary container using a controlled atmosphere packaging
technique.
8. A package of heat transformation product, comprising a container, the product held
within the container, and a film of plastics material sealed to the mouth of the container,
characterised in that the container contains therein a heat transformation product
in an untransformed state and the film of plastics material is in the form of a bag
portion of heat-extensible plastics material lying within the container and substantially
following the interior contour of the container and product therein, the interior
volume of the container and the bag portion being at least sufficient to contain said
product in its transformed state.
9. A package as claimed in Claim 8, wherein a tear-strip or opening strip is incorporated
in the side of the package, the tear-strip preferably comprising a strip of a co-extruded
plastics film comprising a layer of polyester and a layer of heat-seal material.
10. A package as claimed in Claim 8 or 9, wherein the heat-extensible plastics film
is a film of a cast polyester, such as polyethylene terphthalate or polybutylene terephthalate,
or a film of polycarbonate or polypropylene.
11. A package as claimed in any one of Claims 8 to 10 wherein the container is in
the form of a flanged tray or dish and is made of a heat-resistant plastics material
or of a moulded or bonded fibrous material lined with a heat-resistant plastics and
optionally incorporating a susceptor material or a pad of susceptor material between
the fibrous material and the lining.
12. A package as claimed in any one of Claims 8 to 11, wherein the heat transformation
product is a food transformation product and comprises grains of corn (maize) transformable
into "popcorn".