The present invention relates to a method for sealing containers,
notably to a method for sealing containers containing a foodstuff which has been
pasteurised or heat sterilised, so as to preserve the integrity of that container.
BACKGROUND TO THE INVENTION
Many foodstuffs are presented in a cooled or chilled state for sale
to the public. In order to preserve the foodstuff, this is packed in a sealed container.
However, with the current awareness of potential contamination or bacterial degradation
of the foodstuff it is required that the foodstuff be sterilised or pasteurised
by heat or other treatment before the goods are offered for sale. In one method
for achieving this, the foodstuff is passed through a microwave heating tunnel
or chamber where the foodstuff is heated to the desired temperature and retained
at that temperature for a required period.
Whilst this treatment may reduce the problems of contamination with
salmonella, lysteria and other bacteria, physical problems arise in that the foodstuff
must be presented in a container for transport, storage and sale which protects
the foodstuff against fresh contamination after the heat treatment. Typically,
such a container is provided as a rigid or semi-rigid plastics tray in which the
foodstuff lies, and a transparent cover which is heat sealed around its periphery
to the tray. If the foodstuff is packed in the container after the heat treatment,
there is a risk that the foodstuff may pick up new contamination during the packing
operation. On the other hand, if the foodstuff is heat treated when sealed into
its container, problems arise in that the heat treatment will cause the release
of steam from the foodstuff and expansion of air in the airspace around the foodstuff
within the container. This may cause the container to burst, since it is of only
light weight construction, destroying the integrity of the container.
In order to reduce this problem, it has been proposed to fabricate
the container from heavier gauge material so that it will resist the expansion
during the heat treatment. However, this imposes cost penalties and requires the
use of different manufacturing techniques, which are unacceptable for operators
who already have existing foodstuff packing equipment.
We have now devised a simple and effective method by which the problem
of expansion can be reduced and the integrity of the container maintained after
the heat treatment and which does not require additional specialised equipment.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a method for the thermal
treatment of a foodstuff contained in a rupturable container, said container comprising
a basal tray member and an upper cover member secured to the tray member, notably
by heat sealing the upper cover member to the tray member, which method comprises
passing the container through a thermal treatment zone, characterised in that
the container fed to the treatment zone is only partially secured to the tray member
so as to permit expansion or contraction of the contents of the container during
the thermal treatment, and in that the container is sealed whilst within or immediately
adjacent the thermal treatment zone once the contents of the container have been
reached the desired temperature.
As indicated above, it has hitherto been considered essential that
the container of foodstuff should be totally sealed as soon as practical after
the contents have been placed in the container so as to avoid the risk of contamination
of the foodstuff. The method of the invention runs contrary to this established
criterion by only partially sealing the container prior to the thermal treatment
and completing the sealing during or immediately after the thermal treatment.
The invention can be applied to a wide range of foodstuffs and containers
therefor, notably to prepared foodstuffs and meals. For convenience, the invention
will be described hereinafter in terms of the heat treatment of a meal portion,
for example a chicken portion or a portion of chilli con carne, held in a generally
rectangular plan shape container. The container of the invention is rupturable,
that is it would break when subjected to the stress of the expansion or contraction
of the contents and the air space above them when subjected to the thermal treatment.
Such containers are typically made from a sheet plastic or other comparatively
fragile material and metal can or other subtantial and rigid containers are not
included within the term container as used herein. Furthermore, the contents of
the container do not fill the container as is required in a tin can or similar
filling and processing line, but there is an appreciable air space, typically
10 to 50% of the volume of the container, over the contents of the containers for
use in the method of the invention.
The thermal treatment can be carried out using a wide range of methods,
for example by steam autoclaving, by infrared or radiant heating. However, the
invention can be applied to freezing methods where the bacteria are killed by rapid
freezing of the foodstuff within the container, since this can cause contraction
of the air space above the contents, leading to collapse of the container cover.
The invention is of especial application in the heat teatment of the container
using microwave energy in a tunnel or other chamber through which the container
is transported on a rubber or other conveyor mechanism. For convenience, the invention
will be described hereinafter in terms of the use of such a heat treatment method.
The thermal treatment is carried out using conventional techniques
and equipment, except as stated below. Thus, the container will typically be subjected
to microwave radiation at frequencies of from 750 to 2500 mHz, eg. 896 or 2450
mHz. to achieve a temperature of at least sufficient to kill the desired bacteria.
Typically, the temperature achieved will be at least 72°C, preferably 75 to 90°C
and this temperature will typically be maintained for from 1 to 6 minutes. The
container will usually be held in a holding region for a period of from 1 to 10
minutes once the contents have reached the desired temperature. The heating equipment
and its method of operation are those conventionally used in the food processing
The container comprises a conventional basal tray and transparent
cover made from any suitable material. Typically, the tray will be made from a
food grade polyvinyl, polyester or polystyrene resin as conventionally used in
the food industry; and the lid or cover from a similar plastic or from any other
suitable material, for example a polyterephthalate resin. The meal portion is
placed in the tray and the cover then affixed using conventional techniques, for
example by adhesive or crimping the cover and tray together along their contacting
peripheries. Since these are well known in the art, they will not be described
in any detail herein. However, the invention is of especial application where the
cover is secured to the tray member by means of friction or heat welding of the
two components. For convenience, the invention will be described hereinafter in
terms of a cover which is heat sealed to the basal tray member.
The heat sealing is conveniently achieved by locating the rim of
the filled tray in a supporting recess or frame and bringing a suitably shaped
heat sealing bar or die into contact with the rim of the tray so as to trap the
edge of the cover or lid between the bar or die and the edge of the tray. The
bar or die is heated to cause fusion of the cover or lid to the tray. The heating
can be achieved by any suitable method, for example by heating the bar or die
by an air jet, by an infra red heater or by induction heating.
However, as stated above, the cover or lid is not fully sealed to
the tray. For example, the bar or die sealer used to seal one of the edge junctions
between the tray and the cover does not extend for the full length of that junction
or has a break along its length, so that there are one or more discontinuities
in the heat seal. Typically, the discontinuity in the seal between the tray and
the cover will be from 0.5 to 20, preferably 5 to 10, mms long to provide an adequate
vent through which expansion can occur during the heat treatment stage. If desired,
there may be more than one vent to a container, as when the cover is attached
to the tray by a series of tack welds, but it will usually be preferred that there
is only one such vent. The optimum size of the vent aperture will depend upon the
nature of the foodstuff within the container and also upon the rate and level
of the temperature rise during the heat treatment. Thus, for foodstuffs which readily
evolve water vapour, it may be possible to use a vent aperture length of 5 mms
with heating tempertures in the range 72 to 75°C, whereas with a drier foodstuff
the vent aperture may be smaller. In general we prefer to use a vent aperture
length of from 5 to 15 mms for most foodstuffs with heating temeratures in the
range 75 to 95°C.
The vent may be merely an unsealed butt join between the cover and
the tray. However, the cover frequently carries a peripheral flange or shoulder
which locates upon the upstanding rim of the tray or vice versa. In such a case,
the unsealed portion of the rim of the cover can be formed from a flexible material
which adopts a shape closely aligned to that of the rim of the tray it contacts.
The overlap between the tray and cover material will thus act as a temporary loose
seal which can be readily blown open as the contents of the container expand.
For convenience, the invention will be described hereinafter in terms
of a container in which the cover or lid is made from a semi-rigid thermoplastic
material so that there is a positive aperture between the cover and tray at the
The aperture is conveniently formed by cutting away part of a conventional
heat sealing bar or die used to seal the lid or cover to the tray so that heat
sealing between the lid or cover and the tray does not occur at the cut away portion.
This can readily be achieved by filing away part of the bar or die so that conventional
machinery can readily be modified for use in the method of the invention.
The container is fed to and thermally treated in the microwave tunnel
in the conventional manner, at the conventional temperature and for the conventional
time required to pasteurise the meal portion within the container. During such
heat treatment, some steam may be given off from the meal portion and the air in
the head space above the meal portion within the container will expand. Such expansion
will vent through the discontinuity in the seal between the cover and the tray
so that minimal expansion stresses are applied to the comparatively fragile cover,
which thus remains intact. However, once the food portion has reached the desired
treatment temperature, the majority of any venting of the expansion will have taken
place and the container can then be sealed to provide a sterile environment within
The sealing can be achieved by any suitable method, having regard
to the material from which the tray and cover are made and the vent gap to be sealed.
Thus, it will usually be preferred to apply a conventional heated bar sealer to
the unsealed area. However, other sealing methods, for example a hot air sealer
or the application of a thermoplastic sealer to the vent gap can be used. For
convenience, the invention will be described hereinafter in terms of the use of
a conventional heat sealer bar technique in which a heated bar is slid over the
area to be sealed.
The vent gap can be sealed whilst the container is still within the
heating zone. However, this may impose problems due to the interaction of the microwave
energy on any metal components of the heat sealing mechanism. It is therefore
preferred that the heat sealing be carried out when the container is retained in
a holding zone at the desired temperature for the dwell time required to destroy
the bacteria in the meal portion. Such dwell time is usually achieved by carrying
the container out of the immediate range of the microwave heater elements but retaining
them within the thermally insulated tunnel so that comparatively little heat is
lost from the container. The heat sealer can be located within this dwell area
of the tunnel and can be operated in the conventional manner.
Once the container has been sealed and the meal portion has dwelt
at the desired temperature for the desired time, the container is discharged from
the microwave tunnel in the conventional manner. If desired, and adequate sterility
provisions have been made, the heat sealing of the container can take place immediately
after the container is discharged from the tunnel, but before the contents of the
container have begun to contract to any significant extent.
The method of the invention reduces the problems of rupture of the
conventional fully sealed containers during the thermal treatment and requires
the minimum of change to the conventional operations and equipment. The invention
can thus be applied to existing container filling and sealing lines and to existing
thermal treatment operations by simple modification of the container sealers and
by the provision of a container sealer mechanism to a conventional microwave tunnel
or other conventional thermal treatment equipment.
The invention therefore also provides a microwave tunnel which incorporates
a mechanism for sealing a container within the tunnel. Preferably, the sealing
mechanism is a bar heat sealer located within the dwell section of the tunnel.
By sealing the container after the majority of the expansion has
been vented, the sealed container will be under slight vacuum once the contents
have cooled to ambient temperature. Subsequent inspection of the container will
readily establish whether this vacuum has been retained or not. Containers produced
by the method of the invention can thus be readily inspected at the point of sale
by the purchaser to ensure the status of the container without the need for expensive
bacterial testing equipment or specialist expertise.
The invention will now be illustrated by way of example only in the
following examples in which all parts and percentages are given by weight unless
Samples of a range of pre-prepared meals, for example chicken portions
with potatoes and gravy, chilli con carne, and lasagne or spaghetti meals, were
filled individually into rectangular plan shape trays made from crystalline or
amorphous polyester resins. The weights of the samples ranged from 200 to 450
gms. The containers were covered by lids of the polyterephthalate ester resin sold
under the trande mark Melinex 850 and the covered containers were fed to a support
frame where the edges of the trays were supported under a heated die. The edges
of the trays and lids heat were sealed together by applying the heated die thereto.
The die had a section 10mms long cut away from one longitudinal section thereof
so that the resultant heat seal between the tray and cover had a 10 mms vent aperture
The partially sealed containers were then fed in single file on a
conveyor through a conventional microwave tunnel operating at 896 mHz and 20Kw
to achieve a temperature within the range 75 to 85°C within the contents of the
containers. The dwell time within the heating zone of the tunnel was from 2 to
3 minutes. During the heating stage, steam and air vented from the containers through
the vent apertures and rupture of the containers was avoided.
The hot containers passed from the heating zone of the tunnel to
a holding zone continuation of the tunnel where they were maintained at an elevated
temperature for a period of from 2 to 6 minutes to complete the heat treatment
of the contents. Within the holding zone of the tunnel was mounted a heated shoe
sealer which was spring biassed to bear against the partially sealed longitudinal
edge of the containers. The shoe was heated to from 150 to 200°C by a hot air
jet and caused sealing of the vent aperture to take place as the containers passed
under the sealing shoe.
The sealed containers were discharged frommthe tunnel and allowed
to cool in the normal manner. As the contents cooled, a vacuum developed within
the container which could readily be detected by the inward bowing of the container
lid or cover. This provides a simple and effective means for confirming that the
container had not been ruptured during subsequent storage and transport prior to
The invention has been described above in terms of the treatment
of a single container. However, it will be appreciated that the invention can be
applied to the treatment of a series of containers travelling in a row, in which
case the heat sealing means can extend over several containers in each row so that
each heat sealing stage can be carried out on a plurality of containers simultaneously.