Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02611432 2007-09-26
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Method for heat treatment of a product contained in a pack such as a tray
This invention relates to a method for heat treatment of a
product, in particular a food product, contained in a pack such as a tray
having at least one deformable wall, in which the product contained in the
pack is heated, vapour produced by the heating being allowed to escape
through an opening in the pack.
A method of this type is already known through French patent
application number 2 852 921.
Heating the product is useful for helping it keep. It may
concern simple cooking - whether complete or partial - or alternatively
concern pasteurisation or even sterilisation.
As is known, pasteurisation and sterilisation are used for
destroying certain bacteria by heating, in order to push back the use by
date of the product.
The consumable product is in particular a food product, such as
vegetables, fruit or other food, whether fresh, blanched, pre-cooked or
alternatively deep-frozen. It may also concern ready-cooked dishes
including, for example, meat or fish or any food preparation.
It is known that, during the heating of a product of this type,
vapour is produced. This is, in particular, water vapour, produced by the
water which evaporates from the heated product due to the effect of
temperature. In addition, the gas initially contained in the pack is
expanded because of the rise in temperature, the excess gas volume
forming part of the vapour which is allowed to escape from the opening in
the pack at the time of heating to prevent the occurrence in the latter of
overpressure which could lead to bursting or, in any case, to impairment
of the pack and to failure of the heating method.
The above-mentioned French patent application particularly
applies to a pack formed by a tray or by a soft pouch. According to this
patent application, the opening in the pack, through which the vapour can
escape, is formed by an area provisionally not welded between two walls
of this pack and this opening is closed by welding at the end of heating.
At the end of heating, the pack is thus re-closed while its
contents are still hot. During cooling, whether this is sudden or
progressive, the volume of the gas contained in the pack decreases,
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which, as the pack wall is deformable, causes deformation of the latter.
When the pack is formed by a soft pouch, this shrinkage may be an effect
which is sought.
It is not so when the pack is formed by a tray, which must
normally have a substantially parallelepipedic shape. Indeed, the
consumer would be alarmed by the deformation of such a pack.
In order to try and prevent these deformations, European
patent number 1 265 793 recommends finally re-closing the pack only
when the temperature of the product it contains has reached a precise
value and the inside of the pack still has overpressure so that the pack is
not deformed after cooling. Indeed, the volume of gas at overpressure at
the time of closure regains, after cooling, a volume which correctly fills the
normal internal volume of the pack.
This method has a certain benefit but requires, on one the
hand, monitoring the temperature of the product in the pack very
accurately, which is extremely difficult on an industrial scale and, on the
other hand, making sure that the opening in the pack is initially of
dimensions such that a controlled overpressure will be ensured inside the
pack at the time when the latter is re-closed.
Thus, this method has practical implementation difficulties.
The object of this invention is to propose a different solution to
allow a release of vapour during heating and prevent excessive permanent
deformation of the pack's deformable wall, arising from the heating.
This object is achieved due to the fact that the product is
heated so as to be taken to a desired temperature, a temperature-holding
stage is carried out, the vapour being allowed to escape, a gaseous fluid is
introduced into the pack at the end of this temperature-holding stage and
the pack is totally closed.
According to the invention, the heating comprises a
temperature-holding or "temperature adjustment" stage, with which the
desired cooking value (i.e. a value with which the desired cooking can be
carried out), pasteurising value or sterilising value can be obtained and
during which the vapour is allowed to escape. At the end of this stage, a
gaseous fluid is introduced into the pack to cause an increase in its
internal volume by means of a provisional deformation of its deformable
wall and/or to cause increased overpressure. The pack can then be closed
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and, when the gas contained in this pack has regained its "normal"
volume at ambient temperature, the pack will have regained a shape close
to its initial shape, without excessive deformation. As will be seen below,
the volume of gas introduced at the end of the temperature-holding stage
can be determined in such a way that the final shape of the pack is
substantially its initial shape, without appreciable deformation in
comparison with it. This volume can also be determined in such a way
that the final shape is intentionally slightly different from the initial
shape.
For example, the pack can be a semi-rigid tray closed by means of a soft
film and the option can be taken to inject a slightly insufficient volume of
gas so that, after cooling, the outside surface of the film is slightly
concave, thus showing that the product has actually undergone heat
treatment, while the tray walls retain their initial shape.
In other words, with the invention, the pack deformations can
be prevented or controlled. The final shape is appreciated at ambient
temperature and "normal" atmospheric pressure.
For the purposes of this invention, ambient temperature is the
normal temperature for storing the products contained in the pack, i.e.,
for deep-frozen products, of the order of -18 C, for products stored in
cold rooms, of the order of 4 C and for products stored without cooling, of
the order of 20 C.
Advantageously, the gaseous fluid introduced is at a
temperature at least close to the temperature of the pack vapour(the
vapour present in the pack) during the temperature-holding stage.
In this way, a thermal shock between the contents of the pack
and the gaseous fluid introduced into the latter is prevented. In addition,
it is thus ensured that this gaseous fluid corresponds, from the point of
view of its temperature, to the cooking, pasteurisation or sterilisation
criteria sought. The temperature of this fluid can even be turned to
account in order to assist pasteurisation or sterilisation.
Advantageously, the gaseous fluid is a neutral gas, such as
nitrogen, a gaseous mixture of nitrogen/deoxygenate or air,
advantageously filtered. This gaseous fluid is chosen depending on the
products treated.
The introduction of the gaseous fluid is managed in such a way
that, when the pack, once closed, regains ambient temperature, the pack
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wall is substantially free from deformation or deformed in a controlled
manner. It is possible to proceed empirically, by introducing, in the same
introduction conditions, various different volumes of gaseous fluids into
various different packs and then determining therefrom which is the
volume of fluid introduced which leads to the final shape desired, so as to
decide on this volume as the one to be introduced in an industrial manner.
It is also possible to evaluate the volume of vapour which
escapes from the pack during heating and temperature holding and to
introduce a corresponding gaseous fluid volume into this pack. The
evaluation of the volume of vapour which escapes can be carried out by
direct measurement or, indirectly, depending on the length of time for
which the vapour escapes and by taking into account the dimensions of
the opening in the pack.
Then again, it is possible, from tests, to determine the volume
of fluid to be injected depending on the deformation it causes in the pack
wall.
The best conditions for introducing the gaseous fluid having
thus been determined, particularly relating to the length of time for which
this fluid is introduced or the quantity of fluid directly measured which is
introduced, the method can then be industrialised.
To obtain controlled deformation, of limited amplitude and
affecting only certain areas (film) of the pack, a quantity of gas slightly
different from that which would be necessary to prevent deformation and
advantageously slightly less than the latter can be injected to obtain
controlled concave deformation. The difference measured in volume may
be, for example, of the order of 5% to 10%.
The invention will be understood well and its advantages will
appear better when the following detailed description, of an embodiment
shown by way of example without limitation, is read. The description
refers to the appended drawings, in which:
- figure 1 is a vertical section view of a tray containing a product,
prior to heat treatment,
- figures 2 and 3 are plan views of such a tray, for two variants for
forming its opening,
- figure 4 illustrates the stage of heating the product contained in
the pack,
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- figure 5 illustrates the introduction of a gaseous fluid to the inside
of the pack and
- figures 6 and 7 show the final closure of the pack, for the two
variants in figures 2 and 3.
5 The tray in figure 1 comprises a receptacle part 1 and a
covering wall 2, such as a removable film, arranged across the opening of
this receptacle. Food products such as, for example, peeled potatoes 3 are
arranged in this tray. Typically, this tray is made of plastic or aluminium or
a composite such as, for example, card/plastic, card/aluminium or
alternatively aluminium/plastic and it has a relatively thin wall, this being,
for example, 0.5 mm to 2 mm thick. The top wall of this tray is thus
formed by the film 2, which is sealed on the tray edge formed by the
receptacle rim 1A. The materials chosen for the receptacle and the film
are of any types known for this type of tray, for example those based on
aluminium or plastic, in particular polypropylene, possibly loaded as is
recommended by patent application EP 1 313 651.
The film constituting the top wall 2 forms a wall which is easily
deformable. However, in so far as the wall of the receptacle 1 is relatively
thin, in reality all the tray walls are deformable but the film 2 is
deformable to a greater extent.
Before the product 3 contained in the tray is subjected to heat
treatment, this tray is closed in an incomplete way, an opening being left.
In figure 2, this opening is formed by an interruption 4A in the sealing
strip 4 between two walls of the pack, in the case in point between the
receptacle rim 1A and the film 2.
In figure 3, the opening 5 is formed by a hole in one of the
pack walls, in the case in point in the film 2, whereas the sealing strip 4
between the film 2 and the receptacle rim is continuous.
In a general way, the opening through which the vapour can
escape at the time of sealing is of relatively small dimensions so that the
inside of the pack is taken into overpressure at the time of heating, as this
overpressure assists with the heating through of food products.
As indicated above, the vapour which is released comes, on the
one hand, from the vaporisation of the water initially contained in the pack
- particularly in the products which are arranged inside - and, on the other
hand, from the expansion of the gas initially contained in the pack. For the
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purposes of this invention, these two sources of release gas are
designated "vapour".
In figure 4, the tray in figures 1 and 2 is arranged in a heating
enclosure 6. It is, in particular, a microwave tunnel or oven in which
microwaves are emitted by a generator. The tray is carried by a support 8.
In the case where the enclosure is formed by the internal space of a
microwave tunnel, this support 8 can be a conveyor, the path of which
goes through the inside of this tunnel, which has an entrance and exit for
the tray, which are not shown. Of course, several trays can be in the
tunnel at the same time, it being possible to adjust the length of the said
tunnel and the pace of the conveyor travel in order that the duration of a
tray's immobilisation in the enclosure is sufficient to allow the heating
desired.
The tray and its contents are subjected to heating, on the one
hand, to allow a rise in temperature of the product contained in the tray
and, on the other hand, to carry out a temperature-holding stage.
Conventionally, for pasteurisation, the temperature-holding stage is
conducted so that the food product is kept at a temperature of the order
of 80 C to 100 C for a given time, of the order of 3 to 5 minutes for
example. For sterilisation, the product is taken to a temperature of the
order of 120 C to 130 C for the necessary duration, of the order of 3 to 5
minutes for example. The heating and temperature-holding stages can be
carried out in the same microwave enclosure. By way of a variant, two
microwave tunnels, arranged end-to-end, can be used, the first one using
higher power to carry out the temperature rise stage, whereas the second
one only carries out the temperature-holding stage, with lower power.
Of course, heating by means of microwaves is only an example,
it being known that other heating methods can be used, in particular
heating by means of other electromagnetic waves such as high frequency
waves or alternatively heating with steam.
The temperature-holding stage can be carried out as is shown
in figure 4. It is indeed seen on the one hand that, under the effect of the
overpressure generated inside the pack by the vapour produced during
heating, its most deformable wall, in the case in point the film 2, has
become deformed so as to increase the pack volume. The excess vapour V
escapes through the opening 4A.
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At the end of the temperature-holding stage, a gaseous fluid is
introduced into the pack as is shown in figure 5. This introduction can be
carried out in the heating enclosure as is shown in figure 5. However, it is
appropriate to point out that it can also be carried out after the
temperature-holding stage, when the pack comes out of the heating
enclosure.
In the example in figure 5, this introduction of the fluid F is
carried out through an injection nozzle 9 which is engaged in the opening
4A. This nozzle is connected to a gaseous fluid supply system represented
symbolically by a square 10. It is appropriate to point out that the
introduction of the nozzle into the opening 4A is easy as, under the effect
of the release of vapour V, this opening is at least partially open, i.e. the
film 2 is locally kept at a distance from the rim 1A.
The introduction of a nozzle is also easy, from an opening
formed in one of the receptacle walls, like the opening 5 in figure 3. It is
appropriate to point out that, because of the overpressure in the pack, the
film 2 is therefore at a distance from the products contained in the pack,
so that the introduction of the nozzle does not spoil these products.
However, instead of introducing a nozzle into the opening, it is
possible to arrange a gas injection system in the immediate vicinity of this
opening, whether this be the opening 4A or the opening 5, arrangements
being made for the pressure and flow rate of the gaseous fluid to be
sufficient for the latter to enter the pack.
As can be seen by comparing figures 4 and 5, at the time of
introducing the gaseous fluid F into the pack, the film 2 has become more
deformed, increasing the pack internal volume further.
After the introduction of the gaseous fluid, the pack is closed
totally. In figure 6, this closure is carried out by completing the welding
strip 4, as indicated by the hatched area 4B. The opening 4A can, in
particular, be made and re-closed as is recommended in French patent
application 2 852 921.
In figure 7, this closure is carried out by closing the hole 5 by
means of a cap 5'.
Advantageously, as indicated above, the gaseous fluid is
introduced through the opening through which the vapour is allowed to
escape during temperature holding. By way of a variant, another orifice
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can be used. In particular, the vapour can be introduced through a nozzle
forming a needle which,
for introducing the gaseous fluid, pierces one of the pack walls. The hole
thus formed is then re-
closed by, for example, arranging a cap across this hole; the opening through
which the vapour V
has escaped is also closed by sealing or by fitting a cap, for example.
Advantageously, the gaseous fluid which is introduced into the pack at the end
of the
temperature-holding stage is at a temperature between 80% and 130%, preferably
between 90%
and 110%, of the vapour temperature during the temperature-holding stage. As
indicated above,
choosing a gaseous fluid temperature at least close to the vapour temperature
is advantageous.
When even a higher temperature for this gaseous fluid is chosen, in particular
when the latter
temperature is close to 130% of the vapour temperature, the gaseous fluid thus
heated can be
used for assisting with the cooking, pasteurisation or sterilisation.
With tests carried out in various different conditions of introducing vapour
into the pack
at the end of the temperature-holding stage, it is possible to determine what
the optimum gaseous
fluid introduction conditions are so that, once the pack has been closed and
after its contents have
cooled, the balance of pressures between inside and outside the pack is such
that the walls of this
pack remain substantially free from deformation compared with that which is
shown by figure 1
or that they are only slightly deformed in the way desired, only the film 2,
for example, being
slightly concave.
The gaseous fluid introduction conditions are defined by parameters such as:
the
temperature of the gaseous fluid introduced, its composition, the volume of
the fluid introduced
or alternatively, for determining this volume indirectly, the duration of the
introduction stage, the
flow rate of fluid at the outlet from the injection system and entering the
pack, ....
Tests can also be carried out for various different types of pack. openings,
it being known that
the size of the opening obviously has an influence on the volume of vapour
which escapes from the
pack during the temperature-holding stage. This volume of vapour can be
determined indirectly if
the time for which the gaseous release occurs and the dimensions of the
opening are known, in the
knowledge that the pressure of this vapour is generally known from another
source.
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Thus, advantageously, a volume of gaseous fluid corresponding
at least approximately to the volume of vapour which escapes from the
pack during heating and temperature holding is introduced.
As indicated above, these volumes of vapour and gaseous fluid
can be determined by tests, in an empirical manner and by comparison
between various different tests.
It is possible, for example, to evaluate the volume which
escapes and then to introduce a corresponding volume. It is also possible
to go ahead with tests in the same conditions, introducing various
different volumes of gaseous fluid and comparing the initial mass of the
pack with its contents and the mass after closure and cooling which, by
calculation, gives the difference between the volume of vapour which has
escaped and the volume of gas introduced.
Advantageously, the volume of gaseous fluid introduced is
determined in such a way that this volume is substantially equal to the
volume of vapour which escapes from the pack during heating and
temperature holding, preferably being lower than this volume. For
example, the volume of gaseous fluid introduced into the pack can be of
the order of 5% to 10% lower than the volume of vapour which has
escaped from the said pack. With this, it can be arranged that, after the
pack contents have cooled, the most deformable wall of the said pack, in
particular the film 2 which forms the top wall, has a slight concavity, which
assists with the stages of further handling of the pack, in particular the
stacking of several packs of this type. This wall which is kept slightly
concave can also be the bottom of the receptacle. This concavity is
therefore slight enough not to constitute a deformation which is
troublesome for the consumer but, as indicated above, it has certain
advantages. For the purposes of this invention, this concavity is slight
enough for the pack to be considered "substantially free from
deformation" at the end of cooling.
The volume of gaseous fluid introduced into the pack and the
volume of vapour which has escaped from it are compared in conditions
which make this comparison possible, in particular adjusting these
volumes, by calculation, to equivalent volumes at the same temperature,
in particular the ambient temperature.
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By way of a variant, the gaseous fluid is introduced into the
pack, causing deformation of the deformable wall and the pack is closed
when it is found that this deformation has reached a given level.
As has been indicated above, the introduction of the gaseous
5 fluid can cause deformation of the film 2 (see figure 5). The option can be
taken, by means of tests on several identical packs having the same
contents and heated in the same conditions, to close the packs for
different deformations of their walls 2 and then to determine the pack
which has the most favourable geometry at the end of cooling of its
10 contents. The deformation found for this pack will then be decided on as a
criterion for industrialisation of the method. The level of the deformation
is, in particular, reflected in the amplitude of the said deformation.
It has been indicated above that, at the time of heating, this
wall also deforms but to a lesser extent. At the time of the temperature-
holding stage, the flow rate of vapour escaping through the opening 4A or
5 stabilises, with the result that the deformation of the film 2 also
stabilises. The option can be taken to measure the level of deformation at
which the pack is closed in a relative manner, by comparison between the
deformation stabilised at the time of temperature holding and the
deformation obtained just before closure, after introduction of the gaseous
fluid.
Then again, the option can be taken to introduce the gaseous
fluid into the pack for a predetermined length of time, at the end of which
the pack is closed. Here again, with tests it is possible to check, taking
account of introduction conditions (in particular pressure, flow rate and
temperature) and determine which of these tests has led to the best final
geometry of the pack. With this, it is possible to decide on a normal
duration of introduction, which is used when the method is industrialised.
