FR2852921A1 - METHOD AND INSTALLATION FOR HEATING AND PACKAGING A PRODUCT IN A PACKAGE, AND PACKAGE USED - Google Patents

Abstract

A consumable product (14), in particular a food product, is introduced into a package (10) and heated in the latter. After having introduced the product into the packaging, the latter is partially closed while preserving at least one temporarily unsealed area (17) between two walls of the packaging (10A, 10B), the product is heated in the partially closed packaging , then the packaging is completely closed by welding the walls in said temporarily non-welded area.

Description

The present invention relates to a method for heating and

  packaging a consumable product, in particular a food product, in a packaging, process according to which said product is introduced into the packaging and this product is heated.

  Heating the product is used in particular to promote its conservation. It can be a simple cooking, complete or partial, or else pasteurization, or even sterilization.

  It is recalled that pasteurization makes it possible to destroy certain bacteria by heating. During pasteurization, it is sought to avoid, at least to limit, the deterioration of the physicochemical properties of the consumable product. For example, during a pasteurization, this product can be brought to a temperature of 800C to 1000C, for a given time, for example of the order of 3 to 5 min, so as to assure it a expiration date of 20 days after this treatment.

  By carrying out sterilization, this expiry date can be postponed to a much longer period, for example of the order of one year, by bringing the product to a higher temperature, for example of the order of 1200C, for the time required, for example of the order of 3 to 5 min. The consumable product is in particular a food product, such as vegetables, fruit or other fresh, blanched or precooked, or frozen food. It can also be ready meals, taking for example meat or fish, or any food preparation.

  The packaging may in particular be a tray, or else a flexible pouch. It is advantageously made of plastic or composite, allowing heating by microwaves. At the end of the process, the product is packaged in this packaging, in which it can be kept for some time. Depending on the case, it can, when the package is opened, be consumed as a fresh product or as a cooked or precooked product. It can be reheated before consumption, without necessarily being removed from the packaging at the same time.

  When the product contained in the package is heated, a vapor is produced which puts the interior of the package under pressure. Various possibilities are known for preventing this overpressure from leading to a bursting of the packaging and to a failure of the heating process.

  Thus, FR-A-2 635 760 recommends the adaptation on the packaging of a valve formed of a composite resin which becomes porous when the temperature reaches a certain value, thus allowing the evacuation of excess gases during the heater.

  FR-A-2 695 108 recommends making a hole in the packaging and placing on this hole a portion of sheet sealed with a special adhesive, which allows the lifting of this portion of sheet when, during heating, the overpressure in the packaging becomes too great.

  WO 91/01927 proposes to use a packaging, the sealing of which has a zone of partial interruption, covered with a coating, for example of adhesive, which melts during heating, so as to allow a reduction in the overpressure in the packaging, and which solidifies upon cooling so as to close the packaging.

  All these processes are relatively complex and use sealing or bonding systems, attached to the packaging and having very specific physical properties. In the above examples, the valve system, the glue or the adhesive must regain properties after heating (losing porosity, regaining an adhesive effect or becoming solid again) allowing perfect closure of the packaging. It is always possible that this new state is only imperfectly reached or that, before it is reached, the interior of the package has been contaminated.

  The present invention proposes to provide a much simpler and effective solution to the problem of overpressure which occurs inside the packaging during heating.

  This object is achieved thanks to the fact that, after having introduced the product 30 into the packaging, the latter is partially closed while preserving at least one temporarily non-welded zone between two walls of the packaging, the product is heated in the packaging partially closed, then the packaging is completely closed by welding said walls in said zone.

  Thus, according to the invention, one deliberately chooses to enclose the packaging only partially before heating. At the end of the heating, by a welding operation, the area left temporarily welded is sealed between the two walls of the package, and it is very easily ensured that this area is perfectly sealed, so as to completely close packaging. In particular, this weld can extend not only over the area left temporarily unwelded, but also slightly beyond.

  Advantageously, the product is introduced into the packaging through an opening formed between said walls and the packaging is partially closed by welding said walls to a part of said opening and preserving said temporarily non-welded zone on a remaining part of the opening.

  The packaging may for example be a flexible rectangular pocket, welded on three sides, but one side of which, through which the product has been introduced into the packaging, has remained unwelded. A rigid or semi-rigid tray can also be used. Initially, this tray can be opened on one of its faces. After the product has been placed in said tray, a cover (for example constituted by a flexible sheet or by a rigid or semi-rigid plate, possibly formed from the same material as the tray) can be placed across this face, so as to completely cover it, but only to be welded to the rim of the tray over most of its contour, leaving a small part that is not welded, which will only be welded after the product has heated up.

  Advantageously, the dimensions of the temporarily unwelded area (s) are chosen so that the vapor released by the contents of the package during heating causes overpressure inside said package. It is for example advantageous that this overpressure is at least substantially equal to 0.2 bar, preferably 0.4 bar.

  As is known, maintaining a certain overpressure can in fact promote the heating and cooking of the product contained in the packaging. Preferably, the step in which the packaging is completely closed is carried out immediately after heating, while a slight overpressure (advantageously at least 0.2 bar, or even at least 0.4 bar) still prevails in this packaging, so as to avoid any risk of contamination of the contents of the packaging by products located outside of the latter.

  Advantageously, before heating, the consumable product and an additional liquid are introduced into the packaging. It is advantageously an aqueous liquid, pure water or water with one or more additives which can, for example, be chosen from dyes, preservatives, flavoring agents, perfumes, bodies. emulsified fat. A nonaqueous liquid, for example alcohol, can also be used. It is preferably a liquid which vaporizes during heating, the latter generally occurring at a temperature below 1500C.

  During heating, this liquid vaporizes very quickly and the steam thus generated, which quickly creates an overpressure, promotes the cooking of the product contained in the packaging. As indicated below, adding a liquid to the packaging before heating is particularly advantageous when this heating is carried out by microwaves.

  Advantageously, the contents of the packaging are suddenly cooled after having completely closed the latter.

  An abrupt cooling makes it possible to stop the cooking of the product, to obtain exactly the desired cooking, while minimizing the degradation of the organoleptic properties of the product.

  During cooking, depending on the dimensions of the provisionally non-welded area, a certain amount of steam may have escaped from the packaging. The brutal cooling after total closure suddenly condenses the vapor still contained in this packaging, so that the internal pressure decreases very quickly.

  This creates a partial vacuum effect inside the packaging.

  As indicated above, at least one of the walls can be formed by a flexible film. This film tends to deform during heating to allow an increase in the interior volume of the packaging but, during the brutal cooling, it tends to shrink, thus reducing the volume of the packaging and facilitating its subsequent handling.

  According to a particularly advantageous embodiment, to preserve said temporarily non-welded area, said walls are welded along a weld line having an interrupted area.

  Advantageously, to preserve the temporarily non-welded zone, the walls are welded along a first weld line having a first interrupted face and along a second weld line, distant from the first weld line and having a second interrupted face, said first and second interrupted areas being offset.

  The invention also relates to an installation for heating and packaging a consumable product, in particular a food product, in a package, comprising a heating station and drive means capable of driving the product contained in the package into the installation.

  The object of the invention is to propose an installation of this type, making it possible in a simple way to avoid or, at least, to considerably limit, the harmful effects of an excessive overpressure inside the packaging during heating. .

  This object is achieved thanks to the fact that the installation according to the invention comprises, arranged upstream of the heating station in the direction of advancement of the drive means, partial closing means capable of partially closing the packaging in preserving at least one temporarily non-welded zone between two walls of the package and, arranged downstream of the heating station, complementary closing means capable of welding said walls in said zone.

  For example, the packaging containing the consumable product is driven step by step in the various stations of the installation.

  Advantageously, the installation comprises, arranged upstream of the heating station, means for introducing the product through an opening formed between two walls of the packaging, and the partial closing means are capable of welding together said walls according to a part of the opening while preserving said temporarily non-welded zone on a remaining part of the opening.

  The invention also relates to a package containing a consumable product, in particular a food product which has been heated.

  The object of the invention is to propose a packaging making it possible to avoid or, at least, to considerably limit, the harmful effects of an overpressure inside the packaging when the product it contains is heated.

  This object is achieved thanks to the fact that the packaging according to the invention has at least two walls welded along at least one partial weld line preserving an area initially not welded along said line and according to at least one complementary weld area, connected to said partial sealing line and extending across said zone 10 initially not welded so as to completely close the package.

  The complementary weld line is made, across the non-welded area of the partial weld line, after the product contained in the packaging has been heated.

  Advantageously, the non-welded zone of the partial weld line 15 is part of a passage in the form of a baffle capable of allowing, before the completion of the complementary weld line, communication between the interior space of the packaging and the 'outside.

  Thus, advantageously, the packaging has a first and a second partial weld line spaced from each other and preserving, respectively, a first and a second, an area that is not welded along the line considered, and at least one weld area. complementary connected to the first seal line and extending across the first zone so as to completely close the package, said first and second zones being offset.

  The offset between the non-welded zones of the first and second partial weld lines makes it possible to generate a baffle effect. When heating the contents of the package, the overpressure vapor may escape through this baffle, but this evacuation will be difficult enough to maintain a certain overpressure inside the package. The amount of gas escaping will be sufficient to avoid bursting of the packaging, but the positive effects of an overpressure on the heating will be preserved.

  The invention will be clearly understood and its advantages will appear more clearly on reading the detailed description which follows, of an embodiment shown by way of nonlimiting example. The description refers to the accompanying drawings, in which: - Figure 1 shows a packaging of the type of a flexible pouch, in which a food product is about to be arranged for heating; - Figures 2 to 4 illustrate successive stages of the heating method according to the invention, applied to such packaging; - Figure 5 shows the packaging in its final state; FIG. 6 schematically illustrates an installation according to the invention; FIG. 6A is a section along the line VIA-VIA in FIG. 6; - Figure 7 shows, according to a variant, a partial weld of the packaging before heating; - Figure 8 shows this welding completed after heating; - Figure 9 shows a package of the type of a tray which can also apply the method according to the invention; and - Figures 10 and 11 are comparative curves of cooking vegetables with and without liquid additives in the package.

  In Figure 1, the package 10, for example consisting of a flexible bag, has an opening 12 formed between its walls 10A and 10B. A consumable product, for example pieces of vegetables 14, 20 is introduced into this packaging through the opening 12. FIG. 2 shows the pocket 10 partially closed. In fact, the two walls 10A and 10B are welded together across the opening 12 by two weld lines 16A and 16B, between which a temporarily non-welded area 17 is preserved. In the example shown in FIG. 2, The two portions 16A and 16B are two portions of the same rectilinear welded zone, interrupted locally. Of course, not one, but several temporarily non-welded areas could be preserved.

  With the consumable product 14, a liquid 18 was introduced into the pocket. In the example shown, the product 14 and the liquid 18 are introduced through the opening 12, and it is then across this opening that the pocket is then partially welded. It can also be provided that the pocket present in another region, for example in the zone Z indicated in FIG. 1, a local interruption in the welding of its walls, and that, after the introduction of the product 14 and, if necessary , that 35 of the liquid 18, this opening is completely closed by welding.

  In Figure 2, the bag containing the product is ready to undergo heating.

  Figure 3 shows the situation of the bag during heating. Under the effect of heating, the pressure inside the bag increases, in particular due to the evaporation of water due to the increase in temperature. The water which evaporates can come entirely from the consumable product 14 (it can for example be fresh or semi-fresh vegetables having a certain water content) or else it can also come from the liquid 18 when it is an aqueous liquid. The vapor 18 'fills the interior space of the bag and, as the vaporization increases as the temperature rises, the walls 10A and 10B of the bag deform so as to allow its volume to increase. The excess steam escapes through the opening 17A formed between the walls 10A and 10B due to the presence of the temporarily non-welded zone 17.

  Advantageously, the dimension of this zone is small enough that, while allowing evacuation of the excess vapor pressure, it allows the establishment of an overpressure inside the pocket. Indeed, such an overpressure of vapor promotes heating.

  Figure 4 shows the bag after heating. It can be seen that, instead of the temporarily non-welded zone 17, a weld 17 ′ has been made so as to completely close the pocket.

  After this total closure, the bag and its contents have undergone a sudden cooling to arrive in the state shown in FIG. 25. It can be seen that the walls of the bag 10 are pressed against the product 14 contained in this bag, that they enclose between them. In fact, the brutal cooling made it possible to brutally condense the vapor remaining inside the pocket after it had closed, which produced a sudden decrease in pressure. A partial vacuum effect is thus achieved. This vacuum effect promotes the conservation of the product and, moreover, it facilitates the handling of the pocket whose volume has decreased.

  Figure 6 shows an installation for implementing the method of the invention. For example, the package 10 is formed from a film tube 8, retained on a hollow support 20, also tubular. The lower end of the tube 8 is closed; it is in the lower part that the packaging 10 will be formed. Firstly, through the interior of the support 20, the consumable product 14 initially contained in a reservoir 26 or the like, as well as the liquid initially contained in a reservoir 28 are poured into the lower part of the tube 8.

  When a sufficient quantity has been poured in this way, this lower part of the tube 8 can be closed and cut to form a package 10.

  More specifically, the installation comprises partial closing means such as two welding electrodes 34A and 34B capable of pinching the walls IOA and 10B between them, across the opening 12. These 10 welding electrodes have an area interrupted so to, during this welding, preserve an unwelded zone of the type of zone 17 shown in FIGS. 2 and 3.

  When this welding has been carried out, cutting tools 33A and 33B can be activated to cut the lower part of the tube 10, 15 just above the welding produced by the electrodes 34A and 34B, so as to form a package 10. Other closing means such as welding electrodes 32A and 32B can be activated, for example at the same time as the electrodes 34A and 34B, to weld the two walls of the tube 8 against each other, above of the cut made by tools 33A and 33B. Preferably, the welding carried out by these electrodes 32A and 32B achieves a complete closure of what, after cutting by the tools 33A and 33B, will become the new lower end of the tube 8.

  During this cutting, the packaging formed by a pocket 10 detaches from the tube 8 and falls on a slide 25. It is thus conveyed to a first station of the installation. This first station 21 comprises means such as a vibrating conveyor belt 20A, which serves to distribute the contents of the bag homogeneously inside the latter, which is then placed flat. In this station 21, by means not shown, the pocket can also be pivoted by 900 in a horizontal plane so that the weld lines at its two ends which, in the position of the pocket shown in station 21, extend perpendicular to the plane of Figure 6, are oriented parallel to this plane. At the exit from station 21, the bag is taken up by a new conveyor 20B which, advancing in the direction F, drives it into a heating station 36 advantageously comprising an enclosure 37 for heating by microwaves. We can see a microwave generator 38 placed inside this enclosure 37.

  The latter has an inlet and an outlet for the pocket 10 which are not shown. Of course, several pockets can be located simultaneously in the enclosure 37. The length of the latter and the step of advancement of the conveyor 20B can be adjusted so that the duration of parking of a pocket 10 in the enclosure 36 is sufficient to allow the desired heating.

  Downstream of the heating station 36, the installation includes a complementary closing station 10 in which the pocket is driven by the conveyor 20B. This station comprises complementary closing means comprising, for example, welding electrodes 44A and 44B, able to pinch the two walls of the pocket between them across the zone left temporarily not welded by the electrodes 34A and 34B. These two electrodes 44A and 44B can be substantially similar to electrodes 34A and 34B but, unlike the latter, extend over the entire width of the pocket. They may also extend only over the length of the zone which is temporarily left unsealed 17 or, preferably, over a length at least slightly greater.

  It can be seen in the section of FIG. 6A that, on the conveyor 20B, the pocket 10 can be arranged so that its end comprising the temporarily non-welded zone protrudes beyond the edge of the conveyor 20B. The lower electrode 44B of the complementary welding means 25 can therefore be moved without obstacle.

  Downstream of the complementary closing means, the installation includes means for suddenly cooling the contents of the packaging. They include for example a tank 46 containing cold water 48, in which the bag can be immersed. The temperature of the water 48 can be kept sufficiently low by the maintenance of a circulation of water and the use of cooling means not shown.

  For example, tank 46 is simply placed at the end of the conveyor 20, and the packaging 10 naturally falls into this tank at the outlet of station 40.

  While in station 36, the bag was inflated under the effect of the overpressure generated during heating, it retracts into the tank 46.

  It can be seen in FIG. 6A that the total closure is achieved while the interior of the pocket is still under overpressure, steam continuing to escape through the partial opening 17.

  FIG. 7 shows a variant for the partial closure of the pocket 10. It can be seen in this figure that the walls of the pocket are welded along a first weld line 56 having a first interrupted area 57, and along a second line of weld 58, distant from the first line 56 and having a second interrupted area 59. The first and second interrupted areas 57 and 59 are offset from each other. In this case, the weld lines 56 and 58 are rectilinear, are each formed by two portions of weld lines, respectively 56A, 56B and 58A, 58B, and are parallel to each other.

  For example, the bag 10 is capable of containing 2 kg of potatoes, each of its faces forming a rectangle of approximately 40 cm x 30 cm, the two weld lines are spaced approximately 25 mm apart and the length of each interrupted area 57 and 59 is of the order of 20 mm.

  The partial closure means 34A and 34B of the installation are advantageously able to weld the walls of the packaging according to the weld lines 56 and 58 defined above. For this, the welding electrodes 34A and 34B each have two interrupted edge lines in accordance with the zones 57 and 59, the edge lines of the respective electrodes 25 being located opposite one another.

  The conformation of the lines 56 and 58, and the offset between the zones 57 and 59 makes it possible to obtain a chicane effect for the exit of the steam generated in the pocket 10 during heating. Of course, it is possible to provide for a greater number of weld lines and / or to choose a greater number of temporarily non-welded zones while preserving this chicane effect.

  In FIG. 8, the same end portion of the pocket as in FIG. 7 is shown, after it has been completely closed. It can be seen that welds 57 ′ and 59 ′ have been made respectively across zones 57 and 59. Of course, this total closure could be achieved with only one of the two welds 57 ′ and 59 ′.

  FIG. 9 shows that the invention applies to the heating of products contained in containers other than flexible bags. It may for example be a tray 60 whose upper opening is covered by a cover 62 which is welded to the tray on its rim 5 61. However, on a portion 61A of the rim, extending for example on the entire width of the tray 60, the welding between the cover 62 and the rim is carried out so as to spare the temporarily non-welded area. In this case, the weld is similar to that of FIGS. 7 and 8, and there are the same weld lines 56 and 58 as those which have been described with reference to these figures.

  In the example of FIG. 9, the two walls of the package between which the temporarily non-welded zone is formed are therefore formed respectively by the part of the rim 61A and the end of the film 62 disposed on this part of the rim.

  The consumable product and, if necessary, the additional liquid can be placed in the tray 60 before the fitting of the cover 62 and its welding as shown in FIG. 9. It is only after heating that the zones temporarily not welded 57 and 59 are completely closed.

  The packaging can be formed by a flexible pocket made of a film of composite or plastic material; whether for example a flexible bag or a tray, it is advantageously made of a material allowing the heating, by microwaves, of the product which it contains.

  In particular, this packaging can be formed from polypropylene (PP) or tetrafluorinated polyethylene (PET), polyamide / polyethylene or even polyamide / polypropylene, according to multilayer arrangements.

  In general, the walls between which the temporarily non-welded zone is formed are advantageously made of a material comprising plastics and composite materials capable of being welded by application of heat.

  When at least part of the packaging is formed by a flexible film (the two walls 10A and 10B of the pocket 10 or the cover 62 of the cap 60), the latter tends to deform during heating.

  Depending on the material used, and depending on the thickness of the film, this deformation is more or less significant and the film softens under the effect of the rise in temperature. This can be an advantage because, during the sudden cooling step which follows the total closure of the container, the still flexible film can shrink around the product it contains. This is particularly advantageous when this packaging is formed by a flexible pocket.

  We now describe Figures 10 and II which are comparative curves relating to tests.

  For the test corresponding to FIG. 10, 2,000 g of peeled potatoes, 55 g in size, of the firm flesh variety type Agatha, were placed in a pocket, and 90 g of water were added. The weld 10 made across the opening of this pocket is interrupted on a temporarily unwelded area 15 mm in length. Each pocket wall, made of polyamide / polypropylene multilayer film, having a thickness of 90 µm and a grammage of 89 g / m2 (film distributed under the name "CRYOVAC - NOP" forms a rectangle of 40 cm x 30 cm. 15 Cooking was carried out by applying microwaves, with a power of 3.8 kW for the first 150 seconds (temperature rise time) then, for the remaining 180 seconds (time to maintain temperature), with a power of 3 kW During the heating, the ambient temperature inside the bag 20 ("T ambiance" curve) as well as the temperature of a potato located in the center of the bag and that of an apple are measured. of earth located at the edge of the pocket (curves "T produced in the center" and "T produced at the edge).

  It is found that, during the entire heating, the three curves remain close to each other, which reveals a rise in temperature, then a very homogeneous temperature maintenance inside the pocket. The potatoes are therefore cooked in the same way, whether they are in the center or at the edge of the pocket.

  After heating, the bag is completely closed as soon as it leaves the microwave enclosure and immersed in a tank of water at 100C 30 for 1 min. The weight of the sachet then measured is 2001 g, which represents a loss in mass, essentially due to the evaporation of liquid, of 89 g. The potatoes were therefore cooked evenly without drying out.

  For the test corresponding to FIG. 11, 2000 g of potatoes of the same variety are placed in an identical bag, and the latter is welded so as to provide the same area temporarily not welded. The heating is carried out under exactly the same conditions as in FIG. 10 but the water has not been added. The three curves reflect temperature measurements taken in the same way as in FIG. 10. It can be seen that these temperatures are much less homogeneous than in FIG. 10. In particular, the temperature of the product at the edge increases too rapidly compared to that of the product in the center, while the ambient temperature remains low for approximately 100 seconds, the time necessary for the generation of vapor from the evaporation of the water contained in the potatoes.

  After heating, the bag is completely closed and cooled under the same conditions as in the test in FIG. 10. The mass of the sachet then measured is 1875 g, which corresponds to a loss in mass of 125g.

  In the test corresponding to Figure 11, the product has dehydrated (the significant loss in mass comes from dehydration), and its appearance is somewhat degraded since the potatoes at the edge of the pocket are overcooked , while those on the edge are not enough. In the test in FIG. 10, on the other hand, the 20 potatoes appear perfectly cooked in a homogeneous manner. Indeed, cooking by microwaves, with the addition of water in the sachet allows to combine the advantages of microwaves and steam. Microwaves heat the liquid very quickly and therefore generate steam in a very short time. The microwaves also heat the product itself to the core. Steam promotes heat transfer and the temperature to which the potatoes are brought is more uniform over their entire thickness.

  The choice of microwave heating the product to which a liquid has been added, maintaining an overpressure of vapor during heating, while allowing a limited evacuation of vapor (so as to avoid an excess of overpressure) appears therefore particularly advantageous.

  The quantity of liquid added can be optimized so that, after total closure and cooling of the package, practically no more liquid remains in the bag, i.e. most of the liquid has been discharged or impregnates the product contained in the packaging, without remaining in the liquid state as such.

  It has been found that for consumable products such as fresh vegetables, the amount of liquid added advantageously represents, by mass, from 2% to 10% and, preferably, from 4% to 6% of the mass of the products. The method according to the invention is perfectly suitable for heating fresh products, for example mixtures of different vegetables, and for heating frozen products used as fresh products.

  In some cases, you can choose to use a non-aqueous product (alcohol, ...) or a water / oil emulsion when this product vaporizes at heating temperature.

  In general, it is advantageous to heat the product according to a temperature raising step followed by a temperature keeping step, the power used during the temperature raising step being greater than the power used during the temperature maintenance step. The step of raising the temperature makes it possible to very quickly bring the interior space of the packaging (in particular when a liquid has been added to the product contained in this packaging 20 before heating) to the desired temperature for pasteurization or sterilization. The temperature maintenance step makes it possible to maintain this temperature for the time necessary to obtain the desired expiration date. It is important to avoid overheating the product to keep its organoleptic qualities as much as possible. The combined use of microwave and liquid is also advantageous from this point of view.

  Advantageously, the temperature raising step is continued until there is an overpressure inside the packaging.

  When the latter is a flexible pocket, this overpressure is simply observed on seeing the volume of this pocket increase sufficiently.

  The fact that this overpressure is observed makes it possible to ensure that the sachet contains a sufficient quantity of water vapor to carry out the heating under the desired conditions.

Claims (22)

  1. Method for heating and packaging a consumable product (14), in particular a food product, in a package (10; 60), process by which said product is introduced into the package and this product is heated, characterized in that that, after having introduced the product (14) into the packaging (10; 60), the latter is partially closed, preserving at least one temporarily non-welded zone (17; 57, 59) between two walls (10A, 10B; 61A, 62) of the packaging, the product is heated in the partially closed packaging, then the packaging is completely closed by welding (17 '; 57', 59 ') said walls in said zone.   2. Method according to claim 1, characterized in that the product (14) is introduced into the packaging (10; 60) through an opening (12) made between said walls (10A, lOB; 61A, 62) and the packaging is partially closed by welding said walls along a part (16A, 16B; 56A, 56B, 58A, 58B) of said opening and preserving said temporarily non-welded zone (17; 57, 59) on a remaining part of the 'opening.   3. Method according to claim 1 or 2, characterized in that one chooses the dimensions of the temporarily non-welded zone (s) (17; 57, 59) so that the vapor released by the contents of the package ( 10; 60) during heating causes an overpressure inside said packaging.   4. Method according to claim 3, characterized in that the dimensions of the temporarily non-welded zone (s) (17; 57, 59) are chosen so that said overpressure is at least substantially equal to 0.2 bar, preferably 0.4 bar.   5. Method according to any one of claims 1 to 4, characterized in that, before heating, the consumable product (14) and an additional liquid (18) are introduced into the packaging (10; 60).   6. Method according to claim 5, characterized in that the liquid (18) is water.   7. Method according to claim 6, characterized in that the liquid 35 (18) is water with an additive.   8. Method according to any one of claims t to 7, characterized in that the contents of the package (10; 60) are suddenly cooled after having completely closed the latter.   9. Method according to any one of claims 1 to 8, 5 characterized in that, to preserve said temporarily non-welded area (17; 57, 59), said walls are welded along a weld line (16A, 16B; 56A , 56B, 58A, 58B) having an interrupted area.   10. Method according to claim 9, characterized in that, to preserve said temporarily non-welded area (57, 59), said walls are welded along a first weld line (56A, 56B) having a first interrupted area (57) and along a second weld line (58A, 58B), distant from the first weld line and having a second interrupted zone (59), said first and second interrupted zones being offset.   11. Method according to any one of claims 1 to 10, characterized in that said walls (10A, 10B; 62, 61A) between which the provisionally non-welded area (17; 57, 59) is formed are made of a material comprising plastics and composite materials capable of being welded by application of heat.   12. Method according to claim 11, characterized in that the packaging is formed by a pocket (10) of film of composite or plastic material.   13. Method according to any one of claims 1 to 12, characterized in that the product is heated (14) by microwaves.   14. Method according to any one of claims 1 to 13, characterized in that the product is heated (14) according to a temperature raising step followed by a temperature keeping step, the power used during the temperature rise step being greater than the power used during the temperature maintenance step.   15. Method according to claim 14, characterized in that the step of raising the temperature is continued until an overpressure is noted inside the packaging (10; 60).   16. Installation for heating and packaging a consumable product (14), in particular a food product, in a package (10), comprising a heating station (36) and drive means (22) capable of driving in the installation of the product contained in the packaging, characterized in that it comprises, arranged upstream of the heating station (36) in the direction (F) of advancement of the drive means (22), means of partial closure (34A, 34B) capable of partially closing the packaging (10; 60) while preserving at least one temporarily non-welded zone (17; 57, 59) between two walls of the packaging (1OA, 10B; 62, 61A ) and, arranged downstream of the heating station (36), complementary closing means (44A, 44B) 10 capable of welding said walls in said zone.   17. Installation according to claim 16, characterized in that it comprises, arranged upstream of the heating station (36), means (26) for introducing the product (14) through an opening (12) formed between two walls ( 10A, 10B; 62, 61A) of the packaging (10; 60) and in that the partial closure means (34A, 34B) are able to weld together said walls according to a part of the opening while preserving said provisionally unwelded zone (17; 57, 59) on a remaining part of the opening.   18. Installation according to claim 16 or 17, characterized in that it comprises means (46, 48) for suddenly cooling the contents of the packaging (10; 60) located downstream of the complementary closing means (44A, 44B).   19. Installation according to any one of claims 16 to 18, characterized in that the partial closing means (34A, 34B) are capable of welding said walls of the packaging (10A, lOB; 62, 61A) according to a first weld line (56) having a first interrupted area (57) and along a second weld line (58), spaced from said first weld line and having a second interrupted area (59) which is offset from said first interrupted area.   20. Installation according to any one of claims 16 to 19, characterized in that the heating station (36) comprises a microwave generator (38).   21. Package (10; 60) containing a consumable product (14), in particular a food product having undergone heating, characterized in that it has at least two walls (10A, 10B; 62, 61A) welded according to at least one partial weld line (16A, 16B; 56A, 56B, 58A, 58B) preserving an initially non-welded area (17; 57, 59) along said line and along at least one additional weld area (17 '; 57' , 59 '), connected to said partial sealing line and extending across said initially non-welded area so as to completely close the package.   22. Packaging according to claim 21, characterized in that it has a first and a second partial weld line (56A, 56B, 58A, 58B) distant from each other and preserving respectively a first and a second one. non-welded zone (57, 59) along the line 10 considered, and at least one complementary weld zone (57 ', 59') connected to the first weld line and extending across the first zone so as to close completely the packaging and in that said first and second zones (57, 59) are offset.