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(11) | EP 0 768 248 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Bag for microwave cooking |
| (57) Disclosed herein is a self-supporting bag for microwave cooking which can be easily
sealed simply by folding back its upper part, so that it keeps an internal pressure
and temperature that permit uniform heating and good cooking. The bag is made mainly
of heat-resistant plastic film (F), with the opposing front and back walls (1a, 1b)
joined at the side edges (3), and has a bottom (29) which is connected to the lower
edges of the front and back walls (1a, 1b) and is folded between them into an inverted
V shape, said bottom having the side edges of its folded parts (2a, 2b) joined at
4a, 4b to the front and back walls (1a, 1b), the upper part of at least either of
the front and back walls being formed from paper (P). The bottom (2) has the side
edges (2a, 2b) of its folded parts integrally bonded to the front and back walls (1a,
1b) at the part at least above the range in which the bottom is folded, and the bottom
is formed in a shape like a ship's bottom. |
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a bag that can be easily used for cooking food (such as boiled food, steamed food, and broiled food) in a microwave oven.
[0002] It has been common practice in microwave cooking to place food in a special container molded from heat-resistant plastics or in a dish covered with wrapping film. Such a container or dish needs washing after microwave cooking and also needs a large space for storage. Throwaway containers solve these problems but pose another problem with waste disposal.
[0003] There has recently occurred an idea of utilizing a bag of heat-resistant plastic film for microwave cooking. This bag, however, should not be a mere flat bag, because, when it holds food (such as ingredients and cooking liquid), it is not sufficiently self-supporting and hence causes the spilling of cooking liquid. In addition, a flat bag dose not permit sufficient convection of liquid therein during cooking, which leads to uneven seasoning and heating. For this reason, a flat bag is not suitable for microwave cooking.
[0004] There is a possibility of using for microwave cooking a self-supporting bag as disclosed in Japanese Patent Publication No. 44261/1984. A disadvantage of this bag is that food collects in the flat extending lower part of the bag, preventing convection during cooking. Therefore, it is not satisfactory from the view point of heating and seasoning effects.
[0005] No matter what the shape of bag, it is necessary to close the bag during microwave cooking. Otherwise, steam escapes from the bag and hence the bag does not sufficiently inflate and the desired cooking temperature is not maintained. This leads to water loss, scorching, and uneven heating and steaming.
[0006] For this reason, a bag for microwave cooking needs simple sealing which does not impair bag's breathability. This simple sealing may be accomplished by means of adhesive tape, clip, or fastener. Such a closing means, however, has to be provided separately from the bag and is not capable of closing the bag, without complicated operation, in such a way that the bag still has some breathability suitable for cooking. It may be possible to attach a closing means to the opening of the bag itself; however, this will lead to an increased production cost which does not justify throwaway cooking bags.
OBJECT OF THE INVENTION
[0007] The present invention was completed in view of the foregoing. It is an object of the present invention to provide a self-supporting bag for microwave cooking which can be easily sealed, with adequate breathability remaining, simply by folding back its upper part, so that it keeps an internal pressure and temperature required for cooking. The gist of the present invention resides in a self-supporting bag for microwave cooking which is made mainly, of heat-resistant plastic film, characterized in that the upper part of at least either of the opposing front and back walls thereof is made of paper which remains folded once it is folded.
[0008] It is another object of the present invention to provide a bag for microwave cooking which has a bottom which upon inflation takes a shape like a ship's bottom so that food therein is uniformly soaked in the cooking liquid (such as broth and boiled extract) by convection. Therefore, the bag permits uniform heating and cooking. The gist of the present invention resides also in a self-supporting bag for microwave cooking which is made mainly of heat-resistant plastic film, characterized in that the opposing front and back walls are joined together at their side edges, the lower parts of the front and back walls are connected to a bottom which is folded in an inverted V shape between the front and back walls, with both side edges of the bottom being joined to the front and back walls, and at least either of the front and back walls above the space holding food is made of paper which remains folded once it is folded.
[0009] It is another object of the present invention to provide a bag for microwave cooking which is made mainly of plastic film and partly of paper, with the two materials being joined together by heat fusion for easy fabrication.
[0010] The self-supporting bag for microwave cooking according to the present invention is made mainly of heat-resistant plastic film, with the upper part thereof being made of paper which remains folded once it is folded. Therefore, it stands upright when it holds food, without the possibility that the bag falls or bends to cause the spilling of cooking liquid during cooking. Since food is held in the lower part of the bag which is made of heat-resistant plastic film, the upper part of the bag which is made of paper does not come into contact with the cooking liquid and hence is safe from damage.
[0011] The bag for microwave cooking according to the present invention remains sealed once its upper part is folded back, because the upper part is made of paper which remains folded once it is folded. Thus the bag needs no special tool for closing and sealing. The sealed bag, with its upper part folded back, inflates due to increasing internal pressure by steam which is evolved during cooking, but the sealed bag discharges excess steam through the paper and the gap at the folded part and hence keeps its intenal pressure adequately. The adequate internal pressure produces cooking temperature desirable for individual foods. In addition, the internal pressure does not rise excessively to break the bag during cooking.
[0012] The cooking bag having a curved bottom shaped like a ship's bottom permits the food to collect at the center of the bottom and hence permits the smooth convection of the cooking liquid. This, in combination with the adequate internal pressure, contributes to uniform heating and good seasoning.
[0013] Although the cooking bag is made partly of paper, it can be fabricated easily and economically by heat fusion which provides strong sealing between plastic film and paper.
[0014] These and other objects and advantages of the invention may be readily ascertained by referring to the following description and appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Fig. 1 is a partly cutaway front view of an cooking bag as an embodiment of the present invention.
[0021] Fig. 7 is a perspective view of a cooking bag as another embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Figs. 1 to 6 show a cooking bag (A) as an embodiment of the present invention. It is a self-supporting bag made mainly of durable plastic film (F) and is constructed as explained below.
[0025] The opposing front and back walls (1a, 1b) are joined together at their side edges. Their lower edges are integrally connected to the bottom (2) which is folded in an inverted V shape between the front and back walls (1a, 1b). The bottom (2) is composed of folded parts (2a, 2b), whose edges are joined to the front and back walls (1a, 1b), so that the folded parts (2a, 2b) are unfolded when the lower parts of the front and back walls (1a, 1b) are opened. With its bottom (2) unfolded, the cooking bag (A) can stand by itself.
[0026] The edges of the front and back walls (1a, 1b) are joined together at 3, and the folded parts (2a, 2b) are joined to the front and back walls (1a, 1b) at 4a and 4b, respectively.
[0027] At the side edges of the bottom (2), which are on the extension of the joint (3) of the edges, the folded parts (2a, 2b) of the bottom (2) are integrally joined to the front and back walls (1a, 1b) at the upper part of the inverted-V fold of the bottom. This structure prevents the bag's lower part (including the bottom) from bulging but permits the bottom to curve and expand into a shape like a ship's bottom as shown in Figs 4 and 5. The joining part is indicated by 5 (shaded). This joining part (5) increases the strength of the joint of the folded bottom (2) at the joining part (3).
[0028] The side edges of the folded parts (2a, 2b) of the bottom (2) may be joined to the front and back walls (1a, 1b) on the extension of the joining part (3) of their side edges. However, this structure may permit food (B) in the bag to enter the corners of the bottom (2). Therefore, it is desirable that the folded parts (2a, 2b) be joined such that the inner boundary of the joint (4a, 4b) is inclined as shown in Fig. 1. The angle of inclination may be adequately established according to the shape which is formed when the bottom is unfolded. The inner boundary may be curved. In the case shown in Fig. 1, the lower part of the joining part (5) is cut off at 6 so that the joining parts 4a and 4b are left.
[0029] At the lower edge of the bag, the front and back walls (1a, 1b) and the folded parts (2a, 2b) are joined together by fusion bonding at 7a and 7b. This joining is not always necessary if the bottom (2) is integrally connected to the front and back walls (1a, 1b). However, fusion bonding increases the standing stability of the bag and strengthens the lower part of the bag. It is also possible to form the bottom (2) from a film which is different from the one for the front and back walls (1a, 1b).
[0030] The self-supporting bag constructed as mentioned above is characterized in that the upper part of either or both of the front and back walls (1a, 1b), excluding the lower part that holds food, is made of paper (P) which remains folded once it is folded. (In the embodiment shown in the figure, the upper part of the front wall (1a) is made of paper.) This paper (P) is fusion-bonded to the film (F) which constitutes the lower part of the front wall (1a). The joint is indicated by 8.
[0031] The above-mentioned paper (P) will cover 30-65% (preferably 45-60%) of the height of the bag, as shown in the figure, depending on the dimensions and volume of the bag.
[0032] The above-mentioned plastic film (F) as the major constituent of the cooking bag should preferably be a laminate film having heat resistance higher than 140°C. Such a laminate film may be obtained by dry lamination or extrusion coating from thermoplastic base film having both heat resistance and strength and heat-sealable inner film. Examples of the base film include polyester film (particularly crystalline one) such as polyethylene terephthalate film and polybutylene terephthalate film. Examples of the inner film include polypropylene film.
[0033] Any other laminate film and single-ply film may be used so long as they have sufficient heat resistance for cooking. The thickness of the plastic film ranges from tens to hundreds of micrometers depending on the strength of the bag required. The plastic film may be transparent or opaque. Transparent ones are preferable because they make the content visible.
[0034] The heat-sealable laminate film (F) facilitates bag making by fusion bonding at the joints (3, 4a, 4b, 5, 7a, 7b, and 8), although it is possible to use an adhesive for bonding.
[0035] In making a bag from the above-mentioned laminate film, the outer base layers (2a, 2b) come into contact with each other when the bottom (2) is folded into an inverted V shape. Therefore, in order to accomplish fusion bonding, it is necessary to partly remove the base layer so that the inner heat-sealing layer is exposed.
[0036] The above-mentioned paper (P) may be selected from any paper which remains folded once it is folded. The one made of long-fiber pulp is preferable which has adequate breathability for steam and is easily and firmly fusion-bonded to the plastic film (F) through the heat-sealing layer.
[0037] Experimental data show the bond strength (or peel strength) between the paper and the laminate film to be 400-500 g/15 mm. Experimental data also show the tensile strength of the paper and laminate film bonded together to be ca. 4 kg/15 mm. The paper is one which is made of long-fiber pulp and has a basis weight of 73 g/m2 and a thickness of about 106 µm. The laminate film is one which is composed of polyethylene terephthalate film (as base layer) and polypropylene film (a heat-sealing layer). The test piece is 15 mm wide.
[0038] The above-mentioned paper sample has an air permeability of 25 sec/100 cc/645 mm2 (measured according to JIS P8117, Gurley method) and a degree of water repellency of R 8-9 (measured according to JIS P8137).
[0039] It is to be noted that the above-mentioned paper has a much lower air permeability than nonwoven fabric. In other words, the former permits sufficient pressure build-up for cooking in the bag whereas the latter does not.
[0040] The paper (P) does not necessarily need breathability so long as it permits fusion-bonding and remains folded once it is folded. However, it needs a certain degree of water repellency so that it prevents the cooking liquid from oozing out during cooking.
[0041] The thickness and basis weight of the paper (P) should be properly established according to the inflated shape and strength and the ability to remain folded, which are required at the time of cooking. The thickness should be greater than tens of micrometers, preferably from tens to hundreds of micrometers. The basis weight should be 50-80 g/m2, preferably 60-70 g/m2. With the thickness and basis weight lower than specified above, the paper will have low strength and poor dead fold property. With the thickness and basis weight higher than specified above, the paper will be too stiff to permit the bag to inflate into a desired shape.
[0042] The present invention is not limited to the self-supporting bag mentioned in the foregoing example. It may also be applied to a self-supporting bag of other type. Such a bag is illustrated in Figs. 7 and 8. This bag is similar in basic structure to the bag in the foregoing example. The only difference lies in that the bottom (2), which is folded into an inverted V shape, is joined to the front and back walls (1a, 1b) at the downward extension (5) of the joint (3). The folded parts of the bottom are indicated by 2a and 2b. An advantage of this bag is that the bottom (2) does not bulge because the side edges of the bottom are restrained from opening. Therefore, this bag stands by itself, with its bottom inflated in a shape like a ship's bottom, as shown in Fig. 8. In this bag, too, the upper part of, at least, one of the front and back walls (1a, 1b) is made of the above-mentioned paper (P). The same structure and parts as in the foregoing example are given like reference numerals without explanation.
[0043] The above-mentioned embodiment may be modified such that the folded parts (2a, 2b) of the bottom (2) are joined individually to the front and back walls (1a, 1b) by their side edges so that the bottom (2) expands and the upper part of at least one of the front and back walls is made of paper similar to the one mentioned above. The present invention may also include a self-supporting bag of square-bottom type which is made mainly of heat-resistant film, with the upper part of at least one of the front and back walls made of paper.
[0044] The cooking bag (A) shown in Figs. 1 to 6 is used in the following manner. As food (B) to be cooked is placed in the bag, the lower part of the front and back walls (1a, 1b) opens and the folded parts (2a, 2b) of the bottom (2) unfold. Thus the bottom takes a shape like a ship's bottom as shown in Fig. 4 and food (B) rests on the bottom (2) as shown in Fig. 6.
[0045] Then, cooling liquid is placed in the bag, followed by stirring. The upper part (about 30-40 mm from the open end) of the front and back walls (1a, 1b) is folded back, with the paper wall (P) outside, so that the bag is simply sealed. The bag remains sealed because the paper remains folded once it is folded. The bag may be given a mark at which the bag is to be folded back.
[0046] The bag containing food is placed upright on the turntable in a microwave oven, and cooking is carried out for a prescribed period of time. During cooking, the sealed bag prevents steam from escaping, thereby increasing the internal pressure which permits the adequate inflation of the bag and the thorough convection of the cooking fluid.
[0047] The internal pressure reaching a certain level opens up the seal in opposition to the dead fold of the paper. The thus formed opening and the breathability of paper permit excess steam to escape. Therefore, the internal pressure of the bag is kept at an adequate level (usually 1.01 to 1.20 atm) which is required for adequate cooking temperature (e.g., 100-120°C for oil-poor foods and about 10°C higher for oil-rich foods).
[0048] The inflated bottom (2) of the bag which takes a shape like a ship's bottom permits food (B) to collect at the center of the bottom (2), which is desirable for the convection of the cooking liquid. In other words, the inflated bottom causes convection currents to rise evenly along the curved wall of the bag and to fall at the center. Such convection currents contribute to uniform heating and hence uniform seasoning. This effect is similar to that of fall lid.
[0049] The fact that food (B) is held in the lower part of the bag, which is made of heat-resistant film (F), offers the advantage that the cooking liquid, which might boil during cooking, does not come into direct contact with the upper part of the bag, which is made of paper (P), Even though the cooking liquid should come into contact with the paper (P), the cooking liquid will not ooze out if the paper (P) has water repellency.
[0050] The above-mentioned cooking bag (A) permits cooking by boiling, steaming, or broiling without the necessity of controlling heating power as in the case of cooking on a gas range and also without the possibility of food scorching and being over-done.
[0051] In addition, the cooking bag (A) permits cooking without scorching and uneven heating because it is not restricted by the viscosity of cooking liquid, which may be above 1-2 centipoise.
1. A self-supporting bag for microwave cooking which is made mainly of heat-resistant
plastic film, characterized in that the upper part of at least either of the opposing
front and back walls thereof is made of paper which remains folded once it is folded.
2. A self-supporting bag for microwave cooking which is made mainly of heat-resistant
plastic film, characterized in that the opposing front and back walls are joined together
at their side edges, the lower parts of the front and back walls are connected to
a bottom which is folded in an inverted v shape between the front and back walls,
with both side edges of the bottom being joined to the front and back walls, and at
least either of the front and back walls above the space holding food is made of paper
which remains folded once it is folded.
3. A self-supporting bag for microwave cooking as defined in Claim 2, wherein the bottom,
which is folded into an inverted V shape, has its side edges integrally bonded to
the front and back walls at the part at least above the range in which the bottom
is folded, and the bottom is formed in a shape like a ship's bottom.
4. A self-supporting bag for microwave cooking as defined in any of Claims 1 to 3, wherein
the plastic film as the major constituent is composed of a base layer of heat-resistant
plastic film and an inner layer of heat-sealable plastic film, and the paper constituting
the upper part of the bag is made of long-fiber pulp and is joined by fusion bonding.