Microwaveable food package, and method for manufacturing same

Abstract: A microwaveable food package is provided that can retain the quality of food products in an excellent manner, that is highly safe and hygienic, that does not change its properties when irradiated with microwaves, and that can be cooked directly in a microwave oven. A method for manufacturing such a microwaveable food package is also provided. The microwaveable food package is obtained by sealing a food product (3) inside an airtight container (2) together with a sheet-form deoxidizer (1) prepared by kneading an iron-based deoxidizer composition and a thermoplastic resin, and forming a sheet. The method for manufacturing a microwaveable food package comprises a step of kneading an iron-based deoxidizer composition with a thermoplastic resin and forming it into a sheet to yield a sheet-form deoxidizer (1), and a step of placing a food product (3) and the aforementioned sheet-form deoxidizer (1) into an airtight container (2) and then sealing said airtight container (2).

Description: The present invention relates to a microwaveable food package, and to a manufacturing method thereof. More particularly, placing a sheet-form deoxidizer together with the packaged product inside a package yields excellent quality retention and allows cooking to be performed in a microwave oven with the sheet-form deoxidizer remaining in the package. The microwaveable food package pertaining to the present invention can be used in applications aimed at retaining the quality of sterilized cooked rice and other processed grain foods; fried foods; steamed foods; various side dishes; hors d'oeuvres; Japanese and Western confections; various types of noodles; baked potatoes; pouch-packed foods and other semi- or precooked foods; and various other microwaveable foods. Preservation techniques involving the use of deoxidizers have been established in the past as one type of food preservation technique, and their application to various food products has been expanding. A preservation technique involving the use of a deoxidizer is aimed at preventing the oxidation of food fats, preventing discoloration, preserving the taste, preventing insect damage, and preventing the growth of aerobic bacteria by placing a deoxidizer together with the contained product into the food packaging container and sucking out the oxygen inside the container to create an anaerobic state, affording satisfactory retention of the quality of food products. With the recent spread of household microwave ovens, it has become more common to cook or defrost packaged food by irradiating it (in packaged form) in a microwave oven. For this reason, numerous food manufacturers have recently begun marketing various precooked packaged food products for cooking in microwave ovens. Meanwhile, food manufacturers put manufactured packaged food products in cold storage, then defrost the food products by microwave irradiation before shipping in order to be able to balance production and inventory in accordance with fluctuating demand. Techniques involving cooking food products in microwave ovens (by microwave irradiation) have thus become diverse and more widespread. With the spread of such microwaveable food products, a need arose for improved preservability, and the use of deoxidizers with microwaveable food products have gradually gained prominence. It is desirable that a microwaveable food product used together with a deoxidizer be directly placed in a microwave oven without having each time to remove the deoxidizer from the packaging container containing the food product. A commonly used deoxidizer is obtained by placing a particulate deoxidizer composition based on an iron powder into a breathable pouch of packaging material. However, irradiating such a conventional pouched deoxidizer with microwaves results in most of the microwaves passing through the packaging material and being absorbed by the deoxidizer composition based on an iron powder. As a result, eddy currents form in the deoxidizer composition, and heat rapidly evolves. The resulting disadvantage is that the packaging material is burned or the water contained in the deoxidizer composition is rapidly vaporized, causing expansion and pouch rupture. As a result, the deoxidizer spills from the pouch and the packaged material is contaminated. Other problems that have been identified in the past with respect to pouched deoxidizers include obstacles encountered during the filling of the food package and the sensation of a presence of a foreign object. In view of this, various proposals have been submitted in order to address the unsatisfactory microwave resistance of such deoxidizers based on iron powders. In the conventional example described in Japanese Laid-Open Patent Application 63-55075, metal foil is employed as a packaging material for a deoxidizer used in microwave ovens, preventing the contents of the packaging material from heating up. In this conventional example, however, eddy currents are formed in the metal foil of the deoxidizer package by microwave irradiation, generating sparks along the edges of the deoxidizer package. In addition, Japanese Laid-Open Patent Application 2-413 describes a deoxidizer composition for use in microwave ovens that is obtained by mixing an iron powder and a special filler to prevent eddy currents from being generated in the iron powder by diluting the iron powder with the filler. The deoxidizer described in this publication possesses improved microwave resistance but is still disadvantageous in that the contents of the powdery deoxidizer are difficult to handle and that contamination, dusting, and the like readily occur during filling and packaging. Another disadvantage is that the deoxidizer hardens when stored for a long time, losing its microwave resistance. EP 0 481 730 A describes a microwaveable food package, obtained by sealing a food product in an airtight container together with an oxygen absorbing packet comprising an iron-based deoxidizer composition packaged in an air-permeable packaging material and a microwave-proof layer; and a method for manufacturing such a package by packaging an iron-based deoxidizer composition in an an air-permeable packaging material and a microwave-proof layer and placing a food product and the oxygen absorbing packet into a container and then sealing the container to form an airtight container. The document does not describe a sheet-form oxygen absorbing agent prepared by kneading an iron-based deoxidizer composition with a thermoplastic resin. EP 0 367 390 A describes a deoxidizer film having a film body made of a thermoplastic resin in which a deoxidizer composition is dispersed. Microwaveable food packages manufactured using conventional deoxidizers are thus not necessarily satisfactory, and many improvements are still needed. The present invention provides a completely novel microwaveable food package which of course retains the quality of the contained product in an excellent manner, which does not change its properties when placed directly in a microwave oven and irradiated with microwaves, which is highly safe and hygienic, which is easy to manufacture, and which does not create the sensation of a presence of a foreign object when packaged. As a result of painstaking research aimed at overcoming the aforementioned drawbacks of prior art, the inventors devised the present invention after discovering that a deoxidizing sheet obtained by kneading a deoxidizer based on an iron powder with a thermoplastic resin, dispersing the kneaded product, and forming the dispersion into a sheet possesses superb microwave resistance. Specifically, the microwaveable food package of the present invention comprises a microwaveable food package, obtained by sealing a food product inside an airtight container together with a sheet-form oxygen absorbing agent obtained by packaging in a breathable packaging material a product prepared by kneading an iron-based deoxidizer composition with a thermoplastic resin, and forming a sheet. According to a second aspect of the present invention there is provided a method for manufacturing a microwaveable food package, comprising the steps of: kneading an iron-based oxygen absorbing agent composition and a thermoplastic resin and forming them into a sheet to yield a sheet-form oxygen absorbing agent; and placing a food product and said sheet-form oxygen absorbing agent into a container and then sealing said container to form an airtight container. According to a third aspect of the present invention there is provided a method for using a microwaveable food package, comprising the steps of: irradiating a package with microwaves, said package comprising: a food product sealed inside an airtight container together with a sheet-form oxygen scavenger that is obtained by kneading an iron-based oxygen scavenger composition together with a thermoplastic resin and forming a sheet. The product obtained by kneading an iron-based deoxidizer composition with a thermoplastic resin and forming a sheet is referred to herein as "a deoxidizing sheet," and the product obtained by coating at least a portion of the deoxidising sheet or the entire deoxidizing sheet with a breathable packaging material "a sheet-form deoxidizer." The deoxidizing sheet and the sheet-form deoxidizer will hereinafter be collectively referred to as "sheet-form deoxidizers". A deoxidizing sheet obtained by kneading an iron-based deoxidizer with a thermoplastic resin, dispersing the ingredients, forming the dispersion into a sheet, and drawing the sheet possesses excellent microwave resistance and oxygen absorption characteristics, and is thus suitable as the sheet-form deoxidizer for the present invention. The deoxidizer based on an iron powder that is used for the deoxidizing sheet contains the iron powder as its principal component, also contains metal halide salts, and may optionally contain added activated carbon, fillers poorly soluble in water, and other components. Although this deoxidizer based on an iron powder can be a simple mixture of the aforementioned components, it is preferable for a metal halide salt to be applied to or to be dispersed and deposited on the surface of an iron powder, a mixture of an iron powder and activated carbon, or the like. A reduced iron powder, electrolytic iron powder, atomized iron powder, or the like can be used as the aforementioned iron powder. A powder of small particle size is preferred. A powder with a particle diameter of 100 µm or less is commonly used, and a powder with a particle diameter of 50 µm or less is preferably used. Examples of metal halides include sodium chloride, potassium chloride, barium chloride, calcium chloride, and magnesium chloride. A low-density polyethylene, high-density polyethylene, polypropylene, ethylene-α-olefin copolymer, polymethyl pentene, or other olefin-based resin can be used as the thermoplastic resin in which the deoxidizer based on an iron powder is dispersed. A deoxidizing sheet can be easily manufactured by the method described in Japanese Laid-Open Patent Application 2-72851, for example. Specifically, with such a deoxidizing sheet, a deoxidizer based on an iron powder is kneaded with the aforementioned thermoplastic resin in a ratio of 30 to 85 wt% scavenger to between 15 and 85 wt% resin, the kneaded product is melted and formed into a sheet, and the sheet is drawn. It is preferable for the resulting draft ratio to be 1.5 to 10. The optimum thickness value of the deoxidizing sheet can be suitably selected with consideration for the oxygen absorption characteristics, workability, filling properties, and the like, with a range of 0.2 to 3 mm being preferred. In the present invention, the deoxidizing sheet is used as a sheet-form deoxidizer coated completely or at least partially with a breathable packaging material. There are two principal types of such sheet-form deoxidizers: a type in which the deoxidizing sheet is packaged using a breathable packaging material, and a type in which the sheet-form deoxidizer and a breathable packaging material are laminated and coated. Listed below are specific examples of such sheet-form deoxidizers. The present invention is not limited by these examples, however. (1) Type Obtained by Packaging Deoxidizing Sheet in Breathable Packaging Material This is a form in which a deoxidizing sheet is sandwiched between a packaging material having a double-layer structure composed of a polyethylene terephthalate layer and a polyethylene layer, and a packaging material composed of a water-resistant nonwoven fabric (for example, "TYVEK" (trade name) manufactured by Du Pont, or "Luxer" (trade name) manufactured by Asahi Chemical Industry Co., Ltd.) or the like. (2) Label Type Obtained by Combining Deoxidizing Sheet, Breathable Packaging Material, and Adhesive Material This is a label type that is obtained by laminating a base layer having an adhesive layer, an deoxidizing sheet, and a breathable layer in the sequence indicated, and directly bonding the breathable layer to the base layer along the perimeter. An alternative label type is one made integral with an indicator label obtained by laminating a breathable tackifying layer, a breathable packaging material layer, a deoxidizing sheet, an adhesive layer, and an indicator label in the order indicated. (Sheets provided in advance with adhesive layers in such a manner will hereinafter be referred to as "label types. ") (3) Pasteboard Type Obtained by Laminating Deoxidizing Sheet With Breathable Packaging Material This is a pasteboard type obtained by laminating a deoxidizing sheet onto one of the following; a breathable packaging material produced by laminating a perforated film onto paper, a water-resistant nonwoven fabric (for example, "TYVEK" or "Luxer"), any of a variety of breathable packaging materials composed of various microporous films (for example, "Celgard" (trade name) manufactured by Celanese Corp.), "NF Sheet" (trade name) manufactured by Tokuyama Soda Co. Ltd., and "NITFLON" (trade name) manufactured by Nitto Denko Corp.), a breathable packaging material obtained by laminating a perforated film onto such a water-resistant nonwoven fabric or microporous film, or any of the above after they have undergone a water-resistance and oil-resistance treatment. (4) Pasteboard Type Obtained by Packaging Deoxidizing Sheet in Breathable Packaging Material This is a pasteboard type obtained using a deoxidizing sheet to package one of the following: a breathable packaging material produced by laminating a perforated film onto paper, a water-resistant nonwoven fabric (for example, "TYVEK" or "Luxer"), any of a variety of breathable packaging materials composed of various microporous films (for example, "Celgard", "Nf Sheet", and "NITFLON"), a breathable packaging material obtained by laminating a perforated film onto such a water-resistant nonwoven fabric or microporous film, or any of the above after they have undergone a water-resistance and oil-resistance treatment. Because many microwaveable foods usually are products of high water content or fatty products, it is desirable that the packaging material for sheet-form deoxidizers be resistant to solutions. It is also preferable to use, in addition to the aforementioned nonwoven fabrics and microporous films, products obtained by laminating perforated polyethylene terephthalate, polyamide, EVAL, aluminum foil, and other heat-resistant films onto water-and oil-resistant breathable packaging materials in order to prevent the iron powders from eluting. Examples include packaging materials obtained by laminating perforated films having a double-layer structure consisting of a polyethylene terephthalate layer and a polyethylene layer or perforated films having a double-layer structure consisting of a nylon layer and a polyethylene layer, on the one hand, and water-resistant nonwoven fabrics, microporous films, water- and oil-resistant packaging materials, or the like, on the other hand, as well as packaging materials having a three-layer structure obtained by laminating a porous polyolefin resin film based onto one side. The sheet-form deoxidizer used in the present invention is commonly a moisture-dependent type that absorbs oxygen using moisture evaporated by the food product. When the target food product has a low moisture content, a sheet and a water-absorbing resin that retains moisture can be used together as a self-initiating reaction type. Deodorant sheets (such as paper that supports activated carbon or the like), water-absorbing resins, water-absorbing sheets, carbon dioxide-absorbing sheets, and the like can also be appropriately used in accordance with the object. The sheet-form deoxidizer is sealed together with a food product in an airtight container, yielding a food package. When sealed in the aforementioned container, the sheet-form deoxidizer can be fixed in accordance with the object, but simple sealing is occasionally employed. Adhesives, hot cement, double-sided tapes, and the like can be used to fix the sheet-form deoxidizer, and the aforementioned types of sheet-form deoxidizer are adequately selected depending on the fixing method. A container which is at least resistant to oxygen transmission and which can be cooked by transmitting microwaves in a microwave oven is used as the airtight container for storing food products. The airtight container may be in the form of a pouch or a container. It is preferable for the airtight container to have permeation resistance characteristics that correspond to an oxygen permeability of 100 mL/m 2 ·24 Hr·atm or less. Packaging pouches or containers that exhibit a variety of oxygen-permeation resistance levels and that are used for the preservation and packaging of deoxidizers for food products can commonly be employed as such airtight containers. Pouches made of airtight plastic films can be cited as examples of such airtight pouches. Specific examples include pouches made of polyethylene terephthalate, polyamides, polyvinylidene chloride, ethylene-vinyl alcohol copolymers, polyvinyl alcohol, polyvinylidene chloride coat films, aluminum vapor deposited films, aluminum foil, silica vapor deposited films, and other films; composite films ("Triplenylon" (trade name) manufactured by Ozaki Fine Chemical Co. Ltd., and "BARRIALON" (trade name) manufactured by Asahi Chemical Industry Co. Ltd.) obtained by the lamination of the aforementioned films with polyethylene, EVA, ionomers, EAA, EMMA, EEA, and other polyolefin resin films; and various coextruded films. Barrier shrink films and other pouches can also be used. Products obtained by sealing the tops of containers made of polypropylene, ethylene-vinyl alcohol copolymers, polypropylene, and the like with the aforementioned airtight films can also be used as airtight containers. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a cross section depicting the microwaveable food package pertaining to an embodiment of the present invention; Fig. 2 is a cross section depicting the microwaveable food package pertaining to another embodiment of the present invention; Fig. 3 is a cross section depicting the microwaveable food package pertaining to yet another embodiment of the present invention; Fig. 4 is a cross section depicting the microwaveable food package pertaining to still another embodiment of the present invention. Examples of the types of package in which a sheet-form deoxidizer is sealed together with the food product stored in an airtight container will now be described as embodiments of the present invention with reference to drawings. In the package depicted in Fig. 1, a sheet-form deoxidizer 1 is used as a pasteboard, a food product 3 is placed on top of this sheet-form deoxidizer 1, this is introduced into an airtight container 2 consisting of a packaging pouch composed of an airtight film, and the airtight container 2 is then heat-sealed. In the package depicted in Fig. 2, a food product 3 is placed inside an airtight container 2 consisting of an airtight plastic tray, the inside surface is applied with a label-type sheet-form deoxidizer 1, and the airtight container 2 is heat-sealed with an airtight lid 21. In the package depicted in Fig. 3, a venthole 23 is formed; a food product is placed into a container 22, where the breathable surface of a sheet-form deoxidizer 1 is brought into contact with and bonded to the area around the venthole 23; this is then introduced into an airtight container 2 consisting of a packaging pouch composed of an airtight film; and the airtight container 2 is then heat-sealed. In the package depicted in Fig. 4, a food product 3 is placed into an airtight container 2 consisting of an airtight plastic tray, and a lid 21 heat-sealed with the tray. A venthole 23 is formed in the lid 21 on which a sheet-form deoxidizer 1 is mounted in such a manner that the air-permeable face of the deoxidizer 1 covers the venthole 23. The food product 3 sealably held inside the airtight container 2 can be directly cooked in a microwave oven by making holes in the airtight container 2 as needed to break the airtightness of the airtight container 2 without removing the sheet-form deoxidizer 1 from the airtight container 2. Examples of food products 3 packaged in the package of the present invention include semi- and precooked foods and various other microwaveable foods. Specific examples include cooked rice and red beans, a Chinese dish of fried rice, cooked rice, boiled rice with assorted mixtures, and other processed grain foods; hamburgers, nuggets, croquettes, bite-size cutlets, hot dogs, various types of fries, and other fried foods; grilled fish, steamed fish, foil-grilled fish, cubed fish, and other types of processed seafood; fried dumplings stuffed with minced pork, shao-mai, and other steamed foods; boiled beans, egg rolls, cooked food, foods fried without coatings, stir-fried vegetables, and other side dishes; half-dried squid, fried sausage, green soybeans, roasted fish, and other hors d'oeuvres; pies, bean-jam pancakes, steamed dumplings, and other Japanese and Western confections; baked potatoes, chow mein, fried noodles, spaghettis, and various other types of noodles; curries, stews, and other pouch packed foods; and foods easily heated or cooked by irradiation with microwaves. In particular, precooked side dishes, hors d'oeuvres, fried foods, and other food products are often cooked in a microwave oven and served at home, and can thus be effectively used as the food products to be stored in the microwaveable food packing pertaining to the present invention. A more detailed description will now be given with reference to embodiments. Embodiment 1 100 parts iron-based deoxidizer with a mean particle diameter of 70 µm and 150 parts polyethylene were mixed, the mixture was heated and melted at 190°C, and the melt was formed into a sheet by means of an extruder. This sheet was subsequently drawn to a factor of 4 in the longitudinal direction at 50°C, yielding a high-performance deoxidizing sheet. A disk with a diameter of 60 mm  was subsequently punched out from this deoxidizing sheet. A disk with a diameter of 70 mm  was also punched out from a breathable sheet obtained by the hot lamination of a nonwoven fabric ("Luxer") and a perforated film having small holes with a diameter of 0.8 mm at a hole area rate of 7% onto a film produced by the lamination of drawn nylon (thickness: 15 µm) and polyethylene (thickness: 15 µm). The two resulting discoid breathable sheets were superposed in such a way that the Rukusa surfaces faced inward, the aforementioned discoid deoxidizing sheet was sandwiched between them, and the edges were heat-sealed, yielding a pasteboard-shape sheet-form deoxidizer 1 such as that shown in Fig. 1. A grilled rice ball was placed on top of the pasteboard-shape sheet-form deoxidizer 1, and this was introduced into an airtight container 2 made of a packaging pouch composed of an airtight film obtained by laminating vinylidene chloride coat nylon and polyethylene. The pouch opening of the airtight container 2 was subsequently heat-sealed, yielding a grilled rice ball package, This was then kept for 14 days at a temperature of 10°C. A small hole was made in the airtight container 2 of the grilled rice ball that had been standing for 14 days, the container was directly irradiated with microwaves and cooked for 2 minutes using a 500-W household microwave oven (RP-50, manufactured by Mitsubishi Denki), the appearance was observed, and the functionality was evaluated. The results are shown in Table 1. Embodiment 2 A disk with a diameter of 30 mm  was stamped out of a 1-mm deoxidizing sheet manufactured in the same manner as in Embodiment 1, yielding a discoid deoxidizing sheet. A perforated polyethylene film was hot-laminated onto water- and oil-resistant paper, yielding a breathable packaging material. An adhesive based on a synthetic rubber elastomer was also applied to one side of a white polypropylene film (thickness: 35 µm) to form a tackifying layer, and silicon coat paper (release paper) was glued to one side thereof. The same adhesive as above was subsequently applied to the other side of the aforementioned white polypropylene film, yielding a tackifying sheet provided with a separator layer. The aforementioned discoid deoxidizing sheet was subsequently placed on the adhesive side of the tackifying sheet, the aforementioned breathable packaging material was superposed on top thereof to cover the tackifying sheet, and this was compression-bonded to bond the breathable packaging material and the tackifier surface of the tackifying sheet along the edges of the discoid deoxidizing sheet. An area with a diameter of 50 mm  was subsequently stamped out around the aforementioned discoid deoxidizing sheet, yielding a label-type sheet-form deoxidizer. A grilled rice ball was subsequently placed inside the same airtight container as that in Embodiment 1, the separator layer of the label-type sheet-form deoxidizer was peeled off, and this was pasted to the inside of the aforementioned airtight container. The pouch opening of the airtight container was heat-sealed, yielding a grilled rice ball package. This was subsequently kept for 14 days at a temperature of 10°C, and the same tests as in Embodiment 1 were then conducted. The results are shown in Table 1. Embodiment 3 A 1-mm deoxidizing sheet manufactured in the same manner as in Embodiment 1 was bonded to one side of a water-resistant nonwoven fabric ("TYVEK") with the aid of a highly breathable tackifying sheet (manufactured by Nitto Denko Corp.), and a double-sided tackifying tape (width: 10 mm) provided with release paper was bonded to the other side at 40-mm intervals. This was drawn into an angular shape with the aid of a cutting die measuring 60 mm × 50 mm in such a way that a double-sided tape was positioned at both ends. This yielded a sheet with a multilayer structure consisting of a deoxidizing sheet, a breathable tackifying sheet, Tyvek, and a double-sided tackifying tape. A commercial label (60 mm × 50 mm) comprising an adhesive layer and a paper label containing the trade name, bar code, and other information was subsequently bonded to the deoxidizing sheet of the resulting sheet to produce a label-type sheet-form deoxidizer endowed with an identification function. Then, as shown in Fig. 3, a grilled rice ball was placed in a container 22 (length: 120 mm, width: 100 mm, depth: 35 mm) that was equipped with a polypropylene lid provided with an opening 23 measuring 40 mm × 40 mm, and the lid was replaced. The double-sided tackifying tape of the aforementioned label-type sheet-form deoxidizer 1 was subsequently peeled off to expose the tackifying surface, and this label-type sheet-form deoxidizer 1 was bonded and fixed to the lid in such a way that the aforementioned opening 23 was covered. The container 22 was subsequently placed into the same airtight container 2 (15 cm × 18 cm) as in Embodiment 1, and the pouch opening was heat-sealed, yielding a grilled rice ball package. This grilled rice ball package was subsequently kept for 14 days at a temperature of 10°C, and the same tests as in Embodiment 1 were then conducted. The results are shown in Table 1. Embodiment 4 The back surface of a polyethylene terephthalate film (thickness: 12 µm) was imprinted, and a urethane-based adhesive was applied in a thickness of 30 µm to the other side, yielding an airtight sheet (width: 60 mm) having an adhesive on the inner surface and an imprint on the front surface. A fibrous adhesive (manufactured by Nitto Denko) was superposed as a breathable adhesive layer onto a water-resistant nonwoven fabric ("Luxer"), and a 1-mm deoxidizing sheet manufactured in the same manner as in Embodiment 1 was then laminated on top thereof. This yielded a sheet with a multilayer structure consisting of a deoxidizing sheet, a breathable adhesive layer, and a breathable sheet. This sheet was subsequently cut into squares measuring 40 mm x 40 mm, yielding laminates. The aforementioned laminates were subsequently aligned with the surface of the deoxidizing sheet and successively bonded to the surface of the adhesive layer on the aforementioned airtight sheet at 20-mm intervals. Silicone-based release paper (width: 60 mm) was then superposed on top thereof and bonded to the tackifying layer of the airtight sheet exposed along the two side edges and between the aforementioned laminates. This was subsequently half-cut with a cutting die from the side of the airtight -sheet over an area measuring 55 mm × 55 mm around the aforementioned laminates while the release paper was left intact. A continuous article consisting of a plurality of continuously formed label-type sheet-form deoxidizers was thus produced. Then, as shown in Fig. 4, a grilled rice ball was placed inside an airtight container (tray) 2 (width: 100 mm, length; 120 mm, depth: 35 mm) having a multilayer structure composed of polypropylene, an adhesive layer, EVOH, another adhesive layer, and again polypropylene. An opening 23 measuring 30 mm × 30 mm was subsequently formed, and the top of the airtight container 2 was sealed with an airtight film consisting of a laminate of polypropylene and vinylidene chloride coat nylon. One of the label-type sheet-form deoxidizers 1 was subsequently peeled off from the aforementioned continuous article, the breathable surface thereof was pressed against the opening 23 of the airtight container 2 to close this opening, and the exposed tackifying layer was bonded to the entire surface of the top film to fix the label-type sheet-form deoxidizer 1, yielding a grilled rice ball package. This grilled rice ball package was subsequently kept for 14 days at a temperature of 10°C, and the same tests as in Embodiment 1 were then conducted. The results are shown in Table 1. Comparative Example 1 A grilled rice ball package was prepared in the same manner as in Embodiment 1, except that a commercially available deoxidizer (deoxidizing yield: 100 mL; dimensions: 50 mm × 50 mm) obtained by packing a pouch with a particulate deoxidizer based on an iron powder was used instead of the sheet-form deoxidizer. This grilled rice ball package was kept for 14 days at a temperature of 10°C, and the same tests as in Embodiment 1 were then conducted. The results are shown in Table 1. Comparative Example 2 A grilled rice ball package was prepared in the same manner as in Embodiment 1, except that no deoxidizer was used. This grilled rice ball package was kept for 14 days at a temperature of 10°C, and the same tests as in Embodiment 1 were then conducted. The results are shown in Table 1. Appearance of deoxidizer following microwave cooking Taste evaluation Embodiment 1 No abnormalities Satisfactory taste Embodiment 2 No abnormalities Satisfactory taste Embodiment 3 No abnormalities Satisfactory taste Embodiment 4 No abnormalities Satisfactory taste Comparative Example 1 Packaging material of deoxidizer burned Deoxidizer contents dispersed, contents spilled tasting impossible Comparative Example 2 No abnormalities Tasting impossible due to mold formation It can be seen in Table 1 that the microwaveable food packages (Embodiments 1 through 4) pertaining to the present invention have excellent microwave resistance and oxygen absorption characteristics. In particular, a sheet-form deoxidizer has excellent microwave resistance, allowing a consumer who bought the microwaveable food package to irradiate it with microwaves in a microwave oven, to heat it, and to cook it safely and conveniently without causing the pouch to rupture and spill the contents as a result of microwave irradiation, or causing other safety or hygiene problems associated with conventional deoxidizers based on iron powders. In addition, food manufacturers, distributors, and retailers freeze and store food products at low temperatures in the form of the food packages of the present invention, making it possible to balance production and inventory and to cook the products by irradiation with microwaves for rapid delivery to consumers. Furthermore, the sheet-form deoxidizer is easy to manufacture, and a food package can be easily filled with it, increasing the rate of production of microwaveable food packages and making commercial production possible. It is thus obvious that the microwaveable food package pertaining to the present invention has excellent deoxidizing characteristics; can be used in applications aimed at retaining the quality of semi-cooked food products, cooked food products, and a variety of food products designed for cooking in microwave ovens; and can significantly contribute to the spread of microwaveable foods.

Claims: A microwaveable food package, obtained by sealing a food product inside an airtight container (2) together with a sheet-form oxygen absorbing agent obtained by packaging in a breathable packaging material a product prepared by kneading an iron-based deoxidizer composition with a thermoplastic resin, and forming a sheet (1). A microwaveable food package as defined in claim 1, wherein said sheet-form oxygen absorbing agent has been drawn. A microwaveable food package as defined in claim 1 or 2, wherein said sheet-form oxygen absorbing agent is obtained by laminating with a breathable packaging material a product prepared by kneading an iron-based oxygen absorbing agent composition with a thermoplastic resin and forming a sheet. A microwaveable food package as defined in any preceding claim, wherein the surface of said sheet-form oxygen absorbing agent is provided with a tackifying layer. A microwaveable food package as defined in any preceding claim, wherein said food product is supported by said sheet-form oxygen absorbing agent. A microwaveable food package as defined in any preceding claim, wherein said sheet-form oxygen absorbing agent is pasted to the inside of said airtight container. A microwaveable food package as defined in any preceding claim, wherein said iron based oxygen absorbing agent composition contains an iron powder and a metal halide salt. A microwaveable food package as defined in claim 7, wherein said iron-based oxygen absorbing agent composition is obtained by coating the surface of an iron powder with a metal halide salt. A microwaveable food package as defined in claim 7, wherein said iron-based oxygen absorbing agent composition is obtained by dispersing and depositing a metal halide salt on an iron powder. A microwaveable food package as defined in any preceding claim, wherein said thermoplastic resin is an olefin-based resin. A method for manufacturing a microwaveable food package, comprising the steps of: kneading an iron-based oxygen absorbing agent composition and a thermoplastic resin and forming them into a sheet to yield a sheet-form oxygen absorbing agent (1); and placing a food product and said sheet-form oxygen absorbing agent into a container (2) and then sealing said container to form an airtight container. A method for using a microwaveable food package, comprising the steps of: irradiating a package with microwaves, said package comprising: a food product sealed inside an airtight container (2) together with a sheet-form oxygen scavenger (1) that is obtained by kneading an iron-based oxygen scavenger composition together with a thermoplastic resin and forming a sheet.

Inventors: NAGATA MASAKI, TAKAHASHI HIDEYUKI, NAGATA, MASAKI, TAKAHASHI, HIDEYUKI, Nagata, Masaki, c/o Mitsubishi Gas Chem. Comp., Takahashi, Hideyuki, c/o Mitsubishi Gas Chem. Comp
Applicants: MITSUBISHI GAS CHEMICAL CO, MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1996
Main classification: B65D81
IPC codes: B65D81, A23L3