MICROWAVE INTERACTIVE PACKAGE CONTAINING STAINLESS STEEL AND METHOD OF MAKING SAME

Abstract: A flexible package (10) containing a charge of popcorn kernels (12) for cooking in a microwave oven. A problem exists in microwave heating of foods in packages of facilitating and maintaining optimum microwave cooking conditions for foods such as popcorn. The microwave popcorn package of the invention more effectively focuses the energy in the desired locations and consistently yields a greater popped volume as compared to other microwave popping techiques. Inside the package is a preferably discontinuous layer (26), averaging about three atoms thick, of a stainless steel alloy. The alloy is capable of absorbing microwave energy and converting it into thermal energy through magnetic coupling in addition to I<2>R losses.

Description: - 1 - MICROWAVE INTERACTIVE PACKAGE CONTAINING STAINLESS STEEL AND METHOD OF MAKING SAME FIELD OF THE INVENTION The present invention relates to popcorn, and more particularly to food packages contained in packages suitable for use in microwave ovens. BACKGROUND OF THE INVENTION Microwave cooking has become increasingly popular with a resulting demand for a greater variety of food products that are suitable for use in microwave ovens. Some foods, such as popcorn, have been more difficult to adapt for this purpose without sacrificing quality. Popcorn is made by converting the natural moisture content of corn kernels to steam, thereby causing the kernels to expand. An adequate amount and intensity of heat must be applied to expand the kernels rapidly, giving the corn a light fluffy texture and consistency, while scorching must be avoided. A delicate balance must be maintained if all or substantially all kernels are to be popped in this manner, achieving a uniform product and a maximum popped volume. In general, the energy output of microwave ovens used in homes is too low for optimum popping conditions. It is also difficult to determine the required cooking time for popcorn and other foods because microwave ovens have been found to vary significantly in energy output, even as between ovens having the same nominal power rating. Accordingly, there is a need for a food packaging technique that will facilitate the attainment and maintenance of optimum microwave cooking conditions for foods such as popcorn. It is also desirable that a disposable package be provided that enhances shelf stability, is readily and economically manufacturable and protects the food from contamination. SUMMARY OF THE INVENTION The present invention provides a flexible package containing a charge of food such as popcorn kernels, shortening and other ingredients suitable for cooking in a microwave oven. Inside the package is a thin layer of stainless steel alloy contents that is capable of rapidly absorbing microwave energy and converting it into thermal energy. The microwave popcorn package of the present invention more effectively focuses the energy in the desired location and consistently yields a greater popped volume as compared to other microwave popping techniques. The alloy layer is preferably discontinuous and optimally averages about three atoms thickness, making it substantially non-conductive. The alloy can be applied by sputtering or other vacuum deposition techniques employing liquid or vapor phases. The stainless steel alloys of the present invention can include nickel, chrome and iron, being known as austinetiσ stainless steel of the 300 series. 303, 304 and 316 stainless has been found to be particularly effective, 304 being preferred. The contents of these alloys "is applied to the inner layer material to a thickness such that the alloy has a surface resistivity of at least 300 ohms, preferably afc least about 1200 ohms, and not more than 1500 ohms. It can be sandwiched between two layers of other material to avoid contract with the food. When the package of the invention is subjected to microwave energy, the alloy rapidly absorbs this energy and converts it to thermal energy, primarily by magnetic coupling and also through I R losses. The magnetic coupling effect decreases as the temperature increases, thus counteracting the tendency of a high output oven to overheat the charge. In addition, microwave energy falls directly on the charge and heats the charge in the conventional manner. The preferred microwave popcorn package of the present invention generally comprises a conventional standup bag having at least a front panel, a back panel and inwardly foldable gusseted side panels. It may advantageously be a standup bag with a bottom panel on which the charge rests and where the alloy is positioned. The bag can be formed of paper and the entire inside of the bag can be coated with an inner layer, such as polyester, that has low moisture permeability as compared to the paper. It is preferable to avoid depositing the alloy on the rough surface of the paper. Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a bag suitable for containing popcorn and for cooking in a microwave oven; and Fig. 2 is a cross-sectional diagrammatic view of the bag of Fig. 1 taken along the line 2-2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is embodied in a flexible and expandable bag 10 containing a charge of popcorn ingredients 12 and suitable for use in a microwave oven (not shown) . In general, this exemplary bag 10 is of a conventional standup or self-opening configuration, having a flat rectangular bottom panel 14, rectangular front and back panels 16 and 18, and inwardly folded gusseted side panels 20. The bag 10 is folded in the conventional manner, but the bottom panel 14 is shown in Fig. 2 as a single piece to avoid unnecessary complexity in the drawings. It is understood, however, that a variety of other food package constructions are suitable for practicing the invention. The ingredients 12 to be cooked are disposed inside the bag 10 adjacent to on the bottom panel 14 and consist principally of a charge of corn kernels, shortening and salt. Additional seasonings and other ingredients may be added as desired. The preferred shortening is a solid at room temperature so that the product is shelf stable. The bag 10 is constructed from an outer layer of a single sheet 22 of paper, such as bleached kraft paper, that may be treated with a commercially - 5 - available stain inhibitor. The bag 10 also has a non-wicking, heat retaining inner layer 24 that has a low permeability to moisture and grease (compared to the paper layer 22) . The preferred thickness of this inner layer 24 is about 0.4 to 1.0 mils. Suitable inner layer materials include, by way of example, polyesters such as polyethylene teraphthalate, O-L mylar available from DuPont, polymethyl pentene, imid ethers, polysulfonate, polycarbonate and the like. Other types of inner layers may be used provided they are capable of withstanding microwave oven temperatures of about 350 to 450° F without melting or otherwise contaminating or imparting flavor to the ingredients 12. All the seams of the bag 10 are sealed such that the entire inner surface of the bag that is exposed to the ingredients is covered by the inner layer 24. In this way, the ingredients 12 cannot come into direct contact with the outer layer 22 of paper and cause undesirable staining or leakage of the shortening. If popcorn kernels in the charge 12 are all of approximately equal size, a maximum number of kernels will pop within a narrow time frame. For optimum conditions, the kernels should be quickly brought up to this temperature, preferably between 350 and 370° F and maintained there for approximately 1.0 to 5.0 minutes. Cooking the kernels within this time and temperature range will pop the highest proportion of kernels and thereby achieve what is known as the maximum popped volume. Cooking at lower temperatures results in fewer popped kernels and a lower popped volume, and cooking at higher temperatures generally results in scorched or burned popcorn. - 6 - Achieving the maximum popped volume for a particular charge is economically desirable because fewer starting kernels are needed to attain the same number of popped kernels after cooking. Moreover, the consumer, is more pleased with a completely popped product.. Most importantly, however, achieving the maximum, popped volume generally results in popcorn that lias highly desirable organoleptic qualities, such as a * , light and fluffy texture and appearance, a light color, tender kernels, and good flavor. Quickly reaching and maintaining a temperature range of about 350 to 370° F inside the bag helps achieve this goal. In accordance with the present invention, stainless steel alloy contents are included in the popcorn bag 10 and adapted to receive microwave energy and convert it into thermal energy. Austinetic stainless steels of the 300 variety are to be used. It has been found that a stainless steel alloy of the 304 series composed of about 8% nickel, 18% chrome and 74% steel exhibits the desired characteristics. 303 and 316 series stainless steel are also suitable. It has been found that depositing the alloy contents directly on the paper beneath the inner layer 26 is not to be preferred because the irregular surface of the paper creates undesirable irregularity of the surface configuration of the alloy, resulting in arcing that could burn the bag 10. Thus, a very flat, smooth surface is desired for receiving the alloy, and polyester, as used for the inner layer 24, has been found to present such a surface. In the exemplary form of this invention, therefore, an alloy layer 26 is sandwiched between two polyester layers 28 and 30, forming a patch which is then inserted in the bag 10 and positioned over the inner layer 24 on the bottom panel 14. In this way, the alloy layer 26 is - 7 - prevented from coming into contact with the edible ingredients 12 that might otherwise be contaminated. There are, of course, other satisfactory arrangements by which the alloy can be deposited on a smooth surface. The alloy should be as close to the charge 12 as posible, preferably separated by only a single polyester layer of no • more than 1.0 mils in thickness. In the preferred embodiment, the alloy layer 26 is positioned on the bottom panel 14 of the bag 10, and covers a rectangular area approximately corresponding to the size and shape of the charge 12. It is understood, however, that the alloy may be placed in another location or at several locations in the package, particularly if the package is not provided with a bottom panel. Sputtering is • the preferred technique for depositing the alloy, but other known methods, such as vacuum deposition techniques employing liquid or vapor phases, may be used to deposit the alloy on the inner layer to the desired thickness. In the preferred embodiment, the alloy is sputtered onto the bottom panel 14 to form a thin layer 26 averaging approximately three atoms or about 300 to 400 angstroms thickness. The alloy weighs about 0.00014 grams per square meter. The layer 26 is discontinuous, being applied in a pattern characteristic of sputtering. The discontinuity and thickness of the alloy is a factor in obtaining the desired surface resistivity. If the alloy is too thick, or if it is not sufficiently discontinuous, it may become too hot and may promote arcing. Thus, accurate thickness and discontinuity measurements of the alloy layer 26 should be taken and measurements conducted by using optical density and resistance methods have been found to be accurate and reliable for this purpose. In general, the surface resistivity of the alloy layer 26 should be at least 300 ohms, preferably about 1200 ohms, and not more than 1500 ohms (as measured in ohms per square by an ohmmeter, using American Standard Testing Methods) . Lower alloy surface resistivities have been found to cause the alloy and its substrate to become too hot and may therefore present a possible fire danger. The ability of the alloy to produce the desired cooking temperatures quickly at a high resistivity is thus a significant advantage. It is believed that the sputtered alloy contents reconstitute the crystalline form of the alloy on the bag 10, at least in the areas of greater thickness. In areas of lesser thickness, the characteristic crystalline structure of the alloy may not exist. Moreover, the crystalline structure is particularly likely to be lost if the alloy is vaporized before it is deposited on the bag 10. When the bag 10 is ready for use by the consumer, it is placed in a microwave oven with the bottom panel 14 resting on the floor of the oven. The oven is operated in the conventional manner, typically at full or high power, and microwave energy is directed toward the package 10. The alloy rapidly absorbs the microwave energy and is believed to convert it into thermal energy or heat through two phenemona known as magnetic coupling and resistance heating. In magnetic coupling, as microwaves impinge on the alloy, they cause molecular vibration within the alloy which creates molecular friction and thus heat. The microwaves also induce electrical currents within small areas covered by the alloy and not interrupted by discontinuities, thus causing local resistance heating in the form of I 2 R losses. In addition to magnetic coupling and I 2 R losses, heating takes place because the microwave energy impinges directly on the ingredients 12. This manner of cooking by dielectric heating takes place despite the presence of the alloy. It is a characteristic of the stainless steel alloy content layer that the conversion of microwave energy by magnetic coupling decreases as the temperature increases, even at temperatures well below the Curie point at which magnetic coupling substantially ceases. It should be noted that the Curie point of the alloy is over 1200° F, far above the maximum desired cooking temperature of 370 F. Nevertheless, there is a very significant temperature stabilizing effect in the cooking temperature range. This temperature stabilizing effect reduces the tendency of the charge to be overheated in a high output oven and eliminates or reduces charring that would otherwise occur. It will be appreciated from the above detailed description that the present invention provides a convenient and reliable microwave popcorn package for rapidly achieving and maintaining the mimimum temperature range for popping corn, thereby yielding the maximum popped volume. While a particular form of the invention has been described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.

Claims: I CLAIM; 1. A food package comprising a charge of edible ingredients to be cooked by microwave energy and a food container in which said charge is disposed, said container including a sheet folded to form an enclosure for said charge and configured for use in a microwave oven, said container further including a thin layer formed by depositing the content of an austinetic stainless steel alloy on said sheet, said layer having a surface resistivity of at least about 300 ohms, which said layer converts microwave energy to thermal energy through magnetic coupling as well as resistance heating, and which said layer tends to maintain said charge within an optimum cooking temperature range independently of the intensity of energy falling thereon. 2. The package of claim 1 wherein the surface resistivity of said alloy is about 1200 ohms or more. 3. The package of claim 1 wherein said alloy is arranged in a discontinuous pattern formed by sputtering. 4. The package of claim 1 wherein: the surface resistivity of said alloy is about 1200 ohms, or more, and said alloy is arrangedin a discontinuous pattern formed by sputtering. 5. The package of claim 1 wherein said sheet is flexible and said container expandable to accommodate expansion of said charge upon cooking. 6. The package of claim 1 wherein said sheet is flexible and is folded to form gussetts, said container thus being expandable to accommodate expansion of said charge upon cooking. 7. A popcorn package comprising a charge of popcorn ingredients and a food container in which said charge is disposed, said container including a sheet folded to form an enclosure for said charge and configured for use in a microwave oven, said container further including a thin layer formed by depositing the content of an austinetic stainless steel alloy on said sheet, said layer having a surface resistivity of at least about 300 ohms, whereby said layer converts microwave energy to thermal energy through magnetic coupling as well as resistance heating, and whereby said layer tends to maintain said charge within an optimum cooking temperature range independently of the intensity of energy falling thereon. 8. The combination of claim 7 wherein the surface resistivity of said alloy is about 1200 ohms. 9. The combination of claim 7 wherein said alloy is arranged in a discontinuous pattern formed by sputtering. 10. The combination of claim 7 wherein: the surface resistivity of said alloy is about 1200 ohms; and said alloy is arranged in a discontinuous pattern formed by sputtering. 11. The package of claim 7 wherein said sheet is flexible and said container expandable to accommodate expansion of said charge upon cooking. 12. The package of claim 7 wherein said sheet is flexible and is folded to form gussetts, said container thus being expandable to accommodate expansion of said charge upon cooking. 13. A popcorn package comprising a charge of popcorn ingredients and a food container in which said charge is disposed, said container including a flexible sheet folded to form an expandable gussetted enclosure for said charge configured for use in a microwave oven, said container further including a thin discontinuous layer formed by sputtering the content of an austinetic stainless steel alloy onto said sheet, said layer having a surface resistivity of about 1200 ohms, whereby said layer converts microwave energy to thermal energy through magnetic coupling as well as resistance heating, and which said layer tends to maintain said charge within an optimum cooking temperature range independently of the intensity of energy falling thereon.

Inventors: ARCHIBALD WILLIAM E, ARCHIBALD, WILLIAM, E.
Applicants: HUNT WESSON BEATRICE INC, BEATRICE/HUNT-WESSON, INC.
Application year: 1986
Main classification: B65D81
IPC codes: A23L1, B65D81