NL8801407A - BIFUNCTIONALLY ACTIVE PACKAGING MATERIAL FOR FOOD PRODUCTS TO BE TREATED WITH MICROWAVES. - Google Patents

Description

Bifunctional active packaging material for food products to be treated with microwaves.

The invention relates to a new class of microwave responsive materials which can be used in suitable configurations to improve the preparation of many food products in microwave ovens by promoting those conditions that lead to a browned and crispy surface of the food product .

The way in which electromagnetic waves, which also include microwaves, interact with metal surfaces is well known in itself and is described by Maxwell's laws. The reflective properties of metal surfaces for light and microwaves are attributed to the high electrical conductivity of the surface. If the metal of the surface is present in the form of a very thin film, then the surface layer where the interaction with the electromagnetic waves takes place has a significantly higher electrical resistance, as a result of which the reflection is reduced and a significant absorption occurs, which absorption results in a temperature rise. With an even smaller thickness of the metal layer, the surface resistance (generally expressed in units of ohm / ·) increases further until a situation is reached in which the reflection is very small, the absorption is considerable and some of the microwaves are even metal can penetrate. At an even smaller thickness, the resistance of the metal layer becomes so high that there is no longer any electrical interaction between the metal layer and the electromagnetic waves, so that these waves (for example microwaves or light) can penetrate through the layer.

In the past, use has been made of the ability offered by thin metal layers to absorb electromagnetic waves, in materials used during the preparation of food products in a microwave oven to create a hot surface, such as for example the surface of dough products browned and crispy or provide a hot plate which in addition to microwave heating contributes during, for example, popping "popcorn". Such materials are known in the literature as susceptor materials and are described, for example, by Brastad, Seiferth, Turpin and others.

These prior art materials are a laminate made of a thin metallized heat-resistant plastic film adhered to a structural support layer of paper or cardboard, The metallization thickness, generally expressed in terms of optical density ( OD) or surface resistance (ohms / ·) is chosen at such a level that maximum microwave absorption is obtained, resulting in very rapid heating of the laminate in a microwave oven. Due to this laminate surface that heats up very quickly, a rapid heating of the outside of the food product is realized. On the other hand, materials with good absorbing properties will also allow part of the microwaves to pass through, which simultaneously also heats the inside of the food product by the microwaves. By heating the interior of the food product, moisture will migrate to the outside of the food product, which prevents the formation of the desired brown crispy crust on the outside of the food product.

The material of the present invention now offers an important advantage over the existing materials used in the preparation of food products by microwave because the material of the invention can operate in a dual or bifunctional mode. The material according to the invention has a metal layer of a greater thickness than the metal layer in the known materials, such that the metal layer is initially impermeable to microwave radiation (although this does not necessarily mean completely impermeable to the much shorter wavelengths of visible electromagnetic radiation), while the layer is not so thick that all absorbent properties are lost.

More specifically, the invention provides a bi-functional laminate that undergoes irreversible change in a microwave oven, after a period of time and at a temperature dependent on the laminate, from a microwave shielding material with absorbent properties to a microwave-permeable material with a high weaker absorbent properties, which laminate is composed of a film of a plastic material selected for its suitability for contact with food and for its softening temperature which determines the temperature at which the transformation takes place; a metal layer deposited in vacuum on the plastic layer, the metal deposited to a thickness sufficient to provide full shielding for microwave transmission at room temperature (transmittance less than 1%); an adhesive layer and a support layer of paper or cardboard, the mass of which is chosen such that a desired heating time up to the transformation temperature is obtained, which support layer is attached to the metal layer via the adhesive layer.

When such a material is used in a box-shaped configuration, which is preferred according to the invention, a total shielding of the food product contained therein against microwave radiation is initially obtained. During this period, the laminate will be heated, but this heating will take place less quickly than with the materials known in the art because of its lower absorption properties. Finally, a temperature of between 200 and 250 ° C is reached (which temperature can be selected depending on the manner in which the laminate is produced in accordance with the invention, which will be discussed later).

During the first heating phase, the food product is heated on the outside only because it is in contact with the laminate or because thermal radiation from the laminate reaches the food product. As a result, the surface of the food product is already browned and crisp without any microwave heating of the inside of the food product.

Surprisingly, it has been found that when the laminate material reaches a certain high temperature, the material is spontaneously transformed into a material with entirely different microwave interactive properties. The material now no longer serves as a shield against microwaves, but reacts as a transparent window for microwaves, while still having sufficient absorbency to maintain the high temperature achieved. During this second use phase, the outside of the food product is still heated by the surface of the material, so that the conditions necessary to promote browning and crunching of the surface remain, while at the same time the microwaves can penetrate into the food product in which they will be absorbed, resulting in rapid heating and thus preparation of the entire food product.

It is believed that the heat generated by the microwave absorption in the metal leads to thermally induced stresses in the laminate structure due to high thermal gradients and differences in thermal expansion in the metal, plastic substrate and adhesive layers. These stresses can be resisted under the transition temperature, thanks to the solid nature of the laminate. At a certain characteristic temperature, which depends on the materials used in the construction, the laminate will soften and will no longer be able to withstand these thermal stresses.

The thermal deformation takes place on a micro scale and causes the very thin and mechanically weak metal layer to break. As a result, the electrical continuity is broken on a scale significant to the microwave wavelengths, thereby losing the reflective properties of the metal layer.

In this state, the metal layer is irreversibly converted into a microwave-transparent layer, which also has weak absorption properties. This is in contrast to what is described by Brastad in U.S. Patent 4,267,420, in which the heating effect results from currents flowing between high-resistance microspheres formed in the metal layer.

The invention will now be explained in more detail with reference to the accompanying figures.

Figure 1 illustrates a section through a metallized plastic film used for various measurements.

Figure 2 illustrates the transmission, absorption and reflection of aluminum coatings on a plastic film as a function of optical density.

Figure 3 illustrates the transmission, reeling and absorption of aluminum cladding layers on a plastic film in a different manner, again as a function of optical density.

Figure 4 illustrates the same values as Figure 2, but now as a function of surface conductivity.

Figure 5 illustrates the transmitted, reflected and absorbed power, expressed as a percentage of the incident microwave power on a stainless steel layer as a function of the optical density of the stainless steel layer.

Figure 6 otherwise illustrates transmission, reflection and absorption for a stainless steel layer as a function of optical density.

Figure 7 illustrates a section through a laminate according to the invention.

Figure 8 shows various containers manufactured from the laminate according to the invention.

Figure 9 illustrates the conditions under which measurements were taken in practice.

Figure 10 illustrates the result of the measurements obtained with the arrangement schematically described in Figure 9.

To obtain the correct metal coatings for the bifunctional effect of the present invention, the transmission, reflection, and by calculation also the absorption properties of thin metal coatings were measured by directing microwaves toward a film sample mounted in a waveguide. Using a network analyzer, the transmitted and reflected powers were measured on both sides of the material. The film samples were made in the manner schematically illustrated in Figure 1 and consisted of a layer 1, representing a film of a plastic material selected for its ability to come into contact with food products, as well as a layer 2, which represents a thin metal layer. Using a series of such laminates with metallized films of different thickness, the relationships shown in the graphs of Figures 2 through 6 were found. Figures 2, 3 and 4 relate in particular to aluminum cladding layers. The measurement values graphically illustrated in Figures 2 and 3 show that for aluminum with an optical density less than 0.1 corresponding to a surface resistivity greater than 1000 Ω / ·, the materials are totally transparent to microwaves at the frequency used in microwave ovens (2 , 45 GHz).

At an optical density of about 0.2, corresponding to a surface resistance of about 100 Ω / ·, the absorption showed a maximum of about 50% of the incident radiation, with the transmission and reflection distributing the remaining 50% almost equally. Material with these properties has hitherto been selected in the prior art to generate a hot surface, with which food products are browned in a microwave oven during preparation and provided with a crisping surface. At an optical density of 0.6, the microwave transmission has fallen to 0 and the material is mainly a microwave reflector, although about 10 to 20% of the incident microwave radiation is still absorbed. At greater thicknesses, the reflection increases to about 100% with a simultaneous reduction of the microwave absorption and thereby heating of the metal layer.

Figure 4 illustrates microwave energy transmission (T), absorption (A) and reflection (R) for vacuum deposited aluminum coatings as a function of surface conductivity. For the sake of the figure, this surface conductivity is defined as: conductivity = 100 / surface resistance (Ω / ·).

The conductivity is plotted along the horizontal axis on a logarithmic scale. Unfortunately, measuring points for conductivity values less than -0.5 were not available at the time of writing the present application. However, it is clear from the figure that the transmission, reflection and absorption curves show a similar behavior as illustrated in figure 2.

In Figure 5, the transmissions (T), absorption (A) and reflection (R) of microwave energy are illustrated for stainless steel coatings as a function of the optical density of the stainless steel coatings.

In Figure 6, the same values for films coated with a stainless steel layer are illustrated as a function of the surface conductivity (taking into account the above definition thereof).

As shown in Figures 5 and 6, stainless steel coatings also show similar behavior in microwave energy transmission, absorption and reflection. Unfortunately, in Figures 5 and 6, no measurements associated with lower surface conductivities were available at the time of writing this application. There is no doubt, however, that the various curves will exhibit similar behavior as the curves for the aluminum cladding layers in Figures 2 and 4.

The metallized coatings required for the realization of the present invention are those which, in the case of aluminum, have an optical density of about 0.6 OD or, in the case of stainless steel, an optical density of 0, 4 OD. In particular, these are layers which, with microwaves with a frequency of 2.45 GHz, have a transmission of less than 1% and a microwave absorption of between 10 and 20%. Ideally, the coatings should have a surface resistance of 10-20 Ω / onafhankelijk regardless of whether it is aluminum or other metal or an alloy of metals.

The material of the invention was used in a closed box-shaped configuration, as well as in an open-ended enclosure made of a laminate comprising a metallized plastic film, as described above, adhered to a support layer of paper or cardboard. Such a laminate structure is illustrated in Figure 7.

In Figure 7, the layer 3 consists of a plastic film selected for its ability to come into contact with food products and additionally selected for its softening point- The table below lists some of the plastics commonly used in the food packaging industry, together with their softening temperatures.

Table 1

Transitional temperature of various plastic film materials.

J-1 (plastic Softening point (type *) |

Jop cellulose based films below 100eC (1) jPolyvinyl chloride (PVC) 80 ° C (2) | Polyethylenes 108 - 132 ° C (3) | jPolycarbonate 150 ° C (2) (j Oriented polypropylene 170 ° C (2) | | Polyvinylidene dichloride (PVDC) 205 ° C (3) | | Nylon 6 215 ° C (3) | j Polyesterimide 215 ° C (2) j (Polybutylene terephthalate ( PBT) 224 ° C (3) | j "TPX" 245 ° C (3) | (Nylon 6-6 255 ° C (3) | jPolyethylene terephthalate (PET) 260 ° C (3) |

(Polyimide 310 ° C (2) J

I I

* The softening points above are referred to in literature depending on the type by: 1) deformation equipment 2) glass transition temperature 3) crystal melting point.

The thickness of the plastic film can be of any thickness suitable for the intended purpose and will generally be between 5 and 100 microns.

A preferred material is 12 micron thick biaxially oriented polyester.

In Figure 7, the layer 4 is formed by a metal layer which has been brought together on the plastic film to a certain controllable thickness by means of a vacuum deposition technique such as evaporation or sputtering, so that the desired microwave shielding properties are obtained. Depending on the different electrical properties, different thicknesses are also required for different materials. The metal can be any corrosion-resistant metal or an alloy of metals suitable for use in a food packaging material, such as, for example, aluminum, stainless steel or titanium. The thickness required for aluminum corresponds to a layer resistance of about 15 ohms / which is considerably higher than the layer resistance of the coatings described by Seiferth (0.4-8 ohms /).

The optical density of such an aluminum cladding layer is about 0.6 which is considerably higher than the 0.2 to 0.3 used in the prior art hot plate type monofunctional materials.

Layer 5 represents the adhesive layer which is also selected for its suitability for use in food packaging applications. This layer 5 forms the adhesive layer for the subsequent layer 6, which consists of paper or cardboard and serves both to provide structural rigidity to the laminate and to provide the final configuration, as well as to serve in a controlled manner. dampen the heating rate of the laminate when exposed to microwave radiation. If the laminate is not in direct thermal contact with the food product during use, the heating rate is essentially inversely proportional to the thermal mass of this supporting cardboard or paper layer.

Figure 8 illustrates a number of possible configurations of molded material, all of which provide a substantially closed structure that provides the necessary effective initial shielding of the food product from microwave radiation.

a) this embodiment consists of a four-sided box with open ends, suitable for square or rectangular food products with a whole or partial dough coating, such as pastries.

b) this embodiment consists of a box closed on six sides, which may optionally be provided with ventilation openings and which is suitable for preparing fish fingers, potato slices or rectangular pies.

c) this embodiment consists of a hexagonal or tubular sleeve suitable, for example, for sausage rolls or the like.

d) this embodiment consists of a polygonal shallow box suitable for pizzas, round dough products or pies.

e) in contrast to the other embodiments shown which are formed from a flat laminate, in this embodiment the metallized plastic layer structure is applied to a preformed paper or cardboard holder, which holder in this case is designed as a shallow box with an approximately equal shaped lid.

Example

A laminate was prepared by adhering 12 µl of polyester film, metallized with an aluminum coating of optical density of 0.6, which coating did not show microwave transmission, via an adhesive layer to a layer of cardboard of 240 g per m2.

A rectangle of this laminate was folded into a four-sided open-ended sleeve (similar to the embodiment illustrated in Figure 8a) which sleeve was a good package for a sausage roll. Four microwave insensitive temperature sensors were attached at various points of the package and of the food product. Temperature sensor 1 was positioned under the sausage roll and attached to the inner surface of the laminate, i.e. to the inwardly facing plastic film. The temperature sensor 2 was also attached to the inner surface of the casing, but now at the top. The temperature sensor 3 was introduced into the meat filling of the food product. Temperature sensor 4 was inserted directly under the actual surface into the crust of the sausage roll. The positions of these four temperature sensors are schematically illustrated in figure 9. In this figure, the envelope formed by means of the laminate according to the invention is denoted by reference numeral 10, the food product as a whole is denoted by 11 and consists of a crust 11a and a filling 11b. Figure 9 also shows the four temperature sensors as such.

The sausage roll packed in this bifunctional package was placed in a microwave oven for household use, with the temperature sensors positioned in the above manner. The temperature rise of the four sensors was continuously monitored during the heating period. 90 seconds after the microwave oven was turned on, the product was removed from the oven. Both the filling and the crust were hot, which was also reflected in the readings provided by the temperature sensors, and the crust was crispy and brittle.

A similar sausage roll, heated in a microwave oven without the bifunctional package of the invention, remained cold on the surface and, after the product was removed from the oven, had a crust layer much more moist and softer than before the heating process.

The temperatures observed with the aid of the temperature sensors are schematically illustrated in figure 10. The two different functions realized by the laminate according to the invention can be clearly seen from these curves. In figure 10 a distinction is made between three different stages, indicated by I, II and III. During the first stage, indicated by I, the product is shielded from the microwaves by the packaging, so that the filling of the food product is not heated. The temperature sensor 3 therefore does not show a temperature rise. The surface of the laminate (temperature sensor 2) was heated relatively quickly to a high temperature that was about 200 ° C in the laminate used. Since the food product itself forms a relatively large heat sink, the surface of the laminate in contact with the food product, although heated, was much slower than the non-food contact surface. This is indicated by the temperature rise observed with temperature sensor 1. Furthermore, temperature sensor 4 indicates that some heating of the exposed surface of the crust is already occurring.

When the material of the packaging has reached its highest temperature (in that case ± 200 ° C), the material is transformed into a microwave-transparent material which also exhibits weak absorbing properties. As a result, the temperature of the laminate will not rise further, but will be stabilized at the achieved value. Reaching the maximum temperature also marks the beginning of the second stage indicated by II. During this stage II, the temperature of the meat filling of the sausage roll begins to rise as evidenced by the measured value supplied by the temperature sensor 3, while also the temperature of the crust (readings provided by sensors 1 and 4) is still increasing.

Because the crust remains warmer than the filling, the crust will tend to lose water, making the crust crisp and brittle.

Should these measurements be repeated without the coating according to the invention, the filling of the sausage roll will heat up faster than the crust, so that the filling will tend to release water into the crust, resulting in the crust becoming moist and limp and certainly did not achieve the desired crispness.

When the filling of the sausage roll reaches a temperature of about 100 ° C, this temperature will stabilize due to the relatively large amount of water in the filling. At that moment, in fact, the last stage, indicated by III in figure 10, begins. At any suitable arbitrary moment, the preparation process can be ended at this stage, preferably the user choosing a time at which the crust has reached its desired appearance. , in other words, has turned brown and crackling. At that time, the microwave oven is turned off and the food product to be prepared is taken out of the oven.

Claims (8)

1. Bifunctional laminate that undergoes irreversible change in a microwave oven, after a period of time and at a temperature dependent on the laminate, from a microwave shielding material with absorbing properties to a microwave-permeable material with much weaker absorbing properties, which laminate is composed of a film of a plastic material selected for its suitability for contact with food and for its softening temperature which determines the temperature at which the transformation takes place; a metal layer deposited in vacuum on the plastic layer, the metal deposited to a thickness sufficient to provide full shielding for microwave transmission at room temperature (transmittance less than 1%); an adhesive layer and a support layer of paper or cardboard, the mass of which is chosen such that a desired heating time up to the transformation temperature is obtained, which support layer is attached to the metal layer via the adhesive layer. Laminate according to claim 1, characterized in that the metal layer has a surface resistance of 8 to 25 ohms /. Laminate according to claim 1 or 2, characterized in that the plastic material consists of polyethylene terephthalate. Laminate according to any one of the preceding claims, characterized in that the metal layer consists of aluminum with an optical density of 0.5 to 0.7. Laminate according to any one of the preceding claims, characterized in that the metal layer is deposited around the plastic film by evaporation in vacuum or by sputtering. Laminate according to claim 1, characterized in that the metal layer consists of stainless steel or titanium. Laminate according to claim 1, characterized in that the supporting layer of paper or cardboard is preformed into a container. A laminate container according to any one of the preceding claims, which container can be completely closed or has an open end and is intended for enclosing a food product during a treatment process of the food product in a microwave oven, which container is initially provides a high degree of total shielding of the food product from the microwaves.