WO2013006049A1

WO2013006049A1 - Cooking food in a microwave oven

Info

Publication number: WO2013006049A1
Application number: PCT/NL2012/050468
Authority: WO
Inventors: Cornelis Keizer; Ruurd Sibren KEIZER
Original assignee: Cornelis Keizer; Keizer Ruurd Sibren
Priority date: 2011-07-01
Filing date: 2012-07-02
Publication date: 2013-01-10
Also published as: NL1038915C2; EP2725929A1

Abstract

A method for cooking of food, in particular live shellfish, contained in a package characterized in that in order to prevent superheating, the shellfish are kept or brought in a clamped state before and during heating and/or that the shellfish are heated at a pressure being equal to the sum of the equilibrium water vapor pressure corresponding with the actual temperature of the food and the partial pressure of gases being present in the package.

Description

Title: Cooking food in a microwave oven

ABSTRACT

The invention relates to the field of devices for cooking food and in particular the preparation in a microwave oven of fresh shellfish contained in a package. Excellent eating quality can be obtained especially due to a uniform density distribution of the food which is realized by keeping the shellfish in a clamped state, preventing the animals to expand and correspondingly to keep liquid water in-between the valve parts, and/or by imposing a pressure high enough to prevent evaporation and superheating of the water, due to which both dehydration as well as the occurrence of hot spots and cold spots, known as the main drawbacks of microwave heating, can be prevented. The invention may be applicable in general for food products which are sensitive to

dehydration or generally difficult to be cooked in a microwave oven.

BACKGROUND

The present invention relates to the field of devices for cooking food.

More particularly, the present invention relates to a package or container for use in a microwave oven for cooking fresh, thus alive, shellfish. Foods, either or not pre-packaged, are widely cooked by using microwave ovens. Consumers want a convenient and tasty meal that can be preferably prepared in a very short time in the package it was purchased. However, cooking tasty food in a microwave oven is often cumbersome, because foods cooked in a microwave oven may simultaneously show hot spots and cold areas due to uneven heating, show dehydration and/or undesirable changes in textural and sensory properties.

PRIOR ART

Live shellfish may be packaged in a closed, gas and liquid tight, container, for instance known from EP720954 (Keizer). In this known package shellfish can be kept alive for some time so that they remain fit for consumption. Evidently such a package, primarily aiming at prolongation of shelf life during storage and distribution by keeping the valves in a clamped state, that is tightly closed, thus preventing loss of water contained within the valves, is not necessarily adequate for cooking the very shellfish in the same package. The latter would require at least even heating and prevention of dehydration. For the purpose of providing convenience food the contents additionally ought to be able to be heated in the package itself, preferably in a microwave oven.

According to EP 1481920 (Heiploeg Shellfish) uniform heating in a microwave oven of the entire contents of a package can be ensured when the moisture, which may be adhering to the shellfish and the moisture exuded from the shellfish during preparation, is separated from the shellfish. Indeed, all shellfish are then exposed to the same conditions. However, the liquid accumulated at the bottom of the package, either or not collected beneath a liquid-permeable plate or absorbed by a pad at the bottom of the package, will have a local density higher than the product above the liquid or the plate, and will correspondingly be heated slower than the product itself. As a result the liquid at the bottom of the package will thus remain relatively cold, whereas the product above the liquid will be relatively warm and may even be overcooked and dehydrated.

Similar negative effects are obtained by creating a void volume above the shellfish to provide space for expansion of the shellfish during cooking, as is taught by NL1004929 (Van Wezel). It may be remarked here that shellfish can actively close their valves when alive, and comprise resilient means to passively open their valves when either dead or alive, provided a void volume is available. Due to this opportunity for the shellfish to expand, that is to open their valves, the animals will lose the water initially contained within their valves. Again all shellfish will be exposed to the same conditions, and the drained liquid collecting at the bottom will still be cold compared with the shellfish. However, the surface of the meat of the shellfish will, just as in EP1481920, be exposed to the water vapor c.q. steam.

Some packages known in the art apply a subatmospheric pressure in the package for bringing or keeping the shellfish in the clamped state in order to prolong shelf live during storage and distribution.

EP 0 880 899 (Prins) discloses for instance a method for packaging alive shellfish or crustaceans in a hermetically closed (liquid-tight and gas- tight) container or flexible bag, in which the animals are enclosed together with a gas atmosphere consisting of 02 and C02. Due to dissolution of the C02 in water originating from the animals, at refrigerating temperatures that is below 10 ⁰ C, the pressure within the package will decrease, become sub atmospheric, and as a result the shellfish may be clamped together.

However, when this package as such would be heated to prepare the shellfish for consumption the C02 will be desorbed and at 40-50 ⁰ C nearly all C02 will be in the gas phase again and have a partial pressure about equal to the partial pressure at filling the package with the gas. At 50 ⁰ C the total pressure in the package will be higher than the initial pressure just before dissolution of the C02. Due to the partial pressure of the water vapor at 50 ⁰ C, which is about 0.1 bar, the total pressure will amount to more than 1.1 bar and thus has become super atmospheric. Correspondingly the shellfish will not be clamped any more.

EP1063177 (Keizer) discloses another method to prolong shelf life of bivalves by clamping the animals by packaging in air at subatmospheric pressure for instance at about 0.1 bar below the atmospheric pressure. Heating this package with shellfish from 10 ⁰ up to 50 ⁰ C will result in an increase of the partial pressure of the water vapor from 0.01 to 0.1 bar, yielding a total pressure in the package being approximately atmospheric. Similar to the package according to EP 0 880 899 the vacuum being the driving force keeping the bivalves tightly closed is lost too during heating and the animals will lose the water contained by the valves. This kind of packages will not be suitable for heating the shellfish without dehydration and uneven heating.

Another disadvantage of an airtight package is that the pressure during heating will increase due to temperature increase of the gas in the package and moreover the generation of water vapor in the package. The volume of the package correspondingly may, though not necessarily will, increase with increasing pressure. A rigid package evidently will maintain the same volume. A flexible package will show, depending on the rigidness of the packaging material, both an increased pressure as well as an increased volume. The latter will however not occur as long as the pressure within the package, enclosing the shellfish, will be lower than the atmospheric pressure outside the package.

In a closed package the temperature may easily adopt values above 100 ⁰ C when the pressure will become super atmospheric, since the vapor pressure of water increases to 1 bar and more at these temperatures. EP 0971849 (Keller) indicates a temperature of 150 ⁰ C, which means that the pressure amounts to 5 to 6 bar, depending on the initial presence of air. This will, just as in a pressure cooker, speed up cooking time for some food products such as raw vegetables which need time to get done or have to be simmered.

Another aspect of increased pressure is the risk of pack collapse. To prevent the package from bursting or exploding during heating the

hermetically sealed package may be designed to withstand the evolving pressure. In order to avoid too high pressures within the package and the risk of collapse, the package may be equipped with a pressure release opening or a state of the art pressure control valve.

INVENTION

According to our invention a package, which may be similar to for instance those described in EP720954, in which shellfish can be kept alive during storage and distribution, may be very well applied for cooking the very shellfish in a microwave oven without preliminary removal from the package, provided that specific boundary conditions required for microwave heating are taken into account.

As a first condition to be satisfied the package obviously ought to comprise microwave transparent material.

A further requirement to be adhered to according to our invention relates to the dimensions of the package.

It is known that the penetration depth of microwaves in water and most high-moisture food products typically amounts to between 1 and 1 ½ cm: about 70 % of the energy of the microwaves is converted to heat within this distance. This means that products which are both dielectrically homogeneous and which have a uniform density distribution and which are being radiated from all sides will show volumetric heating, that is, will show a more or less uniform temperature, if at least one of the spatial dimensions of the product will be limited to about twice the penetration depth of the food involved. In case of shellfish there is a void volume in-between the individual animals (interstitial void) which, depending on the size of the animals, can amount to about 30 % of the total volume occupied by the shellfish. When correcting for this void fraction the penetration depth can be calculated at between (1/(1-0.3) and (1.5/(1-0.3) times the penetration depth for high moisture food, thus approximately respectively 1 ½ and 2 ½ cmas the contents of the shellfish is mainly water and meat, this can be seen as high moisture food. The microwaves can pass the voids substantially unhindered.

The commercially available packages containing only fresh shellfish, often 1 or 2 kg packages, can be considered rather voluminous (respectively at least 1 ½ and 3 liters) compared to common one-person prepared convenience meals (most often less than 1 liter) meant for microwave heating. The minimal thickness of the commercial packages with shellfish, commonly the height of the package, can, depending on the shape, amount to more than 10 cm, substantially exceeding twice the penetration depth of microwaves. At these conditions even heating cannot be guaranteed or, rather, uneven heating is to be expected.

According to our invention heating of fresh shellfish in a microwave oven requires a spatial dimension in at least one direction of the package, for instance the thickness, to be limited to approximately twice the penetration depth of microwaves, thus preferably between about 3 and 10 cm, more preferably between 3 and 6 cm. In embodiments the height can be about 5-6 cm, in order to allow even heating and to avoid hot spots and cold spots. In embodiments with sachets 8 as to be discussed above and/or below the animals said height may be slightly bigger, for example correcting for the thickness of the sachets when compressed in unheated state.

The question may be raised to which temperature the shellfish have to be heated and for how long to yield a delicious food having the desired textural and sensory properties and lacking hot and cold spots. Nutrients react differently to heat. Proteins coagulate, solidify, at temperatures higher than 60 ⁰ C, whereas starches present in the meat of the shellfish gelatinize, that is absorb water, swell and soften, already at 40-50 ⁰ C. The edible part of shellfish consists for the larger part of water and additionally of protein and starch (glycogen) and a minor amount of fat.

It is a further aspect of our invention, as appeared from our experiments, that heating the shellfish to a temperature in-between 80° -90 ⁰ C is sufficient to yield an excellent eating quality.

Heating of shellfish does not have to be prolonged like other food such as raw vegetables or other meats which need, depending on the temperature, time to get done. EP 0971849 (Keller) teaches for instance to raise the temperature to 150°C to get raw vegetables done fast. For shellfish it may may sufficient to just bring them up to the said temperature of between 80° -90 ⁰ C. When heating liquid water this water will initially be increased in temperature while simultaneously some water is evaporated. The partial pressure of the vapor increases with the temperature of the liquid which is, at equilibrium conditions, equal to the temperature of the vapor. However, when the pressure in a system, for instance a package, is limited, for instance because the package has an opening to ambient pressure , the at least one opening not seriously restricting the flow of gas and correspondingly will be substantially at atmospheric pressure, the temperature will be limited to about 100 ⁰ C. Thus, when the liquid is further heated the temperature will continue to be at about 100 ⁰ C and the remaining liquid water will be evaporated, that is the water is boiling. When all liquid has been converted into vapor, this vapor will be superheated, that is the vapor will be further increased in temperature. The temperature at which water is boiling depends on the pressure: when the pressure is higher the boiling temperature will be higher. Boiling can only occur when the pressure in a system, for instance a package, will be equal to or less than the pressure of the water vapor which is related to the temperature of the liquid water. Thus water will, at equilibrium conditions, not boil and will not be superheated when contained in a hermetically closed package in which the pressure is not limited. This observation may, surprisingly, be successfully applied in cooking an egg in a high pressure (for instance 10 bar) resistant container which is, apart from the one or more eggs, completely filled up with liquid water. Due to the

uncontrolled, not restricted, pressure there will not be a pressure build up in the egg which exceeds the pressure outside the egg there will not be any damage to the egg.

The pressure in a package is not only determined by the (partial) pressure, thus the temperature, of the water (vapor) but will depend as well on the pressure of other gases also present in the package. When both water and for instance air, at atmospheric pressure (1 bar), are contained in the package the total pressure in the package evidently will increase to 2 bar when the temperature is increased to 100 ⁰ C. Apart from water this obviously is also true for high moisture food as for instance shellfish meat. Therefore, when a package, containing shellfish and air at atmospheric pressure, will be heated to about 85 ⁰ C, the temperature at which the shellfish already shows an excellent eating quality, the total pressure in the package will have been similarly increased to about 1.5 bar, since the partial pressure of the water vapor at 85 ⁰ C amounts about 0.5 bar. In case no air is present the total pressure evidently will be equal to the partial pressure of the water vapor and amount to about 0.5 bar. When some air or other gas is present at a certain (partial) pressure the total pressure after heating evidently will, in general, amount to the sum of this partial pressure of the air/ gas and the partial pressure of the water vapor at this temperature. The final pressure after heating to 85 ⁰ C will thus increase to the atmospheric pressure when the package is evacuated/vacumized before heating to a pressure of about 0.5 bar. A further increase in temperature will cause an expansion of the volume of the hermetically closed package and correspondingly forms a visual sign that the food is ready for consumption. To avoid that the user/ consumer denies this sign the package preferably ought to be equipped with a state of the art safety valve to prevent the package from bursting.

The concept of this description may apply to alive shellfish. It may even apply to packages according to for example EP 1063177 and EP 0 880 899 in which a sub atmospheric pressure just (for example about 0.1 bar) below ambient pressure is created to keep the bivalves clamped to prolong shelf life, provided that the package is designed to be able to resist a pressure of at least 1.5 bar and preferably will be equipped with a safety valve opening to prevent pack collaps in case the pressure exceeds this value.

It may also be relevant to pre-cooked meals contained in a package which is after heating closed and subsequently refrigerated as is described in US7051762B2 (Haamer). The latter document stresses the importance to evacuate as much oxygen as possible to increase shelf life of the pre-cooked food, thus to create a near to zero pressure of the air. It does not mention the control of this sub atmospheric pressure at a predetermined value, which means that aforementioned sign that the food is ready for consumption does not apply.

When the liquid and the vapor are not at equilibrium condition, as may be the case when parts of the liquid will locally be at different

temperatures when heated in a microwave oven, then local boiling and even local superheating, in the absence of liquid water, may occur. Since water vapor is more susceptible to microwaves than liquid water or moisture food, that is, will be heated more easily, this vapor will easily be superheated and correspondingly will obtain a property stimulating dehydration of the food product and will deteriorate textural and sensory characteristics. Heating the shellfish with superheated water vapor, causes the meat to become tough because it will dehydrate and shrink.

According to our invention a further advantage may be obtained by bringing or maintaining the bivalves in the clamped state, not only to extend shelf life of alive shellfish before heating but also to assure eating quality during heating.

By keeping the valve parts of the shellfish clamped together the loss of liquid enclosed by the shellfish will be prevented. During heating the meat of the shellfish will thus stay surrounded with water, in-between the two valve parts, and correspondingly the meat will be heated and depending on the pressure in the package either or not be boiled. Due to the presence of this water and its uniform distribution over the volume to be heated the meat will be more evenly heated and certainly not be superheated thus not be steamed. Moreover, local boiling and local superheating, thus dehydration, will be largely prevented when the liquid is kept in-between the valve parts. As long as liquid water is present near to the surface of the meat of the shellfish and/or simultaneously the pressure in the package is not fixed at a maximum value the water vapor will not be superheated during heating. Contrary to what is taught by NL1004929 the void volume available for expansion of the shellfish during heating is preferably held as small as possible to enforce the animals to maintain clamped together i.e. the volume of the package preferably is just large enough to contain the animals, thus show no extra volume enabling expansion of the bivalves. Allowing the shellfish to expand, not being tightly clamped, will result in drip loss and subsequent uneven heating and dehydration. It may be remarked here that with a void enabling expansion is meant an additional volume compared with the interstitial void, which is about the smallest possible volume in-between the shellfish as is defined before.

It is another aspect of our invention that, though it has been established that heating the shellfish to a temperature in-between 80° -90 ⁰ C is sufficient to yield an excellent eating quality, there is no drawback of applying a higher heating temperature, provided the shellfish are not being boiled or superheated.

According to a further aspect of our invention prolonged heating does not remarkably affect the eating quality in a negative way, provided that the shellfish are kept surrounded with liquid water and/or that the pressure in the package containing the shellfish is not limited.

Correspondingly the cooking results are little affected by a difference of 5 to 10 minutes in cooking time, when the shellfish are kept surrounded with liquid water contrary to common microwave cooking where a prolonged cooking time of 1- 2 minutes may already result in dehydration while simultaneously hot spots and cold spots may still be present.

It is known that the useful heating capacities of microwave ovens may largely differ from one another, since the gross power requirement differs but also due to the decreasing efficiency of the conversion of electric energy to microwave radiation during the lifetime of the microwave oven.

Correspondingly one may expect that it is difficult to prescribe an adequate heating time, resulting in the desired tasty food. For reasons of convenience it would be more appropriate if the user/ consumer would not be bothered with precisely setting the adequate cooking time and/or with continuously checking whether the food is ready for consumption. It would be ideal if the user could set a fixed time, which is independent of the net capacity of the heating device, for instance a microwave oven, but which is not critical with respect to the resulting quality of the cooked product. Obviously a minimum time will be required to heat the shellfish. Correspondingly a fixed time may consist of a minimum time and in addition to this some excess time to be sure that the food is sufficiently cooked. The shellfish will not be overcooked however provided the conditions for prolonged heating, especially the prevention of superheating, will be adhered to, as described herein.

Nevertheless convenience would even be increased when the user/ consumer would be able to observe and ascertain that the food is ready for consumption and subsequently switch of the heating device before the end of the fixed cooking time which had been set. An indication that the food is ready may consist of sound, a visual sign or otherwise.

Evidently shellfish kept in a clamped state in a rigid package, thus having a constant volume, will maintain being clamped.

According to a further aspect of our invention shellfish may, during heating, be maintained in a clamped state in a flexible package as well when the increase of the void volume of the package during heating would be prevented or eliminated. Prevention of an increase in void volume may be realized by for instance application of an elastic band, or similar collar, on the top cover or around the package to obtain a pseudo-rigid c.q. pseudo-reinforced package. The term pseudo is used here since it does not refer to the packaging material itself which may be flexible. Elimination of an increase in void volume could be realized by for instance the occupation of this volume in the package by one or more bodies accompanying the shellfish.

Such a body could be realized as a flexible sachet, bag or packet (from now on sachet) with a predetermined volume and/or pressure that contains a gas and/or a liquid, which will be designed such that it expands if necessary to fill any excess void space. The sachet may preferably though not necessarily be placed under or on top of the shellfish. When under or on top of the shellfish it is favored to prevent contact of the shellfish with respectively the top foil or the bottom of the package by selecting a surface area of the sachet which is large enough. When appropriate the sachet may be microwave transparent and may contain air as the gas and/or water as the liquid.

This sachet may be an airbag, i.e. a sachet containing gas only, for example air, at atmospheric or super atmospheric pressure. In a manner of application, described by way of example only, the package can be filled with shellfish such that the top level of the shellfish is below the top edge of the package. Subsequently a sachet is positioned on top of the shellfish such that the top level of these contents is higher than the top edge of the package when at atmospheric pressure. As an alternative the sachet may be first inserted on the bottom of the package and subsequently topped up with shellfish such that the top level of these contents is higher than the top edge of the package.

Before sealing a top cover on the package, comprising the shellfish and the sachet, the volume of the hermetically closed sachet is decreased and this goes along with a simultaneous increase of the pressure in the sachet.

At atmospheric pressure during packaging the contents of the package may be subjected to a mechanical force of for instance a piston to realize a decrease of the volume of the sachet to an extend that the height of the contents will be equal or below the top edge of the package.

Alternatively the decrease of the volume of the sachet may be realized by placing the package in a chamber, i.e. a hermetically enclosed space, in which a super atmospheric pressure is applied in order to bring the contents of the package below or at the height of the top edge of the package.

The package may also be placed in a hermetically closed space in which a sub atmospheric pressure is applied. Evidently the volume of a sachet containing a gas pressurized at atmospheric or super atmospheric pressure will increase when it is subjected to a sub atmospheric pressure outside the sachet. In order to realize a decrease of the volume of the sachet the latter may be subjected to a mechanical force of for instance a piston.

Obviously more than one such sachet can be provided in any package.

Alternatively the at least one sachet may be an integral part of the package and/or may be provided with means for pressurizing it from outside the package, preferably after filling the package with the food, especially the shellfish and closing of the package. This allows the package to be filled and closed at for example ambient pressure and thereafter pressurizing the at least one sachet, which can for example be a partly deformable chamber of the package.

After sealing a top cover on the package, comprising the shellfish and the pressurized sachet, the latter obviously will try to expand when the package is subsequently subjected to the atmospheric pressure. However, the expansion of the sachet will be counteracted by a force exerted by the top cover; the degree of expansion evidently depends on the pressure in and the stiffness of the sachet as well as the stiffness of the package, especially of the top cover. Because of the balance of forces the same force exerted by the top cover will also act on the shellfish, and if large enough this force will bring or keep the animals in a clamped state and the bivalves will retain the water within the valves.

The pressure in the package containing both the shellfish and the sachet may be atmospheric, sub atmospheric or super atmospheric but at all of these conditions the shellfish will remain clamped and therefore will not be dehydrated because of being exposed to superheated water vapor.

In case of an atmospheric pressure the package does not need to be gastight and may contain one or more openings. This offers some advantages: firstly an extension of the shelf live of the living animals since these openings afford the exchange of oxygen (02) entering the package and expulsion from the package of carbon dioxide (C02) and other gases produced by the shellfish or present bacterial flora. A second advantage is that the temperature of the shellfish can be limited to a maximum of 100 ⁰ C. Though this temperature is higher than the optimal cooking temperature of 80-90 ⁰ C this will only slightly affect the cooking quality provided that the opening area in the package is large enough to prevent pressure build up. Evidently the pressure in the package will become super atmospheric when the opening area in the package is too small to vent all the steam generated and correspondingly the maximum attainable temperature will be higher than 100 ⁰ C and the required cooking time will be shorter. Since the shellfish are kept closed there is no risk of overheating. A third advantage will be that the package can be designed to only withstand the force exerted by the sachet and does not have to be designed to be able to withstand a high steam pressure. Disadvantages of cooking at atmospheric pressure are that heating may be slower and less efficient because of the loss of steam/ heat right from the start of heating.

In case one prefers to choose to store the shellfish at a sub atmospheric or super atmosphericpressure in the package the latter obviously has to be gastight. Upon heating, either or not in a microwave oven, the water surrounding and adhering to the bivalves will evaporate and the vapor pressure of the water in the package will increase. Due to the higher, finally super atmospheric, pressure in the package the top cover will (further) expand and simultaneously the volume of the sachet may be decreased. Evidently precautions have to be taken to prevent collapse of the package and/or to control the pressure.

According to another aspect of our invention a sachet on top of the shellfish presses against the top foil containing a vapor release opening which is so covered and thus kept closed by the sachet but, at increasing pressure due to which the volume of the package increases and the volume of the sachet decreases, this opening will become accessible for the gas in the package including the water vapor and correspondingly the gas will be (partly) released.

As a consequence of the release of gas the pressure in the package will be decreased and the top cover will shrink again and simultaneously the volume of the sachet will increase with as a result that the release opening will be closed again. This process of pressure build up, release of gas, pressure decrease and closing the pressure release opening will be repeated as long as heat is supplied, which means that the sachet is functioning as a pressure control device. Repeated opening and closing of the vapor release opening, if the latter is large enough to eliminate the excess pressure, may according to our invention be considered as a sign that the shellfish are ready for

consumption. Alternatively such a sign may be created in case the vapor release opening is chosen too small to quickly release the excess pressure and correspondingly the release opening will remain constantly open, indicating that the food is ready for consumption.

To avoid excess pressure in a package which is hermetically closed before heating the user/ consumer may create one or more holes in the package by simply punching the package just before heating. The advantages and disadvantages described before with respect to atmospheric cooking apply here as well, at least to some extend, for example depending on the combined surface area of the openings created and the pressure build up in the package. Evidently a hermetically closed package may be equipped with a state of the art pressure control or pressure relieve or safety valve which may be provided with a sticker which is removed manually before heating or automatically during heating, the latter because of for instance weakening of adhesive binding the sticker to the top cover due to heating and/or excess pressure in the package. It will be evident that the eating quality of the shellfish will not be negatively affected by punching the package when the bivalves are forced to keep the valves clamped. Instead of at least one sachet comprising a gas only, one or more sachet may be applied containing an amount of liquid water only. The amount of water is preferably predetermined and may be tiny, having a nearly negligible volume relative to the total volume of the package. Upon heating, either or not in a microwave oven, the amount of water will evaporate and the sachet will expand. At 100 ⁰ C only 1 g of water will typically yield about 1 liter of vapor. Due to the very small, restricted amount of water the volume of the sachet after heating will be restricted: when all liquid has been

evaporated, some superheating may occur without a noticeable further expansion. Due to the controlled expanded volume of the sachet the shellfish cannot expand and will be forced to keep their valves closed.

Correspondingly the animals will retain the water within the valves, i.e. the shellfish meat will remain enclosed with water, and will be boiled instead of being steamed, thus preventing dehydration. Evidently, precautions have to be taken to prevent the sachet from exploding. The sachet may be made of material able to withstand the pressure increase or may be equipped with a state of the art pressure control device or be made of material with several tiny punctures, still inhibiting liquid water flowing out of the sachet though allowing a gradual release of the water vapor. The sachet has to be liquid tight though not necessarily gas tight. By allowing the water vapor to leave the sachet the pressure in and the volume of the sachet will decrease when all water has been evaporated and according to our invention the sachet losing volume or being empty may be considered as a sign that the food is ready for consumption.

Instead of only water the at least one sachet may alternatively contain both some air and some water, and should be both gas and liquid tight then. Additionally or alternatively the at least one sachet may contain even vegetables and/or beer or wine, for instance according to common recipes in preparing shellfish dinners. The vegetables may be precooked. Since

vegetables and fresh shellfish require completely different preservation conditions it is an advantage to package both food items separate from each other, which can be realized by packaging the vegetables in a sachet. Since raw, especially hard, vegetables require a longer cooking time than the shellfish it is a further advantage to apply precooked vegetables. The latter is not a necessity however when the raw vegetables are packaged in an hermetically sealed sachet allowing a super atmospheric pressure thus enabling cooking temperatures remarkably exceeding 100 ⁰ C which reduces the required cooking time.

In case the sachet is positioned on top of the shellfish it will protect the cover or foil sealed on top of the package from undesired penetration by sharp edges of the very shellfish, thus avoiding leakage of the package.

Simultaneously the shellfish is protected. Evidently the material of which the sachet is made of ought to be resistant to puncturing parts of the shellfish.

In case the sachet is positioned beneath the shellfish it will be easier to obtain a smooth top cover or foil, which may be important with respect to presentation and/or stacking of the product. It is advantageous to select a relatively stiff top cover/foil compared with more flexible material of the sachet. Correspondingly the sachet adapts its shape to shellfish easier than the top cover/ foil, which thus remains substantially smooth. A disadvantage of positioning the sachet on the bottom of the package could be that the sachet cannot be used as a pressure control device. This can be overcome, at the expense of a higher consumption of packaging material, by applying two sachets, one beneath and one on top of the shellfish the latter functioning as a pressure control device, or by providing a pressure release opening at or near the bottom of the package.

The sachet may be introduced in the package as a separate device or may be integrated with the package. In the latter case the sachet may be sealed to the top edges of the package instead of sealing a foil on top of the package containing a separate sachet. Another way of creating a sachet may exist of first sealing a (first) foil on top of the package containing shellfish only at sub atmospheric pressure and subsequently sealing a second foil on top of the package at super atmospheric pressure. At subsequent exposure to ambient pressure the sachet will exert the desired force on the shellfish. When desired the package may be further provided with opening(s) to allow atmospheric pressure in the package, while maintaining the shellfish being kept clamped, and yielding the advantages of storage and heating at atmospheric pressure as described before.

Evidently the second top foil may be of a more rigid material to obtain a more attractive appearance and a package which can be stacked more easily.

Though the method of cooking shellfish described here primarily aims at preparation of shellfish by the consumer, at home or in restaurants, etc., it may be applied as well in industrial precooking followed by ultimate cooking just before consumption.

Detailed description of drawings.

Fig. l refers to packaging shellfish with a separate sachet on top of the animals.

Fig.1 A: shows that the top level of the contents of the package, the shellfish and the sachet, at atmospheric pressure, is above the upper edge of the package.

Fig. IB: depending on the pressure in the chamber the top level of the contents may be lowered to or below the upper edge of the package by applying a super atmospheric pressure and/or the force of a piston and subsequently sealed.

Fig. 1C shows the package brought back at ambient pressure again due to which the sachet has increased in volume. The pressure in the package may be freely selected:

Fig. 1C1: atmospheric pressure the package has an opening to ambient, or Fig. 1C2: sub atmospheric or super atmospheric pressure in the package: the package has no opening to ambient or an opening which is for instance initially covered with a sticker. The sticker may be manually removed before heating. Alternatively the sticker may be automatically separated from the foil for instance due to the creation of a predetermined pressure in the package and/or due to the loss of adhesive strength when the temperature has reached a predetermined value. As a result the uncovered opening can function as a vent.

Fig. ID shows the package when heated in for instance a microwave oven.

Fig. 1D1: some vapor release is allowed but the pressure in the package still may increase until a predetermined pressure.

Fig. 1D2: the pressure in the package has adopted the

predetermined value at which the sachet becomes separated from the top foil and the vapor release is increased until the pressure in the package becomes below the predetermined value and the sachet shuts off the opening opposite the sachet again; this process will be repeated during continued heating i.e. the sachet functions as a pressure control valve.

Fig.2 refers to packaging with a separate sachet beneath the animals.

Fig.2A and 2B: conditions correspond with Fig. 1A and IB.

Fig. 2C1: the pressure inside the package is atmospheric before and during heating, which means that the temperature during heating cannot be higher than 100 ⁰ C.

Fig. 2C2: the pressure inside the package before heating can be freely chosen at either atmospheric, sub atmospheric or super atmospheric conditions. To be able to control the pressure in the package during heating a second sachet may be inserted on top of the shellfish.

Fig.3 refers to packaging with an sachet which is used as a top cover before (Fig. 3. A) and after (Fig.3.B) sealing the sachet to the package. The package itself may be equipped with one or more openings, either or not covered with a sticker.

Fig.4 refers to a package which is first vacumized and sealed with a first top foil and subsequently exposed to a super atmospheric pressure and equipped with a second top foil as shown in fig. 4A, B and C. Thereafter the package may or may not be equipped with an opening in the package beneath the first top foil to realize respectively an atmospheric pressure or maintain a sub atmospheric pressure.

In this description embodiments of a method and package according to the present disclosure shall be described, by way of example only, in elucidation of the inventive concepts. In these embodiments and description the same or similar parts shall be indicated by the same or similar reference signs. In this description embodiments are shown and described of packaging shellfish such as bivalves in a package. Bivalves can be but are not limited to mussels. However, also other foods can be packaged accordingly.

In this description reference made to temperatures are related to substantially average temperatures of the meat of the food itself. In case of shellfish such as mussels temperatures mentioned relate to the meat inside the shells. The shellfish can contain at least shellfish meat and water inside the shell. Pressures are measured preferably as average pressures inside the package, outside shells of shellfish.

Fig. 1 shows a package 1, for example comprising a plastic tray 2, which can be partly or entirely made of a microwave radiation transparent material, such as a transparent plastic. Such type of tray 2 is known from the art and is commonly used for microwave meals and the like. In the

embodiment shown the tray 2 comprises a bottom 3 and a peripheral wall 4, which can slope slightly outward in a direction away from the bottom 3. The free end of the wall 4 is provided with an outward reaching flange 5, again in a known fashion, against which for example a closure 6 can be sealed or otherwise attached, such as for example a foil, which can be more flexible than the tray..

In fig. 1 a tray 2 is shown having a height H as a first spatial dimension, which can for example be between 3 and 10 cm, more specifically between 3 and 6 cm. In an embodiment where the tray can be used in a microwave oven, the material of the tray 2 and/or of the closure 6 can be made at least partly of microwave transparent material. Microwave ovens generally operate at a frequency of between about 900 and MHz 3 GHz, wherein standard household microwave ovens generally operate at a frequency of about 2.4 to 2.5 GHz, for example about 2.45 MHz, with a wavelength of around 122 mm, and industrial microwaves can operate at for example about 915 MHz, with a wavelength of about 328 mm. With high moisture content food stuff in domestic microwave ovens this may normally lead to a penetration depth D of about between 1.5 and 2.5 cm. For microwave radiation coming from different, and preferably all sides in an oven, due to inter alia reflection, this means that the radiation will penetrate fully into the food stuff. Especially with shellfish such as bivalves this will be true since there will be about 30% of the volume defined by void between the shells, which will not hinder radiation

penetration. The shell fish meat such as but not limited to mussel meat will have a high moisture content, for example above 50%, such as for example above 60%, more specifically about 65% of or higher. In mussels the weight percentage of moisture, especially water, can for example be about 75%. These percentages and higher are considered representatives of high moisture and high moisture food. For mussels or such bivalves when packed in an at least partly and preferably largely or even entirely microwave transparent package this can mean that between about one and three layers of such shellfish and/or a height of the package of between 3 and 10 cm may still lead to full penetration of the microwaves into the heart of the package, at least sufficient for an even heating, without local superheating or nucleus forming. In the embodiment as shown in fig. 1 the package 2 is filled with about three layers of live mussels 7, for example of average size, the lower mussels being placed on the bottom 3. The top layer of mussels 7 lies just below the upper edge or flange 5. It should be noted that other numbers of layers of mussels could be provided, for example with smaller or larger mussels and/or trays. A body 8 such as a sachet or bag of at least partly flexible material, further to be called a body or sachet 8, is placed on top of the mussels 7. The sachet 8 can be filled with gas, such as air, can contain a small amount of liquid, for example one or a few grams of water, or a mixture thereof. The sachet can in stead or additionally contain other food stuff, such as but not limited to vegetables, beer, wine or other ingredients that are commonly used with shellfish. Then a closure such as a foil 6 is placed over the mussels 7 and sachet 8, to be sealed or otherwise adhered to the flange 5 of the tray 2. This may be realized in a chamber either at atmospheric, sub

atmospheric or super atmospheric pressure.

As can be seen in fig. 1A the sachet 8 rests on top of the mussels 7 and extends partly above the edge or flange 5 of the tray 2. For closing the package 1 the foil 6 is placed over the sachet 8 and underlying mussels 7. Then in this embodiment a piston 11 having a seal beam 12 is pushed onto the foil 6, pushing this with a peripheral edge against the flange 5 and sealing the edge and flange together.

In embodiments where the closing of the package is done at atmospheric of sub atmospheric pressure, in pushing the foil 6 down the piston 11 simultaneously pushes the sachet 8 down against the mussels 7 and compresses the sachet 8, to a position as shown in fig. IB. This means that the pressure in the sachet 8 may be increased and the mussels 7 are pushed closed against each other and/or the tray 2. The sachet 8 has an internal volume prior to compression, such that when at least partly compressed it fills at least the void above the mussels 7, between the mussels 7 and the foil 6. when closed at for example super atmospheric pressure the sachet may already be partly or fully compressed when entered into the tray, onto the mussels, prior to closure.

The top foil 6 may have a vent opening 9 with a restricted area to allow gas exchange during storage and distribution of the shellfish 7. In case such a vent opening 9 is provided the pressure in the package 1 obviously will be atmospheric during storage. This vent opening 9 is designed however to be too small to allow all of the water vapor/steam generated during heating to be vented, so that the pressure in the package 1 increases. In order to control the pressure in the package during heating the top foil 6 can be equipped with a second opening (or multiple openings) 10 to allow pressure control by means of a sachet 8 repeatedly covering and uncovering this second opening 10 opposite the sachet 8.

Fig. 1C shows the package 1 at ambient pressure due to which the sachet 8 has increased in volume. The pressure in the package 1 may be freely selecte:

Fig. 1C1: atmospheric pressure; the package has an opening 9 to ambient, or

Fig. 1C2: sub atmospheric or super atmospheric pressure in the package: the package 1 has no opening to ambient or an opening 9 which is for instance initially covered with a sticker 13, closing off the opening 9. The sticker 13 may be manually removed before or during heating. Alternatively the sticker 13 may be automatically separated from the foil 6, for instance due to the creation of a predetermined pressure in the package 1 and/or due to the loss of adhesive strength between the sticker and the foil, or part thereof, when the temperature within the package 1 has reached a predetermined value. As a result the uncovered opening 9 can function as a vent.

Fig. ID shows the package 1 when heated in for instance a microwave oven. Fig. 1D1: some vapor release is allowed through an opening 9, but the pressure in the package 1 still may increase until a predetermined pressure. As can be seen in fig. 1D1, a secondary opening 10 is closed by the sachet 8, allowing pressure to build within the package. Initially the sachet will follow any increase in the volume of the package 1 due to heating and expansion of the foil 6 and/or tray 2. At a given pressure and/or temperature within the package the sachet 8 will no longer grow in volume, or at least insufficiently to follow the foil, leading to release, i.e. opening of the secondary opening 10 or openings 10, thus relieving pressure, at least in part, sufficient for the foil to reposition itself over and against the sachet 8 sufficiently to again close the opening 10 or openings 10.

Fig. 1D2: the pressure in the package 1 has adopted the

predetermined value at which the sachet 8 becomes separated from the top foil 6 and the vapor release is increased until the pressure in the package 1 becomes below the predetermined value and the sachet 8 shuts off the opening 10 opposite the sachet 8 again; this process will be repeated during continued heating i.e. the sachet 8 functions as a pressure control valve.

Fig.2 refers to a packaging 1 with a sachet 8, such as a separate sachet 8A, beneath the animals such as mussels 7. The sachet 8A can be the same or similar to the sachet 8 above the animals 7 as shown in fig. 1, and perform the same function of pressing the shells of the shellfish such as mussels 7 against each other and/or the tray 2 and/or foil 6, to close and/or keep closed the shells. The sachet 8A can be provided as an alternative to or additional to the sachet 8 above the animals 7.

Fig.2A and 2B: conditions correspond with Fig. 1A and IB.

Fig. 2C1: the pressure inside the package 1 is atmospheric before and during heating, which means that the temperature during heating cannot be or become higher than 100 ⁰ C. Excess pressure can be prevented by the opening or openings 9. Fig. 2C2: the pressure inside the package 1 before heating can be freely chosen at either atmospheric, sub atmospheric or super atmospheric conditions, due to the or each opening 9 being closed, at least between the stage of packaging and prior to cooking. The or each opening 9 can be closed by a sticker 13, at least until directly prior to or during heating. To be able to control the pressure in the package 1 during heating a second sachet 8 may be inserted on top of the shellfish 7 as discussed before.

Fig.3 refers to a packaging 1 with an sachet 8 which is used as or combined with a top cover 6. Fig. 3 shows the package before (Fig. 3. A) and after (Fig.3.B) sealing the sachet 8 and/or top cover 6 to the tray 2 for closing the package 1. The package 1 itself may be equipped with one or more openings 9, 10, either or not covered with a sticker 13. The or each opening 9, 10 may be provided in the tray 2, for example a wall 4 or the bottom 3 of the tray 2. The package 1 may be formed again in any suitable manner, for example as described with reference to fig. 1 or 2. Additional sachet 8A or sachets 8A can be provided, for example between the animals 7 and the bottom 3 and/or wall 4.

Fig.4 refers to a package 1 which is first vacumized and sealed with a first top foil 6A. The animals 7 are enclosed between the foil 6A and the tray 2. Subsequently a second top foil 6B can be sealed against the first foil 6A and/or the tray 2, enclosing a space between the foils 6A, B, effectively forming a sachet. The space between the foils 6A, B may be exposed to a super atmospheric pressure during closure. Thereafter the package 1 may or may not be equipped with an opening 9, 10, for example in the package 2 beneath the first top foil 6A, to realize respectively an atmospheric pressure or maintain a sub atmospheric pressure. The or each opening 9, 10 may again bbe initially closed by a sticker 13. As shown in fig. 4B and C, alternatively a sachet 8 can be connected to the second foil 6B, to be enclosed between the foils 6A, B. In fig. 4B the sachet 8 is shown as applied at super atmospheric pressure. In fig. 4C the sachet 8 is shown being placed at atmospheric pressure. Alternatively the tray can be provided with a sachet that can be pressurized from outside the package 1, for example after filling and closure thereof. Thus the package can be closed easily and then be pressurized by the sachet. The or each sachet 8 can be an integral part of the tray 2 and/or foil 6. Again additional sachets 8 can be provided between the shellfish 7 and/or the shellfish and the tray 2 and/or top foil 6.

In embodiments the package can have a content of about 1 liter or less, or of about 1 kg of food product or less, especially 1 kilo of shellfish or less. In other embodiments the content can be larger. The package can comprise more than one individual space for containing food products, for example as if a number of packages as shown is combined into one.

In the embodiments discussed here above the sachets 8, 8A are discussed as being filled with a gas, such as but not limited to air.

Alternatively or additionally food products, such as but not limited to vegetables, wine and/or beer could be provided in at least one of the sachets 8, 8A, preferably partly or fully pre cooked. The pressure build up during heating in the sachet 8 can provided for (further) cooking in the time necessary for heating the shellfish. Alternatively or additionally to the gas and/or food products, a fluid may be provided in the sachet, which can be chosen such in both amount and type that it will generate the desired amount of pressure inside the sachet during heating for keeping the shells of the animals 7 clamped and, if desired, repeatedly opening and closing of one or more vent opening 10.

The invention is by no means limited to the embodiments of a package or method as described here above with reference to the drawings. Many amendments and alterations are possible within the scope of the claims as enclosed. Especially all embodiments are considered to have been disclosed which are or can be derived by combining embodiments or parts thereof of the examples given. Moreover other food products can be provided in the package and/or sachets therein. The sizes and dimensions of packages can be amended, for example according to microwaves used and the food products to be heated. For example in industrial microwaves a higher capacity radiation is used and therefore penetration depth may be higher than in home micro wave ovens. Accordingly the said at least one spatial dimension may be increased maintaining about twice the penetration depth.

These and many such variations, including all combinations, are considered falling within the disclosure.

Claims

1. A method for cooking of food, in particular live shellfish, contained in a package characterized in that in order to prevent superheating, the shellfish are kept or brought in a clamped state before and during heating and/or that the shellfish are heated at a pressure being equal to the sum of the equilibrium water vapor pressure corresponding with the actual temperature of the food and the partial pressure of gases being present in the package.

2. A method according to claim 1, wherein the food is cooked in a microwave oven, in a package measuring in at least one spatial dimension less than approximately twice the penetration depth of the microwaves

corresponding with the food in the package

3. A method according to Claim 1 or 2, wherein the food cooked in the package comprises at least shellfish, wherein the package containing the shellfish measures in at least one spatial dimension less than approximately 5- 6 cm, respectively 1-3 layers of shellfish.

4. A method according to one of the previous Claims wherein the package containing shellfish measures less than 1 kg in weight.

5. A method according to one of the previous Claims wherein the package is equipped with a safety valve opening at a predetermined pressure, wherein the package is further pressure tight, and is preferably not equipped with a vent opening or pressure control device.

6. A method according to one of the previous Claims wherein heating of the shellfish is limited to a preferred temperature in-between about 80° -100

⁰ C (Hoe en waar wordt die gemeten).

7. A method according to one of the Claims 1-4 wherein a maximum temperature of the shellfish can be and preferably is limited to about 100 ⁰ C by limiting the pressure in the package to the atmospheric pressure.

8. A method according to one of the Claims 1-4 wherein a maximum temperature of the shellfish is limited to about 85 ⁰ C by limiting the pressure in a hermetically closed package to increase to a maximum of the sum of the equilibrium pressure corresponding with a temperature of the food at 85 ⁰ C, which preferably amounts to about 0.5 bar, and the partial pressure of air and/or other gases being present in the package, wherein preferably:

the maximum temperature of the shellfish is limited to about 85 ⁰ C by limiting the pressure in a hermetically closed package to increase to a maximum of about 1.5 bar; and/or

- wherein partial pressure of air and/or other gases in the package is first decreased to a value of not more than 0.5 bar by evacuating air from the package before heating and that the maximum temperature of the shellfish is limited to about 85 ⁰ C by limiting the pressure in a hermetically closed package to increase to a maximum of about the ambient pressure.

9. A method according to one of the previous Claims wherein a clamped state of shellfish in the package is maintained by preventing increase of void volume of the package during heating by applying an elastic band, or similar collar, on the top cover or around the package to obtain a pseudo-rigid c.q. pseudo-reinforced package exerting a force on the shellfish.

10. A method according to one of the previous Claims wherein a clamped state of the shellfish in the package is maintained by preventing of an increase in void volume of the package during heating by the occupation of this volume in the package by one or more bodies accompanying the shellfish, wherein preferably at least one occupying body is used consisting of or comprising a flexible sachet, bag or packet, that contains a gas and/or a liquid, which is designed such that it expands if necessary to fill any void space excess, preferably at a predetermined pressure.

11. A method according to Claim 10 wherein the at least one body is a bag containing at least one of: gas only, for example air, at atmospheric or super atmospheric pressure; or

an amount of liquid water only; or

both air and water; or

- vegetables and/or beer or wine for instance according to common recipes, wherein the vegetables preferably are at least partly precooked.

12. A method according to one of the Claims 9 - 11 wherein the at least one bag/sachet is positioned on top of the shellfish and/or at a bottom of the package.

13. A method according to any one of the previous claims, wherein a package and/or a top foil sealed to the package is /are used which is/are liquid tight and gas permeable due to which the pressure in the package after sealing the package will be or will become about atmospheric when exposed to ambient pressure.

14. A method according to one of the previous Claims wherein a package and/or a top foil sealed to the package are be equipped with at least one of a safety valve opening at a predetermined valueand a control valve maintaining the pressure in the package in-between a predetermined pressure range, wherein preferably the pressure in the package is controlled by a sachet according to Claim 10 which repeatedly covers and uncovers due to pressure changes, both within the package and within the sachet, during heating a vent opening being provided in the package or the top foil opposite the sachet.

15. A method according to one of the previous Claims wherein the package is covered and sealed with one or more sheets and/or a sachet according to Claim 10.

16. A method according to one of the previous Claims wherein the package before or during heating is punctured with a device which has a predetermined opening area allowing a controlled release of gas, for instance air, and/or water vapor during heating.

17. A method according to one of the previous Claims wherein the food comprises or is pre-cooked shellfish and/or one or more eggs.

18. Package, at least partly made of microwave transparent plastic material, comprising a tray and a closure, wherein the tray is filled at least partly with shellfish, packed and clamped against each other, wherein within the tray and/or the closure at least one body is provided for pressurizing the shellfish in the package, which at least one body is designed to fill voids occurring in the package during heating of the shellfish and water inside said package.