Abstract:
A device for improving steam cooking of microwave food is described. A container  10  defines a compartment  22  for holding food. A first member  14  is sealed to a second member  16  to form the compartment  22  in the container  10 . The members  14, 16  are composed of two different materials having different coefficients of thermal expansion. The members  14, 16  expand at different rates when heated forming a vent in the seal between the members  14, 16 . The venting enables steam to escape at a gradual rate improving the steam cooking of the food.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/507,195, filed Sep. 29, 2003, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to the field of cooking devices, and, more particularly, to cooking containers for use in a microwave oven for cooking meats, vegetables, and other food products.  
         [0003]     Consumers often prefer to cook food in a microwave oven rather than conventional ovens because of the reduced cooking time required to heat foods in a microwave oven. Consumers also want to be provided with the opportunity to cook pre-packaged food products in the package in which they were purchased without the hassle of transferring the food from one container to the next.  
         [0004]     Unfortunately, foods cooked in a microwave oven tend to be tough and/or dry in texture and consistency, rather than tender and moist. When liquid is added to the food in an attempt to retain moisture, the food can become soggy and undesirable. In addition, microwave ovens do not evenly distribute heat to the product being cooked. This results in a cooked food product that may be very hot in one area, but cold in another area. Because of these problems, many people consider microwave cooking to be problematic and generally undesirable.  
         [0005]     One method for improving the texture and consistency of food cooked in a microwave oven is to use steam generated by the heated food product to assist in cooking the food. Cooking with steam not only provides moisture for the food being cooked, but also results in more consistent heating throughout the food product.  
         [0006]     Specifically, a container using the steam generated by the heated food product to assist in cooking the food takes advantage of the ideal gas law, a distillation of several kinetic theories including Boyle&#39;s Law and Gay-Lussac&#39;s Law. More specifically, such containers take advantage of the proportional relationship between pressure and temperature when volume and number of gas molecules remain constant. This proportional relationship can be expressed as a mathematical equation, (P2/P ) (T2/T,), where P, is the initial pressure, P2 is the final pressure, T, is the initial temperature, and T2 is the final temperature. Accordingly, any increase in pressure will result in a proportional increase in temperature that would not occur at ambient pressures. For example, if the pressure was to increases 1.2 fold (e.g., from 1 to 1.2 atmospheres), the temperature would also increase 1.2 fold (e.g., from 275 K to 330 K, which is an increase from 35° F. to 134° F.).  
         [0007]     In order to steam cook a food product in a microwave oven, the steam must be retained within a cooking container; accordingly, the container must be at least partially sealed. When a sealed container is used to heat a food product contained therein, pressure rapidly builds as steam is generated from the heated food product. As heating continues, this pressure will continue to escalate until the container ruptures in some fashion, thereby relieving the pressure.  
         [0008]     This relief often comes in the form of an explosion forcing an opening of the container and resulting in food being ejected therefrom. Not only does such an explosion create a mess, but it also undermines the attempt to use steam to cook the food product because the explosion causes a rapid release of the collected steam from the no longer sealed cooking environment.  
         [0009]     It is therefore the paramount object of the present invention to provide a microwave cooking container with a lid design that allows for controlled venting of the container.  
         [0010]     This and other objects and advantages of the present invention will become apparent upon a reading of the following description.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a perspective view of a microwave cooking container with a lid, which allows for controlled venting of the container, made in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     The present invention is a microwave cooking container  10  for steam cooking food product. With reference to  FIG. 1 , the container  10  generally includes a base  14 , having at least one compartment  22 , and a lid  16 . Because the container  10  is being used to steam cook a food product, the steam must be retained within a container  10  while the food product is being prepared; accordingly, the container  10  must have a sealed cooking environment. A sealed cooking environment is one which has a substantially fixed volume. Specifically, after food product has been delivered to the compartment  22 , the lid  16  is sealed to the base  14 , thereby substantially fixing the volume of the container  10 .  
         [0013]     There are various manners in which the lid  16  may be sealed to the base  14 . For example, the base  14  could be a substantially rigid tray, and the lid  16  could be snap-on lid. Specifically, in the embodiment of the container  10  depicted in  FIG. 1 , the base  14  includes a rim  18  and the lid  16  includes a channel  26  adapted to snuggly receive the rim  18  of the base  14 , thereby sealing the lid  16  to the base  14  and creating a substantially fixed volume within the container  10 . Of course, the embodiment depicted in  FIG. 1  is merely exemplary and it is contemplated that a variety of bases and lids may be used without departing from the spirit and scope of the present invention.  
         [0014]     The container  10  of the present invention is designed to maintain the fixed volume while being heated such that pressure builds within the volume, allowing for efficient steam cooking of the food product. However, as heating continues, this pressure will continue to escalate until the pressure is relieved. Without a controlled venting mechanism, this relief would come in the form of an explosion forcing the covering from the base of the container, result in food being ejected from the container. The container  10  of the present invention avoids this problem by controlling the venting of the steam and the release of pressure.  
         [0015]     First, it is important to recognize that the physical properites of a material give certain indications of the manner in which that material will respond to certain conditions. For example, the Coefficient of Thermal Expansion (CTE) of a material will provide an indication of how the length of the material will change in response to being heated, specifically, the change in length per unit length per unit rise in temperature. The CTE of a material can be calculated using the following formula: CTE=(ΔL)/[(L 0 )(ΔT)], where ΔL is the change in length, L 0  is the original length before heating, and ΔT is the change in temperature.  
         [0016]     The higher the CTE, the greater the degree with which the material expands when it is subjected to heat. To exemplify this correlation, generally speaking, polymers have a CTE which is approximately 5 to 10 times higher than that of metals, reflecting the greater degree with which polymers expand in response to being heated, as compared to metals. Polymers also have CTEs which vary from one another, reflecting the varying degrees with which different polymers expand when subjected to heat. For example, certain polypropylenes have a CTE of approximately 60×10 −6  to 110×10 −6 K −1  (0.00006 to 0.00011 K −1 ) while polyester terephthalate (PET) has a CTE of approximately 20×10 −6  to 80×10 −6  K −1  (0.00002 to 0.00008 K −1 ), where K is Kelvin. As such, when subjected to the same amount of heat, polypropylenes expand more than PET.  
         [0017]     With respect to the controlled venting of the container  10  of the present invention, the materials with which the container  10  is constructed are specifically chosen, not only because they are transparent to radiant energy, such as energy from a microwave oven, but also for their ability to allow for controlled venting. Specifically, the lid  16  and the base  14  of the present invention are constructed from materials having different CTEs. For example, and with reference to  FIG. 1 , the base  14  of the present invention may be constructed from a first material, such as crystallized polyester terephthalate (C-PET), having a CTE which is less than that of a second material, such as polypropylene, from which the lid  16  may be constructed. As such, the channel  26  of the lid  16  would expand more rapidly than the rim  18  of the base  14 , which originally fit snuggly within the channel  26 , thereby breaking the seal between the lid  16  and the base  18  and allowing the container  10  to vent in a controlled manner.  
         [0018]     Pairs of materials may be chosen for their varying CTE differentials. The greater the differential, the less heat which must be applied before venting occurs. In this manner, the container  10  may be customized to provide an optimum cooking environment for an optimum period of time for a particular food type. It is additionally contemplated that the base  14  of the container could include multiple compartments  22 , each compartment having an associated lid  16  and each lid  16  being constructed from a material having a different CTE, allowing venting from the various compartments  22  of the container  10  to occur at various times.  
         [0019]     It is further contemplated that a custom designed lid  16  could be made for an existing base  14  such that the custom designed lid  16  could be sealed to the existing base  14 , the custom designed lid  16  being constructed from a material having a CTE which is different than that of the existing base  14  to allow for controlled venting.  
         [0020]     Although it is not necessary, it is preferred that the container  10  of the present invention additionally include one or more venting assemblies (not shown) designed to specifically urge the seal between the base  14  and the lid  16  to be broken at one or more predetermined location to allow for further control over the venting of the container  10  of the present invention. For example, the venting assembly (not shown) may include at least one lifter, which is a substantially V-shaped indentation along the top of the rim  18  of the base  14  with the “V” ending before the outer edge of the rim  18 , thus creating a weakened portion in the seal, at which portion venting will preferentially initiate.  
         [0021]     It will be obvious to those skilled in the art that other modifications may be made to the invention described herein without departing from the spirit and scope of the present invention.