Abstract:
A microwave heating apparatus including a cavity for heating food, a first antenna, which projects into the cavity and is arranged to pass into or through the food in order to irradiate the food internally, and a second antenna arranged to supply microwave radiation into the cavity in order to irradiate the food externally. The heating apparatus further includes at least one magnetron for supplying microwave energy to the first and second antennae, and control means for allowing a user to independently control the levels of microwave energy irradiated by the first and second antennae.

Description:
[0001]    This application claims priority to Great Britain Patent Application No. 0007033.4, filed Mar. 23, 2000.  
           [0002]    The invention relates to microwave heating apparatuses, and methods of heating articles using such apparatuses.  
         BACKGROUND OF THE INVENTION  
         [0003]    When an object is defrosted in a conventional microwave oven, the initial microwave heating effect causes thawing of ice a small distance into the item to be defrosted, producing regions of free water molecules. Because the absorption of microwave energy is much higher in water than in ice, this causes localised heating. In extreme cases it is possible to fully cook the product where the ice has initially melted, while leaving the remaining ice frozen. In the case of a food product which must be stored frozen, and served hot to a customer, for example a burger, this can lead to the situation where the customer is presented with a food product which is apparently correctly cooked and heated, but where certain areas of the product have not attained the legally required temperature before serving. FIG. 1 demonstrates such a situation.  
           [0004]    The conventional methods of attempting to overcome this problem come in two forms: introducing a time delay into the thawing process, or shaping the product to maximise the surface area and thus the absorption of microwave energy.  
           [0005]    The main benefit quoted for microwave heating is the increase in speed over conventional heating methods. If the time delay method is used to overcome the problem mentioned above, time is allowed during the heating process for thermal conduction to transfer some of the heat from the thawed regions to those which are still frozen; i.e. thawing by conduction as in any conventional method. The delay which is introduced into the heating process is usually performed by operating the magnetrons supplying the microwaves at a reduced duty cycle, i.e. pulsing the magnetrons on and off. A typical ratio of “on” to “off” time is eight seconds “on” followed by twelve seconds “off”, which gives an effective reduction to only 40% of the available microwave power, and thus increases the time required to defrost the product by a factor of approximately 2½ times. Particularly in commercial “fast food” applications, this time delay is unacceptable.  
           [0006]    In a domestic situation, much use is made of ring shaped cooking containers, the large diameter hollow center allowing the microwaves to penetrate the product from two sides. This toroidal shape does indeed minimise the problems of ice formation, but at the cost of ease of putting the product into the cooking container. This also has the effect that the product is bulky to store whilst frozen.  
           [0007]    One process which has heretofore been considered largely unsuitable for microwave heating is that of “tempering” foodstuffs, i.e. raising the temperature of the product from “deep frozen” (usually considered to be −18° C.) to a “softer” frozen temperature of about −4° C. A particular example of this is the tempering of blocks of meat products to allow mechanical operations, such as slicing to produce evenly thin slices of meats for use in ethnic food preparation. This process usually highlights all of the inadequacies of conventional microwave heating, as the localised melting mentioned above proves disastrous in such a case. Once thawed or tempered, the food product may also then require raising in temperature to a serving condition, possibly also with the addition of extra heating by a conventional means for cosmetic “browning” purposes, without further intervention from the operator of the microwave apparatus.  
           [0008]    These problems are alleviated by ovens described in the applicant&#39;s earlier British Patent Application No. 9915368.6, filed Jul. 2, 1999, which describes the use of separate internal and external antennae for irradiating food both internally and externally.  
           [0009]    British Patent No. 1,470,408  
           [0010]    describes a microwave oven in which food is heated internally by a rod which passes through the food, and externally by a plate member, both of which are connected to a single magnetron. However, this may result in the food cooking too quickly from the inside relative to the outside, or vice versa, particularly when different sizes, shapes and types of food are cooked in the oven. It should be understood that “food” in the present specification includes any type of food or drink. Furthermore, “antenna” in this specification includes any article or part of an article from which radiation is emitted, and includes for example part of a magnetron from which microwave radiation is emitted.  
         SUMMARY OF THE INVENTION  
         [0011]    According to the invention there is provided a microwave heating apparatus and assembly, and a method of heating food, as set out in the accompanying claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Some preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0013]    [0013]FIG. 1 shows a conventional microwave oven, and illustrates the effect of heating a frozen product in such an oven;  
         [0014]    [0014]FIG. 2 shows a container used in a known method of attempting to overcome the problem of thawing at the edge of a product;  
         [0015]    [0015]FIG. 3 shows a microwave heating apparatus in accordance with an embodiment of the present invention;  
         [0016]    [0016]FIG. 4 shows four stages in a process for defrosting a frozen article according to a preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 5 shows a second embodiment of a microwave heating apparatus in accordance with the invention;  
         [0018]    [0018]FIG. 6 shows a control panel for controlling the embodiment of FIG. 3, or FIG. 5; and  
         [0019]    [0019]FIG. 7 shows a combination food product suitable for heating using the embodiment of FIG. 3, or FIG. 5. 
     
    
     DETAILED DESCRIPTION  
       [0020]    [0020]FIG. 1 shows the usual method of heating a product  2  in a microwave oven  4 . The product  2  is placed in the microwave oven  4  and the oven  4  switched on. The microwaves  6  penetrate the outer surface  8  of the product  2 , causing a localised increase in temperature. As the local temperature rises, the absorption of microwaves by the outer region of the product  2  increases, leading to a “runaway” effect where only the warmer regions in the product  2  increase in temperature. This leads to surface melting, while the inner parts  10  of the product  2  remain substantially “deep frozen”.  
         [0021]    [0021]FIG. 2 shows a container  12  with a central hollow tube  14 , which thereby reduces the thickness of the product  16  required to be heated. However, because of the small diameter of this tube  14 , no appreciable amount of microwaves can penetrate inside the tube, so the amount of heating from inside the tube is minimal. If the central tube is made larger, as mentioned above, products can become very bulky and inconvenient to store whilst frozen.  
         [0022]    An embodiment of the present invention is shown in FIG. 3. This comprises a magnetron  18  , which is coupled via a waveguide  20  to a tuned antenna  22 , a lower part of which is within the waveguide and acts as a pick-up for the microwave energy, and an upper part  26  of which is within a tempering cavity  28  and acts as a re-radiator of the microwave energy. In one embodiment this cavity  28  is of substantially cylindrical form, but it may be any convenient shape. The magnetron will typically emit microwaves of frequency 2.45 GHz. It should be appreciated that any suitable microwave source may be used instead of a magnetron, including a solid state microwave source.  
         [0023]    The product (not shown in FIG. 3) is placed into the cavity  28  in a container  12  similar to that shown in FIG. 2, having a central hollow tube  14  extending upwards from its base  40 . The tuned antenna  22  is arranged in such a way that, when the product, in its container, is placed in the tempering chamber, the re-radiating section  26  of the antenna  22  protrudes into the central hollow tube  14  of the product to be heated. The antenna  22  is located centrally in an opening in the waveguide  20  by means of an insulating component  30  made from a material which has a low dielectric constant at microwave frequencies, such as a ceramic, or PTFE, or polypropylene. An additional magnetron  32 , which is conventional in its application, also supplies microwaves to the cavity  28 , and is attached in the present embodiment to the cavity door  34 , in order to heat the product from the outside. Magnetron  32  is provided with antenna  33 . It is important to note that the magnetrons  18  and  32  are independently controllable, as will be described below.  
         [0024]    [0024]FIG. 4 shows four stages (A, B, C and D) in the operational sequence of the embodiment shown in FIG. 3. At stage (A) the cavity  28  is empty. At stage (B) the frozen product  16  in its container  12  is placed into the tempering cavity  28 , and the cavity door  34  is closed. The antenna  22  protrudes into the central hollow tube  14  of the container. When the system is switched on, at stage (C), microwaves  36  and  38  are emitted from the source  32  and antenna  22  respectively. This means that the product is irradiated by microwaves from inside the hollow tube  14  and from the outside at the same time. Although the surface will still thaw, the surface area irradiated by microwaves is greatly increased compared to the example shown in FIG. 1, and the thickness of product between the thawed surfaces is greatly decreased. At stage (D) the product has been evenly defrosted.  
         [0025]    At this stage it would be possible to apply additional microwave heating to the food product to increase the temperature to a suitable temperature for serving, i.e. soup or a similar product could therefore be taken from “Deep frozen” to serving temperature in one continuous operation. The apparatus described may be used in conjunction with conventional heating means, for example hot air or infrared heating, to meet a specific need such as raising the surface temperature to cause cosmetic browning.  
         [0026]    [0026]FIG. 5 shows an alternative embodiment in which the antenna  22  and antenna  33  are both supplied by magnetron  18 . The same reference numerals are used for parts which correspond with FIG. 3. Antenna  33  is connected to waveguide  20  by a coaxial cable  40 . The magnetron  18  is positioned on waveguide  20  between two adjustable waveguide shutters  42  and  44 . Shutter  42  controls the supply of microwave energy to antenna  22 , and shutter  44  controls the supply of microwave energy to antenna  33 . The shutters  42  and  44  can be controlled either manually, or electrically. U.S. Pat. Nos. 5,451,751, 4,449,026 and 3,697,894 describe other means for determining the direction of the microwave energy.  
         [0027]    [0027]FIG. 6 shows a suitable control panel  46  for allowing a user to independently control the two magnetrons  18  and  32  shown in the embodiment of FIG. 3. The control panel  46  is provided with a keypad  48 , a visual display  50 , and separate “INNER” and “OUTER” buttons  52  and  54  for allowing independent control of magnetrons  18  and  32  respectively. The user can thus control the rates at which the food is heated both internally and externally, and the microwave oven may also be provided with suitable preset programmes providing different levels of internal and external heating for different types, sizes and shapes of food. The control panel  46  is also suitable for controlling the embodiment of FIG. 5. In this case, the INNER button  52  controls shutter  42 , and the OUTER button  52  controls shutter  44 .  
         [0028]    [0028]FIG. 7 shows an example of a combination food product  60  comprising a layered construction of two food types with different dielectric properties. The example shown in FIG. 7 is that of a filled bread roll comprising a meat inner layer  62  and a bread outer layer  64 . In this case, more microwave energy is required to be supplied from inside the product than from the outside. However, it should be appreciated that even in the case of a homogeneous food product it may be necessary to vary the relative power levels of the internal and external sources in order to ensure an even temperature distribution throughout the food product.  
         [0029]    It will be appreciated that there are other possibilities for working the invention. For example, the antenna need not be coupled to the microwave source via a waveguide; microwaves could be supplied via a coaxial cable. The preferred embodiment shows the antenna  22  permanently attached to the cavity, but it may be removable therefrom.  
         [0030]    It will also be appreciated that the invention is suitable for use with many different shapes of container. For example, the cylindrical container  12  shown in FIG. 2 may be replaced by a frustroconical container.