Patent Grant
7214914
1. Field of the Invention
The present invention relates to a system for controlling the operation of a microwave oven and determining the cooking time for a food item, and more particularly to a system for monitoring the change in color of a thermochromic element associated with a microwave food package.
2. Description of the Related Art
Owners of microwave ovens often find it difficult to assess whether food is properly cooked. It is hard to know whether the inside of food is hot enough—or overcooked—even when correctly using guidelines on timing. Because the air inside a microwave oven does not heat up significantly compared to the food or packaging being heated, it is difficult to measure temperature of food within a microwave oven. The appearance of the food alone does not help to tell whether the inside of food is cooked or heated adequately because of the penetrative way in which the microwaves cook the food.
Many microwavable foods are purchased and supplied in packaging which can be directly placed inside a microwave and heated. In heating such packaged food items, it is likewise difficult to assess when the food is properly heated. In such a case, direct observation of the food is difficult or impossible to do and it is necessary to rely on guidelines for timing supplied with such food item. However, due to variations in a number of factors, such instructions are not always appropriate for proper heating. It is frequently the practice that heating or cooking of such food items is stopped and the oven door is opened so that the temperature of the food item may be tested, for example, by stirring or prodding the food item sometimes directly by finger. This is inconvenient and slows the cooking process, which can lead to a deterioration in the taste or texture of heated foods.
Various systems for directly sensing the temperature of food items heated in a microwave have been proposed. Published UK Patent Application GB 2 312 954 discloses a system of monitoring the temperature of a microwave container for heating food where the container is provided with a thermochromic composition which changes color over a desired temperature range and the color of the composition is monitored, visually or spectroscopically, during the heating process. In this way, it is possible to detect when the package or container has received sufficient microwave energy.
Swedish Patent SE 0302030 pertains to a device which provides temperature controlled heating of primarily solid food stuff in a microwave oven by monitoring the color changes in a heat sensitive, readable color (thermochromic) marking provided on a food stuff micro-wavable package. Since the temperature of the surface of the food stuff container during heating is not the same as the actual temperature of the food stuff, the color marking is located on the surface of a block of a polymer material, which is thermally insulated from the food stuff, and which is arranged to be subject to the same microwave radiation as the food stuff inside the oven, and which, through the choice of suitable dielectric properties, is subject to heating corresponding to that of the food stuff. The color change may be monitored by a detector.
While the prior proposed systems provide a partial solution to the problem of temperature measurement within a microwave oven, they do not address the practical realities of operating within a modern microwave oven. There is a need for a system which monitors the temperature of a food items being heated on a rotating microwave turn table.
One embodiment of the invention is a method of operating a microwave oven and determining the correct cooking time for a food item placed within the oven. In this embodiment, a food package having a thermochromic element which change color over a predetermined temperature range is placed in the microwave oven. Microwave energy is supplied into the oven. The food package (28) is rotated within the oven cavity and the color of the thermochromic element is monitored as it rotates past an opto-electronic device or detector capable of detecting the color change of the thermochromic element such that a plurality of signal pulses are generated having a value corresponding to the color of the element. The supply of microwave energy into the oven is terminated when the rate of change between successive pulses decreases to a predetermined value.
The present invention will be more apparent from the accompanying drawings, which are provided by way of non-limiting example and in which:
With reference to
During use, the food package 28 with the thermochromic disc 26 is placed in the oven cavity 12 for heating. As the disc is heated, due to its thermochromic properties, it will change color. The dielectric properties of the disc material are tailored to give a specific power dissipation when the disc is subject to microwaves. The thermochromic element 26 may be mounted to a small block or plate of a polymeric material which is thermally insulated from the food stuff, and which is arranged to be subject to the same microwave radiation as the food stuff inside the oven, and which, through the choice of suitable dielectric properties, is subject to heating corresponding to that of the food stuff, such as disclosed in Swedish patent SE 0302030. The thermochromic element 26 may be connected or associated with food packaging 28 in many different ways and may be formed as part of a packaging label for the food package 28.
The detector 24 may be of any type but is preferably, an opto-electronic device, capable to detect the color change of the disc. The detector unit 24 may consist of a high intensity LED and a light detector tuned to the same light wavelength. There may also be a filter in front of the detector tuned to the LED wavelength. The light source and the light detector can be mounted together as a unit. The detector 24 may be positioned perpendicular to a radius of the turntable 14 and at a horizontal angle looking towards the centre of the turntable 14. The detector operates a light source (e.g. LED) to emit a beam of light and receive the reflected signal. In order to overcome the problem of interfering light sources, the light source may be pulsed and a synchronous rectifier may be used to discriminate between the measuring signal and interfering light sources.
Upon initiation of heating, the turntable 14 is rotated causing the package 28 and thermochromic disc 26 and food items to rotate as well. This rotation of food items is well know in a microwave oven and provides for even heating of food items. The rotation of the food and the disc 26 is an important aspect of the detection technology. When the disc 26 is not in front of the detector 24, a zero level for the signal can be established. When the thermochromic disc 26 passes the detector, a pulse will be detected. This helps in detecting abnormal conditions, such as a missing disc. This condition can then be communicated to the user through the display 20 or audible signal.
The diagram shown in
When peak values are successfully measured, these values are compared to previous values and the rate of peak value change is measured, steps 52 and 54. When the rate of change decreases to a predetermined level approaching zero, this indicates that the color change has been accomplished. By correlating this to the proper temperature of the food item, this condition can be used to identify when the food has reached the desired temperature and cooking can be terminated, step 56. In the event peak values do not decrease over a pre-set time, the system indicates to the user an error indication, steps 58 and 50.
The present system provides a method of using the thermochromic disc 26. Varying alignment of the disc 26 with the detector 24 leads to a varying level of reflection. A system seeking an absolute or predetermined reflection value is susceptible to failure due to misalignment. In the present invention, it is variation or rate of change in the reflection signal due to rotation of the turntable that is monitored to determine the proper end cooking period. It can be understood that any color change can be used.
Apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ from those that have been described in the specification and description. It should be understood that we wish to embody within the scope of the patent warranted hereon all such modifications within the scope of our contribution to the art.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5036172 | Kokkeler et al. | Jul 1991 | A |
| Number | Date | Country |
|---|---|---|
| 0272089 | Jun 1988 | EP |
| 2312954 | Dec 1997 | GB |
| 0302030 | May 2004 | SE |
| 2005004543 | Jan 2005 | WO |
