Electric heating – Microwave heating – Field modification
Reexamination Certificate
2001-03-20
2002-08-27
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
Field modification
C219S697000, C219S746000, C219S717000
Reexamination Certificate
active
06441355
ABSTRACT:
This application claims priority to Great Britain Patent Application No. 0007033.4, filed Mar. 23, 2000.
The invention relates to microwave heating apparatuses, and methods of heating articles using such apparatuses.
BACKGROUND OF THE INVENTION
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.
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.
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.
In a domestic situation, much use is made of ring shaped cooking containers, the large diameter hollow centre 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.
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.
These problems are alleviated by ovens described in the applicant'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.
British Patent No. 1,470,408 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
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.
REFERENCES:
patent: 2909635 (1959-10-01), Haagensen
patent: 3493708 (1970-02-01), Johnson
patent: 3867607 (1975-02-01), Ohtani
patent: 4460814 (1984-07-01), Diesch et al.
patent: 4803324 (1989-02-01), Fujii et al.
patent: 4937418 (1990-06-01), Boulard
patent: 5451751 (1995-09-01), Takimoto et al.
patent: 5512736 (1996-04-01), Kang et al.
patent: 1470408 (1977-04-01), None
patent: 2284133 (1995-05-01), None
patent: 2344501 (2000-07-01), None
patent: 1-194288 (1989-08-01), None
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Great Britain Search Report for Application No. GB0007033.4 dated Jul. 25, 2000.
Leung Philip H.
Merrychef Limited
Ohlandt Greeley Ruggiero & Perle LLP
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