Food or edible material: processes – compositions – and products – Direct application of electrical or wave energy to food... – Heating by electromagnetic wave
Reexamination Certificate
2001-07-12
2004-07-20
Yeung, George C. (Department: 1761)
Food or edible material: processes, compositions, and products
Direct application of electrical or wave energy to food...
Heating by electromagnetic wave
C099S325000, C219S703000, C219S708000, C426S524000
Reexamination Certificate
active
06764702
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods of processing frozen food in a microwave oven and to a microwave oven therefor.
2. State of the Art
Traditionally, frozen food has been thawed by supplying heat from its outside. One problem associated with this technique is that it takes a long time since heat is supplied to the interior of the foodstuff by means of heat conduction only. A further problem is that when a surface layer of the food has thawed it acts as an insulating layer since thawed food has considerably lower heat conductivity than frozen food.
Microwave ovens are generally used for heating both thawed and frozen food. Microwave ovens heat the food by means of microwaves at a frequency of 2.45 GHz. Using a microwave oven for thawing food makes it possible to supply energy to the central parts of the frozen foodstuff since the microwaves propagate through the food even though they decay.
A problem associated with thawing food in microwave ovens is that the foodstuff may be heated unevenly so that some parts become extremely hot while other parts of the foodstuff remain frozen. This results in the thawed food being heated and burned.
U.S. Pat. No. 4,453,066 describes a method and a device for thawing frozen food in an oven cavity. The method is divided into several steps, the first of which involves feeding continuous microwave energy into the oven cavity, at a wattage of between 450 and 600 W, for a time period which depends on the weight of the foodstuff. The first step is followed by a second step during which no microwave energy is fed into the oven cavity. During the second step, the temperature in the foodstuff evens out. In a third step, microwave energy of considerably lower average power is fed into the oven cavity for a time period which depends on the weight of the foodstuff.
The PCT application PCT/JP98/00065 describes a method of thawing food in a microwave oven. The method is characterised in that the microwave energy is pulsed irregularly over time at least at the phase transition between ice and water. The average power of the microwaves is low in order to avoid overheating the food.
A problem associated with the prior art is that the thawing takes a fairly long time. For example, it takes more than 10 minutes to thaw 500 grams of minced meat by means of the method according to the above-mentioned U.S. patent. Users of microwave ovens have expressed the wish that thawing should be quick. Accordingly, there is a need for methods of thawing food in a microwave oven which are quicker than the present methods. At the same time, it is necessary to avoid hot areas in the foodstuff.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide quick thawing of frozen food in a microwave oven, while avoiding overheating certain areas of the foodstuff.
This object is achieved by methods and a microwave oven exhibiting the features stated in the appended claims.
A method and a microwave oven according to the invention relate to processing of frozen foodstuffs preferably weighing more than 0.1-0.2 kg.
One basic idea of the invention is to feed as much microwave energy as possible into the food before the surface thaws.
A starting-point for the present invention was the insight that the frozen foodstuff is heated partly because of absorbed microwave energy and partly because the warmer ambient air heats the surface of the foodstuff.
A further basic idea of the invention is to supply a great deal of high power microwave energy during two time intervals so that a substantial part of the foodstuff will be thawed by the end of the second interval.
Surprisingly, it has been found possible and advantageous to supply a great deal of microwave energy over a short time, a considerable amount of energy thereby being absorbed inside the food before the surface layer has thawed.
The microwaves have a substantially shorter depth of penetration in thawed food in comparison with frozen food. Consequently, when the surface layer has thawed it absorbs a large part of the incoming microwave energy, resulting in the heating of the surface layer. Accordingly, it is important that the foodstuff be frozen when the thawing begins and particularly important that the surface layer of the foodstuff be frozen.
The inventors have come to realise that, using present microwave ovens with uniform field distribution, it is possible to feed a great deal of microwave energy into the foodstuff over a short time without overheating it locally.
Then invention enables considerably shorter thawing times, especially for food weighing up to a limit weight of 0.4-0.6 kg.
However, the invention enables a considerable time saving at other weights as well.
A microwave oven for thawing frozen food comprises a microwave source for generating microwaves, and oven cavity, and a control unit.
According to one aspect of the invention, a method for thawing frozen food, having a weight in a range from a lower weight, which is 0.1-0.2 kg, to the limit weight, comprises the steps of
providing the control unit with an input signal containing information about the weight of the foodstuff and preferably also about its type;
the control unit causing the microwave source to feed microwaves, having an average power of more than 400 W, preferably more than 600 W, and advantageously more than 800 W, into the oven cavity during a first time interval during which the total microwave energy supplied to the oven cavity exceeds 50 J per gram of food, preferably exceeds 80 J per gram of food, and advantageously exceeds 120 J per gram of food;
the control unit causing the microwave source to be shut off during a waiting period subsequent to the first time interval;
the control unit causing the microwave source to feed microwaves, having an average power of more than 400 W, preferably more than 600 W, and advantageously more than 800 W, into the oven cavity during a second time interval during which the total microwave energy supplied to the oven cavity exceeds 40 J per gram of food, preferably exceeds 60 J per gram of food and advantageously exceeds 90 J per gram of food.
It has been found disadvantageous from the point of view of thawing for the average power of the microwaves to be excessively high during the first and second time intervals. According to a preferred embodiment, the average power of the microwaves during the first and the second time intervals is a maximum of 2 kW, preferably a maximum of 1.5 kW, and advantageously a maximum of 1.2 kW.
In the light of the invention, the person skilled in the art will appreciate that it is necessary to carry out experiments in order to optimise the method for a specific oven. Accordingly, in order to obtain an optimal thawing result, it may be necessary to adapt lengths of the first and the second intervals to the specific oven to be used.
Even when using an oven with a relatively uniform field distribution, it is advantageous to turn the foodstuff over subsequent to the first time interval in order to even out the effects of any lack of spatial uniformity of the microwave field. By turning the food over, it is possible immediately to begin a new time interval during which high average power is fed into the oven cavity from the microwave source.
Consequently, a method according to a preferred embodiment of the invention also comprises the steps of
emitting a turning signal at the end of the first time interval; and
the control unit detecting during the waiting period whether the foodstuff has been turned over.
According to a second aspect of the invention, it is advantageous to turn the food over subsequent to the first time interval when its weight is above the limit weight in order to make it possible to supply high power microwaves without overheating the foodstuff. Consequently, in connection with foodstuffs whose weight exceeds the limit weight a method according to the invention always comprises the steps of
emitting a turning signal at the end of the first time interval;
the control unit detecting during the
Kidblad Birgitta Jna
Törngren Per Åke
Colligan John F.
Krefman Stephen
Rice Robert O.
Whirlpool Corporation
Yeung George C.
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