Electric heating – Microwave heating – With control system
Reexamination Certificate
2002-04-16
2003-05-06
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
With control system
C219S686000, C426S241000, C426S523000
Reexamination Certificate
active
06559429
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to defrosting technology designed to prevent dripping and the loss of quality in the defrosted products. In particular, the present invention relates to defrosting technology in which high-quality defrosting is performed in an extremely short amount of time by carrying out low-energy microwave heating at reduced pressure. Further, the defrosting technology of the present invention can be used in various industries, including the food industry, pharmaceutical industry, cosmetic industry, cattle raising industry, marine products industry, machine manufacturing industry and home electronics manufacturing industry.
2. Description of the Prior Art
In prior art defrosting methods that use microwave heating at reduced pressure, microwave heating is carried out after the pressure has been reduced to a prescribed level (e.g., 25 torr) in order to prevent the product temperature from becoming too high during defrosting, and the progress of the defrosting process is confirmed by relaxation of the reduced pressure level.
In defrosting methods that use a microwave oven, the microwave radiation is emitted intermittently.
Further, there exists a tempering method that uses microwave radiation, in which frozen products are evenly irradiated with microwave radiation while being conveyed on a conveyor in an open atmosphere until defrosting is completed at a minus temperature near 0° C.
In the meat selling industry, frozen meat at a temperature of −40° C. is defrosted by being placed into a refrigerator and left to stand for about two days.
Further, in the expensive fish meat selling industry related to tuna and the like, frozen tuna at −60° C. is defrosted by being immersed in warm salt water at 40° C.
Now, in the prior art defrosting method which uses microwave heating at reduced pressure and in the prior art defrosting method which uses a microwave oven, a slight drip is created during defrosting. As soon as this drip begins to flow, the microwave radiation will concentrate at such location, thereby causing the region where the drip is occurring to be overheated even though temperature inside the frozen products is −10° C., and this results in a marked loss in the quality of the frozen products.
On the other hand, in the tempering method where microwave radiation is used under an open atmosphere, because the temperature of the frozen products is stopped at a minus temperature slightly below 0° C., if a strict uniform microwave irradiation of the frozen products is carried out, there will be fewer occurrences of the kind of problem described above for the other defrosting methods. However, uniform irradiation is difficult to achieve with frozen products that have irregular shapes and sizes, and there is the further difficulty involved in accurately establishing microwave irradiation time when the frozen products have various shapes. Accordingly, the problem of dripping can frequently occur when defrosting is carried out to a relatively high temperature such as −1° C. or −2° C.
Furthermore, neither the method of letting frozen products stand over time in a refrigerator nor the method of immersing frozen products in warm salt water can avoid the problem of dripping, and for this reason, frozen products defrosted by these methods will suffer a loss in quality.
SUMMARY OF THE INVENTION
In order to overcome the problems of the prior art and make it possible to obtain defrosted products having a quality higher than that achieved with prior art defrosting methods, it is an object of the present invention to provide a method and apparatus for carrying out high quality defrosting in a short amount of time which creates only a small temperature difference between the inside and outside of the frozen products, with very little oxidation of the frozen products, and without generating a drip from the frozen products regardless of the shape and temperature of the frozen products.
In this regard, the problem usually associated with microwave heating is knowing when to properly terminate the microwave heating. The first way the present invention deals with such problem is to provide an electrical discharge generating mechanism to generate microwave-induced electrical discharges during microwave heating of frozen products previously placed inside a pressure reducing chamber while the pressure is being reduced. In this way, when an electrical discharge due to microwave radiation in the reduced pressure environment is observed to take place during the defrosting process, a proper termination of microwave heating can normally be carried out.
Namely, when a microwave-induced electrical discharge is generated during the microwave heating of frozen products under reduced pressure at a proper microwave output selected in accordance with the weight of the frozen products so as to avoid overheating thereof, the time of such electrical discharge indicates the time for a proper termination of microwave heating. Accordingly, when a microwave-induced electrical discharge is detected in the reduced pressure environment, if the emission of microwave radiation is terminated immediately after the detection of such electrical discharge, it is possible to achieve defrosting without generating a drip. Further, the inside of the pressure reducing chamber is equipped with metallic elements which have single or plural number of sharp edges which include acute angled portions that normally generate precise electrical discharges.
Further, one cause of the generation of dripping is due to a temperature difference between the inside portion and the outside portion of the frozen products. In this connection, when the frozen products are being heated with microwave radiation, the outer portion of the frozen products receive more microwave heating than the inner portion, and this results inevitably in the outer portion having a higher temperature than the inner portion. Further, because microwave radiation penetrates into frozen products from the outside portion thereof, the risk of the outside portion of the frozen products changing into liquid water must normally be taken into consideration. This problem can be solved by making the temperature of the inside portion of the frozen product and the temperature of the outside portion of the frozen product as close as possible. The present invention achieves this by generating sublimation at the outer port-ion of the frozen products at a minute level that reduces the temperature of the outer portion of the frozen products without affecting the product quality. By repeating this process, the temperature difference between the inner portion and the outer portion of the frozen products can be made gradually smaller.
A method of defrosting frozen products using microwave heating under reduced pressure in combination with a pressure returning step are repeated a plurality of times, the precise time the pressure reducing step is terminated can be controlled by continually detecting pressure level changes at prescribed time intervals. Namely, the pressure reducing step is terminated and the pressure returning step is started when the change in the pressure level reaches a prescribed pressure level. Now, assuming the vacuum pump has no clearance error, if there is no sublimation from the frozen products, each pressure reducing step can simply be terminated at the time a prescribed pressure level is reached. However, vacuum pumps with no clearance error so not exist, and in the present invention sublimation from the frozen products is utilized to reduce the temperature difference between the inside and the outside of the frozen products in order to carry out defrosting without loss of product quality. Consequently, because there is a change in pressure level that needs to be achieved due to the amount of sublimation being generated from the frozen products, there is no way accurate control can be carried out based on the pressure reaching the prescribed pressure level. However, by measuring
Shibata Kazuo
Yagi Shunichi
Ellie Corporation
Hedman & Costigan ,P.C.
Leung Philip H.
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