U.S. patent number 3,835,280 [Application Number 05/328,585] was granted by the patent office on 1974-09-10 for composite microwave energy perturbating device.
This patent grant is currently assigned to The Pillsbury Company. Invention is credited to Lawrence C. Brandberg, Larry D. Gades, Roger A. Gorman.
United States Patent |
3,835,280 |
Gades , et al. |
September 10, 1974 |
COMPOSITE MICROWAVE ENERGY PERTURBATING DEVICE
Abstract
For the purpose of concentrating microwave energy to a central
point or other desired location in a microwave oven, a composite
signal perturbating device is placed in the oven. It consists of a
sheet of a low loss dielectrical material together with a layer of
microwave reflecting material. The device is placed on the floor of
the oven and on the opposite side of the product heated from the
source of microwave energy. The low loss dielectric consists, for
example, of a flat sheet of dielectric material having two sheets
of aluminum foil imbedded therein as concentric rings positioned in
a plane parallel to the floor of the oven chamber and spaced from
about 3/4 of an inch to one inch below the product that is to be
heated.
Inventors: |
Gades; Larry D. (Minneapolis,
MN), Brandberg; Lawrence C. (Minneapolis, MN), Gorman;
Roger A. (Minneapolis, MN) |
Assignee: |
The Pillsbury Company
(Minneapolis, MN)
|
Family
ID: |
23281580 |
Appl.
No.: |
05/328,585 |
Filed: |
February 1, 1973 |
Current U.S.
Class: |
219/728; 423/234;
426/111; 426/234; 219/732; 219/745 |
Current CPC
Class: |
A23L
7/187 (20160801); B65D 81/3469 (20130101); B65D
2581/3421 (20130101); B65D 2581/3489 (20130101); B65D
2581/3472 (20130101) |
Current International
Class: |
A23L
1/18 (20060101); B65D 81/34 (20060101); H05b
009/06 () |
Field of
Search: |
;219/10.55 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3490580 |
January 1970 |
Brumfield et al. |
|
Primary Examiner: Truhe; J. V.
Assistant Examiner: Jaeger; Hugh D.
Attorney, Agent or Firm: Hermon; James V. Ellwein; Michael
D. Lund; Ronald E.
Claims
We claim:
1. A composite microwave energy perturbating device useful in
concentrating microwave energy in a zone located in approximately
the center thereof comprising a sheet of a low loss dielectric
material of an appropriate size to fit within the oven and to rest
on the bottom thereof during operation, said sheet including upper
and lower substantially parallel surfaces and vertical side edges
which are substantially less than the width and breadth of the
sheet and a plurality of concentric rings of electrically
conductive microwave reflective sheet material mounted upon the
perturvating device and positioned in a plane substantially
parallel to the upper surface thereof thereby concentrating the
microwave energy at a point near the center of the concentric
rings.
2. The apparatus of claim 1 wherein the concentric rings of said
sheet material are embedded within the dielectric material in
spaced relationship from both of the upper and lower surfaces
thereof.
3. A microwave concentrator for facilitating the heating of food
products placed within a microwave oven containing a source of
microwave energy, an oven chamber and a means for supplying
microwave energy from the source to the chamber, said concentrator
comprising:
a. a body of low loss dielectric material having a mass on the
order of more than one pound and being resistant to damage by
microwave energy when subjected thereto,
b. said dielectric material comprising a sheet of a dielectric
substance having flat upper and lower surfaces of substantial
width, the height of the sheet between the upper and lower surfaces
being substantially less than the width thereof,
c. a sheet of conductive microwave reflective material bonded to
said dielectric material,
d. said sheet of conductive microwave reflective material
comprising a ring-shaped endless strip of reflective material
located in a plane parallel to the bottom of the oven and to the
upper and lower surfaces of the dielectric sheet material,
e. said microwave reflective material being bonded to the
dielectric sheet and having its center located approximately in the
center of the dielectric sheet material.
4. The concentrator of claim 3 wherein there are a plurality of
said ring-shaped strips of reflective material positioned
concentric with one another and each is bonded to the dielectric
material.
5. The apparatus according to claim 3 wherein the microwave
reflective material comprises a sheet of relatively thin metal foil
lying in a plane substantially parallel to the upper and lower
surfaces of the dielectric sheet.
Description
FIELD OF THE INVENTION
The present invention relates to microwave cooking and more
particularly to an improved energy concentrator for facilitating
the cooking of foods in microwave ovens.
THE PRIOR ART
In spite of the demand for a popcorn product which can be freshly
popped in a microwave oven and served while it is piping hot, some
of the commercially available ovens have been unsuited for this
purpose. For example, numerous attempts have been made by us to
satisfactorily pop packages of unpopped popcorn in the Model 500
oven manufactured by the Litton Industries, 360 North Cresent
Drive, Beverly Hills, Calif., 90210. It was found by us that only
about 25 percent of the kernels would pop on the average while the
other 75 percent remained unpopped. This kind of performance is, of
course, totally unsatisfactory for commercial acceptance.
It has been proposed to package food products such as TV dinners
and frozen foods and the like in boxes formed in part of a
microwave reflective material such as aluminum foil having holes in
selected areas. The microwave energy appears to enter the holes and
is reflected about within the package by the aluminum foil thereby
facilitating the heating of the food product. In addition to
increasing the cost of the package, the use of perforated aluminum
foil layers or strips as a part of the package itself was evaluated
by us and found to be unsatisfactory for the purposes of the
present invention. U.S. Pat. No. 3,271,552 describes a heating
apparatus for concentrating microwave energy at desired points in a
microwave oven and includes spaced parallel strips of conductive
material formed into the side walls of the package containing the
food product to be heated. These strips are one half wavelength in
length to obtain maximum re-radiating characteristics. In another
form of the invention, an insulating block of a low dielectric
constant material is as mounted in the oven. Vertical re-radiating
elements such as metal rods are placed on each side of the block.
We found that the re-radiation of energy is unsatisfactory in some
instances because of localized heating and the possibility for
arcing to occur. In addition, it is expensive to provide conductive
material as a part of the package containing the food product.
OBJECTS OF THE INVENTION
The primary objects of the invention are to provide (a) a simple
and reliable method of concentrating microwave energy in a
microwave oven such as the Litton Industries Inc. 500 oven whereby
products which are difficult to properly prepare because they are
relatively sensitive to the distribution microwave energy can be
cooked satisfactorily e.g., so that when popcorn is popped 80
percent of the kernels pop when the invention is used in a Litton
Model 500 oven, (b) to provide a durable and unitary article which
functions both as an energy concentrator and as a support for the
popcorn or other material being heated, (c) a device which is
capable of functioning by two different methods of operation to
concentrate the microwave energy at the point where it is most
needed, (d) a compact, unitary microwave energy concentrator and
combination shelf which is capable of operating successfully over
extended periods of time without reflecting excessive amounts of
microwave energy back to the magnatron and (e) to provide an energy
concentrator of the type described which can be left in the oven at
all times--even during the heating of foods which are less critical
than popcorn concerning the concentration of microwave energy.
THE FIGURES
FIG. 1 is a side elevational view partly in section of a microwave
oven and concentrator in accordance with the invention showing a
popcorn product in the oven as it appears after being popped.
FIG. 2 is a partial horizontal sectional view taken on Line 2--2 of
FIG. 1 on a slightly reduced scale.
FIG. 3 is a partial perspective view of the concentrator and food
product before heating.
FIG. 4 is a partial vertical sectional view taken on Line 4--4 of
FIG. 3 on a slightly enlarged scale.
SUMMARY OF THE INVENTION
A composite microwave signal perturbating device is provided which
consists of a low loss dielectrical element p.g., a plane or sheet
together with microwave reflecting body bonded thereto. The device
is placed in the oven and on the opposite side of the produce being
heated from the source of microwave energy. The device is
preferably composed of a sheet of low loss dielectric material
having the layer of microwave reflecting material imbedded therein
e.g., as a series of concentric rings positioned in a plane
parallel to the floor of the oven chamber and spaced preferably
from about 3/4 of an inch to 1 inch below the product which rests
on the upper surface of the dielectric material.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As seen in FIGS. 1 and 2 the composite energy concentrator 10 is
generally rectangular as seen in plan view and consists of two
sheets of a low loss dielectric material such as a plastic resin
which can be subjected to microwave energy without damage such as
melting, warpage, or carbonization. Polypropelene is an example of
such a material. For convenience the low loss dielectric material
will be referred to as a ballast. The ballast is composed of an
upper rectangular plate 12 and a lower rectangular plate 14. The
ballast is preferably slightly smaller than the door of the oven so
that it can be introduced and removed as needed. A sheet eleven
inches square is typical. When polypropelene is used, the upper
sheet is 3/4 of inch thick and the lower sheet is 3/8 of an inch.
Thus, the package 30 resting on the ballast 10 is held at an
elevation of about 1 1/8 inches above the floor of the oven. We
have found that during a typical heating period of 2 minutes the
temperature of the composite sheet is raised from about 70.degree.
to about 80.degree. F.
While the reason for the effectiveness of the ballast for improving
the concentration of microwave energy is not known with certainity,
it is believed to improve the matching of the impedance of the
cavity with the impedance of the magnatron thereby assuring maximum
energy absorption within the chamber. The dielectric constant of
the ballast should be that of low loss dielectric. While a variety
of substances can be used, some of the most suitable are
dimensionally stable polemeric resinous materials such as
polypropelene and teflon. The dielectric constants of these
materials range from about 2 to about 3.2. Polyethylene has also
been found suitable but does not have quite as good dimensional
stability under typical conditions of use. Polystyrene appears to
be effective electrically but does not have the desired temperature
stability. The same is true of acrylic resins such as
polymethylmethacrylate. Glass has too high a dielectric constant
and will not function for the purposes of the invention. Other
suitable materials will be apparent to those skilled in the art.
When polypropelene is used, the entire ballast 10 should weigh
about 5 pounds when used in a Litton 500 oven.
As best seen in FIGS. 2 and 4, there is laminated between the
sheets 12 and 14 a pair of rings 24 and 26 of microwave reflective
material formed from 10 mil. aluminum foil sheets each 1 inch wide
and having outside diameters of 6 and 10 inches respectively.
Sheets 12 and 14 can be bonded together in any suitable manner as
for example by a resinous adhesive having a dielectric constant
about the same as the material from which the sheets 12 and 14 are
constructed. One example is an epoxy adhesive.
Refer now to FIGS. 1 and 2 for a description of the oven itself.
The oven indicated generally by the numeral 40 includes an outside
housing 42, a microwave generator including a magnatron (not shown)
connected by means of a waveguide 50 to a distributing device for
uniformly supplying the microwave energy to the interior of the
oven, in this case a mode stirer 48 which communicates with the
interior oven chamber through an inlet 49. The operation of the
oven is regulated by a means of exterior control 52 of a suitable
known construction. As mentioned above, the oven itself is entirely
conventional and is best exemplified by oven model 500 manufactured
by the Litton Industries Inc. of Calif.
The concentrator 10 has general application but is especially
useful for popping frozen packages of popcorn in ovens as low as
500 watts measured output. In this application the popcorn package
usually includes a flexible and expandable container body such as a
gussetted bag formed from two plys of paper. The charge of popcorn
in the package is uniformly mixed with about 1 to 5 parts by weight
of shortening for each eight parts of corn. The shortening can
include any edible cooking oil or plastic fat, whether solid or
liquid and includes both hydrogenated and non-hydrogenated
shortenings. Any of a variety of edible animal or vegetable oils or
plastic fats can be used with those of vegetable origin being
preferred because of their lower melting points. The package also
contains salt for flavoring.
The flexible popcorn package which is designated 39 consists of
paper walls with longitudinally extending gussets on each side to
provide a sizeable expansion volume. The bottom and top is sealed
tightly by transverse seals 34 and 36. The package preferably
consists of two layers of flexible sheet material. One preferred
outer sheet material is bleached kraft paper. A suitable liner (not
shown) consists of glasine paper.
After the package has been filled with a charge of popcorn, the top
is sealed at 36. The charge of unpopped corn and shortening 32 at
the center portion of the package 30 can be best seen in FIGS. 3
and 4. The block of popcorn and shortening 32 is made of about one
to five parts of a shortening for each eight parts of corn.
After the package is filled and sealed as shown in FIG. 3, it is
preferably placed in refrigerator or frozen storage until it is
ready to be used. When the popcorn is to be popped, the package is
placed on the center of the concentrator 10 within the cooking
chamber 41 of the microwave oven 40. As microwave energy is
supplied to the chamber, the package 30 is expanded by steam and
then becomes filled with the popped corn 39 as shown in FIG. 1.
It is believed that the outstanding performance results made
possible through the invention are due in large part to the
concentration of microwave energy in the vicinity of the popcorn.
During heating, the shortening provides a heat transfer medium for
conducting heat evenly between the individual kernels in spite of
the presence of hot or cool spots in the package. The package, in
addition to being flexible and expandable is relatively leak proof
at least during the period of time the product is being cooked.
While the invention has been described in connection with popping
popcorn, it is applicable to a variety of other foods. The
invention will be better understood by reference to the following
example:
Example 1
A concentrator was made of about 5 pounds of polypropelene having
the dimensions given above and was placed in a Litton 500 oven. A
gussetted paper bag as depicted in the figures is formed from Kraft
paper and includes a glasine paper lining having the dimensions 7
.times. 10 inches with 2 inch deep gussets. The bag is filled with
120 grams of a uniform mixture of popcorn, coconut oil and salt.
Forty grams of the shortening and salt mixture consisting of 30
grams coconut oil, 10 grams of super fine granulated salt and 80
grams of yellow hybrid popcorn. After the corn and shortening is
placed in the pouch it is sealed transversely at the top. The
package is then frozen. Later upon being placed in a microwave oven
of a capacity of 1,200 watts running at about 950 watts for one
minute and 45 seconds using the concentrator described above, about
22 percent or less of the popcorn will remain unpopped. The
resulting popcorn will be crisp and will have a very appealing
flavor and texture. The amount of burned kernels will be less than
5 percent and the corn will not be appreciably scorched. The bag
can be handled immediately and the volume of popped corn will be
about 2,800 cc. or more with a volume ratio of popped to unpopped
kernels of about 35 or above.
In a comparative test, otherwise identical except without the
concentrator 10, the volume of popped corn was 400 cc. and the
percent of unpopped kernels was 80 percent.
The mechanism by which the energy density is concentrated above the
center of the unit 10 is not known with certainty but it appears to
result in part from the mass of low loss dielectric material and in
part from the reflective material. As a result of tests run in the
development of the invention, the best results were obtained in
most Litton model 500 ovens when the concentrator had an inner foil
ring with an outside diameter of about 6 inches and about 60
percent of the center is cut out. Optimum results were also
obtained with the top of the polypropelene about 1 and 1/8 inches
above the floor of the oven and reflective rings about
three-fourths of an inch below the upper surface of the plate
10.
It has been found that when operating below 700 - 750 measured
Watts of power output, the conductive strips 24 and 26 become more
important. Above that level of power the plastic ballast alone is
effective to enhance the heating effect so that the corn will pop
to the 80 percent popped level. It has also been found that when
the concentrator shown in the figures is used, the ratio of
transmitted power to reflected power is improved, i.e., raised
which is desirable not only from the standpoint of heating the
product but also because it would appear to reduce damage to the
magnatron which results from reflected energy.
* * * * *