Note: Descriptions are shown in the official language in which they were submitted.
~ 3 ~
SUSCEPTORS HA~ING DISRUPTED ~EGIONS FOR
DIF~ERENTIAL. HEATING IN A MICROWAVE OVEN
In the past, difficulties have been experienced in
various attempts to brown or crispen foods in a microwave
oven. A microwave oven heats foods differently from a
conventional oven. Generally speaking, food substances
are heated in proportion to their tendency to absorb
microwave radiation, which may result in considerably
different heating patterns from those which e~ist in a
conventional oven. Also, microwave radiation penetrates
into most foods in a way which results in considerably
different heating patterns from those which would other-
wise be present in a conventional oven. In most cases,microwave energy will heat foods faster than in a conven-
tional oven. ~or example, a food substance which might
require 30 minutes to properly "cook" in a conventionàl
oven, may take only 3 or 4 minutes to "cook" in a micro-
wave oven. In a conventional oven, the oven atmosphere isheated to relatively high temperatures to transfer heat to
the food surface resulting in the surface always being the
hottest area in the food. In a microwave oven, the oven
atmosphere is generally not heated; the food itself heats
and transfers heat to the surrounding air resulting in the
'. ~
-2- ~ 3 2 ~t~3~ ~
food surface being cooler than the interior. These
differences significantly affect one's ability to brown or
crispen a surface of a food in a microwave oven.
Many attempts have been made to brown or crispen the
surface of a food in a microwave oven. One such attempt
has involved the use of packaging cornponents called
susceptors. Suitable susceptors may contain microwave
absorbing coatings which are deposited upon a microwave
transparent support layer. These susceptors heat when
exposed to microwave radiation. A susceptor may achieve
temperatures high enough to brown or crispen the surface
of a number of food products. The susceptor may be placed
in close proximity to, or in direct contact with, the
L5 surface of the food product. A typical, commercially
available susceptor contains a thin film of vacuum
deposited aluminum on polyester which is then adhesively
laminated to paper or board.
The use of susceptors, however, has resulted in addi-
tional problems. Available susceptors typically do not
heat uniformly. As a result, such susceptors may not
crispen or brown the food substance uniformly. For
example, the outer region of a susceptor may become much
hotter during microwave irradiation as compared to the
center region of the susceptor. As a result, the outer
portion of the food substance may tend to become brown or
crisp, but the center portion will not do so without over-
cooking the outer portion. This is a particular problem
in food substances which have large surface areas, for
example, the baked crust of a large frozen pizza. When a
susceptor pad is used, for example, to crispen several
fish sticks arranged side by side on the susceptor, micro-
wave heating may typically result in fish sticks on the
ends of the susceptor which are crisp, but fish sticks in
-3- ~3~
the center o~ the susceptor pad may not be adequately
crisp.
An additional problem of heating foods using suscep-
tors is the lack of control of the heating profile across
the susceptor surface. It is often desirable to adjust
the amount of heat output in sections of a susceptor to
accommodate different food characteristics. This is a
particular problem when two or more foods with varying
browning/crisping requirements are placed in conjunction
with a common susceptor. When heated, one food's contact
surface may become overcooked while an adjacent food's
contact surface may remain soggy.
When a food substance is cooked by microwave radi-
ation, particular attention must be paid to the overall
energy balance achieved during the heating process. If an
attempt is made to simply increase the strength of the
microwave radiation or cooking time in an effort to brown
or crispen a particular area of the surface of the food
substance, this may result in overheating or overcooking
of other surfaces and/or the interior of the food
substance itself. In other words, if one seeks to achieve
browning or crisping of the center area of a pizza crust
by simply increasing the heating time or by increasing the
strength of the microwave radiation, the likely result
would be an overcooking of the outer surface of the pizza
and/or an overcooking of the pizza toppings.
Heating foods in a microwave oven, particularly where
susceptors are employed, usually involves a complex
balancing of energy which i5 absorbed throughout the food
substance. Although the use of susceptors has resulted in
some improvement in the browning or crisping of food
substances in a microwave oven, the need has existed for
solving the problem of susceptors which do not heat
2 ~
uniformly. Some means ~or browning or crispening food
products uniformly with a susceptor has been needed. The
need has further e.Yisted for some means to achieve uniform
browning and crispening without disturbing the complex
energy balance necessary to properly heat all portions of
the food substance. Also, the need has existed to differ-
entially brown or crispen various types of food products.
Examples of attempts to achieve crispening of food
products is shown in U.S. Patent No. 4,267,420, issued to
Brastad, and U.S. Patent No. 4,23a,924, issued to Brastad
et al. Rrastad attempted to produce flexible wrapping
material which was wrapped completely around a fish stick
to brown the surface of the fish stick. However, Brastad
did not address the problem of nonuniform crispening of
the food surface. Brastad did not disclose how to compen-
sate for nonuniform heating caused by the flexible
wrapping material.
Another example i5 U. S . Pal:ent 4,641,005, issued to
Oscar E. Seiferth. A thin film susceptor is disclosed for
heating foods. However r Seiferth did not address the
problem of nonuniform heating of the susceptor surface.
Seiferth did not disclose how to compensate for food loads
or how to compensate for susceptor preferential edge
heating.
It will be apparent from the above discussion that
prior art attempts to achieve crispening of the surface of
a food substance in a microwave oven have not been
altogether satisfactory. The use of susceptors has often
resulted in nonuniform crispening of the food surface and
undesirable nonuniform heating patterns.
In accordance with the present invention, a system
for heating a food substance in a microwave oven is
_ 5 - ~3~
provided which may be used to achieve more uniform heating of
the surface of a food substance. The system includes susceptor
means which comprises variable sized conductive areas. The
size of the conductive areas is adjusted to compensate for
undersirable nonuniform heating patterns which would otherwise
exist.
In one aspect, this invention provides a packaging
system including susceptor heating means having selective
responsiveness to microwave radiation, comprising a susceptor
for heating in response to microwave radiation. ~he susceptor
is adapted to brown or crispen the surface of a food substance
placed in close proximity thereto. The susceptor has a
conductive film formed upon a support. The conductive film has
a first region and a second region. The first region is
operative to heat responsive to microwave radiation. The
second region is less responsive to microwave radiation than
the first region. The second region and the first region are
separated by a break in the conductive film. The second region
comprises a region with conductivity breaks in the conductive
film to reduce the responsiveness of the second region to
heating due to microwave irradiation.
The susceptor means is located in close proximity to,
or indirect contact with, the surface of the food substance
which is to be crispened or browned. The susceptor means
generally comprises a sheet with a conductive coating,
typically a metalized film, which absorbs microwave energy
during exposure to microwave fields. The susceptor means
there~ore heats in xesponse to microwave radiation. In
accordance with the present invention, the conductive coating
is divided into a plurality of regions having susceptor areas
which may be of a different sixe in each region. The susceptor
areas may be formed, for example, by scoring, cutting, etching,
stamping, printing, or other methods to disrupt the conductive
132 ~3~,ll
- 5~ -
coating of the susceptor means. At least one region has its
responsiveness to the heating effects of microwave radiation
altered by the disruptions in the conductive coating.
Portions of the susceptor which would otherwise tend
to overheat may be provided with small susceptor areas which
are comparably less responsive to microwave radiation.
Portions of the susceptor means which would otherwise tend to
underheat are provided with larger susceptor areas which are
comparably more responsive to microwave radiation. By
adjusting the size of the susceptor areas within the limits of
this invention, it is possible to compensate for nonuniform
heating patters which might otherwise exist on a susceptor.
More uniform crispening and browning of a food substance may
thereby be achieved. Alternatively, when a nonuniform
crispening or browning
0299G/3-4
-6- ~32~
pattern is desired, a susceptor means may be designed in
accordance with the present invention to provide a
specific desired heating pattern.
For a fuller ~lnderstanding of the present invention,
reference should be had to the following detailed descrip-
tion taken in conjunction with the drawings, in which:
FIG. 1 is a top view of a susceptor pad having two
regions, each being provided with different sizes of
susceptor areas formed thereon.
FIG. 2 is a graph showing the heating profile of a
susceptor pad that was not constructed in accordance with
lS the present invention.
FIG. 3 is a graph showing the temperature profile of
a similar susceptor pad, but which used regions with
different sized susceptor areas in accordance with the
present invention.
FIG. 4 is a bar graph illustrating the effect of
different sized susceptor areas formed in accordance with
the present invention on crisping.
FIG. 5 is an image of a temperature pattern achieved
during microwave irradiation of a susceptor that did not
have variable sized susceptor areas in accordance with the
present invention. The image was created by an infrared
camera.
FIG. 6 is a similar image of the heating pattern of a
susceptor which had variable sized susceptor areas in
accordance with the present invention.
-7- ~32~3~
FIG. 7 is a top view of a susceptor having variable
susceptor areas in accordance with the present invention
for use in crisping the surface of food products such as
pizza.
FIG. 8 is a graph representing the temperature
profile of a round pizza susceptor which did not have
graduated sized areas in accordance with the present
invention.
FIG. 9 is a graph showing a temperature profile of a
similar susceptor, but which did have graduated sized
susceptor areas as shown in FIG. 7.
FIG. lO is a bar chart illustrating the effect of
variable sized susceptor areas using a susceptor pad
constructed in accordance with FIG. 7 on browning.
FIG. 11 is an image of the heating pattern of a
susceptor constructed in accordance with FI&. 7. The
image was created with an infrared camera.
FIG. 12 is a graph depicting the temperature reached
during microwave heating as a function of the size of the
susceptor areas.
FIG. 13 is a graph showing the percent power
absorbed, transmitted, and reflected as a function of the
size of the susceptor areas.
FIG. 14 is a graph of capacitance reactance as a
function of the size of the susceptor areas.
FIG. 15 is a top view of an alternative embodiment of
a susceptor utilizing a maze pattern to decrease the
-8- 1 3 2 a ~
microwave heating effect upon a disrupted region of the
susceptor.
FIG. 16 is a top view of a susceptor demonstrating
the effect upon microwave heating by disruption of the
conductive sheet without cutting.
FIG. 17 is an infrared image of the microwave heating
effects upon a susceptor without any disruptions.
FIG. 18 is an infrared image of the microwave heating
effects upon the susceptor illustrated in FIG. 16.
FIG. L9 is a top view of a susceptor having four
different regions of responsiveness formed using the
present invention.
FIG. 20 i5 a graph depicting the temperature profile
during microwave heating of the susceptor constructed in
accordance with FIG. 19.
FIG. 21 is a top view of a susceptor using the
principle of "directed flow."
FIG. 22 is an infrared picture of the microwave
heating effects upon a susceptor without being modified,
which was used as a control example for comparison.
FIG. 23 is an infrared picture depicting microwave
heating of a susceptor constructed in accordance with FIG.
21.
FIG. 24 is a top view of a susceptor constructed in
accordance with FIG. 21, and subsequently modified with
additional cuts to disrupt electrical conductivity between
the center region and the strips of susceptor.
~32~
FIG. 25 is an infrared image depicting microwave heating of
the susceptor constructed in accordance with FIG. 24.
FIG. ~6 is a top view of an e~ample of a susceptor
utilizing the principle of ~directed flown.
FIG. 27 is a top view of a susceptor constructed in
accordance with the present invention having a spiral cut
therein to achieve ~directed flow".
FIG. 28 is a top view of a susceptor constructed in
accordance with the present invention having a square spiral
cut in order to achieve "directed flow".
FI~. 29 is a top view of an alternative embodiment of a
round susceptor using ffdirected flow".
In order to crispen the surface of a food substance, a
susceptor pad 10 may be used. The Eood substance which is to
be crispened may be placed in close proximity to, or in direct
contact with, the susceptor pad 10. The susceptor pad 10 may
include a layer of metallized polye~ster, composed of a layer of
polyester which has a thin film of metal such as aluminum
desposited thereon. The layer of polyester serves as a support
or the thin film of metal. The conductive layer of metal may
be deposited on the polyester substrate by a process of vacuum
vapor deposit;on. The metallized polyester layer is preferably
adhesively bonded to a supporting face, such as paper.
:~20~
-- 10 --
Further disclosure of susceptor pad is contained in U.S. Patent
No. 4,641,005 issued to Oscar E. Seiferth entitled "Food
Receptacle For Microwave Cookingn.
I~ has been found that if a susceptor pad havîng a
continuous metallized layer is e~posed to microwave radiation,
an uneven heating pattern will typically result as shown in
FIG. 5. The heating effects often will be most pronounced at
the edges o~ the susceptor pad, while the center may not be
adequately heatedD When such a susceptor pad is used, for
e~ample, to crispen a plurality of fish sticks arranged side by
side on the susceptor pad, microwave irradiation may typically
result in fish sticks on the ends which are crisp, but fish
sticks in ths center of the susceptor pad may not be adequately
crisp. At least the fish sticks will not be uniformly
crispened.
It is desirable to have some means to compensate for the
nonuniform heating which may result when a susceptor pad is
exposed to microwave radiation. In accordance with the present
invention, variable si~ed susceptor areas are provided which
compensate for the otherwise undesirable nonuniform heating
charact~ristics of a susceptor pad. As illustrated in FIG. 1,
susceptor pad 10 is provided with larger sized susceptor areas ~-
in center region 11 and relatively smaller sized susceptor
areas in end regions 1~. The relatively more microwave
responsive center region 11 contains larger susceptor areas
13. The less microwave responsive end regions 12 contain
relatively small susceptor areas 14.
In this illustrated e~ample, the conductivity of the
metalliæed film is broken by cuts or scores 15. The
-11- 132~ L/~;~
scores 15 may be cuts in the metallized layer made by a
sharp implement such as a razor blade. Or the scores 15
may be formed by stamping a sharp die on the s~sceptor pad
10. Any means Eor forming disruptions or conductivity
breaks between the metallized layer of one susceptor area
13 and an adjacent susceptor area 13 should provide satis-
factory results. For example, conductivity breaks may be
formed by etching, scoring, cutting, stamping, or photo
resist methods. It has been surprisingly found that it is
only necessary to disrupt the metallized layer, which can
be sometimes done by drawing a line with a ball point pen
across the surface of the susceptor pad 10. Generally,
any procedure which disrupts electrical continuity in the
thin film of metal has been found to be effective. The
scores 15 similarly form conductivity breaks in the
metallized film between a small susceptor area 14 and an
adjacent small susceptor area 14.
The smaller susceptor areas 14 are formed suffi-
ciently small so that the susceptor areas 14 are less
responsive to microwave radiation than the larger suscep-
tor areas 13. Thus, when the susceptor pad 10 is exposed
to microwave radiation, the smaller susceptor areas 14
will be less responsive to the heating effects of the
microwave radiation than would be the case if the scoring
15 was not provided on the susceptor pad 10. The smaller
susceptor areas 14, in effect, "detune" the responsiveness
of the end region 12 to microwave radiation.
The larger susceptor areas 13 are comparatively more
responsive to heating effects of microwave radiation. The
larger susceptor areas 13 are believed to have less of a
"detuning" effect upon the center region 11. The larger
susceptor areas 13 also improve the uniformity of heating
of the center region 11. Without having the susceptor
areas 13 cut in the center region 11, some edge heating of
-12- ~ 32~3~ ~
the center region ll could occur in this example. The
susceptor areas 13 may be formed so that no "detuning"
effect is achieved. In some applications, it is only
important that the relative heating of one region 12 be
less than another region 11.
The configuration of the small susceptor areas 1~ and
the larger susceptor areas 13 illustrated in FIG. 1 tends
to compensate for the tendency of the end regions 12 to
overheat as compared with the center region ll.
FIG. 2 illustrates the heating profile of a susceptor
pad used to heat fish sticks in a microwave oven. FIG. 2
involves a susceptor pad which did not have variable sized
susceptor areas 13 and 14. The temperature of various
positions on the horizontal center line of the susceptor
pad were measured using an infrared camera. Line 16
represents the temperature profile of a susceptor pad
after exposure to microwave radiation for 30 seconds.
Line 17 represents the temperature profile of the same
susceptor pad after exposure to microwave radiation for 60
seconds. Line 18 represents the temperature profile of
the same susceptor pad after exposure to microwave
radiation for 210 seconds.
As shown in FIG. 2, the temperature of the center of
the susceptor pad quickly heated to a relatively high
temperature within 30 seconds, and then dropped by the
time that the 60 seconds temperatures were measured. The
temperature of the center of the susceptor pad remained
low through 210 seconds of microwave irradiation, as shown
by line 18. ~Iowever, the edges of the susceptor pad
remained at relatively high temperatures. The result was
that fish sticks at the end regions of the susceptor pad
were more crisp than fish sticks located in the center
re-gion of the susceptor pad. Nonuniform crispening of the
13 ~3~
fish sticks was observed. The total cooking time for the
fish sticks was approximately 3-1/2 to 4 minutes.
The temperatures were measured with an infrared
camera. The temperatures are "uncorrected" because the
infrared camera was aimed through a wire mesh shield. The
wire mesh shield was used to prevent leakage of microwave
energy from the microwave oven. The wire mesh probably
resulted in lower average temperature readings on the
infrared camera. However, here the relative temperature
differences are of primary interest. Thus, although the
actual temperatures measured may not be precisely
accurate, the relative temperatures are believed to be
accurately portrayed.
FIG. 3 represents temperature profiles of a susceptor
pad 10 constructed in accordance with FIG. 1. As shown in
FIG. 3, the relative heating of the center region 11 as
compared with the end regions 12 was effected by the use
of smaller susceptor areas 14 on the end regions 12.
Line 19~ represents the temperature profile of the
susceptor pad 10 after 30 seconds of exposure to microwave
radiation. Line 20 represents the temperature profile of
the susceptor pad 10 after exposure to microwave radiation
for 60 seconds. Line 21 represents the temperature
profile after exposure to microwave radiation for 210
seconds. The temperature of the end regions 12 remained
relatively low at the 30 seconds measurement represented
by line 19, and at the 60 seconds measurement represented
by line 20. The temperature of the end regions 12 did
rise toward the end of the heating period, as shown by
line 21.
FIG. 4 is a graph representing the effect of the
smaller susceptor areas 1~ on the end regions 12 and the
~ ~ 2 ~
larger susceptor areas 13 in the center region 11 upon the
crispness of fish sticks. A plurality of fish sticks was
arranged on the susceptor pad 10 illustrated in FIG. 1.
The fish sticks were placed parallel to each other in
side-by-side relationship. The length of the fish sticks
was oriented vertically in FIG. 1. In other words, the
length of the fish sticks was oriented in the same direc-
tion as the width of the susceptor pad 10. Thus, some
fish sticks lay entirely in contact with the end regions
12, while other fish sticks lay in contact entirely with
the center region 11.
In a test using a trained sensory panel, ~ish sticks
in contact with the center region 11 averaged about a 10
change ~increase) in breading crispness, as shown by bar
graph 2~ in FIG. 4. Fish sticks prepared on standard
susceptors and on scored susceptors were compared by the
panel. Fish sticks in contact with the end regions 12 of
the susceptor pad 10 experienced a ~ change (decrease)
in breading crispness, as shown by bar graph 23 in FIG. 4.
The percentage changes were based upon a comparison with
fish sticks cooked on a susceptor pad which did not con-
tain variable susceptor areas 13 and 14. Thus, the use of
variable susceptor areas 13 and 14 resulted in an increase
in the crispness of fish sticks on the center region 11,
and a decrease in the crispness of fish sticks on the end
regions 12. The variable sized susceptor areas 13 and 14
therefore compensated for nonuniform heating which would
have otherwise resulted during microwave irradiation of
the combination of the susceptor pad and fisn sticks.
The effect of the variable sized susceptor areas 13
and 14 is further illustrated by a comparison of FIG. 5
with FIG. 6. FIG. 5 represents an image taken with an
infrared camera during microwave irradiation of a suscep-
tor pad which did not include variable sized susceptor
-15-
~2~
areas 13 and 14. FIG. 6 illustrates the temperature
profile of a susceptor constructed in accordance with FIG.
1. The infrared image of FIG. 6 was taken after 30
seconds of e~posure to microwave radiation. FIG. 6 corre-
sponds with the 30 second temperature profile shown in
FIG. 3 ~represented by line 19). FIG. 6 shows that the
microwave heating of the end regions 12 was greatly
reduced as compared with the center region 11.
In the susceptor pad 10 illustrated in FIG. 1, the
small susceptor areas 14 are formed in the shape of
squares which are approximately 1/16 inch on each side.
In other words, the small susceptor areas 14 are formed in
the sha~e of squares having a height and width of 0.0625
inch.
The large susceptor areas 13 in the center region 11
of the susceptor pad 10 illustrated in FIG. 1 are formed
in the shape of rectangles having a length of 1-1/4 inches
and a width of 7/8 inch. In other words, the large
susceptor areas 13 have a length of 1.25 inches and a
width of 0.~75 inch.
The overall length of the illustrated susceptor pad
10 was 6-1/2 inches. The overall width was 3-3/4 inches.
Each end region 12 was about 2 inches by 3 3/4 inches.
The center region 11 was about 2-1/2 inches by 3-3/4
inches. The scores 15 used for separating the small and
large susceptor areas 14 from each other and from adjacent
large susceptor areas 13 were the width of a razor blade
cut in the metallized polyester layer.
FIG. 7 illustrates a round susceptor pad 24 used for
browning the crust of a pizza or the like. In the case of
a round susceptor for use in heating pizza, it has been
found that the outer perimeter of the susceptor pad tends
-16- ~3~
to heat much more than the center region of the susceptor
pad. This often results in a browning of the outer
surface area of the pizza crust, while only 50-60~ of the
center area of the pizza crust is browned. This is shown
by the information depicted in FIG. 8 and FIG. lO, which
will be explained in more detail below.
It is desirable to have some means for reducing the
heating of the outer region 26 of the susceptor pad 24,
while increasing the relative heating of the center region
25 of the susceptor pad 24. In the present invention,
this is accomplished by providing conductivity breaks or
scoring 27 in the outer region 26 of the susceptor pad 24.
The scores 27 may be in the form of cuts made with a razor
blade or the like. It is sufficient if the scores 27 are
made in any manner which disrupts or breaks the electrical
conductivity of the metallized layer of the susceptor pads
24.
The scores 27 define small susceptor areas 28 in the
outer region 26 of the susceptor pad 24. The center
region 25 defines a larger susceptor area 29. The small
susceptor areas 28 are less responsive to the heating
effects of microwave radiation, as compared with the large
susceptor area 29. This has the effect of reducing the
level of heating in the outer region 26 of the susceptor
pad 24, where the susceptor 24 would otherwise tend to
overheat. The provision of variable susceptor areas 26
and 29 has the effect of increasing the temperature of the
center region 25 relative to the outer region 26.
FIG. 8 is a graph illustrating temperature profiles
of a round susceptor pad used for browning the crust of a
pizza. Line 30 represents temperature measurements at
various horizontal positions of the susceptor pad after 30
seconds of exposure to microwave radiation. Line 31
-17- ~ $ 2 ~
represents temperature measurements at the same locations
after exposure to microwave radiation for 120 seconds.
Line 32 represents temperature measurements after exposure
to microwave radiation for 300 seconds. Line 33 depicts
temperature measurements after 390 seconds of exposure to
microwave radiation. FIG. 8 shows that the outer portion
of the susceptor pad became much hotter than the center
portion of the susceptor pad.
FIG. 9 illustrates the temperature profile of a
susceptor pad 24 constructed in accordance with the
embodiment illustrated in FIG. 7. Line 34 shows tempera-
ture measurements at various horizontal positions on the
susceptor pad 24 after exposure to microwave radiation for
30 seconds. Line 35 depicts temperature measurements
after 120 seconds of exposure to microwave radiation.
Line 36 shows temperature measurements after 30~ seconds
of exposure. Line 37 depicts temperature measurements
taken after 390 seconds of exposure to microwave
radiation.
A comparison of FIG. 9 with FIG. 8 shows that the use
of variable susceptor areas 28 and 29 dramatically change
the temperature profile of the pizza susceptor 24. The
center region 25 became much hotter after 390 seconds of
exposure, than did the center region of a susceptor pad
which was not constructed in accordance with the present
invention. The temperature of the outer region 26 was
reduced, while the temperature of the center region 25 was
increased.
FIG. 10 is a bar chart illustrating the effect upon
browning of the pizza crust as a result of the use of
different sized susceptor areas 28 and 29 on the susceptor
pad 24. The bar chart represents the percentage of crust
area which was browned after microwave heating.
~32~'.11
Bar 38 in FIG. 10 represents the percentage of crust
area which was browned using a susceptor pad that did not
have different sized susceptor areas. Slightly less than
80~ of the piæza crust area was browned in this instance.
More than about 85% of the area of the outside of the
pizza crust was browned, as shown by bar 40 in the bar
chart of FIG. 10. However, less than 60~ of the center
area of the pizza crust was browned, as shown by bar 39 in
FIG. 10.
Using a susceptor pad 24 having different sized
susceptor areas 28 and 29, as shown in FIG. 7, the amount
of browning which occurred in the center region 25 was
greatly increased, while the amount of browning which
occurred in the outer region 26 was greatly decreased.
Bar 42 represents the amount of browning which occurred in
the center region 25. About 95~ of the area of the crust
in the center region 25 was browned in this instance.
Only about 5~ of the area of the crust in the outer region
26 was browned, as shown by bar 43 in FIG. 10. The total
percentage of the area of the crust which was browned was
less than 30%, as shown by bar 41 in FIG. 10.
FIG. 11 is an image taken with an infrared camera
depicting the heating pattern of a susceptor pad 24
constructed in accordance with the embodiment illustrated
in FIG. 7. The infrared image was taken at a point during
the heating period corresponding to three hundred ninety
seconds of exposure to microwave radiation. The infrared
image of FIG. 11 corresponds with line 37 depicted in the
temperature profile graph of FIG. 9. The areas corre-
sponding to the center region 25 and the outer region 26
are marked in FIG. 11.
In the particular susceptor pad 24 illustrated in
FIG. 7, the diameter of the susceptor 24 was nine inches.
~32~3~
The diameter of the center region 25 was about 4.5 inches.
The small susceptor areas 28 were formed generally as
squares having a height and width of about 1/16 inch, or
0.0625 inches. The scores 27 were formed by razor blade
cuts in the metallized layer of the susceptor pad 24.
When one region 26 of a susceptor 24 is made less
responsive to microwave heating, the amount of heating of
a nondisrupted region 25 may be increased. This phenome-
non is referred to as "load sharing." It is believed thatwhen one region 26 is made less responsive to microwave
heating, there is more energy available to heat other
regions 25.
FI~. 12 is a graph depicting the heating effect of
small susceptor areas 14 as a function of the size of the
area. In this case, the susceptor areas were formed as
squares. The indicated dimensions are the height and
width of the squares.
FIG. 12 shows that the responsiveness of small
susceptor areas 14 to the heating effects of microwave
radiation rapidly decreases when the squares 14 are made
smaller than 0.625 inches on a side where the metallized
susceptor pad 10 has a relatively large resistivlty of
1650 ohms per square. For lower resistivities on the
order of eighteen ohms per square, the responsiveness of
the small squares 14 to the heating effects of microwave
radiation decreases when the squares are made smaller than
0.3125 inches on each side.
In FIG. 12, line 44 depicts the temperature as a
function of size for small squares 14 where the resis-
tivity of the metallized layer of the susceptor pad 10 is
eighteen ohms per square. Line 45 depicts the temperature
as a function of size of squares 14 where the resistivity
-20- ~32~3~
of the metallized layer of the susceptor pad 10 was sixty
ohms per square. Line 46 depicts the temperature as a
function of size for susceptor areas 14 where the resis-
tivity of the metallized layer was 1650 ohms per square.
These temperatures have not been corrected for the
differences in emissivity of the susceptor surface. The
relative temperatures along each line (44, 45, 56) are
correct. The comparative heating between susceptors of
different resistivities is affected by emissivity
differences of the susceptor surfaces and has not been
corrected in FIG. 12.
FIG. 13 depicts data taken with a network analyzer
for the susceptor pad 10 which was 60 ohms per square, and
which formed the basis for the measurements depicted in
FIG. 12 by line 45. A 5-inch square uncut susceptor pad
10 provided reflectance, transmission and absorption
measurements which are shown on the far right-hand portion
of the graph of FIG. 13. For the uncut pad, the absorp-
tion was measured at about 30%. The reflection wasmeasured at about 68%. The transmission was measured at
about 2%.
FIG. 13 shows that the reflection, transmission and
absorption of a susceptor pad 10 are affected by disrup-
tions or conductivity breaks in the susceptor surface.
The curves begin to change significantly when the size of
the squares 14 created b~ the disruptions or breaks in
conductivity were made 0.625 inch on a side, or smaller.
The percentage power absorbed decreased significantly for
squares which were 0.625 inch on a side, or smaller. An
absorption of about 33~ was measured for squares 1~ having
a width of 0.625 inch~ An absorption cf about 27% was
measured for squares 14 having a width of about 0.3125
inch. An absorption of about 20~ was measured for squares
14 having a width of about 0.1563 inch. An absorption of
-21- ~32~
about 11~ was measured for squares 14 having a width of
about 0.0781 inch.
All measurements were taken by the network analyzer
prior to heating of the susceptor pad 10 in a microwave
oven. This technique, i.e., using network analyzer data,
may be used to determine the reduced responsiveness of
susceptor pad regions which have disruptions or conductiv-
ity breaks that form complex patterns which may not define
simple squares 14 as depicted in the above examples.
Thus, it should be appreciated that reduced responsiveness
to microwave heating can be achieved using disruption
patterns or conductivity breaks of various configurations,
in addition to the illustrated example of squares 14.
The effect of disruptions or conductivity breaks in
the susceptor surface may be better understood with
respect to FIGo 14~ FIG. 14 is a graph depicting the
effect upon the reactive component of the impedance of a
susceptor pad when small squares 14 are formed in the
susceptor surface. The data plotted on FIG. 14 was
measured with a network analyzer, using the same susceptor
pad which had an initial resistivity of 60 ohms per
square. More specifically, the impedance of the susceptor
~5 pad was essentially all resistive prior to cutting, as
shown by the point at the upper right-hand corner of the
graph, measured for the uncut 5-inch square susceptor pad.
Conductivity breaks in the surface of the susceptor
pad created a negative reactance, i.e., a capacitive
reactance. The total impedance Zs of the susceptor pad
may be e~pressed as:
-22-
~ 3 2 ~
S S jxs
where Rs is the resistance component of the impedance, and
Xs is the reactance component of the impedance. If Xs is
S positive, then the reactance is inductive. If Xs is
negative, then the reactive component is capacitive. When
the surface of the susceptor is discontinuous, as a result
of disruptions or breaks in the conductivity of the
susceptor pad surface, the susceptor typically demon-
strates a capacitive reactance.
Measuring the reactance of the susceptor surfaceprovides an indication of the magnitude of the discontinu-
ity or disruption of a region of the susceptor surface.
This is proportional to the extent to which the
responsiveness of that region to heating during microwave
irradiation wi~l be affected by the discontinuity or
disruption in the susceptor pad surface.
The relative difference in the capacitive reactance
of various regions of the susceptor pad 10 resulting from
disruptions in the susceptor surface may be used as a
means of determining whether one region will be less
responsive to the heating efects of microwave radiation
as compared to another region of the susceptor pad 10.
Thus, complex patterns may be used to create disruptions
in the susceptor pad surface. Measurements with the
network analyzer may be used for determining the changed
responsiveness of a region of the susceptor pad to the
heating effects of microwave radiation as a result of any
complex pattern of disruptions.
FIG. lS illustrates an embodiment of a susceptor pad
surface having a complex "ma~e" pattern forming disrup-
tions in the susceptor pad surface. For complex patternssuch as shown in ~IG. 15, network analyzer measurements
-23-
may be used for determining the relative responsiveness o~
various regions to microwave radiation.
FIG. 15 shows a first region 47 of the susceptor pad
having discontinuities or disruptions in the form of a
maze pattern. The disruptions in the first region 47
render it less responsive to the heating effects of micro-
wave radiation than would be the case if the disruptions
in the susceptor surface were not present in the first
region 47. A second region 48 is also s~own, in this
example as a center rectangle of susceptor material.
Disruptions in the susceptor surface do not neces-
sarily have to take the form of cuts in the surface. The
susc~ptor surface may be disrupted, for example, by
drawing lines using a ball point pen. An example of the
ability to achieve less responsiveness by disruptions
created, for example, with a ball point pen, is shown in
the experiment illustrated in FIG. 16. A square susceptor
pad 49 was used in this experiment. A grid pattern cover-
ing a first region 50 was drawn on the susceptor pad ~9
using a ball point pen. Three circular regions 51 were
arbitrarily selected, and were not provided with disrup-
tions. The relative heating of two susceptor pads is
shown in FIGS. 17 and 18, without the grid pattern and
with the grid pattern illustrated in FIG. 16,
respectively.
FIG. 17 shows an image formed with an infrared camera
showing the heating effects upon a susceptor pad without
any disruptions. This susceptor pad was used as a control
for the experiment.
FIG. 18 is an infrared image of the heating effect
upon a susceptor pad 49 having a grid pattern drawn on it
using a ball point pen. The relative difference in the
-24- ~ ~2 Q~
heating of the three circular regions 51 which did not
have the susceptor pad surface disrupted is clearly appar-
ent from the infrared image of FIG. 18. This experiment
demonstrated the effectiveness of disruptions in affecting
the heating response of a region of a susceptor pad.
Thus, actual cuts in the susceptor pad surface are not
required. Disruptions may be created by pressing or
stamping the susceptor pad surface. Disruptions may be
created which are virtually invisible. However, the
effect of disruptions can be revealed by measurements
taken using a network analyzer.
FIG. 19 shows a susceptor pad 52 which has a first
region 53, a second region 54, a third region 55 and a
fourth region 56, each having different patterns of
conductivity breaks in the surface of the susceptor pad
52. In this example, squares 57 were formed in the fourth
region 56. The squares 57 had a width of 1/2 inch. The
squares 57 were formed by making cuts 61 in the surface of
the susceptor pad 52 uslng a ra:zor blade.
The third region 55 had smaller squares 58 formed by
cuts 61, which had a width of about 1/4 inch. The second
region 54 had even smaller squares formed therein which
had a width of about l/8 inch. The first region 53 had
the smallest squares 60 formed by cuts 61~ which had a
width of about l/16 inch.
FIG. 20 illustrates the temperature profile of the
susceptor pad 52 constructed in accordance with FIG. 19.
The heating effects of the microwave radiation on the
fourth region 56 was much greater than the heating effects
upon the other regions 53, 54 and 55. The smaller the
size of the squares in the region, the less heating was
observed. Temperatures were measured using an infrared
camera.
_~5_ ~ 3 ~
Cuts or disruptions in the surface of the susceptor
may be used to create an effect which may be referred to
as "directed flow." This Inay be illustrated with refer-
ence to the experiment depicted in FIGS. 21-25.
FIG. 21 illustrates a susceptor pad 62. Parallel
cuts 63 were made in the surface of the susceptor pad 62.
A center uncut region 64 was leEt in the middle of the
susceptor pad 62. The parallel cuts 63 defined strips 65
on the surface of the susceptor pad 62. Tllere was no
conductivity break or disruption between the end of each
strip 65 and the center region 64 of the susceptor 62.
FIG. 22 is an image taken with an infrared camera
showing the heating pattern of an uncut susceptor. This
was used as a control for the experiment. FIG. 23 is an
image taken with an infrared camera showing the heating
pattern of the susceptor 62 constructed in accordance with
FIG. 21. Intense heating of the center region 64 is
apparent. The strips 65, which are connected without
disruption to the center region 64, appear to enhance
heating of the center region 64.
FIG. 24 shows a susceptor pad 66 constructed in
accordance with FIG. 21, with t~e exception that addi-
tional cuts 67 were made to disrupt or break the continu-
ity between the strips 65 and the center region 64. FIG.
25 is an image taken with an infrared camera showing the
heating pattern of the susceptor pad 66 constructed in
accordance with FIG. 24. The heating of the center region
64 is not as pronounced as in the example shown in FIG.
21.
FIG. 26 illustrates an alternative embodiment of a
susceptor pad 68 utilizing the principle of "directed
flow." In this example, the susceptor pad 68 was a
11 32~3~
circular susceptor, for example, sultable for use with
pizza and the like. The susceptor pad 68 illustrated in
FIG. 26 has radial cuts or disruptions 69. The cuts 69
define strips 70 extending radially inwardly toward a
center region or target area 71. The strips 70 are
connected without disruption to the center region 71. It
will be appreciated that the target area 71 may be located
at a position other than the center of the susceptor 68.
Secondary cuts 72 may be provided to extend only
partially toward the center region 71. A secondary region
73 is defined by the region extending radially outward
from the center of the pad 68 to the ends of the secondary
cuts 72. This results in a relatively hot center region
71. The secondary region 73 will be generally warmer than
the outermost region 74 of the susceptor pad 68.
Generally, the more cuts 69 which are provided in the
susceptor pad 68, the hotter the center region 71 will be.
It has also been observed in practice that the uniformity
of the heating of the outermost region 74 of the susceptor
pad 68 is improved by providing an increased number of
cuts 69 in the susceptor pad 68.
A circular cut could be macle around the center region
71 to break electrical conductivity between the center
region 71 and the strips 70O The center region 71, in
such an example, has been observed to get preferentially
hot during microwave heating, but not as hot as compared
to an example where the center region 71 is connected to
the strips 70 without disruption, as shown in FIG. 26.
An alternative embodiment of a round susceptor 75 is
shown in FIG. 29. The illustrated example has a plurality
of cuts 76 extending from the outer perimeter radially
inwardly toward a center region 77. The cuts 76 define a
-
- 27 -
pluarality of strips 78 extending radially from the center
region 77. In this case, all of the cuts 76 e~tend from the
perimeter of the susceptor 75 to the edge of the center region
77. All other things being equal, the center region 77 of the
example illustrated in FIG. 29 would get hotter than the center
region 71 of the e~ample illustrated in FIG. 26.
A variety of geometries have been used to demonstrate the
principle of ~directed flown. For e~ample, round spirals, as
shown in FIG. 27, squared spirals, as shown in FIG. 28,
pinwheel-shaped cuts, cross-shaped re~ions, etc. have been
tried. All of these various geometries demonstrate the ability
to generate a xelatively hot center region which is connected
without disruption to various shaped strips.
The center region generally has been observed to have a
ma~imum size at which the principle of ~directed flow" will
work most effectively. If the area of th~ center region is
made too large, the center region will not get as hot. The
maximum si~e of the center region is believed to be a function
of the resistivity of the susceptor pad material. The lower
the resistivity, the larger the center region may be and still
effectively result in pronounced heating of the center region.
Generally speaking, the smaller center region the hotter or
more intense will be the heating effect on the center region.
The susceptor may be constructed where the susceptor
surface is initially constructed having disruptions or breaks
in the conductive layer. Additional disclosure is contained in
an application entitled ~Microwave Heater and Method of
Manufacture"~ by Terpin et al.
-28- ~ 32~
In the above description, measurements of resistiv-
ity, reflectance, transmission, absorbance, etc., were all
taken at room temperature (21 C) unless otherwise
specified.
In the above descriptions, measurements taken with a
network anal~zer all involved the procedure described
below. A Hewlett Packard Model No. 8753A network analyzer
in combination with a Hewlett Packard Model No. 85046A S-
parameter test set were used. A11 measurements were madeat the microwave oven operating frequency of 2.45 GHz.
All measurements were made at room temperature, unless
otherwise specified. All measurements are made using WR-
282 waveguide. Measurements of reflectance, transmission
and absorption were made without the presence of a food
item.
Measurements are preferably made by placing a sample
to be measured between two adjoining pieces of waveguide.
Conductive silver paint is preferably placed around the
outer edges of a sample sheet which is cut slightly larger
than the cross-sectional opening of the waveguide.
Colloidal silver paint made by Ted Pella, lnc. has given
satisfactory results in practice. The sample is prefer-
ably cut so that it has an overlap of about S0/1000 inch(0.127 cm) around the edge. The waveguide is calibrated
according to procedures specified and published by Hewlett
Packard, the manufacturer of the network analyzer.
Scattering parameters, Sll, S12, S21 an 22
measured directly by the network analyzer. These measured
parameters are then used to calculate the microwave power
reflectance, power transmittance, and power absorbance.
The reflectance looking into port 1 is the magnitude
of Sll squared. The reflectance into port 2 is the magni-
~ ~ 2 ~
- 29 -
tude of S22 squared. The transmittance looking into port 1
is the magnitude of S21 squared. The transmittance looking
into port 2 is the magnitude of S12 squared. The power
absorbance, looking into either port 1 or port 2, is equal to
one minus the sum of the power reflectance and the power
transmittance into that port.
The complex surface impedance of an electrically thin sheet
is obtained from the measured scattering parameters using
formulas presented in "Properties of Thin Metal Films at
Microwave Frequencies", by R.L. Ramey and T.S. Lewis, puhlished
in the Journal of Applied Physics, Vol. 39, No. 1, pp. 3883-84
(July 1968), along with the information in J. Altman, Microwave
Circuits, pp. 370-71 (1964). For undisrupted susceptor
material, the impedance is essentially all resistive.
Disruptions or conductivity breaks introduce a capacitance
reactanca component into the impedance.
The infrared images and temperture measurements made with
an infrared camera were taken using a Thermovision 870 scanner
(infrared camera). The infrared was used in conjuction with a
TIC 8000 Thermoimage Computer. Image analysis was accomplished
CATS software, (ver~ion 1.04 ). The infrared camera, computer
and software are commercially available from Agema Infrared ~~~~~
systems A.B., with opposites in Danderyd, Sweden.
The original infrared images of FIGS. 5, 6, 11, 17, 18, 22,
23 and 25 were in colorO For convenience, black and white
copies have been used herein. The color originals are not
believed to essential matter. However, the color originals are
hereby incorporated herein by raference.
2280b~1 -4
-30- ~ 3 2 ~ ~.ql
The above disclosure has been directed to a preferred
embodiment of the present invention. The invention may be
embodied in a number of alternative embodiments other than
those illustrated and described above. A person skilled
in the art will be able to conceive of a number of modifi-
cations to the above described embodiments after having
the benefit of the above disclosure and having the benefit
of the teachings herein. The full scope of the invention
shall be determined by a proper interpretation of the
claims, and shall not be unnecessarily limited to the
specific embodiments described above.
