Technical Field
[0001] This invention relates generally to improvements in a microwave oven, and more particularly,
pertains to a new and improved method of cooking thin meat in a microwave oven.
Background of Prior Art
[0002] Those concerned with microwave cooking of a thin piece of meat in a microwave oven
have long recognized the need to accurately determine the temperature of the meat.
The present invention fills this need.
[0003] Past prior art devices have been extremely-unreliable in determining the surface
and internal temperature of meat. Initial prior art attempts at determining the internal
doneness of meat was to insert a thermometer into the piece of meat being cooked which
was not affected by the microwave energy radiation, but the disadvantage was that
the thermometer only indicated the internal temperature of the meat at one particular
location and was not a true overall indication of the equilibrium internal doneness
of the meat.
[0004] Another current prior art device for measuring the internal temperature of meats
being cooked is to insert a temperature probe into the meat while in the microwave
oven heating cavity which connects to the control circuitry of the microwave oven.
While the temperature probe is accurate in indicating the temperature of the meat
being cooked by microwave energy, the probe only senses the temperature of the immediate
area surrounding the probe and does not take into account the equilibrium state of
the internal temperature of the meat. If the temperature of the meat is not uniform
and the probe is placed at a hot spot in the meat, the readings from the temperature
probe are not indicative of the internal doneness of the meat.
[0005] Further, the probe which is inserted into the meat is cumbersome and bulky for the
cook who is trying to monitor the internal doneness of the meat during the microwave
cooking in the microwave oven heating cavity. For a thin piece of meat being approximately
equal to or less than one wavelength, it is difficult for a cook, if not impossible,
to insert a temperature probe into the meat as the thickness is approximately no greater
than one centimeter.
[0006] This invention, a method of cooking thin meats in a microwave oven, overcomes the
disadvantages of prior art by providing an accurate method for determining the temperature
of thin meat.
Brief Summary of the Invention
[0007] The present invention obviates the foregoing disadvantages of the prior art by providing
a method of cooking thin meat in a microwave oven.
[0008] According to the preferred embodiment of the present invention, there is provided
a method for cooking thin meat in a microwave oven wherein the time dependent "in-situ"
humidity and temperature environmental conditions of a microwave oven heating cavity
are sensed and sampled, the absolute humidity is determined from the sampled "in-situ"
humidity and temperature, the absolute humidity is determined at a plurality of points
on a "characteristic humidity curve", and the temperature of the meat is determined
from the slope of the sampled points of the "characteristic humidity curve".
[0009] A significant aspect and feature of the present invention is a method which provides
for the determination of the temperature of a piece of thin meat being cooked in the
microwave oven. Since the meat is thin, that is being approximately equal to or less
than one skin depth which for 2450 megahertz is one centimeter, the surface temperature
of the meat is equal to the internal temperature of the meat.
[0010] Another object of the invention is to provide a method of determining when the thin
piece of meat is cooked to doneness by sensing the time dependent "in-situ" humidity
and the temperature environmental conditions of the microwave oven heating cavity.
The term "in-situ" as used in this application is defined as the actual time dependent
environmental conditions which exist in the environment surrounding the food product
such as thin meat which is located in and cooked in the microwave oven heating cavity.
Although in the present invention, a humidity sensor and a temperature sensor may
be positioned outside of the microwave oven heating cavity, the sensors are configured
to sense the "in-situ" environmental conditions of the microwave oven heating cavity
and provide "in-situ" signal information of the environmental conditions of the microwave
oven heating cavity to a programmable controller controlling the microwave oven.
Brief Description of the Drawing
[0011] Other objects and many of the attendant advantages of this invention will be readily
appreciated as the same becomes better understood by reference to the following detailed
description when considered in connection with the accompanying drawings, in which
like reference numerals designate like elements throughout the figures thereof and
wherein:
FIGURE 1 illustrates a flow chart of the method of cooking thin meat in a microwave
oven in accordance with the present invention, and;
FIGURE 2 illustrates a typical "characteristic humidity curve" for the present invention.
Detailed Description of the Invention
[0012] Figure 1 illustrates a flow chart 10 of the method of cooking thin meat in accordance
with the present invention. The flow chart provides for the solving of the equation
![](https://data.epo.org/publication-server/image?imagePath=1979/10/DOC/EPNWA1/EP78100793NWA1/imgb0001)
where equation 1 is a "characteristic humidity curve" for thin meat having a thickness
approximately equal to or less than one skin depth. h
o is the ambient absolute humidity, h
x is the peak absolute humidity, and B is a coefficient, characteristic of meats having
units of 1/seconds
2.
[0013] The flow chart 10 starts at 12 where the "in-situ" relative humidity of the microwave
oven heating cavity is sampled and stored 14, the "in-situ" temperature of the microwave
oven heating cavity is sampled and stored 16, the saturated humidity 18 is determined,
and the absolute humidity 20 is determined from the multiplication of the relative
humidity times the saturated humidity.
[0014] The program after waiting K seconds 22, then sets n equal to 1 at 24 and initiates
by sampling data 26 at four points to compute the slope of the "characteristic humidity
curve" for the algorithm of equation 1 for the method. Unitl n equals five 28 condition
is satisfied, n is incremented by one 30 waiting m seconds 32 between each sample.
Data 26 is sampled at four points satisfying conditions 28 and 30 waiting m seconds
32 between each sample. When n equals five 28 condition cccurs, the slope 34 is determined
by solving the simultaneous equations 36 for h
x and B. From determining the values of h
x and B, t
x is determined 38 and t
done is equal to a percentage of t
x 40 as the "characteristic humidity curve" proceeds to infinity and a cook is only
interested in the plateau of the "characteristic humidity curve".. When t
real less than or equal to t
done YES condition 42 exists, then the program recycles itself after waiting K seconds
to again begin sampling data at four points to determine the slope 34, solve for t
x, etc. This continues to repeat until a t
real equal to or less than t
done NO condition 42 occurs at which time the microwave oven is turned off 46 and a done
indication is given to the cook.
[0015] The method of cooking thin meat in a microwave oven is premised on the sensing concept
based on the most fundamental cooking principles. That is, as microwave energy is
converted to thermal energy, the internal temperature of the meat increases. As the
internal temperature of the meat increases, some of the thermal energy is used to
break the bonding forces holding the water molecules to the food's cell structure.
When the latent heat of vaporization has been added to the food, these free water
molecules are vaporized and released locally. Thus, the rate of evaporation is directly
proportional to the rate of temperature rise. By monitoring time dependent "in-situ"
environmental conditions of the microwave oven heating cavity with sensors, the meat's
internal temperature, and surface temperature for a thin piece of meat being approximately
less than or equal to one skin depth, are determined.
[0016] The "characteristic humidity curve" 48 of absolute humidity in grams per cubic meter
versus cooking time in seconds of Figure 2 for the thin meat equation 1 is determined
by sensing the time dependent "in-situ" humidity and temperature environmental conditions
of the microwave oven heating cavity during microwave cooking of thin meats. The time
dependent "in-situ" environmental conditions are defined as the sensed humidity and
temperature of the microwave oven heating cavity by humidity and temperature sensors.
Each particular cut of thin meat has its own particular "characteristic humidity curve".
h is the initial absolute humidity and h
x is the peak absolute humidity at the time t of the "characteristic humidity curve"
48.
[0017] Figure 2 depicts a characteristic humidity curve derived from a plot of absolute
humidity versus elapsed cooking time. The changes in slope of the curve are indicative
of the rate of evaporative water loss from the thin meat body effecting the absolute
humidity in the cooking cavity. For purposes of the present method, the point of peak
absolute humidity, h
x, indicated at 60, is of primary interest. The "characteristic humidity curve" 48
plateaus at point 60 indicating that surface temperature of the meat has reached equilibrium
at one-hundred degrees centigrade.
[0018] For small meat samples or with thicknesses small compared to the microwave depth
of penetration, that is, being approximately equal to or less than one skin depth,
the internal temperature is approximately and for all practical considerations, the
same as the surface temperature from equation 1 for any point of the "characteristic
humidity curve" of Figure 2.
[0019] The basic premise of the method is that as the relative humidity and temperature
are sampled and stored, h and B are computed by solving simultaneous equation 36 to
predict when the one hundred degree centigrade point 60 occurs. The temperature is
assumed linear between h and one hundred degrees centigrade so that once t is computed,
the corresponding time for any other temperature may be computed such as to determine
when t
done is reached.
[0020] The method may be implemented in accordance with the flow chart 10 of Figure 1 as
an algorithm stored in a programmable controller such as an Intel 8080 Microprocessor
in the microwave oven. This algorithm of Figure 1 determines the temperature for a
thin piece of meat for any point on the "characteristic humidity curve" of Figure
2.
[0021] An aluminum oxide humidity sensor, such as a Thunder Scientific TC-2000 Humidity
Measurement Module, and a temperature sensor, such as a National Semiconductor Corporation
LX 5700 Temperature Transducer, are positioned adjacent to and by the exit ventilation
port to the exterior side of the microwave oven heating cavity for way of example
and for purposes of illustration only to sense the time dependent "in-situ" environment
conditions of the microwave oven heating cavity. The sensors can be positioned anywhere
as long as the time dependent "in-situ" environmental conditions of the microwave
oven heating cavity are sensed and the sensors are electromagnetically protected from
the microwave oven heating cavity. The humidity sensor and the temperature sensor
connect to the programmable controller having the steps of the algorithm of Figure
1 stored in the memory of the programmable controller. The programmable controller
connects to control the microwave power source power supply and the air exchange circuit
and turns the oven off when a NO condition exists at the t
real is less than or equal to a predetermined t
done decision point 42.
[0022] Various modifications can be contempleated for the method of cooking thin meat in
a microwave oven of the present invention without departing from the apparent scope
of this invention.
[0023] Having thus described the invention, what is claimed is:
1. A method for cooking thin meat bodies in a microwave oven having means for measuring
the humidity of the microwave oven cooking cavity environment associated therewith
comprising the steps of:
(a) energizing a microwave energy source to provide microwave energy to said cooking
cavity whereby at least a portion of said energy is absorbed by a thin meat body located
in said cooking cavity;
(b) measuring the humidity of said cooking cavity environment at a plurality of time-spaced
intervals and storing said measurements in a microprocessor forming part of the controls
for said oven;
(c) calculating the slope of a humidity-time curve described by said measurements;
(d) comparing said calculated slope with the slope of a characteristic humidity-time
curve and calculating from said comparison the time when said thin meat body will
reach thermal equilibrium at 100°C;
(e) calculating the time said thin meat body will reach a preselected doneness as
a function of said thermal equilibrium time, and;
(f) de-energizing said microwave energy source at said calculated doneness time.
2. The method of claim 1 wherein said means for measuring the humidity of the microwave
oven cooking cavity environment comprises means for sensing the cavity environment
relative humidity and temperature and wherein said humidity measuring step includes
the calculation of the absolute humidity of said cavity environment from the sensed
relative humidity and temperature.
3. The method of claim 1 wherein said characteristic humidity-time curve is described
by the equation:
wherein h = absolute humidity at time interval N
ho = absolute humidity at time interval N=0
hx = peak absolute humidity at thermal equilibrium
B = a coefficient, characteristic of said thin meat body
4. The method of claim 3 wherein said characteristic humidity-time curve equation
is stored in said microprocessor.
5. The method of claim 1 wherein said calculated doneness time is calculated as a
percentage of said thermal equilibrium time.
6. The method of claim 1 wherein said preselected doneness is stored in said microprocessor.
7. The method of claim 1 wherein steps (b) through (e) are repeated in consecutive
sequences and wherein said calculated doneness time is updated in each sequence.