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08327823.1990.11688124

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Journal of Microwave Power and Electromagnetic Energy
ISSN: 0832-7823 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/tpee20
Measurement of the Complex Permittivity of
Bread Dough by an Open-Ended Coaxial Line
Method at Ultrahigh Frequencies
J. Zuercher, L. Hoppie, R. Lade, S. Srinivasan & D. Misra
To cite this article: J. Zuercher, L. Hoppie, R. Lade, S. Srinivasan & D. Misra (1990) Measurement
of the Complex Permittivity of Bread Dough by an Open-Ended Coaxial Line Method at Ultrahigh
Frequencies, Journal of Microwave Power and Electromagnetic Energy, 25:3, 161-167, DOI:
10.1080/08327823.1990.11688124
To link to this article: http://dx.doi.org/10.1080/08327823.1990.11688124
Published online: 14 Jun 2016.
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Date: 26 October 2017, At: 01:41
M E A S U R E M E NOTF T H EC O M P L E XP E R M IT T IV IT Y
O F B R E A DD O U G HB Y A N O PEN -EN D ED
C O A X IA L
LINE M ETH O DA TU L T R A H IGFHR E Q U E N C IE S
J. Zuercher, L. Hoppie, R. Lade,
Downloaded by [UNSW Library] at 01:41 26 October 2017
S. Srinivasan, and D. Misra
This paper presents the results of a study on the dielectric
characteristics of commercially available bread dough and
yeast under various conditions. An open-ended coaxial line
is used as an in-situ sensorfor the measurements. These results
may lead to the development of more efficient microwave
systems and sensors for household and industrial baking.
Key Words:
Bread dough. Yeast, Complex permittivity, Coaxial sensor.
icrowave cooking and heating applications have
_become a norm in household and commercial food
processing [Osepchuk, 1984;Decareau, 1984;Mudgett, 1986].
While microwave cooking of convenience consumer foods is
an established application, new food processing applications
of microwave energy will soon develop in the area of industrialcooking, pasteurization, and sterilization [Bengtssonand
Ohlsson, 1979; Cone and Snyder, 1986; Mudgett and Schwartzberg, 1982; Ohlsson and Bengtsson, 1975; Rosenberg
and Bogl, 19871.
The interaction of microwave energy with a material
medium is highly dependent upon the dielectric properties of
that material. The reflection and transmission of an electromagnetic wave from a material provide vital informationabout
the electrical characteristics of that material. Variations in
the behavior of resonant cavities may be used to determine
the dielectric properties of materials, but in the present study,
an open-ended coaxial line method developed recently by
Staebell and Misra [1990], has been used. This method has
several advantages over others, including its broad-band
capabilities and on-line monitoring with virtually no special
preparations needed for the samples.
Dielectricpropertiesof various food materials determined
by the waveguide or cavity methods are available in the literature [Bengtsson and Risman, 1971; Roebuck and Goldblith, 1972; Singh and Misra, 1981]. However, the available
data on bread dough and other materials related to the baking
industry do not cover those aspectsneeded for an efficientuse
of microwaves in sensing the various aspects of the baking
process. The purpose of this study is to fill in some of these
gaps.
IT
Theoretical
ABOUT THE AUTHORS:
Joseph C. Zuercher, Lyle 0. Hoppie, and Robert W. Lade are affiliated with the Corporate Research and Development Center, Eaton
Corporation, 4201 North 27th Street, Milwaukee, WI 53216.
Sandeep Srinivasan and Devendra K. Misra are affiliated with the
Department of Electrical Engineering, P.O. Box 784, University of
Wisconsin, Milwaukee, WI 53201.
© International Microwave Power Institute 1990
International Microwave Power Institute
Background
The aperture admittance of an open-ended coaxial line terminated by a linear, isotropic, homogeneous, and nonmagnetic
medium is given by [Misra, 1987],
Y
+ C
(1)
where C, and C, are constants dependent on the operating
frequency and the radii of the coaxial line. £. is the complex
161
relative permittivity of the medium. An admittance equivalentparameter,y, canbe definedas follows [Staebelland Misra,
1990]:
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y=
e* + C
2
(Y3
Y2)
(Y4 - Y2) (Y1 - Y3)
841 832
842 813
(3)
where yi are the admittance parameters with the In' material
terminating the coaxial line. 8, represents the difference in
the measured reflection coefficients r -
Experimental
Procedure
Four materials with known perrnittivities are needed in this
methodto calibratethe system. The right-hand sideof Equation
(3) is determined after measuring the reflection coefficients
of the coaxial aperture terminated by the standard materials.
The left hand side of this equation is evaluated using Equation
(2). Since the permittivities of these materials are known, the
constant C, unknownso far, can be evaluated. Next,the coaxial
probe is terminated by the sample under test and the reflection
coefficient is recorded. The corresponding admittance parameter is evaluated as follows:
8m 1832Y 2Y 3+8m 2813Y 1Y 3+8m 3821Y 1Y(4)
2
m18 32y 1+5 m2813 y2 +8 m3021y 3
162
= [- 1
(2)
The constant C isevaluated bydetermining y fora material
with known e*. Further, the discontinuities at various points
and other imperfections in the measuring system are modelled
by a two-port network connected between the coaxial aperture and the measuring point of the circuit. The parameters
of this network are evaluated by using three different materials of known permittivities. The details are available in the
literature [Staebell and Misra, 19901 so they are omitted here
for the sake of brevity. The final relations between the
measured reflection coefficients and the admittance parameters is given by
(Y 4 Y1)
The complex relative permittivity of the sample under
test is determined by solving the quadratic Equation (2) as
follows:
+ 4 c ym
/ 2C
(5)
Of the two solutions for C m ,one is selected that has its
real part as a positive number and its value is close to y m .
The experimental setup used in measurements is shown
in Figure 1 . The desired microwave signal was derived from
the source (HP8620C main-frame equipped with HP86222A
RF plug-in). The microwave source was connected to a
reflection/transmissiontest set (HP 85044A) which had a semirigid coaxial line at its test-port. A vector voltmeter (HP
8508A) was connected from this reflection/transmission testset which displays the reflection coefficient of the test-port
(magnitude as well as the phase).
The system was calibrated using an open-circuit (e* = 1),
short circuit (e* = -j
water, and the methanol as four
standards. The complex relative permittivities of water and
methanol at a given frequency f G H Ewere
, determined from the
following formulas:
Ow. = 5 + 73 / (1 +j f01,/19.7)
(6)
e * m m ,m=,m5.7
, + 27.4 / I _1+j0.333 fc/J
(7)
and,
For the temperature-dependent measurements (such as
those illustrated in Figure 6), the sample was placed inside an
oven with a coaxial probe continuously monitoring the reflection coefficient and a thermocouple monitoring the sample
temperature. A saturated aqueous NaCl solution in an airtight glass container was used (see Figure I) to create a 75%
humidity [Griffin, 1944].
Results
Figure 2 depicts the complex relative permittivity (the dielectric constant and the loss-factor) of the bread dough ("Pipin'
Hot" by Pillsbury) at two different frequencies. Each data
point shown represents the average of twenty measurements
obtained by probing the dough at various locations. For the
normal sample (as available commercially), the complex
permittivity at 1 GHz is found to be 24.2 - j 12.4. When extra
water is added to the sample, its permittivity increases. For
6% (by weight) extra water, the complex permittivity changes
to 27.7 -j 16.6. The permittivity value decreases with increasing flour in the sample. For example, when 7% (by weight)
Journal of Microwave Power and Electromagnetic Energy
Vol. 25 No. 3, 1990
vector
voltmeter
xi al
Cont rolled
relative
humidity
m her
Downloaded by [UNSW Library] at 01:41 26 October 2017
source
reflection
transmission
test set
sample
oven /ir
FIGURE 1: Experimental setup.
extra flour is added to the normal dough, its permittivity
reduces to 21.3 - j 10.8. Similar characteristics are also
observed at 2 GHz with a relative overall shift in the characteristic curves.
Figure 3 illustrates the dielectric characteristics of the
bread baked for 10,20, and 30 minutes, respectively, over the
frequency range of 600 MHz to 2400 MHz. The complex
permittivity of this sample after baking for 10 minutes is found
to be 23.1 - j 11.85 at 600 MHz. It reduces to 12.17 - j 4.54 for
the signal frequency of 2.4 GHz. If this bread is baked for 20
minutes (not done yet), its permittivity reduces to 16.78 -j 6.66
at 600 MHz and 9.53 - j 3.12 at 2.4 GHz. After baking it
completely (which takes about 30 minutes), the permittivity
is found to be further diminished to 8.64 - j 2.51 at 600 MHz
and 4.54 - j 1.22 at 2.4 GHz. As may be seen in this figure,
the dielectric constant as well as the loss factor of the bread
dough reduce consistentlywith increase in thesignal frequency
or the baking time.
Since some variations in a standard recipe for bread
making are always expected, several experiments were carried out to study the behavior of such samples under 75%
humidity at 95°F. These conditions were considered to bevery
close to the ones used in the proofroom of a bakery [Kotschevar
1974]. Figure 4 shows some of these results at a signal frequency of! GHz. In one case, extra water (2.74% by weight)
was added to the commercially-available "Pipin' Hot" bread
International Microwave Power Institute
dough (Pillsbury) while in the other case, extra flour (2.52%
by wt.) was added. If one bakes these samples, the first one
results in a gooey bread while the other becomes hard and
flaky. The complex permittivities of these two samples were
measured as 7.9-j 4.2 and 5.2-j 2.1, respectively. The permittivities of these samples were monitored in the above-mentioned humid atmosphere over atime period of about 3 hours.
It may be observed from Figure 4 that the permittivity of the
sample with extra water tends to go down, while for the other
sample (with extraflour), itincreases.This phenomenon seems
consistent with the dissipation and absorption of water by the
samples. It was anticipated that the "extra water" and "extra
flour" curves would approach each other, rather than cross
each other as shown by the data. It is believed that this is a
result of having made one measurement per data point, rather
than having averagedmany measurements foreach data point.
Yeast is one of the most important ingredients in bakery
products. A number of experiments were carried out to determine its microwave characteristics_ Figures 5 and 6 depict
selected results of this study. Figure 5 shows the complex
relative permittivity of yeast for two different cases. First, the
permittivity of a commercially available block of yeast (raw
sample) was determined over the frequency band of 600MHz
to 2.4 GHz. In the second case, a piece of yeast was placed
in a microwave oven for about 2 minutes and its temperature
was linearly increased to about 220°F in about 90 minutes.
163
complex
permittivity
imag part (at 101Hz)
— real pert (at 10Hz)
IK
real part (at 20Hz)
imag part (at 20Hz)
Real Part
30
Imaginary Part
0
.0( • • ■ 5
•
15
- -10
71=
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- -15
-15
6% extra water
0.7% extra water
normal
sample
1.26% extra flour
20
7% extra flour
Sample Materials
FIGURE 2: Complex relative permittivity of the dough as a function of composition and frequency.
Imag. Part
Real Part
Complex
Permittivity
- 20
real (after 10 min)
Imag.(after
10 min)
-1(-- real (after
20 mln)
-9-
Imag.(after
20 mm)
real (after
30 min)
0
imaglafter
30 min)
0
-0
- -10
- 10
600
0
1200
1800
2400
Frequency in MHz
FIGURE 3: Complex relative permittivity of the bread dough baked for different time periods, as afunction of frequency.
164
Journal of Microwave Power and Electromagnetic Energy
Vol. 25 No. 3, 1990
Real Part
Imaginary
Part
complex permittivity
real (extra water)
4
-
irnag.(extra water)
real (extra flour)
Downloaded by [UNSW Library] at 01:41 26 October 2017
0
imag.(extra
flour)
26
50
75
Time
100
125
150
175
(sec.)
FIGURE 4: Complex relative permittivity ofdough samples inside
the humidity chamber (proof simulation), as a function of time.
The yeast is expected to die before it attains such a high
temperature. At 600MHz, the real part of the permittivity was
found to change from 47.78 (for the raw sample) to 9.55 (for
the cooked yeast). The corresponding imaginary parts of the
permittivities were found as -22.01 and -1.6, respectively. In
both cases, the permittivity decreases with an increase in the
frequency.
Figure 6 illustrates the complex relative permittivity of
the yeast as a function of temperature in °F. At a room temperature of about 75°F, it is found to be 44.76 - j 14.74. The
real as well as the imaginary parts of the permittivity decrease
with an increase in temperature,except fora distinct localeffect
around 150°F. Specifically, at about 147°F, the permittivity
value shows a sharp secondary peak of about 28.88 - j 17.23
and then falls down to 5.46 - j 2.4 around 212°F. This peak
in the permittivity characteristic corresponds to the approximate temperature at which yeast dies.
International
Microwave Power Institute
Conclusion
The dielectric constant and loss factors of bread dough and
yeast were investigated via microwave techniques.For a given
sample, the complex permittivity tends to decrease with
increase in the microwave signal frequency. This behavior is
similar to the characteristicsof water over the frequencyrange
used in theseexperiments.Various resultsreportedin thispaper
may be useful to the baking industry for the development of
a microwave system capable of monitoring and controlling
the baking processes.
Acknowledgment
The research reported in this paper was supported by the Eaton Corporation,Corporate Research and Development, Milwaukee Center. The authors would liketo thank Dr. C.G.Chen
for his support and interest in the research.
165
50
Complex Permittivity
real (raw sample)
40
— 4 — imag.(raw sample)
real (baked sample)
—8—
Imag.(baked sample)
20
10
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0
600
1
'
'
'
25
900 1200 1500 1800 2100 2400
Frequency (MHz)
FIGURES: Complex relative permittivity of raw and baked yeast samples as a function of frequency.
60
Real part
Imaginary
Part
40 L
complex
permittivity
30
-10
------ real part
imaginary
part
20
-15
10
0
75
100
125
I
I
1
160
175
200
-20
225
Temp. in deg F
FIGURE 6: Complex relative permittivity of yeast in response to a temperature ramp of about 1.5 °F/min.
166
Journal of Microwave Power and Electromagnetic
Energy
Vol. 25 No. 3, 1990
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Cone, M. and Snyder, T. 1986. Mastering microwave
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Decareau, R.V. 1984. Microwave in food Processing. Food
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Griffin, Roger C. 1944. Relative humidity over salt solution.
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Misra, D.K. 1987. A quasi-static analysis of open-ended
International Microwave Power Institute
coaxial lines. IEEE Transactions on Microwave Theory
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167
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