International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 11, November 2013
ISSN 2319 - 4847
Frequency and temperature dependence of
dielectric properties of fresh potato juice at
microwave frequencies
Ravika Vijay, Ritu Jain and K. S. Sharma
Department of Physics, The IIS University, Jaipur, India
Abstract
Frequency and temperature dependence of the dielectric properties, viz., dielectric constant (ε‫ )׳‬and loss factor (ε‫ )׳׳‬for potato juice
as obtained from PNA network analyzer are reported for the first time in the frequency range 1 to 50 GHz, for temperatures 100C
to 700 C. Presence of phenomenan like molecular resonances are reflected from the peaks and dips in the ε‫׳‬-f and ε‫׳׳‬-f curves
respectively, at different temperatures.
Keywords: Dielectric Constant, Dielectric loss factor, Frequency, Temperature
1. INTRODUCTION
Potato (Solanum tuberosum) juice is considered to be a wonderful anti-inflammatory agent which work well for arthritis
and all other forms of inflammatory diseases, aches and pains. It is alkaline and alkalizes the body, which prevents
diseases, like cardiovascular diseases and even cancer. As such, investigation of various properties of potato may help in
further extending uses of potato.
Dielectric properties are important in product development and food process engineering because they influence the way
in which electromagnetic fields associated with the microwave energy interact with the food material. The dielectric
properties of practical importance are the dielectric constant (ε′) and dielectric loss factor (ε′′), expressed as real and
imaginary components of the complex permittivity, i.e.,
εr = ε′- j ε′′
Where εr is relative complex permittivity. ε′ represents the capability of the dielectric for storing electrical energy of the
electric field, while ε′′ is associated with the dissipation of electric energy in the material by its conversion in to heat. The
loss tangent, (tan δ= ε′′/ ε′) is an index of energy dissipation or loss in a material on being exposed to microwave
radiation [1], [2].
The dielectric properties of materials depend on intervention parameters like, frequency of radiation and temperature,
moisture density, grain size and other physical condition of the material.
Knowledge of the relationship between frequency of radiation and dielectric properties of material is helpful in
developing heating processes or grading techniques based on electromagnetic energy [3] – [6] and also helps in selection
of proper cooking utensils and in the design of microwave frequency heating equipments [7].
Dielectric properties of fresh fruits and vegetables have been explored by several researchers [8] – [13]. Microwave
permittivity, or dielectric properties of fresh fruits and vegetables has been considered for potential use in non-destructive
sensing of their quality, such as maturity of peaches and chilling injury in sweet potatoes etc. [14]. Vegetables have quite
high absolute permittivity in accordance with their high water content viz., for cooked peas and mashed potato the value
of ε′ ranges from about 54 to 65 and for cooked carrots from 61 to 76 at temperatures from 3°C to 6°C and frequency
2.8 GHz [15]. For dried vegetables, dielectric constant has low values as the water content in them is low. However,
measurements at a single frequency do not show much promise for quality detection in fruits and vegetables, such as
detecting peach maturity or hardcore condition in sweet potatoes [14]. Therefore, broadband permittivity measurements
were initiated to study the dielectric properties of fruits and vegetables over the frequency range from 200 MHz to 20 GHz
[10]. Dielectric properties of potato, carrot, apple, and peach tissues were also measured at low frequencies from 1 MHz
to 40 MHz [16].
In an attempt to develop theoritical models for dependence of dielectric properties on intervention parameters like
temperature, moisture and ash, Sipahioglu and Barringer developed predictive models describing the dielectric properties
at 2.45 GHz as a function of temperature (5- 1300C), ash and moisture content for a variety of 5 fruits and 10
vegetables[17]. The dielectric properties of some vegetables such as yam, white potato and spinach did not fit within the
predictive models. The problem was attributed by them to the transition in the physical properties of the carbohydrate
components in these vegetables during heating.
Guan et al, developed regression relations for dielectric properties of mashed white potatoes, with a range of moisture
and salt contents subjected to pasteurization and sterilization, measured by radio-frequency and microwave processes in
Volume 2, Issue 11, November 2013
Page 355
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 11, November 2013
ISSN 2319 - 4847
the frequency and temperature ranges 1 to 1800 MHz and 20 to 1200C respectively [18]. However, the major constituents
in starchy vegetables, such as starch and sugar contents were not considered in the developed regression equations.
Dielectric properties of Beauregard sweet potato puree as affected by microwaves in the frequency range 900 to 2500
MHz and by temperature within a limited range of 5 to 800C were reported by Fasina et al, [19]. Both temperature and
frequency were found to have significant effects on the dielectric properties of the sweet potato puree. Brinley et al.,
developed a process for rapid sterilization and aseptic packaging of sweet potato puree using a continuous flow
microwave system operating at frequency 915 MHz and temperature ranging from 150C to 1450 C. In these researches
dielectric constant was found to decrease with increasing temperature, while loss factor increased quadratically [20].
However, no reliable data for dielectric properties of fresh juice of potato at microwave frequenc ies is available in
literature, as such it was decided to investigate dielectric properties of potato juice at microwave frequencies.
The objective of the present research was to determine the dielectric properties of fresh potato juice in the frequency range
1 to 50 GHz and temperature variation from 100 C to 700C, by using the PNA network analyzer and a co-axial probe.
2. MATERIAL
Fresh sample of potato as needed for the present research was obtained from the local super market. For experimental
study juice of the potato was extracted by using a juicer and filtered by using a fine grain filter paper.
3. METHOD
The dielectric constant (ε') and loss factor (ε'') of the fresh juice of the potato were measured by using a E8364C PNA
network analyzer in the frequency range 1 GHz to 50 GHz .The test probe consisted of an open ended coaxial probe
system (HP85070B). The system software calculates the dielectric parameters from the phase and amplitude of the signal
reflected at the interface between the open-ended coaxial line and the sample to be analyzed. The instrument was
calibrated using three different loads: (i) air (ii) a short circuit with the metal contacts and (iii) pure water at room
temperature. After calibration, the instrument was tested by taking a measurement on a standard liquid of known
dielectric properties (methanol in the present case), to confirm the accuracy. Sample temperature control was provided by
circulating cold / hot water through the jacket surrounding the sample cup from a Haake B3 Constant Temperature water
circulator with a digital temperature control module. Observations over the frequency range 1 GHz to 50 GHz were taken
thrice, so as to ensure consistency of the experimental data.
4. RESULTS AND DISCUSSION
Figures 1 and 2 show variation in dielectric constant (ε') and loss factor (ε'') respectively with frequency (1 to 50 GHz) at
different temperatures (100C to 700C) for fresh juice of potato as obtained from the present work. It is observed from Fig.
1 that for potato juice, ε' first increases with increasing frequency, reaching its maximum value and then decreasing with
increasing frequency. It is observed that the frequency for the peak is shifted towards the higher frequency side as the
temperature is increased. The appearance of a peak in ε'-f curve suggests the presence of a phenomenon like molecular
resonance at this frequency, which makes ε' to acquire the highest value at a particular frequency. Frequency about 45
GHz, all curves merge together, showing that at this frequency and probably above this frequency dielectric constant of
potato juice shows only feeble temperature dependence.
It is observed from Fig. 2 that for potato juice ε'', decreases with increasing frequency up to 5 GHz, showing a dip at about
5 GHz and then increasing with increasing frequency up to about 13 GHz. Between 13 to 20 GHz a shallow dip like
structure is obtained at temperatures above 200C.
Above 20 GHz a slight decrease with frequency is observed, and the curves at temperatures 500C to 700C show secondary
peaks at about 40 GHz. It is also observed that at low frequencies losses are higher at low temperatures, whereas at higher
frequencies higher losses are obtained for higher temperatures. The occurance of peaks in ε′-f curves and dips in ε′′-f
curves suggest presence of molecular resonances at certain frequencies in potato juice.
100
10 C
20 C
30 C
40 C
50 C
60 C
70 C
90
Dielectric Constant [']
80
70
60
50
40
30
20
10
0
10
20
30
40
50
Frequency [GHz]
.
Figure: 1 Variation in dielectric constant of potato juice with frequency at different temperatures
Volume 2, Issue 11, November 2013
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 11, November 2013
ISSN 2319 - 4847
60
Dielectric loss factor ['']
50
40
30
20
10
20
30
40
50
60
70
10
0
-10
0
10
20
30
C
C
C
C
C
C
C
40
50
Freq uency [G H z]
Figure: 2 Variation in dielectric loss factor of potato juice with frequency at different temperatures.
Figures 3 and 4 show variation in dielectric parameters (ε' and ε'') with temperature (100 C to 700C) at different
frequencies (1 GHz - 50 GHz) respectivelly. It is observed from Fig. 3 that at 1 GHz, as temperature increases, ε'
decreases slightly, but at frequencies 10 to 40 GHz, ε' increases with increasing temperature. At 50 GHz ε′ shows slight
increase up to 400C and thereafter it decreases up to 500C and then shows almost a constant value up to 700C. It is
observed from Fig. 4 that at low frequencies (i.e., at 1, 10, and 20 GHz) ε'' decreases with increasing temperature, but at
higher frequencies (i.e., at 30, 40 and 50 GHz) ε'' increases with increasing temperature. Therefore losses are higher at
higher temperature and higher frequencies.
100
90
1 GHz
10 G Hz
20 G Hz
30 G Hz
40 G Hz
50 G Hz
Dielectric Constant (')
80
70
60
50
40
30
20
10
10
20
30
40
50
60
70
0
T em p e ra tu re ( C )
Figure: 3 Variation in dielectric constant of potato juice with temperature at different frequencies.
55
1 GHz
10 G H z
20 G H z
30 G H z
40 G H z
50 G H z
50
Dielectric loss factor ('')
45
40
35
30
25
20
15
10
5
10
20
30
40
50
60
70
0
T e m p e ra tu re ( C )
Figure: 4 Variation in dielectric loss factor of potato juice with temperature at different frequencies.
5. CONCLUSION
The dielectric parameters ε′ and ε′′ for potato juice show appreciable dependence on frequency of radiation and
temperature of the sample. The peaks obtained in ε′-f curves and dips in ε′′ -f curves at different temperatures suggest
presence of molecular resonances at certain frequencies.
Volume 2, Issue 11, November 2013
Page 357
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 11, November 2013
ISSN 2319 - 4847
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