chapter 5 effect of moisture on dielectric properties...
TRANSCRIPT
Effect of Moisture on dielectric properties....... 115
CHAPTER 5
EFFECT OF MOISTURE ON DIELECTRIC
PROPERTIES OF FOOD GRAINS AT MICROWAVE
FREQUENCIES
5.1 Introduction
Permittivity of granular materials has an innate relationship with moisture
content. Moisture content is an important component and determines the quality of
agricultural products; as such the processes like harvesting, storage, packaging and
trading depend upon moisture content. Various techniques have been developed to
monitor moisture content of crops, one of them is through measurement of the
permittivity of granular materials. Nelson (1991) and Kraszewski and Nelson (1996)
have published reviews of progress in this field of study, which provide useful
information on aquametry of food products and suitability of moisture level in food
grains for various operations. Different techniques to monitor moisture through
measurement of dielectric permittivity of food grains can be classified into following
groups: (a) wave guide measurements, (b) cavity measurements, (c) measurement of
single kernel of grain, (d) open resonators (e) coaxial probe method (f) capacitance
measurement and (g) non contact scattering measurements. Processes like dielectric
heating and moisture content determination by using electrical methods have got
practical importance, and help in understanding of phenomena like the molecular
structure, the dynamics of living matter, and the mechanism of water binding by
living tissues. These are some of the areas of scientific interest for physicists,
chemists and biologists. Non destructive sensing of moisture content in crops is now
possible because of the high correlation between material permittivity and water
content of the material as was also observed by Kraszewski (1991).
Moisture content of cereal grains determines suitability of the crop for
harvesting and storage, and it must be measured whenever grains are traded. If the
moisture content of these commodities is too high, there are chances of their
spoilage and hence they must be dried properly to avoid spoilage and to ensure safe
storage (Nelson and Trabelsi,2011). Since the standard and reference methods for
Effect of Moisture on dielectric properties....... 116
determining moisture content of food grains involve tedious laboratory procedures
and long oven-drying periods, rapid methods for moisture measurement are essential
in the grain trade (Nelson and Trabelsi, 2011). Electrical, Near-InfraRed (NIR) and
Nuclear Magnetic Resonance (NMR) methods have been explored for rapid sensing
of moisture content in food grains (Lu 2007; Buning and Diller, 2000). However,
equipment used in NIR and NMR techniques are quite expensive and are limited to
laboratory testing; they generally require long time for sample preparation and in
performing experiment. The electrical measurements on the other hand are simple
and cheaper, provide results of adequate accuracy in much less time, and can also be
performed outside the laboratory, even on the site of crops and hence they are of
practical importance. They have been therefore employed for a long time as rapid
reliable techniques for grain and seed moisture testing. It was discovered early in the
20th century that there was a logarithmic increase in resistance of wheat as moisture
content decreased (Briggs, 1908; Nelson, 1991). Grain moisture meters were
subsequently developed based on this principle. Later, the use of capacitance
measurements was made for moisture determination in grains, and moisture meters
were developed that utilize relationships between the instrument readings and %
moisture content as presented by reference methods of moisture determination
(Burton and Pitt, 1929).
Since dielectric properties of materials are highly correlated with the amount
of water contained in them, measuring the dielectric properties can be used for rapid
measurement of moisture content in materials, such as agricultural products and food
materials (Soltani et al. ,2014). The moisture-dependence of dielectric properties in
specific frequency ranges can be used for developing online moisture meters (Nelson
et al., 1992;Nelson and Trabelsi,2005), which can be used not only for monitoring
moisture in drying processes but also in other unit operations in the food industry.
Several investigations on moisture dependence of dielectric properties of agricultural
products have been reported. The influence of frequency, temperature and moisture
variation on dielectric properties of chickpea flour in compressed form was studied
by Guo et al. (2008). It was observed that the dielectric constant (ε') and loss factor
(ε'') of the sample decrease with increase in frequency at all temperatures and
Effect of Moisture on dielectric properties....... 117
moisture levels. Guo et al.(2010) also measured dielectric properties of flour samples
from four legumes (chickpea, green pea, lentil and soybean) at four different
moisture contents, frequencies ranging from 10 MHz to 1800 MHz and temperatures
20°C to 90°C by using open ended coaxial probe method. The dielectric constant (ε')
and loss factor (ε'') of the legume samples were observed to decrease with increasing
frequency but both these parameters increase with increasing temperature and
moisture content. Sacilik et al. (2006) studied the effects of moisture content,
frequency and bulk density changes on dielectric properties of flax seeds in the
frequency range 50 KHz to 10 MHz .
A pronounced dispersive behaviour in some biological materials, with the
sequences of dielectric relaxations depending on molecular, macromolecular,
subcellular and cellular relaxation phenomena was observed by Grant et al (1978)
and Petbig and Kell (1987). Similar relaxations are expected in grain kernels and
also in the bulk of the grains. The complex structure of grain kernels (changing with
stage of maturity and storage conditions), the dependence of their dielectric
properties upon factors like moisture content, grain density and temperature, and,
finally, the unknown character of the dielectric relaxations, all contribute to the
complexity of the dielectric behaviour of grains, as was also observed by Krazewski
and Nelson (1989).
The study of moisture dependence of dielectric properties of the food grains
is therefore quite useful as it yields valuable information on the suitability of grains
for storage and germination of seeds and also describes the behaviour of these seeds
under the influence of high frequency electric fields or when subjected to dielectric
heating. In the present work, the effect of moisture content on the dielectric properties
of wheat, pearl millet and green gram has been investigated, as these are the major
crops in India and every year large quantities of these grains are spoiled due to
improper storage in ware houses. The measurements were taken at six different
moisture levels.
Effect of Moisture on dielectric properties....... 118
5.2 Procurement of Materials and Sample Preparation
Wheat (RAJ 3077) and pearl millet (HHB 62) grains required for the present
study were obtained from Durgapura Agriculture Research Station of Rajasthan
Agriculture University, Bikaner. Green gram of brand „Mani‟ was procured from the
local market. These grains were grinded and converted into flour by a grinder and
samples of grain size 250-300 microns were obtained using sieves of mesh sizes 300
and 250 microns respectively. To obtain the samples of different moisture contents,
wheat kernels (9.20% moisture content, wet basis), pearl millet grains (10.66%
moisture content, wet basis) and green gram (12.86 % moisture content, wet basis)
were taken and grinded. The grinded samples are kept over distilled water in covered
dessicators at room temperature for different periods of time as suggested by Guo et
al (2008). Grinded flour samples are allowed to absorb moisture in the desiccators at
room temperature so that longer the time they are kept, larger the moisture they
absorb. The flour samples are stirred at intervals of one hour by a glass rod to ensure
that the moisture absorption is uniform. After keeping the flour in desiccators for a
few days, the desired moisture levels are achieved in the samples. Different samples
are kept in the dessicator for different number of hours or days, so that they acquire
different moisture levels. The samples are then sealed in plastic bags and equilibrated
at room temperature. The moisture content in the samples is measured by using a
moisture analyzer.
5.3 Experimental Study : Two Point Method
Two point method (Behari, 2005), a technique involving measurement of
reflection coefficient of a solid block of material placed at the end of a wave guide
and backed by a short circuiting conducting plate, was used in the present study to
determine dielectric constant (ε') and dielectric loss (ε'') of food grains in powder
form at different moisture levels. In order to use this method for powders, the wave
guide was bent through 90° by means of a E-plane bend and terminated by a
dielectric cell in which powder sample was filled up. The powdered material filled
in the dielectric cell is compacted by using a hydraulic press,so that it can be
considered close to a solid block of material. The experimental set-up used in this
method for measurement of dielectric properties of powders is shown in Fig. 5.1.
The position of the voltage minimum was located by probe position DR in the slotted
Effect of Moisture on dielectric properties....... 119
section for an empty short-circuited wave guide dielectric cell. The sample prepared
is taken out of the dessicator, equilibrated at room temperature and then a small
fraction of it was placed in the moisture analyzer shown if Fig. 5.2 and moisture
content was measured. The sample was then filled in the waveguide dielectric Cell
and compressed by applying certain pressure by using a hydraulic press, such that the
cell can withstand the applied pressure. The height of the sample in the dielectric cell,
lε, is measured and the new position of voltage minima is located by probe position D
in the slotted section. Then, the cell was filled with the sample upto another height lε'
and the position of voltage minima was again noted in slotted section.
Fig 5.1: Experimental set up for determination of dielectric
properties in powder form
Fig. 5.2 : Moisture Analyzer used to determine the moisture content
Effect of Moisture on dielectric properties....... 120
In two point method, the complex dielectric constant (ε*) of the material of
the sample can be obtained from the solution of the transcendental equation given by
j
j
1 e1 tan XC
j l 1 e X
(5.1)
The transcendental equation provides several solutions for X θ, which in the
present work were found by using a mathematical tool MATLAB. The experiment
was repeated with a different length of the sample and the common root was chosen
for evaluation of the admittance. The normalized admittance (Yε ) of the material of
the sample was calculated by using the relation
X
Y 2( 90 ) G jSl
where Gε and Sε are respectively the normalized conductance and normalized
susceptance of the sample.
The values of Gε and Sε are obtained by separating equation (5.2) in to real
and imaginary parts, from which the formulas for the dielectric constant ( ε') and
loss factor (ε'') of the sample are obtained in the following form:
2
g
2
g
G ( / 2a)'
1 ( / 2a) (5.3)
2
g
S''
1 ( / 2a)
(5.4)
The accuracy of measurements for dielectric constant (ε') was estimated to be
within 5% and for dielectric loss (ε'') was estimated to be within 10%.
Conductivity measurements can also be used to measure moisture contents in
materials, particularly food grain products. These properties are useful in selection
of processing conditions and in deciding the quality of foods. Conductivity of the
material is is usually considered as the property which measures the ease with which
electric charges can be transferred through the material on application of an electric
(5.2)
Effect of Moisture on dielectric properties....... 121
field .The conductivity (ζ) and relaxation time (η) in the material of the sample are
obtained by using the following relations:
ζ = ω εo ε '' (5.5)
η = ( ε'' / ω ε') (5.6)
where,
ω = 2π f, and
ε0 = 8.85 x 10 -12
F/m is the permittivity of the free space..
5.4 Results and Discussion
The dielectric constant (ε') and dielectric loss factor (ε'') of wheat, pearl millet
and green gram in powder form were determined at four microwave frequencies lying
respectively in C, J, X and Ku bands, for different moisture levels at room temperature
and the values obtained are displayed in tables 5.1, 5.2 and 5.3 respectively.
5.4.1. Moisture Dependence of Dielectric Properties of Wheat
The dielectric constant (ε') and dielectric loss factor (ε'') of wheat as
determined at four microwaves frequencies (lying in C, J, X and Ku bands) for
different moisture levels at room temperature (28 °C) are displayed in Table 5.1.
Table 5.1 : Variation of dielectric properties of wheat (RAJ 3077) in powder
form with Moisture content at four microwave frequencies
Moisture
(%)
C band
(4.65 GHz)
J Band
(7.00GHz)
X Band
(9.35 GHz)
Ku Band
(14.92 GHz)
ε' ε'' ε' ε'' ε' ε'' ε' ε''
9.20 4.60 ±
0.17
0.21 ±
0.01
4.09 ±
0.14
0.19 ±
0.01
3.47 ±
0.13
0.18 ±
0.01
1.39 ±
0.06
0.12 ±
0.01
13.80 4.79 ±
0.19
0.39 ±
0.01
4.28 ±
0.14
0.36 ±
0.02
3.66 ±
0.15
0.34 ±
0.01
1.57 ±
0.05
0.28 ±
0.01
15.97 4.88 ±
0.14
0.48 ±
0.02
4.37 ±
0.17
0.44 ±
0.02
3.75 ±
0.15
0.42 ±
0.02
1.66 ±
0.07
0.36 ±
0.02
18.40 4.97 ±
0.17
0.57 ±
0.02
4.47 ±
0.13
0.52 ±
0.02
3.85 ±
0.16
0.48 ±
0.01
1.76 ±
0.08
0.45 ±
0.02
20.70 5.07 ±
0.20
0.66 ±
0.03
4.56 ±
0.15
0.61 ±
0.02
3.94 ±
0.17
0.59 ±
0.02
1.85 ±
0.09
0.53 ±
0.02
24.30 5.21 ±
0.22
0.79 ±
0.02
4.71 ±
0.14
0.74 ±
0.03
4.09 ±
0.17
0.70±
0.03
2.00 ±
0.08
0.66 ±
0.03
Effect of Moisture on dielectric properties....... 122
From Table 5.1, it is observed that for all the four frequencies, values of
dielectric constant (ε') and dielectric loss factor (ε'') increase with increasing level of
moisture content in wheat powder. With increase in moisture content the electric
polarization increases due to dipolar nature of water molecules. The increase in
dipole polarization results in an increase in both, the dielectric constant( ') and
dielectric loss ( ''). It is also observed that both dielectric constant and loss factor
decrease with increase in frequency. The observed behaviour of dielectric properties
of food materials may be accounted for by free water dispersion, bound water
dispersion, and ionic conduction within a broad frequency range as was observed by
Feng et al. ( 2002).
Water is the major absorber of microwave energy in the food. When the
frequency is increased, water molecules are not able to keep up with the changes of
the direction of the electric field, because of their inertia, that is described in terms
of the relaxation time , which is defined as the time in which the dipole moment of
the sample reduces to 1/e of its maximum value when the electric field is removed.
The presence of free moisture in a substance greatly affects its dielectric properties
since the dielectric constant of free water is quite high (78 at room temperature and
2.45 GHz). The moisture-dielectric relationship is consistent in that higher moisture
leads to higher values of both the dielectric constant (ε') and the loss factor (ε'').
Water can exist in either the free or bound state in food systems. Free water
is found in capillaries but bound water is physically adsorbed by the surface of dry
material. The bound water and free water behave differently in contributing to the
dielectric properties of food grains. The dielectric loss factor is influenced by the
losses in free and bound water but since relaxation of bound water takes place below
microwave frequencies, its effect are small in microwave processing and it is the
free water that is responsible for dielectric losses at microwave frequencies, as
observed by Calay et al. (1995) and Serdyuk(2001).
Figure 5.3 shows the variation of dielectric loss with moisture content, as
observed by Sahin and Sumnu (2006b). It is clear from the figure that loss factor is
constant in the bound region (region I) upto a critical moisture content (Mc) and then
Effect of Moisture on dielectric properties....... 123
increases rapidly for higher moisture contents. Therefore the effect of bound water
on dielectric properties is negligible. The interaction of water with food components
is a significant factor in affecting their dielectric properties. The increase in water
content increases the dielectric polarization, which in turn increases the dielectric
constant and dielectric loss in food materials.
Fig. 5.3 : Variation of loss factor with moisture content
(Sahin and Sumnu,2006a)
It is now well established that the water content of seeds affect their
physiological activities (Singh et al, 2008). In seeds, water binds with varying
strengths at different water concentrations and therefore has different
thermodynamic properties (Pozeliene and Lynikiene, 2009). The main components
of wheat are carbohydrates and proteins. Carbohydrates (mainly starch) and proteins
are important for water retention, and proteins contain more polar sites for attraction
of water molecule than carbohydrates. Hence they can adsorb a large amount of
water. For low moisture contents, dielectric constant of food materials is
comparatively low while for high moisture contents it is relatively high. It may be
because at high moisture contents more water dipoles contribute to polarisation, as
water molecules can easily follow up the applied field variations. Free water present
in the food materials is therefore responsible for dielectric polarization.
The variation of dielectric constant (ε') and dielectric loss (ε'') with moisture
content (w.b.) at room temperature (28°C) is shown at four different frequencies for
Effect of Moisture on dielectric properties....... 124
wheat in Fig. 5.4. The variation of dielectric constant with moisture content (9.20%
to 24.30%) for wheat is depicted in Fig 5.4 (a). It is clear from the graph that at all
frequencies, the value of ε' increases with moisture content almost linearly. The
slope of these lines is also approximately the same, showing that as moisture
increases the dielectric constant increases almost at the same rate at all the four
frequencies. Further, at a particular moisture content, highest value of dielectric
constant (ε') is obtained for the lowest value of frequency i.e. 4.65 GHz.
Fig 5.4 (b) shows the variation of dielectric loss with moisture content (MC)
(9.20% to 24.30%) at four microwave frequencies for wheat in powder form. Almost
parallel lines obtained for ε'' – MC curves show that the variation is almost linear at
all the four frequencies, but the slopes of the ε'' – MC lines are steeper showing that
the change in dielectric loss with increase in moisture content takes place at a faster
rate as compared to the change in dielectric constant. However, the change in loss
factor with frequency at a particular value of moisture content is not so prominent.
At low moisture content, the ε'' – MC curves for different frequencies are closer to
each other, whereas at higher moisture contents, separation between them increases
showing that at higher moisture contents the effect of frequency change on loss
factors is better visible as compared to the low moisture content. The rate of change
of dielectric loss with moisture content is almost the same at all the four frequencies,
as can be inferred from approximately equal slopes of ail the four lines. A comparison
of Fig. 5.4 (a) and 5.4 (b) reveals that ε'' – MC curves are much steeper than ε' – MC
curves, showing that dielectric loss (ε'') of food materials is more susceptible to
change in moisture content as compared to dielectric constant (ε')
Effect of Moisture on dielectric properties....... 125
(a)
(b)
Fig. 5.4 : Variation of (a) dielectric constant (ε') and (b) dielectric loss (ε'') for
wheat in powder form at four microwave frequencies
0.00
1.00
2.00
3.00
4.00
5.00
6.00
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Die
lect
ric
con
sta
nt
(ε')
% Moisture content
Variation of dielectric constant (ε') with moisture
content for wheat
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Die
lect
ric
loss
(ε''
)
% Moisture Content
Variation of dielectric loss with moisture
content for wheat
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
Effect of Moisture on dielectric properties....... 126
5.4.2 Moisture Dependence of Dielectric Properties of Green Gram
The dielectric constant (ε') and dielectric loss facto (ε'') of green gram were
determined at four microwave frequencies (in C, J, K and Ku bands respectively) for
different moisture levels at room temperature (28°C) and the values obtained are
displayed in Table 5.2.
Table 5.2: Variation of dielectric properties with Moisture content of green
gram in powder form at four microwave frequencies
Moisture
Content (%)
C band
(4.65 GHz)
J Band
(7.00 GHz)
X Band
(9.35 GHz)
Ku Band
(14.92 GHz)
ε' ε'' ε' ε'' ε' ε'' ε' ε''
12.86 4.04 ±
0.11
0.33 ±
0.02
2.87 ±
0.09
0.28 ±
0.01
2.50 ±
0.07
0.26 ±
0.01
1.93 ±
0.07
0.10 ±
0.005
16.92 4.21 ±
0.15
0.49 ±
0.02
3.01 ±
0.10
0.49 ±
0.02
2.68 ±
0.09
0.49 ±
0.02
2.09 ±
0.07
0.24 ±
0.01
19.86 4.33 ±
0.12
0.59 ±
0.02
3.13 ±
0.14
0.59 ±
0.02
2.80 ±
0.09
0.59 ±
0.02
2.21 ±
0.08
0.35 ±
0.01
21.22 4.59 ±
0.16
0.64 ±
0.03
3.38 ±
0.12
0.64 ±
0.03
3.06 ±
0.08
0.64 ±
0.03
2.47 ±
0.08
0.40 ±
0.01
23.30 4.67 ±
0.09
0.72 ±
0.03
3.47 ±
0.10
0.72 ±
0.03
3.14 ±
0.10
0.72 ±
0.03
2.55 ±
0.08
0.47 ±
0.02
24.86 4.74 ±
0.18
0.77 ±
0.03
3.60 ±
0.15
0.77 ±
0.03
3.22 ±
0.08
0.77 ±
0.03
2.64 ±
0.09
0.53 ±
0.02
It is observed from Table 5.2 that both the dielectric constant (ε') and
dielectric loss (ε'') increase with increase in moisture content at all frequencies. It is
also apparent from the table that the dielectric parameters (ε' and ε'') of green gram
decrease with increase in frequency. The variation of the dielectric properties with
moisture is shown pictorially in Fig 5.5. It is found that the general behaviour of
moisture dependence of ε' and ε'' for green gram is similar to that of wheat, i.e., both
ε' and ε'' at a particular frequency increase with moisture, the increase in '' with
moisture being rapid as compared to ε'. Further whereas now ε'' – MC curves are
linear as for wheat but ' – MC curves show slight curvature upwards showing that
ε''increases more rapidly with moisture as compared to the linear regression .On the
other hand for wheat the ε' – MC curves are essentially of linear character. It is clear
from Fig. 5.5 (a) which shows the variation of dielectric constant (ε') with moisture
content w.b. (12.86% - 24.86%), that the dielectric constant (ε') shows a regular
increase with moisture content at all frequencies. The relation between dielectric
Effect of Moisture on dielectric properties....... 127
constant (ε') and moisture is quadratic in nature. The ε'- MC curves follow the same
pattern for all the four frequencies, ε' values for a particular MC decreasing with
increase in frequency. The highest value of dielectric constant is obtained at highest
value of moisture content,i.e., for 24.86% w.b.at frequency 4.65 GHz. Fig 5.5(b)
depicts the variation of dielectric loss factor (ε'') with moisture content for green
gram in powder form for moisture content 12.86% - 24.86% w.b. Here, the variation
is linear for all the four frequencies. From the slope of the lines, it is clear that the
rate of change of dielectric loss with moisture content is greater than that of
dielectric constant. Similar results were reported by Jiao et al. (2011) for dielectric
properties of black-eyed pea and mung bean flours at four moisture content levels,
on the basis of measurements carried out by them with an open-ended coaxial probe
and impedance analyser at frequencies from 10MHz to 1800 MHz and temperatures
from 20°C - 60°C. A similar conclusion was also made by Guo et al (2008) who
found that both the dielectric constant (ε') and loss factor (ε'') of chickpea flour
increase with density and moisture content for density variation from 1.265 g/cm3
to 1.321 g/cm3
and moisture content variation 1.9% to 20.9%.
Singh et al (2006) stated that at higher moisture levels, more water dipoles
contribute to the polarization, due to high water mobility, showing that the water
dipoles follow the applied field variations more easily as compared to other molecules
in food. At low moisture, because the water is in strongly bound state (monolayer),
the distance between the water molecule and cell wall is very small and hence the
force of attraction on water molecules is very large and hence the water dipoles
cannot easily follow up the variations in electric field. Therefore, the dielectric
properties of materials with low moisture contents are small.
Green gram is a legume which is rich in proteins. Proteins and carbohydrates
are highly water retentive. Thus the interaction of proteins and carbohydrates with
water is a significant factor, which affects their dielectric properties. Since the
binding forces of proteins and carbohydrates on water molecules are strong causing
the free water content in the system to decrease, therefore the values of dielectric
constant (ε') and loss factor (ε'') in green gram are low. Thus, for low values of
moisture content the dielectric constant and dielectric loss factor of green gram are
of small magnitudes and as the moisture level increases, the values of dielectric
constant (ε') and loss factor (ε'') of green gram increase.
Effect of Moisture on dielectric properties....... 128
(a)
(b)
Figure 5.5 : Variation of (a) dielectric constant (ε') and (b) dielectric loss for
green gram in powder form at four frequencies
0.00
1.00
2.00
3.00
4.00
5.00
6.00
0 5 10 15 20 25 30
Die
lect
ric
con
sta
nt
(ε')
% Moisture content
Variation of dielectric constant (ε') with moisture
content for green gram in powder form
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0 5 10 15 20 25 30
Die
lect
ric
loss
(ε'
')
Moisture content (%)
variation of dielectric loss (ε'') with moisture content
for green gram in powder form
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
Effect of Moisture on dielectric properties....... 129
5.4.3 Moisture Dependence of Dielectric Properties of Pearl Millet
The dielectric constant (ε') and dielectric loss factor (ε'') of pearl millet were
investigated at four microwave frequencies, viz., 4.65 GHz, 7 GHz, 9.35 GHz and
14.92 GHz, using microwave benches for appropriate frequency bands, i.e., C, J, X
and Ku bands for different moisture levels at room temperature (28°C) by using two
point method of dielectric studies. The results obtained are displayed in Table 5.3. As
may be observed from the table, low values of ε' and ε'' are obtained for low values
of moisture contents. This is in agreement with the observations made by Guo et
al.(2010); according to them low moisture inhibits the mobility of charged ions,
resulting in low dielectric loss values. As the moisture content increases, both the
dielectric constant (ε') and the dielectric loss (ε'') increase for all the four frequencies.
For moisture content values below 10.66%, the dielectric constant changes
very slowly with the moisture content. This shows that below the critical moisture
content the water in the sample is in tightly bound state and cannot be easily
polarized (Sahin and Sumnu, 2006a).
Table 5.3: Variation of dielectric properties with Moisture content of pearl
millet (HHB 62) in powder form at four microwave frequencies.
Moisture
Content(%)
C band
(4.65 GHz)
J band
(7.00 GHz)
X band
(9.35 GHZ)
Ku Band
(14.92 GHz)
ε' ε'' ε' ε'' ε' ε'' ε' ε''
0 3.98 ±
0.13
0.58 ±
0.02
3.66 ±
0.12
0.49 ±
0.02
2.65 ±
0.10
0.37 ±
0.01
2.04 ±
0.07
0.03 ±
0.001
10.66 4.31 ±
0.16
0.65 ±
0.02
4.03 ±
0.15
0.58 ±
0.02
3.03 ±
0.10
0.41 ±
0.01
2.48 ±
0.08
0.03 ±
0.001
15.39 4.90 ±
0.14
0.69 ±
0.03
4.62 ±
0.13
0.62 ±
0.02
3.32
±0 .11
0.43 ±
0.01
3.10 ±
0.11
0.09 ±
0.004
16.90 5.32 ±
0.15
0.74 ±
0.02
5.00 ±
0.13
0.68 ±
0.02
3.91 ±
0.14
0.44 ±
0.02
3.59 ±
0.12
0.12 ±
0.010
18.65 5.96 ±
0.15
0.78 ±
0.02
5.46 ±
0.19
0.73 ±
0.03
4.64 ±
0.13
0.47 ±
0.01
4.39 ±
0.15
0.16 ±
0.012
19.2 6.38 ±
0.20
0.80 ±
0.03
6.02 ±
0.22
0.77 ±
0.02
4.93 ±
0.15
0.50 ±
0.02
4.57 ±
0.16
0.22 ±
0.016
27.16 7.50 ±
0.19
0.82 ±
0.02
6.98 ±
0.18
0.79 ±
0.03
6.68 ±
0.21
0.55 ±
0.03
5.76 ±
0.2
0.36 ±
0.014
Effect of Moisture on dielectric properties....... 130
Calay (1995) also reported that due to bound water the dielectric relaxations
are small in comparatively dry dielectrics and hence in microwave processing the
loss factor is almost constant below critical moisture level (water present in bound
form) and it increases with moisture content above the critical moisture level.
(a)
(b)
Figure 5.6 : Variation of (a) dielectric constant (ε') and (b) dielectric loss (ε'')
for pearl millet in powder form at frequencies
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0.00 10.00 20.00 30.00
Die
lect
ric
con
sta
nt(
ε')
% Moisture Content
Variation of Dielectric constant (ε') with moisture
content for pearl millet
4.65 GHz
7.00 GHz
9.35 GHz
14.98GHz
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Die
lect
ric
loss
(ε''
)
% Moisture content
Variation of dielectric loss (ε'') with moisture content
for pearl millet
4.65 GHz
7.00 GHz
9.35 GHz
14.98GHz
Effect of Moisture on dielectric properties....... 131
From these diagrams, it is apparent that critical moisture content lies in the
range 15 % to 20%, where the ε' – MC and ε'' – MC curves change their slopes. This
region of moisture content may be treated as transition region where water in free
state starts becoming available. Below this region water remains in bound state and
hence its contribution to ε' and ε'' is limited.
As discussed in section 5.4.1, proteins adsorb a larger amount of water as
compared to carbohydrates. At a particular frequency, the dielectric constant (ε') for
low moisture contents has very low values, while for high moisture content it is
high. It may be because at high moisture content more water dipoles contribute to
polarisation, as water molecules in free water can easily follow up the applied field
variations. It can also be seen that both ε' and ε'' have low values below 10.66%
moisture content. This is because of strong bound water state (monolayer) where
distance between the water molecule and the cell wall is very small, and force of
attraction is very large. This strong force prevents water molecules from aligning
with the varying electric field. Therefore, the dielectric constant (ε') and loss factor
(ε'') are small at MC <10.66 %. As we increase the moisture content beyond 10.66%,
the values of dielectric constant(ε') and loss factor (ε'') increase because of the
changes in bound water state from the first (monolayer) to the second (multilayer)
type (free water). At high moisture contents the water molecules play a greater role
in dielectric polarization, the ionic conductivity also increases causing dipole effects
to further increase and hence the values of dielectric constant (ε') and dielectric losses
(ε'') are quite high. The effect of ionic conductivity are more prominent at lower
frequencies, and hence the values of ε' and ε'' increase as we lower the frequency.
5.4.4 Relaxation Time and ac Conductivity
Conductivity ( ) and Relaxation time ( ) are calculated using equation (5.5)
and equation (5.6) respectively. The values of dielectric constant and dielectric loss
factor obtained by using two point method for wheat, green gram and pearl millet
are used for this calculation.
Tables 5.4,5.5 and 5.6 show the variation of values of conductivity ( ) and
relaxation time ( ) with moisture content for wheat, green gram and pearl millet
respectively. It is obvious from Table 5.4 that the values of conductivity( ) and
relaxation time ( ) for wheat in powder form increase with increase in moisture
Effect of Moisture on dielectric properties....... 132
content at all the four frequencies. Increase in conductivity with moisture can be
explained by the fact that as moisture increases, number of polar molecules per unit
volume also increases. When polar molecules per unit volume are more in number,
then under the influence of high frequency electromagnetic field, the rotatory motion
of the polar molecules of a system is not rapid enough to attain equilibrium with the
field. The polarization then acquires a component out of phase with the field and
displacement current acquires a conductance component in phase with the field,
resulting in thermal dissipation of energy. Thus, the dielectric loss is proportional to
the a. c. conductivity. The increase in relaxation time ( ), with increasing value of
moisture content may be considered due to increasing hindrance to the process of
polarization in presence of electric field or to the process of relaxation on removal of
electric field.
Table 5.4 : Variation of conductivity( ) and relaxation time ( ) with moisture
content for wheat in powder form at microwave frequencies.
Moisture
Content
(%)
C band
(4.65 GHz) J band
(7.00 GHz) X band
(9.35 GHz) Ku band
(14.98 GHz)
(S/m) x sec S/m x sec S/m) x sec S/m) x sec
9.20 0.0543 0.0016 0.0739 0.0011 0.0935 0.0009 0.0995 0.0009
13.80 0.1008 0.0028 0.1401 0.0019 0.1767 0.0016 0.2322 0.0019
15.97 0.1241 0.0034 0.1712 0.0023 0.2183 0.0019 0.2985 0.0023
18.40 0.1473 0.0039 0.2023 0.0026 0.2494 0.0021 0.3732 0.0027
20.70 0.1706 0.0045 0.2373 0.0030 0.3066 0.0026 0.4395 0.0031
24.30 0.2042 0.0052 0.2879 0.0036 0.3638 0.0029 0.5473 0.0035
Graphical representation of the variation of conductivity and relaxation time
with moisture content for wheat is shown in Fig. 5.7. From Fig. 5.7 (a) it is apparent
that at any frequency the variation of conductivity ( ) with moisture content (MC) is
almost linear; at lower values of MC the values are not much different for
different frequencies, but at higher values of MC the dependence of conductivity on
frequency of e.m. radiation is better visible - its value being higher at higher
frequencies. Variation of relaxation time with MC is shown in Fig. 5.7(b) for different
frequencies, which also indicates linear dependence of on MC at any frequency, but
now for a particular MC, the values are higher for lower frequencies. Relaxation
Effect of Moisture on dielectric properties....... 133
time η is a measure of the mobility of the molecules (dipoles) that exist in a material.
As the moisture content increases, the dipoles increase and hence both conductivity
and relaxation time increase.
(a)
(b)
Fig. 5.7: Moisture dependence of (a) conductivity ( ) and (b)relaxation time ( )
of wheat at indicated frequencies and room temperature (28°C)
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25 30
Con
du
ctiv
ity (
)
% Moisture content
Variation of conductivity ( ) of wheat
with moisture content
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
0.0000
0.0010
0.0020
0.0030
0.0040
0.0050
0.0060
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Rel
ax
ati
on
tim
e (
)
Moisture Content (%)
Variation of Relaxation time ( ) of wheat with
moisture content
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
Effect of Moisture on dielectric properties....... 134
The conductivity ( ) and relaxation time ( ) values for green gram obtained
from the present research for MC from 12.86 % to 24.86 % at four microwave
frequencies are displayed in Table 5.5 and relevant diagrams for dependence of
and on MC are shown in Fig. 5.8. Diagrams 5.8 (a) and 5.8(b) show that the
general behaviour of variation of and with MC for green gram is similar to that
of wheat. However, now - MC lines for green gram are more steeper than wheat,
particularly at 14.92 GHz.
Table 5.5: Variation of conductivity ( ) and relaxation time ( ) with moisture
content for whole green gram in powder form at microwave frequencies.
Moisture
Content
(%)
C band
(4.65 GHz) J band
(7.00 GHz) X band
(9.35 GHz) Ku band
(14.98 GHz)
(S/m) x sec S/m x sec S/m) x sec S/m) x sec
12.86 0.0853 0.0028 0.1089 0.0022 0.1351 0.0018 0.0829 0.0006
16.92 0.1266 0.0040 0.1673 0.0032 0.2079 0.0025 0.1990 0.0012
19.86 0.1525 0.0047 0.2101 0.0039 0.2598 0.0030 0.2902 0.0017
21.22 0.1654 0.0048 0.2295 0.0040 0.2858 0.0031 0.3317 0.0017
23.30 0.1861 0.0053 0.2591 0.0044 0.3274 0.0034 0.3897 0.0020
24.86 0.1990 0.0056 0.2801 0.0045 0.3534 0.0038 0.4395 0.0022
Table 5.6 : Variation of conductivity and relaxation time with moisture content
for pearl Millet in powder form at four frequencies.
Moisture
Content
(%)
C band
(4.65 GHz) J band
(7.00 GHz) X band
(9.35 GHz) Ku band
(14.98 GHz)
(S/m) x sec S/m x sec S/m) x sec S/m) x sec
0.00 0.1499 0.0050 0.1906 0.0030 0.1923 0.0024 0.0167 0.0001
10.66 0.1680 0.0052 0.2256 0.0033 0.2131 0.0023 0.0250 0.0001
15.39 0.1783 0.0048 0.2412 0.0031 0.2235 0.0022 0.0749 0.0003
16.90 0.1912 0.0049 0.2646 0.0031 0.2286 0.0019 0.1415 0.0005
18.65 0.2016 0.0046 0.2840 0.0030 0.2442 0.0017 0.2165 0.0006
19.20 0.2068 0.0040 0.2996 0.0028 0.2598 0.0017 0.3080 0.0009
27.16 0.2119 0.0038 0.3073 0.0026 0.2858 0.0014 0.3996 0.0009
Effect of Moisture on dielectric properties....... 135
(a)
(b)
Fig. 5.8: Moisture dependence of (a) conductivity ( ) and (b) relaxation time ( )
of green gram at indicated frequencies and room temperature (28°C ).
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Con
du
ctiv
ity
()
% Moisture content
Variation of Conductivity ( ) of green gram with
moisture content
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
0.0000
0.0010
0.0020
0.0030
0.0040
0.0050
0.0060
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Rela
xa
tio
n t
ime (
)
% Moisture content
Variation of relaxation time ( ) of green gram
with moisture content
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
Effect of Moisture on dielectric properties....... 136
The dependence of and on moisture content for pearl millet in powder
form at four microwave frequencies as obtained from the present research is shown
in Table 5.6. The relevant diagrams showing - MC and - MC variation for pearl
millet at four microwave frequencies are shown in Fig. 5.9 (a) and (b).The nature of
these diagrams is different from those obtained for wheat and green gram. For lower
frequencies (4.65, 7.00 and 9.35 GHz) the - MC curves are almost linear on two
sides of the region of MC (15 – 20 %), the slopes of lines on two sides of this region
being slightly different. For MC > 20% the slope is lower for 4.65 and 7.00 GHz
than that below MC = 15%. However, for 9.35 and 14.92 GHz the slopes of - MC
lines above 20% MC is greater than those below 15 % moisture. The curve at 14.92
GHz is quite complex, showing that at higher frequencies dielectric polarization
become prominent for MC > 15%. Fig. 5.7(b) also indicates that the moisture region
(15% < MC < 20%) is quite crucial. For MC < 15% the variation in with MC is
almost linear at all the frequencies and for MC > 20%. - MC curves are again
linear, being almost horizontal to % MC axis for 4.65, 7.00 and 14.92 GHz, but the
line bending downwards at 9.35 GHz. In the region 15% to 20%, the relaxation time
shows complex behaviour for change with moisture for all the frequencies, which
may be attributed to the state of water changing from bound water to free water on
increasing moisture content.
Effect of Moisture on dielectric properties....... 137
(a)
(b)
Fig. 5.9: Moisture dependence of (a) conductivity ( ) and (b) relaxation time ( )
of pearl millet at indicated frequencies and room temperature (28°C)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Con
du
ctiv
ity
()
Moisture Content (%)
Variation of Conductivity of pearl millet with
Moisture content
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
0.0000
0.0010
0.0020
0.0030
0.0040
0.0050
0.0060
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Rel
axa
tio
n t
ime
()
% Moisture Content
Variation of Relaxation time ( ) of pearl millet with
moisture content
(4.65 GHz)
(7.00GHz)
(9.35 GHz)
(14.92 GHz
Effect of Moisture on dielectric properties....... 138
5.5 Conclusion
It can be concluded that moisture content affects the dielectric properties of
food grains to a large extent. The dielectric properties can be correlated to moisture
content and can be used to monitor the moisture levels in crops and for quality
assessment of food grains. It can be further concluded that for higher levels of
moisture content ε' is higher for higher frequencies, whereas for low value of MC
the frequency dependence of ε' does not show definite trends.The dielectric loss also
increases with moisture content at all the frequencies, where as it decreases with
increase in frequency, for any moisture level in the samples of food grains, like,
wheat, green gram and pearl millet.