CHAPTER 6 TEMPERATURE DEPENDENCE OF DIELECTRIC …shodhganga.inflibnet.ac.in/bitstream/10603/39222/12/12_chapter 6.pdf6.3.1 Temperature Dependence of Dielectric Properties of Barley

Temperature dependence of dielectric properties....... 139

CHAPTER 6

TEMPERATURE DEPENDENCE OF DIELECTRIC

PROPERTIES OF BARLEY, CHICKPEA AND

MUSTARD SEEDS IN POWDER FORM AT

MICROWAVE FREQUENCIES

6.1. Introduction

The dielectric properties of food materials are important in order to understand

their behaviour when they are subjected to high frequency electromagnetic fields in

the process of microwave cooking or in other processes which involve radio

frequency (RF) or microwave dielectric heating. These properties also help in

assessment of quality of food materials by RF and microwave instruments. The most

relevant example is the moisture meters designed for rapid determination of the

moisture content of cereal grains and other food materials. Temperature of a material

has a significant influence on the dielectric properties. The dependence of dielectric

properties on temperature is a function of dielectric relaxation processes occurring

under the particular conditions of existing RF or microwave fields and the frequency

being used. Wang et al (2003) observed that bound water plays a major role in

dielectric heating in foods with low moisture content in the frequency range 20 MHz

to 30,000 MHz .

Dielectric materials, such as food materials, convert electric energy at RF

and microwave frequencies into heat. The rise in temperature is proportional to the

loss factor ( '') of the material in addition to electric field intensity, frequency and

time of exposure (Komarov et al, 2005). At radio and microwave frequencies,

dielectric properties of granular materials depend upon frequency, bulk density,

moisture content and temperature (Nelson, 1981; Nelson and Stetson, 1976). Water

plays the most influential role at these frequencies due to its polar nature. Effect of

temperature and bulk density both are related to water effects (Trabelsi et al, 1999).

A change in temperature affects the thermal energy of molecules as such the energetic

status of water molecule is also changed, which brings about a change in their

response to the electromagnetic fields. Several studies on the dielectric properties of


agricultural products have been reported in the past, covering a wide range of

frequency, temperature and moisture contents. (Nelson, 1965; Jiao et al., 2011;

Ohlsson et al., 1974).

Dielectric constant ( '), dielectric loss ( '') and conductivity of some oil seeds

were measured over the range of temperature 15°-45°C within the frequency range 5

kHz to 10 MHz by Singh et al. (2006). Nelson (1978) studied the variation of

dielectric constant of shelled corn with temperature for three different frequencies

and two different moisture contents. Dependence of the dielectric properties of

ground wheat having 11.2% moisture content on frequency and temperature in the

frequency range (10 MHz to 1.8 GHz) and temperature range (25°C to 95°C) was

observed by Nelson and Trabelsi (2006). Trabelsi and Nelson (2006) investigated the

dielectric properties of wheat, corn, and soybeans by measuring the scattering

transmission coefficient S21 in free space at frequencies ranging between 2 and 3

GHz. They studied the variations in the dielectric properties of above mentioned

samples with frequency and physical properties such as bulk density, moisture

content, and temperature. Both dielectric constant (ε') and the loss factor (ε'') were

found to decrease with frequency and increased linearly with bulk density, moisture

content, and temperature. The relative complex permittivity(ε*), consisting of the

dielectric constant and loss factor, were measured for samples cut from three fresh

fruits, viz. Apple, avocado and banana, over the frequency range from 10 MHz to

1.8 GHz at temperatures ranging from 5°C to 95°C by Nelson (2003). Measurements

of the dielectric properties of unshelled and shelled peanuts at 6 GHz were made by

Trabelsi et al (2009) for the temperature range from 0°C to 60°C.

From the literature survey it is apparent that temperature dependence of the

dielectric properties of food grains like mustard seeds, barley and chickpea have not

been reported at microwave frequencies particularly in the range 3 GHz -10 MHZ.

As this is the frequency range in which dispersion effects are maximum both for ε'

and ε'', it was considered relevant to study the temperature dependence of the

dielectric properties of these samples at some of the frequencies in this range.


6.2 Material and Method

Chickpea (RSG 888) and Barley grains (RD2508) were obtained from

Durgapura Agriculture Research Station of Rajasthan Agriculture University, Bikaner.

The samples of mustard seeds (MAYA) were obtained from the National Mustard

Research Centre, Sewar, Bharatpur. The grains were grinded to bring them in

powder form. The method used in the present work is a method proposed by Yadav

and Gandhi (1992) described in detail in Chapter 3. This method has been employed

because it is simple and can be used by attaching a slotted section and liquid

dielectric cell in the microwave bench. The dielectric cell has a Jacket to circulate

hot / cold water around the sample so as to keep it at different temperatures. In the

present work, the temperature of the sample powder filled in dielectric cell was

varied by circulating hot water around it. The measurements were performed at

different temperatures by using a constant temperature water bath fitted with a

digital temperature controller having temperature stability of the order of ± 0.1˚C.

6.2.1. Experimental Details

The method used for the study of temperature variation of dielectric properties

of food grains in powder form is the method proposed by Yadav and Gandhi (1992).

The experiment was carried out at three frequencies viz., 4.65 GHz,7.01 GHz and

9.42 GHz on C band, J band and X band microwave benches respectively.

The experimental arrangement is shown in Fig. 6.1. Microwave power was

obtained from a microwave source, viz. Klystron tube and was allowed to form

standing waves in the slotted waveguide section after being reflected from the short

circuiting plunger in the liquid dielectric cell, which was initially kept at its lowest

position in the cell. The probe in the slotted waveguide section was accurately

adjusted at the node of the standing waves, as adjudged by the position of the

minima in the indicating meter. The temperature of the cell was maintained by

circulating hot water in the jacket surrounding the cell from a water bath by means

of a motor. Then, a small quantity of food grain powder was introduced in the

dielectric cell and the plunger was brought over it by moving the micrometer screw

till a proper contact was established. The height „h‟ of the powder in the dielectric

cell was determined from the difference of readings on the scale of micrometer screw


Fig. 6.1: Experimental Arrangement for Yadav- Gandhi (1992) method used to

observe temperature dependence of the dielectric properties of food grains in

powder form.

taken with and without food powder in the cell. The powder was added slowly in the

dielectric cell till for a height „h‟ of the powder in the cell, the position of minima in

the slotted section is the same as for the empty cell. For this position,

dh2

Or d 2h

(6.1)

The value of λd. was determined by using equation ( 6.1). The phase factor in

the dielectric (βd) is then given by

βd = 2π / λd (6.2)

In order to calculate the value of attenuation constant in the dielectric (α), the

plunger was kept at the bottom of the empty cell and the probe was located at one of

the maximas, i.e., at the position of a voltage antinode in the waveguide slotted

section, and reading „x1‟ of the indicating meter is noted. The food powder (flour of

the food grains in the present case) was then added slowly in the dielectric cell and

position of the maxima in the slotted section is noted every time. The height of the

powder column in the dielectric cell (h') was accurately adjusted so that the probe

position locating the maxima in the slotted section was again the same as with the

empty cell. The deflection „x2‟ of the indicating meter in this state, for such height

„h' of the powder column, was noted.. The value of the attenuation constant in the

dielectric ( αd ) is then given by


1

d

2 1

x2.303log

2h ' 2 x x (6.3)

On using the measured values of αd and βd,the values of ε' and ε'' of food

grain powder were calculated by the following equations

2 2 2

0 0 d

c d d

' 1

(6.4)

2

0 d

d d

" 2

(6.5)

where λ0 is the guide wavelength for air dielectric and λc is the cutoff wavelength

given by λc = 2a,where a is the width of the waveguide.

The same procedure was repeated for determination of dielectric properties

at different temperatures. The experiment was performed three times for each

temperature and the mean values of dielectric constant and dielectric loss factor

were obtained.

6.3 Results and Discussion

6.3.1 Temperature Dependence of Dielectric Properties of Barley

The values of dielectric constant (ε') and dielectric loss factor (ε'') computed

for barley in powder form at different temperatures are displayed in table 6.1. It is

observed from this table that at any frequency both the dielectric constant and

dielectric loss show a slight increase in their respective values with increase in

temperature.

At a fixed temperature, the dielectric values (ε' and ε'') are observed to vary

with frequency. Both the dielectric constant and dielectric loss decrease with increase

in frequency, which is as expected, as discussed in chapter 4. It has been stated by

Sahin and Sumnu (2006a) that free and bound water and the ionic conductivity

affect the rate of change of dielectric constant and loss with temperature .If bound

water is present, the increase in temperature leads to increase in the values of the


dielectric parameters. But in the presence of free water, the increase of temperature

causes a decrease in the values of the dielectric properties (ε' and ε'') . Since barley

powder is taken in dry form, it contains the moisture in bound form, hence the

dielectric properties increase with increase in temperature. Water molecules that are

chemically bound have less influence on the dielectric properties of food powder

than free water molecules, since free water molecules can rotate freely when an

electric field is applied, while bound water molecules cannot (Içier and Baysal,

2004).The stronger the binding force between water and other constituents, the

smaller is the value of dielectric constant and dielectric loss factor. As the

temperature increases, the binding forces become weaker and the water dipoles can

orient more easily.

Table 6.1: Temperature dependence of dielectric constant (ε') and dielectric

loss factor (ε'') of barley at three frequencies

Temperature

(°C)

4.69GHz 7.01 GHz 9.32 GHz

ε' ε'' ε' ε'' ε' ε''

30 4.82 ±

0.19

0.44 ±

0.02

3.93 ±

0.16

0.33 ±

0.03

2.30 ±

0.14

0.15 ±

0.03

40 4.93 ±

0.21

0.47 ±

0.02

4.01 ±

0.15

0.36 ±

0.04

2.46 ±

0.12

0.17 ±

0.04

50 4.98 ±

0.23

0.51 ±

0.02

4.07 ±

0.18

0.39 ±

0.02

2.60 ±

0.15

0.19 ±

0.05

60 5.08 ±

0.19

0.56 ±

0.02

4.12 ±

0.17

0.43 ±

0.03

2.67 ±

0.12

0.22 ±

0.04

70 5.17 ±

0.19

0.62 ±

0.02

4.22 ±

0.16

0.49 ±

0.03

2.80 ±

0.13

0.26 ±

0.04

80 5.30 ±

0.21

0.68 ±

0.03

4.36 ±

0.17

0.54 ±

0.03

2.87 ±

0.16

0.31 ±

0.06

As temperature increases, the molecular mobility increases and relaxation

frequency which is strongly related to the molecular mobility decreases (Barrow,

1988). Hence, the values of ε' and ε'' show a increase. Increase in temperature also

increases the ionic conduction, leading to an increase in dielectric loss factor. Thus,

both ε' and ε'' increase as the temperature is increased.


(a)

(b)

Fig. 6.2: Variation of (a) dielectric constant and (b) dielectric loss with

temperature for barley

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 20 40 60 80 100

Die

lect

ric C

on

stan

t

Temperature ( C)

Variation of dielectric constant with temperature

for barley

9.32 Ghz

7.01 GHz

4.69 GHz

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 20 40 60 80 100

Die

lect

ric l

oss

(ε'

')

Temperature ( C)

Variation of loss factor with temperature for barley

9.32 Ghz

7.01 GHz

4.69 GHz


Figures 6.2 (a) and (b) show temperature dependence of dielectric constant

(ε') and dielectric loss factor (ε'') for barley in the temperature range of 30° C to

80°C respectively. From the Fig. 6.2 (a), it is apparent that the linear regression

provides good estimates for the temperature dependence of dielectric constant .The

values of regression coefficients for the lines shown in this figure are given below in

Table 6. The rate of change of ε' with temperature is approximately the same at all

frequencies. This behaviour is similar to that observed by Nelson (1978) for shelled

yellow dent corn at frequency 2.45 GHz and temperatures 25°C to 60°C.

Fig. 6.2 (b) shows the variation of dielectric loss factor (ε'') with temperature.

From this diagram, it can be inferred that the dependence of loss factor (ε'') on

temperature is also almost linear. The values of linear regression coefficients (R2)

for

the three cases are shown in Table 6.2. From this table it is clear that the rate of

change ε'' with temperature is slightly higher at frequencies 4.69 GHz and 7.01 GHz

as compared to 9.32 GHz.

It may be concluded that the variation of both ε' and ε'' with temperature is

gradual at all the frequencies at which measurements are made, no irregular jumps

are observed.

Table 6.2 : Linear Regression coefficients for variation ε' and ε'' of barley with

temperature

Frequency

(GHz)

ε' ε''

Coeff. of det.

(R2)

Linear

equation

Coeff. of det.

(R2)

Linear

equation

4.69 0.98 y = 0.009 x +

4.540 0.98

y = 0.004 x +

0.279

7.01 0.96 y = 0.008 x +

3.673 0.97

y = 0.004 x +

0.190

9.32 0.98 y = 0.011 x +

1.997 0.96

y = 0.003 x +

0.043


6.3.2 Temperature Dependence of Dielectric Properties of Chickpea

The values of dielectric constant(ε') and dielectric loss (ε'') of chickpea as

obtained by Yadav-Gandhi method (1992) in powder form at different temperatures

are displayed in table 6.3.It is apparent from this table that at a particular frequency,

the values of ε' and ε'' increase with increase in temperature. This indicates that the

water exists in chickpea in bound form. A change in temperature causes the energetic

status of the molecules to change, with the result that the capability of the molecules

to rotate under the influence of electric field also changes. As such, when temperature

is changed, there is a change in the contribution of water molecules to polarisation,

giving rise to a change in the value of complex permittivity of the dielectric.

Increase in dielectric loss factor with increasing temperature often results in a

well-known phenomenon, commonly referred to as „„thermal runaway‟‟ (Metaxas

and Meridith, 1983), in which a preferentially heated subject in an EM field

accelerates heating as its temperature rises. Stuchly and Stuchly (1980) investigated

the effect of moisture content on the dielectric properties of granular solids at 9.4

GHz over a wide range of temperature and moisture contents and reported that at

low moisture contents the temperature dependence of ε' and ε'' was not appreciable

for dried solids but it increased dramatically at higher moisture contents. Under the

influence of electromagnetic fields, a material at high temperatures has low value of

relaxation time (η) and therefore the peak of dielectric loss shifts to the higher

frequency (ω = 1/ η) side. They observed that the temperature effects on dielectric

properties also depend on the relaxation frequency of the material. The dielectric

constant increases and dielectric loss either decreases or increases with increasing

temperature depending on whether the operating frequency is lower or higher than

the relaxation frequency.


Table 6.3 : Variation of dielectric constant (ε') and dielectric loss (ε'') of

chickpea with temperature, at three microwave frequencies.

Temperature

(°C)

4.69 GHz 7.01 GHz 9.32 GHz

ε' ε'' ε' ε'' ε' ε''

30 5.4 ±

0.24

0.42 ±

0.05

3.64 ±

0.17

0.35 ±

0.05

2.75 ±

0.16

0.32 ±

0.06

40 5.46 ±

0.23

0.45 ±

0.07

3.68 ±

0.19

0.38 ±

0.06

2.84 ±

0.15

0.36 ±

0.04

50 5.53 ±

0.28

0.47 ±

0.05

3.74 ±

0.15

0.42 ±

0.08

2.94 ±

0.12

0.36 ±

0.06

60 5.58 ±

0.21

0.51 ±

0.08

3.79 ±

0.17

0.47 ±

0.09

3.17 ±

0.11

0.39 ±

0.05

70 5.64 ±

0.16

0.57 ±

0.06

3.84 ±

0.23

0.52 ±

0.06

3.21 ±

0.13

0.44 ±

0.04

80 5.76 ±

0.18

0.68 ±

0.07

3.94 ±

0.2

0.58 ±

0.1

3.29 ±

0.14

0.52 ±

0.06

In a food system, the change of dielectric properties with temperature depends

on other factors like frequency, bound water to free water ratio, ionic conductivity,

and composition of the material (Tang et al, 2002 ). At microwave frequencies, the

contribution of bound water to polarization causes both the dielectric constant (ε')

and the loss factor (ε'') to increase with temperature, whereas on the other hand, the

presence of free water in foods causes both ε' and ε'' to decrease with increase in

temperature.


(a)

(b)

Fig. 6.3: Variation of (a) dielectric constant(ε') and dielectric loss factor (ε'') of

chickpea with temperature.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 20 40 60 80 100

Die

lect

ric co

nst

an

t (ε

')

Temperature (θ C)

Temperature dependence of dielectric constant of chickpea

9.32 Ghz

7.01 GHz

4.69 GHz

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80 100

Die

lect

ric l

oss

(ε'

')

Temperature (θ C)

Temperature dependence of dielectric loss for chickpea

9.32 Ghz

7.01 GHz

4.69 GHz


It is clear from these figures that both the dielectric constant (ε') and dielectric

loss factor (ε'') increase almost linearly with increase in temperature. For loss factor

(ε'') at 4.69 GHz deviations from linear dependence are observed. As such, curve

fitting has been used in this case. At other frequencies linear relationship is found to

be quite satisfactory. This shows that variation of dielectric constant (ε') at low

frequency is faster as compared to other frequency. The increase in dielectric

parameters with temperature at lower frequency denote the predominant effect of

ionic conduction as well as molecular mobility. For lower moisture content the

variation of ε ' and ε'' with temperature is almost linear.

It may be observed from Fig. 6.3 (a) and (b) that slope of ε'-θ lines is smaller

than the slopes of ε''-θ lines at all the frequencies, showing that the dielectric

constant is less affected by temperature than that of the dielectric loss factor because

the increase in ionic conduction at higher temperature produces additional effect on

dielectric loss factor, whereas the dielectric constant is least or not at all affected by

the increase in ionic conduction (Khan and Chandel, 2011).

Table 6.4 : Linear Regression coefficients for variation ε' and ε'' of chickpea

with temperature

Frequency

(GHz)

ε' ε''

Reg.coeff.

(R2)

Linear equation Reg.coeff.

(R2)

Linear equation

4.69 0.97 y = 0.006 x + 5.186 0.96 y = 0.004 x + 0.271

7.01 0.97 y = 0.005x + 3.452 0.98 y = 0.004 x + 0.198

9.32 0.96 y = 0.011x + 2.398 0.99 y = 0.004 x + 0.196

6.3.3 Temperature Dependence of Dielectric Properties of Mustard Seeds

The values of dielectric constant (ε') and dielectric loss factor (ε'') for

mustard seeds were measured in crushed form at different temperatures by

employing Yadav – Gandhi method (1992) and the results obtained are displayed in

Table 6.5.


Table 6.5 : Variation of dielectric constant(ε') and dielectric loss factor (ε'') of

mustard seeds with temperature at three microwave frequencies

Temperature

(°C)

4.69 GHz 7.01 GHz 9.32 GHz

ε' ε'' ε' ε'' ε' ε''

30 5.22 ±

0.22

0.43 ±

0.09

4.38 ±

0.21

0.32 ±

0.07

2.48 ±

0.20

0.19 ±

0.06

40 5.19 ±

0.19

0.46 ±

0.12

4.35 ±

0.19

0.35 ±

0.06

2.44 ±

0.19

0.21 ±

0.05

50 5.15 ±

0.23

0.49 ±

0.09

4.29 ±

0.17

0.36 ±

0.05

2.41 ±

0.20

0.25 ±

0.06

60 5.09 ±

0.17

0.53 ±

0.08

4.25 ±

0.23

0.42 ±

0.06

2.39 ±

0.17

0.29 ±

0.05

70 5.05 ±

0.21

0.58 ±

0.11

4.22 ±

0.21

0.49 ±

0.09

2.38 ±

0.25

0.32 ±

0.06

80 5.15 ±

0.20

0.51 ±

0.10

4.31 ±

0.19

0.41 ±

0.10

2.41 ±

0.17

0.22 ±

0.07

It is observed from Table 6.5 that as the temperature is increased from 30°C,

the dielectric constant (ε') decreases and the value of dielectric loss (ε'') increases till

70° C, above which a reverse trend is observed i.e. the value of ε' increases and the

value of ε'' decreases at all frequencies. This behaviour is different from the behaviour

shown by barley and chickpea. A similar trend where the value of' ε' decreases with

increase in temperature and ε'' increases with increase in temperature was also

observed for mustard oil by Bansal et. al. (2001). This suggests that the oil content

of the mustard seeds is responsible for the different behaviour of oil seeds as

compared to other food grains. As temperature increases the protein denaturation

occurs, which further increases the structural deterioration of proteins, the resulting

charge asymmetry may be considered to be responsible for increase in dielectric

constant (ε') at higher temperatures. A similar trend for variation of dielectric

properties of other seeds was also reported by Khan et al. (2012). An increase in

temperature leads to denaturation of proteins. This then leads to a changed protein

structure and rearrangement of the charge distribution of the protein, which results in

an amplified dipole moment and greater polarization of proteins, that brings about an

increase in the dielectric properties of such materials (Sahin and Sumnu, 2006b).


(a)

(b)

Fig. 6.4: Variation of (a) dielectric constant (ε') and (b) dielectric loss factor

(ε'') with temperature for mustard seeds.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 20 40 60 80 100

Die

lect

ric c

on

stan

t (ε

')

Temperature ( C)

Temperature dependence of dielectric constant (ε')

of mustard seeds

9.32 Ghz

7.01 GHz

4.69 GHz

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 20 40 60 80 100

Die

lect

ric l

oss

Temperature ( C)

Temperature dependence of dielectric loss factor

of mustard seeds

9.32 Ghz

7.01 GHz

4.69 GHz


However, if water becomes bound during the denaturation process, the

dielectric properties will decrease and vice versa if water is released during

denaturation. Carbohydrates are also present in the oilseeds. A strong binding exists

between bound water and carbohydrates which results in low value of dielectric

constant (ε') and dielectric loss factor (ε'') (Sahin and Sumnu, 2006b). It is this

binding between bound water and proteins and bound water and carbohydrates and

consequent decrease in free water content that is responsible for a decrease in ε' and

increase in ε'' with increase in emperature. When denaturation takes place and water

is in bound state, the mobility of the ions present is also reduced, causing the

dielectric properties to reduce (Sahin and Sumnu, 2006 b).

The variation of dielectric constant (ε') with temperature is depicted in Fig.

6.4 (a). It is observed from this figure that the value of ε' shows a linear dependence

on temperature upto 70°C with a negative slope, but above 70°C the slope of the

curves become positive at all the three frequencies. The change in ε' with

temperature is found to be quite gradual. The variation of dielectric loss factor (ε'')

with temperature is shown in Fig.6.4 (b).It is observed from this figure that linear

regression does not apply to this case. Now the dielectric loss factor (ε'') increases

with temperature almost linearly till 70°C and above 70°C it decreases till 80°C,

showing a peak at about 70°C at all three frequencies. This change in the trend can

be attributed to protein denaturation and the change in viscosity of the oil content of

the mustard seeds. Protein denaturation is not observed in the case of chickpea

though they are rich in proteins. This may be due to the fact that chickpea has high

proteins as well as carbohydrates while mustard seeds have high content of fats and

proteins.

It may also be observed that the general trend of the present results for

dependence of dielectric constant (ε') and dielectric loss factor (ε'') on temperature

for barley and chickpea agree well with the results of Trabelsi et.al.(2009) at 6 GHz

for temperature dependence of ε' and ε'' of shelled and unshelled peanuts. However,

the results for ε' and ε'' of mustard seeds show variation from the general trend, as

can be inferred from Fig. 6.4.


6.4 Conclusion

It can be concluded that the dielectric properties of barley, chickpea and

mustard seeds vary with the change in temperature. The change in dielectric constant

(ε') with temperature is very small since the moisture content in these grains and

seeds is quite low. It can be inferred that both dielectric constant (ε') and dielectric

loss factor (ε'') increase with increase in temperature for cereals and pulses while the

dielectric constant decreases with increase in temperature for mustard seeds which

represents the oilseeds. The knowledge of temperature dependence of dielectric

properties helps in design of microwave processes and control and also in material

characterization.

Documents

CHAPTER 6 TEMPERATURE DEPENDENCE OF DIELECTRIC …shodhganga.inflibnet.ac.in/bitstream/10603/39222/12/12_chapter 6.pdf6.3.1 Temperature Dependence of Dielectric Properties of Barley