chapter 5 effect of moisture on dielectric properties...

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


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


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.


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


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


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)


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


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


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


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')


(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


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


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.


(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


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


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


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


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


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


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)


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)


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


(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


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.


(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


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.

chapter 5 effect of moisture on dielectric properties...

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