CHAPTER - 4 DIELECTRIC PROPERTIES OF FOOD GRAINS ...

Dielectric properties of food grains, pulses....... 79

CHAPTER - 4

DIELECTRIC PROPERTIES OF FOOD GRAINS,

PULSES AND OIL SEEDS AND THEIR FREQUENCY

DEPENDENCE

4.1 Introduction

The dielectric properties of foods and biological products are important in

food engineering and technology. These properties have fundamental importance in

predicting the rate of heating and describing the behaviour of food materials when

subjected to high-frequency radiation. The dielectric properties of food grains and

seeds are used in many applications, such as moisture content determination, killing

insects in stored grains, dry fruits by selective heating, preparation of microwaveable

food, dielectric heating, sorting of food grains etc. (Nelson, 1977). So, for the

development of microwave process and control systems, it is important to have the

knowledge of dielectric properties of the materials. Many factors, such as frequency

of the applied radiation, temperature and moisture content affect the dielectric

properties of agricultural products and food materials (Venkatesh and Raghavan,

2004). Knowledge of the relationship between frequency and dielectric properties is

helpful in determining the suitable frequency range in which the material under study

has the desired dielectric characteristics for intended applications (Nelson, 2005).

Dielectric properties (electrical characteristics) of agricultural products have

been of interest for many years. The first quantitative data on the dielectric properties

of food grains were reported by Nelson et al. in 1953 for barley in the frequency

range 1 to 50 MHz. Dielectric properties of chickpea flour in compressed form were

determined by Guo et al. (2008) and it was observed that dielectric constant and loss

factor of the sample decreased with increase in frequency at all temperatures and

moisture levels. Guo et al. (2010) measured dielectric properties of four legumes

(chickpea, green pea, lentil and soybean) in the form of flour at four different

moisture contents, frequency ranging from 10 to 1800 MHz and temperatures 20°C

to 90°C by using open ended coaxial probe method. The frequency and moisture

dependence of the dielectric properties of hard red winter wheat has been reported

by Nelson and Stetson (1976).


Although a lot of work has been done on the dielectric properties of food

grains outside India, there are not many reports of dielectric study of food grains of

Indian variety. Therefore, it was considered to be of importance to study dielectric

properties of Indian varieties of food grains, pulses and oilseeds and investigate their

frequency dependence at room temperature. The dielectric properties of five samples

of cereals and grains, viz., wheat, rice, barley, sorghum and pearl millet, three

samples of pulses, viz. chickpea, green gram and split green gram and have been

determined in powder form for grain size 250-300 microns at four different

frequencies by using microwave benches in X, C, J and Ku bands and employing

two point method. The dielectric properties of oilseeds viz. mustard seeds and

soybean were investigated in crushed form at the four frequencies. For the sake of

comparison, dielectric properties of one sample of grains were determined in whole

grain form using two point method and dielectric mixture equation. The sample

selected for this study was wheat since it is the most widely used grain.

In this chapter, dielectric constant (ε') and dielectric loss (ε'') values for the

above mentioned samples of food grains, pulses and oilseed are reported at four

different frequencies viz 4.65 GHz,7.00 GHz,9.35 GHz and 14.98 GHz.

4.2 Materials

The foods which we consume daily are classified as cereals, legumes, nuts

and oilseeds, vegetables, fresh fruits and vegetables, milk and milk products and

flesh foods. A person needs a wide range of nutrients to perform various functions in

the body and to lead a healthy life. These nutrients include proteins, fats,

carbohydrates, vitamins and minerals. Depending on the relative concentration of

these nutrients, foods are classified as protein rich foods, carbohydrate rich foods

and fat rich foods. The food grains in the present study were selected from these

three groups. These food grains constitute the major part of our diet and are locally

available. They are more commonly used as compared to fruits and vegetables. They

show distinct variation in nutrient composition and therefore it is possible to establish

a relationship between their dielectric properties and composition. For such a

relationship, the selection of samples is made so that mainly one nutrient varies from


sample to sample, whereas the other nutrients remain approximately constant.

(Gopalan at el, 2007). The food grains used in the present study are:

i. Cereals and grains : Wheat, Rice, Barley, Pearl Millet, Sorghum

ii. Pulses and legumes : Chickpea, Green gram

iii. Nuts and oilseeds : Mustard seeds, Soybean

The studies on food grains and pulses were carried out in powder form,

whereas studies on nuts and oilseeds were made in crushed form for the sake of

uniformity of stuff.

An attempt has been made to compare the dielectric properties of the grain in

powder form and in whole grain form .The values are determined by using two point

method for both the samples. The sample selected for this study is wheat. The

dielectric properties of wheat in powder form is determined by two point method.

For whole wheat, dielectric constant and dielectric loss is determined for air-wheat

mixture by two point method and then dielectric properties are estimated using

dielectric mixture equation.

4.2.1 Cereals and Grains

The grains are the main source of energy in Indian diets, contributing

approximately 70-80% of daily energy intake by majority of Indians. They are the

cheapest and widely available source of energy. Cereals are plants belonging to the

'grass family' which are cultivated for their edible grains. In most of the countries,

cereal grains are grown in large quantities as staple food. When the grains are used

in whole grain form, most cereals are rich in vitamins, minerals, carbohydrates, fats,

oils and proteins. Millets are a group of highly variable small-seeded grass, widely

grown around the world as cereal crop and millet grains are used both as human

food and fodder (Amadou et al.,2013).


4.2.1.i Wheat

Hindi Name Gehun

English name Wheat

Botanical name Triticum aestivum

Family name Poaceae

In the realm of food crops in the world, wheat occupies the number one

position. Wheat is the major food component of most of the people worldwide, as it

is rich in carbohydrates and in dietary proteins, being only next to the pulses in

protein contents. India is one of the principal wheat producing and consuming

countries in the world. Its importance in Indian agriculture is second to only rice.

Wheat flour based products, such as the bread (chapati) is part of the staple diet in

most of the parts of India - particularly in northern India. Wheat products are used to

prepare different food items, like breads, biscuits, cookies, cakes, breakfast-cereal,

pasta, noodles, couscous etc. Wheat by way of its fermentation is also used for items

like beer, alcohol, vodka, bio-fuel etc. Wheat, in its natural unrefined state, features

a host of important nutrients. Indian Wheat (whole grain) contains in every 100

grams of it, 71.2 grams of carbohydrates, 11.8 grams of proteins, 1.5 grams of total

fat, 12.8 grams of moisture, 1.2 grams of crude fiber and 1.5 grams of minerals

(Gopalan et al., 2007).


4.2.1.ii Rice

Hindi Name Chawal

English name Rice

Botanical name Oryza sativa L.

Family name Poaceae

Rice is one of the most important cereals cultivated worldwide, constituting

the basic food for large number of human beings, sustaining two-thirds of the world

population (Zhout et al.,2002). Rice is the seed of Oryza sativa plant which is a

monocot plant It is the most widely consumed staple food for a large part of the

world's human population, especially in Asia. In India, rice consumption is generally

accomplished in various forms like whole cooked grain, as dish meal, where rice is

served normally in two ways, white rice and parboiled grains. It is the main base for

preparation of many indigenous fermented food products (like idli, dosa, uttapam),as

sweets (anarasa, khir), and in khichadi, pulav, puffed and extruded (Ghadge and

Prasad, 2012). There is a large number of rice varieties such as long-grain, basmati,

Arborio etc. but only a few of them are grown widely. Rice straw is used as cattle

feed, for thatching roof and in cottage industry, for preparation of hats, mats, ropes,

sound absorbing straw board and as litter material. Rice husk is used as animal feed,

for paper making and as a fuel source. Rice bran is used in cattle and poultry feed

and as defatted bran which is rich in protein (Prasad et al, 2011). Rice bran oil is

used in soap industry. Refined rice bran oil can be used as a cooking medium like

cotton seed oil and corn oil. Rice bran wax, a byproduct of rice bran oil is used for

various industrial applications (Sabale et al,2009)


4.2.1.iii Pearl Millet

Hindi Name Bajra

English name Pearl Millet

Botanical name Pennisetum glaucum

Family name Poaceae

Pearl millet is the most widely grown cereal crop. It survives in soils with

high salinity, soil with low fertility and under drought conditions. It is grown in bulk

in African and Indian sub continents. It has been a staple diet for Indians since pre-

historic times. Rajasthan is the largest producer state of pearl millet in India. Pearl

millet is consumed in the form of chappatis and bhakris, as porridges and boiled or

steamed food or Khichdi in Indian states like Gujrat, Rajasthan, Maharashtra.

Nutritive Values of Pearl Millet

It is high in protein as compared to other cereals. It contains all essential

amino acids and is particularly high in lysine, methionine, and cysteine. It is rich in

foliate, potassium, magnesium, copper, zinc, vitamin E and B-complex. It is also

rich in calcium and iron. It helps maintain cardiovascular health and helps reduce

acidity problems. It contains high amount of antioxidants and is beneficial for

overall health and well being (Nambiar et al., 2011)


4.2.1.iv. Barley

Hindi Name Jau

English name Barley

Botanical name Hordeum vulgare

Family name Poaceae

It is available as barley whole grain, barley flour and barley flour mixed with

bengal gram flour. Barley grains look like wheat grains. It is a crop which easily

thrives in excess heat and saline water. It is highly drought tolerant. When the

fibrous outer hull is not removed it is called as the hulled or covered barley. Once

removed, it is called de-hulled or pot barley. De-hulled barley is considered as a

whole grain, has its bran and germ intact and is rich in fiber. Pearl barley is de-hulled

barley which has been further processed to remove the bran. Although pearl barley

is rich in fiber but ground whole barley definitely has the benefit of extra fiber.

De-hulled or pearl barley is used to make barley products like barley flour,

barley flakes and grits. Barley soup, stew, porridge made of barley flour are

commonly eaten throughout the world. Barley flour is primarily used in combination

with other flours to make multigrain breads (Mahdi et al, 2008). It is approximated

that about 85% of the world's barley production is intended for feeding animals,


while the rest is used for malt production, seed production and as food by human

beings; also used for the production of starch either for use as food or for the

chemical industry.

Nutritional Benefits of Barley

Barley contains eight amino acids which are essential for our diet. It is a

great source of elements like magnesium, potassium, selenium, phosphorous. People

wanting to lose weight need to incorporate this millet in at least one of their main

meals. It is beneficial for diabetics, people with high cholesterol and high blood

pressure (Mahdi et al, 2008). Barley contains high amounts of beta-glucan, which is

a form of soluble fiber. Eating barley can regulate blood sugars for up to 10 hours

after consumption, as compared to wheat.

4.2.1.v. Sorghum

Hindi Name Jwaar

English name Sorghum

Botanical name Sorghum vulgare

Family name Poaceae


Sorghum is available as whole grain, flour, and as multigrain flour.

Sorghum, like barley is extremely resistant to drought. It is usually grown in dry

parts of the world.

Nutritional Benefits of Sorghum

It is a very good source of proteins. It contains essential nutrients like iron,

calcium, potassium, and phosphorous. It contains good amounts of B-vitamins like

thiamin and riboflavin. Sorghum is rich in phytochemicals including tannins,

phenolic acids and anthocyanins. The phytochemicals have gained importance due

to their antioxidant activity, Cholesterol lowering property and other potential health

benefits. Studies have shown that sorghum can reduce the risk of certain types of

cancer in humans. The phytochemical levels are so high in this millet that it has

shown potential usefulness in reducing obesity as well (Awika and Rooney,2004) .

Sorghum is also known to be useful for heart.

4.2.2 Pulses

Pulses are the edible seeds of leguminose plants.The word „pulse‟is derived

from latin „puls‟ meaning forage (Salunkhe 1982; Salunkhe and Kadam, 1989).

These are also known as legumes. The word legume is derived from the latin word

„legumen‟ which means seeds harvested in pods. Legumes are members of pod

bearing plants, belonging to the family of Fabaceae or Leguminoseae. Pulses offer a

relatively cheaper source of proteins as compared to animal proteins which are

valuable for the purpose of protein rich diet in developing countries (Singh and

Singh, 1992). The food value of pulses is high, they have about the same caloric

value per unit weight as the cereals. Their protein contents are high. They contain

about 20 to 30% proteins, around 2 to 5% fats and 50% or more carbohydrates.

These are good sources of dietary fiber ( Chopra et al,2009). Because pulses are low

in fat and rich in proteins, fiber, minerals and vitamins, they have a low glycemic

index and can contribute to improved blood glucose level.


4.2.2.i Chickpea

Hindi Name Chana

English name Chickpea

Botanical name Cicer arietinum

Family name Leguminose

Chickpea is an important legume or pulse crop grown and consumed all over

the world, especially in the countries of Africa and Asia. It is a good source of

carbohydrates and proteins, and the protein quality in chick pea is considered to be

better than other pulses. Chickpea has significant amounts of all the essential amino

acids, except types containing sulfur, which can be complemented by adding cereals

to daily diet. The carbohydrate content of chickpea consists of starch as the major

component followed by dietary fiber, oligosaccharides and simple sugars like

glucose and sucrose (Jukanti,2012). Chickpea has been present in human diet since

ancient times owing to its good nutritional properties. In the Indian subcontinent,

chickpea is split (cotyledons) as dhal and grinded to make flour (besan) that is used

to prepare different types of snacks (Chavan et al, 1986; Hulse,1991). Moisture

dependent physical and mechanical properties of chickpea were studied by Ayman et

al. (2010).


4.2.2 .ii Green Gram

Hindi Name Moong

English name Green Gram, mung bean

Botanical name Vigna radiate

Family name Leguminosae

Green grams or Mung beans are green in colour and light yellow in colour

when their husk are removed. Mung beans are small and ovoid in shape. .It is a store

house of nutrients. They are rich in Vitamin B, Vitamin C,Proteins, Manganese and

a lot of other essential nutrients required for effective functioning of the human

health. Green gram seeds are high in carbohydrates (>45%) and proteins (>21%);

fair source of calcium, iron, vitamins A and B, but deficient in vitamin C. Sprouted

mung beans are a good source of vitamin B. Raw green gram contains trypsin

inhibitor, which gets destroyed on cooking. Various physical properties of green

gram were evaluated as a function of moisture content in the range of 8·39 to 33·40%

d.b. by Nimkar and Chattopadhyaya (2002). The average length, width, thickness

and thousand grain mass were 4·21 mm, 3·17 mm, 3·08 mm and 28·19 g at moisture

content of 8·39% d.b. The geometric mean diameter increased from 3·45 to 3·77

mm, whereas sphericity decreased from 0·840 to 0·815.


4.2.3 Oil Seeds

India is the leading oilseed producing country in the world. Some of the

oilseeds grown in India are Groundnut, Mustard seed, rapeseed, sesame, linseed,

soybean etc. Oilseeds occupy the second place after food grains as a farm commodity.

They form an important export item. Vegetable oil is a necessary part of our diet.

Oilcake (the by-product after the oil is extracted from oilseeds) is used as cattle feed

and fertilizer. Oils like that of linseed oil are in great demand for industrial purposes

such as lubricants, varnishes and paints. Oilseeds are the largest source of vegetable

oils even though most oil bearing tree fruits provide the highest yields (eg. Olive,

palm and coconut) (Gunstone, 2002). Oilseeds are also used as animal feed because

of their high protein content. Their seeds contain energy for their sprouting embryos,

mainly as oil as compared to cereals, which contain the energy in the form of starch

(Lucas, 2000).

4.2.3.i Mustard Seeds

Hindi Name Sarson

English name Mustard seeds

Botanical name Brassica juncea

Family name Cruciferae

Mustard Seeds are tiny round seeds, having an interesting aroma and exotic

flavor. These seeds are used in Indian cooking as a spice. There are three types of


mustard seeds, white, black and brown. All have different uses, taste and preservative

qualities. Mustard seeds are famously used for tempering or giving tadka in dal or

lentil recipes, they add extra flavor and aroma in any type of dal. Whole mustard

seeds are used for making variety of Indian pickles. Mustard plasters or poultices are

medically proven best and they are applied to the chest to aid in clearing the sinuses

and decongest the lungs. Mustard seeds contain sulphur, that has been used as an

effective treatment for skin diseases. Mustard seeds not only helps in stimulating the

appetite, but they also contain digestive, laxative, and antiseptic properties. Rapeseed

and mustard play an important role in oil seed production, as they form the major

group of winter oilseed Crops that contribute majorly in domestic edible oil

production in India.

Though used as regular cooking oil in rural India and certain other parts of

the country particularly for cooking fish, the rape-seed and mustard oil are not used

in regular cooking due to the presence of higher contents of erucic acid and

glucosinolates in them. Glucosinolates are sulphur containing compounds that occur

pre-dominantly in brassica spp. These substances can lower rapeseed cake palatability

and thus produce a range of nutritional disorders in farm livestock. Even then,

oilseed rape (Brassica napus L.) has become one of the most important oil crops and

at present, is the third largest source of vegetable oil all over the world. Conventional

rapeseed and mustard varieties impose health concerns due to the presence of erucic

acid in oil and glucosinolate in meal (Manaf and Hassan, 2006).

4.2.3.ii Soybean


Hindi Name Soyabean

English name Soybean

Botanical name Glycine max

Family name Fabaceae

The soybean is the most important and cheapest bean which provides

vegetable proteins for millions of people throughout the world and is an ingredient in

hundreds of chemical products. It is highly nutritious and easily digested food of the

bean family. It is a staple in the diet of people and animals in numerous parts of the

world today. Soybeans are a good source of proteins for diabetics since they contain

no starch. In East Asia the bean is extensively consumed in the forms of soybean milk,

a whitish liquid suspension, and tofu which is a curd somewhat resembling cottage

cheese. Soybeans are also sprouted for use as a salad ingredient or as a vegetable and

may be eaten roasted as a snack food. Soy sauce, a salty brown liquid, is produced

from crushed soybeans and wheat that undergo yeast fermentation in salt water.

4.3 Material Procurement

Grains of wheat (UP 2382), Pearl millet (HHB 62), Chickpea (RSG 888),

Barley (RD 2508) required for the present studies were obtained from Durgapura

Agriculture Research Station of Rajasthan Agriculture University, Bikaner whereas

the samples of rice (Parmal PR 11), green gram (Mani), sorghum(CH5) and soybean

were purchased from the local market. The samples of mustard seeds(MAYA) were

obtained from the National Mustard Research Centre, Sewar, Bharatpur.

4.4 Sample Preparation

4.4.1 For Powder Form

It is difficult to measure the dielectric properties of the food grains in whole

grain form because of the irregular shape of the grains and large air gaps formed

between them. The measurement errors are reduced by using a grinded sample of

seeds (Nelson,1991; Trabelsi and Nelson, 2006). To determine dielectric properties

of food grains accurately, powder of food grains or flour is therefore taken. Sample

of particular grain size is then obtained with the help of sieves. The grinded flour is


placed in the dielectric cell specially constructed for powder and compressed by

using a hydraulic press to a nominal pressure which the cell can safely withstand.

In the present study, samples of all food grains were prepared by grinding the

food grains and sieving the same through sieves with mesh size 300 micrometers

and then through mesh size 250 micrometers so as to obtain sample with grain size

250 μm and 300μm. The sample of mustard seeds was prepared by crushing the seeds.

4.5 Experimental Setup for Determination of Dielectric Properties

The dielectric constant (ε') and loss factor (ε'') of the samples prepared as

above of five varieties of cereals, two varieties of pulses and legumes and two

varieties of nuts and oilseeds were determined at four microwave frequencies lying

respectively in X, C,J and Ku bands by two point method. The experimental details

for the same are given below:

4.5.1 Dielectric Cell

For the present study, dielectric cells for microwave benches of different

frequency bands (viz., X,C, J and Ku bands) were fabricated by closing a wave guide

piece of certain height for the relevant band at one end by a short circuiting metallic

plate and attaching wave guide flange at the other end so as to connect it to the slotted

section by means of a E-plane bend. The powder of food grains can be compressed in

the cell by means of a plunger of almost the same cross section as the wave guide, by

using a hydraulic press. Wall thickness of the cells was increased so that it could

withstand pressure applied by the hydraulic press to compress the food powder. The

interior walls of the dielectric cell were silver plated so as to ensure good conductivity.

Fig 4.1: Specially designed Dielectric cell for X band


4.5.2 Experimental Details

4.5.2.i For Samples in Powder Form

The dielectric constant (∈') and dielectric loss factor (∈") of food grain

samples were measured at four frequencies 4.65 GHz (C-band), 7.00 GHz (J band),

9.35 GHz (X-band) and 14.98 GHz (Ku band) in the microwave range, using

microwave benches for these bands and employing the two-point method. The reflex

Klystrons are used to generate microwave power at frequencies in X, J and C bands,

while a Gunn diode is used to generate microwave power at Ku band frequencies.

The experimental set up using Klystron tube and Gunn diode is shown in Fig (4.2)

and Fig. (4.3) respectively.

Fig. 4.2: Experimental set up for two point method at X band

microwave frequency using klystron.

Fig. 4.3: Experimental set up for two point method at Ku band

microwave frequency using gunn diode.


4.5.2.i.a Theoretical Formulation and Procedure

First, with no sample in the dielectric cell, the position of the first minimum

DR in the slotted line was noted . Now the food grain sample in powder form was

filled in the cell and compressed by using the hydraulic press, the height of sample

in the dielectric cell being l , and the sample touching the short-circuited end over the

full cross section. To obtain consistent and reproducible results, the upper surface of

the powder in the cell was maintained correctly horizontal in the vertical position of

the cell and the height of the sample was measured accurately. Then with powdered

sample in the cell, the position of the first minimum D on the slotted line was noted

and the corresponding VSWR was measured (Figure 4.4) using a VSWR meter.

(a) (b)

Fig. 4.4: Position of minima with and without sample

The readings were taken in triplicate and mean value along with standard

deviation were calculated. In two point method, the complex dielectric constant is

given by

j

j

1 e1 tan XC

j l 1 e X

(4.1)

This transcendental equation when solved, provides several solutions for

X θ, which were obtained by using a mathematical tool, MATLAB. The experiment

was repeated with a different length of the sample and the common root was chosen

from the two sets of solutions for evaluation of the admittance. The normalized

admittance (Yε ) of the material of the sample is given by

XY 2( 90 ) G jS

l (4.2)


Where Gε and Sε are respectively the normalized conductance and normalized

susceptance of the sample.

The values of Gε and Sε were obtained by separating equation (4.2) in to real

and imaginary parts, from which the values of ε' and ε'' can be calculated by using

the following relations:

2

g

2

g

G ( / 2a)'

1 ( / 2a) (4.3)

2

g

S''

1 ( / 2a) (4.4)

The accuracy of results for measurement of dielectric constant(ε') by this

method was is estimated to be within 5% and for dielectric loss factor (ε''), it was

found to be within 10%.

4.5.2. i.b. Frequency Dependence of ε' and ε''

For frequency (f) dependence, the values of ε' and ε'' of each sample are

determined at four different frequencies in C, J, X and Ku microwave bands and

then ε'- f and ε''-f graphs are plotted for each sample to learn the effect of frequency

variation on the dielectric properties of the above mentioned three classes of materials.

4.5.2.ii For Samples in Whole Grain Form

The complex permittivity (ε*) of the mixture is determined by using two

point method. First, with no sample in the dielectric cell, the position of the first

minimum DR in the slotted line was noted. Now, whole wheat is filled in the cell and

compressed by using the hydraulic press, such that the height of wheat sample in the

dielectric cell is l , and the sample is touching the short-circuited end over the full

cross section. The new position of minima is noted and the shift in minima position

is calculated. The value of VSWR is also measured. This procedure is repeated with

wheat filled upto a different height lε'. The values of dielectric constant (ε') and

dielectric loss (ε'') for wheat – air mixture is then calculated using equations

(4.2),(4.3) and (4.4) .


In the present work two component dielectric mixture equation viz., complex

refractive index equation is used to determine the dielectric properties of wheat in

whole form, in which ε represents the complex permittivity of the mixture, ε1 is the

complex permittivity of the medium (air in the present case, for which ε1 = 1 _

j0), in

which particles of the solid material having complex permittivity ε2 are dispersed, v1

and v2 being the volume fractions of the medium (i.e., the air in this case) and the

solid material (i.e. grains of wheat in this case) respectively, such that v1 + v2 = 1

Complex Refractive Index Mixture Equation:

1/ 2 1/ 2 1/ 2

1 1 2 2( ) v ( ) v ( )

(4.5)

In order to determine the volume fractions v1 and v2 required for the above

equation, the dielectric cell was fully filled with whole grains of wheat. These grains

were weighed and the volume occupied by them was determined by the liquid

displacement method (Guo et al., 2008).Toluene (C7H8) was used as the immersion

liquid, instead of water, to avoid absorption by the samples. Toluene also fills

shallow dips in a seed due to its low surface tension (Ogut,1998). Since, toluene

(C7H8) shows little tendency to soak into the sample, flows smoothly over the sample

surface, and has stable specific gravity and viscosity, it was used as the displacing

liquid. The volume occupied by the wheat grains was measured by immersing them

in 100ml of toluene and observing the displacement. These measurements were

replicated three times and the mean volume (V2) occupied by wheat grains was

calculated. Total volume (V) of the dielectric cell was calculated using the parameters

„a‟, „b‟ and „h‟ of the respective dielectric cell where „ a‟ is the waveguide width, „b‟

is waveguide breadth and „h‟ is the height of the dielectric cell. The difference

between the total volume and volume of wheat grains was considered to be the

volume occupied by air (V1) .The volume fractions are then calculated as v 1 = V1/V

and v2= V2 /V for air and wheat grains respectively.

4.6 Results and Discussion

The Dielectric properties of the three classes of different food grains,viz., (a)

cereals and grains : barley, rice, sorghum, pearl millet and wheat,(b) pulses and


legumes : chickpea, green gram and split green gram,and (c) nuts and oilseeds :

mustard seeds and soyabean were determined at room temperature by employing a

specially designed dielectric cell as mentioned above, connected to microwave

benches of X, C, J and Ku bands by means of E-plane bends. Food powder samples

were filled and compressed in the dielectric cell by means of a piston of appropriate

cross sectional area and the hydraulic press. Two point method was used for

determination of dielectric properties of food grain powder.

4.6.1 Cereals and Grains

The results obtained for five species of cereals for grain size 250 to 300

micrometers are displayed in Table 4.1.

Table 4.1: Frequency dependence of dielectric properties of Cereals at pressure

(19.6 x 104 Newton / m

2 ) and at room temperature (28°C)

Frequency

Food Grains

4.65(GHz) 7.00(GHz) 9.35(GHz) 14.98 (GHz)

ε' ε'' ε' ε'' ε' ε'' ε' ε''

Barley

(RD 2508)

4.55 ±

0.09

0.40 ±

0.02

3.89 ±

0.02

0.35 ±

0.02

2.28 ±

0.07

0.28 ±

0.01

1.86 ±

0.05

0.20 ±

0.010

Rice

(Parmal PR11)

2.34 ±

0.07

0.25 ±

0.01

1.91 ±

0.04

0.17 ±

0.01

1.60 ±

0.05

0.13 ±

0.01

1.39 ±

0.04

0.04 ±

0.004

Sorghum

(CH 5)

4.23 ±

0.07

0.34 ±

0.02

3.85 ±

0.10

0.27 ±

0.01

3.16 ±

0.08

0.24 ±

0.01

2.68 ±

0.07

0.16 ±

0.010

Pearl Millet

(HHB 62)

4.31 ±

0.15

0.64 ±

0.03

4.03 ±

0.06

0.58 ±

0.02

3.03 ±

0.06

0.41 ±

0.01

2.48 ±

0.07

0.03 ±

0.005

Wheat

(UP 2382)

4.28 ±

0.08

0.32 ±

0.01

3.97 ±

0.12

0.20 ±

0.02

3.63± 0

.09

0.18 ±

0.01

1.37 ±

0.05

0.08 ±

0.005

From the table, it is apparent that for all the samples, both the dielectric

constant (ε') and dielectric loss (ε'') decrease with increase in frequency, which shows

that these materials exhibit dielectric dispersion in these material at microwave

frequencies. Higher values of dielectric constant (ε') at lower frequencies may be

attributed to higher polarizability of the dipoles at the lower frequencies and the

higher values of dielectric loss (ε'') at lower frequencies can be attributed to increase

in ionic and surface conductivity at these frequencies.This view is supported by the


fact that at higher frequencies penetration depth decreases, causing the effect of

ionic conductivity in the bulk of the material to decrease. Increase of dielectric loss

(ε'') due to increase in ionic conductivity at lower frequencies is apparent from the

definition of loss factor (ε'') given by Magario and Yamaura (1988)

ε''=ε''d + ζi / ωε0

The ionic loss is inversely proportional to frequency and becomes critical at

lower frequencies. At higher frequencies, the dipolar energy dissipation is the

predominant loss, which dominates the ionic loss. Thus, the combination of ionic

and dipolar polarization losses result in decrease of the numeric value of losses at

higher frequencies.

Fig 4.5 (a) shows the effect of frequency variation on dielectric constant (ε')

of barley at room temperature (27°C). The dielectric constant decreases with

increase in frequency. The dielectric constant (ε') bears a quadratic relationship with

frequency with coefficient of determination R2 = 0.953. Frequency variation of

dielectric loss factor (ε'') of barley is depicted in fig 4.5(b) .The variation of loss

factor with frequency is observed to be linear with R2 = 0.974, showing that the

losses decrease with increase in frequency, which may be attributed to decrease in

the penetration depth with increase in frequency. The frequency dependence of the

dielectric constant(ε') of rice is presented in fig 4.6(a). It is observed that the best fit

to the variation can be represented by a quadratic curve with R2

= 0.943. Fig 4.6 (b)

shows the variation of dielectric loss factor (ε'') with frequency for rice in powder

form. The trend of variation is again linear with R2 = 0.965. Similar behavior is also

shown by ε' and ε'' of sorghum and pearl millet as shown by Fig 4.7 and 4.8

respectively for these cereals. The ε' varies with frequency in a quadratic manner

with R2 =0.943 both for pearl millet and sorghum, whereas the variation of ε'' with

frequency f is linear for both of these cereals with R2

= 0.961 and 0.981 respectively

for the two cases. The variation of dielectric constant (ε') with frequency for wheat is

depicted in Fig 4.9 (a). Here also the variation is observed to be second order

function of frequency with R2= 0.998,but the curvature is found in this case to that

of inverted parabola, whereas for other cereals the ε' – f curves have the shapes like


an arc of a normal parabola. The variations of dielectric loss (ε'') with frequency for

wheat is shown in Fig. 4.9 (b),from where it is observed that whereas the behavior of

dielectric loss factor (ε'') for change with frequency is observed to be linear in all

other cereals, it is found to be almost parabolic for wheat, where the variation is

given by a quadratic function with R2= 0.990. This change in the behavior observed

in wheat may be attributed to the compositional difference which exists among all

the cereals. It is obvious from these graphs that out of the five cereals studied, pearl

millet shows the sharpest change in dielectric loss (ε'') with frequency, whereas the

slowest change is observed for rice, ε'' for wheat showing a quadratic decrease with

frequency. On the other hand, on comparing ε' – f curves for the five cereals, it is

observed that the quadratic fall is sharpest for barley and slowest for wheat for

which an inverted parabola is obtained.

The present results for five cereals are in qualitative agreement with the

frequency dependence of ε' and ε'' for corn reported by Nelson (1977) in the

frequency range 1 to 11 GHz. Harvey and Hoekstra (1972) and Wang et al. (2003)

observed that bound water plays a major role in dielectric heating in the frequency

range between 20 GHz and 30 GHz at room temperature (20°C) for a medium with

low-moisture . Water molecules bound to the surface of polar materials in

monolayers or multilayers have much longer relaxation times than those for free

water molecules. For example, the relaxation time of bound water in different food

materials at 20 °C has been observed to be between 0.98 and 2.00 ns, corresponding

to a peak in ε''-f curve at 100 MHz, whereas the relaxation time of free water in

those foods at 20°C has been reported to be between 0.0071 and 0.00148 ns,

corresponding to a peak in ε''-f curve at around 16000MHz ( Mashimo et al,1987).

This is confirmed by the present observations, as no peak is observed in the ε'' – f

graphs for any of the five cereals in the frequency range 3 GHz to 15 GHz. It may be

expected that if we extend the frequency range on the two sides or perform the

experiment in the wide range of frequencies by using a coaxial test probe and

Network Analyzer technique, we may certainly obtain resonance peaks due to bound

water and free water present in the cereals under investigation.


(a)

(b)

Fig. 4.5 : Variation of (a) dielectric constant (ε') and (b) dielectric loss (ε'') with

frequency (f in GHz ) for barley in powder form

R² = 0.9538

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

Co

nst

an

t (ε

')

Frequency (f) in Ghz

Barley(RD 2508)

R² = 0.9744

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

Die

lect

ric

loss

(ε'

')

Frequency (f) in GHz

Barley (RD 2508)


(a)

(b)


frequency (f in GHz) for barley in powder form

R² = 0.9435

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

Co

nst

an

t(ε'

)


Rice ( Parmal PR11)

R² = 0.9658

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

loss

(ε''

)


Rice ( Parmal PR11)


(a)

(b)


frequency (f in GHz) for sorghum in powder form

R² = 0.9435

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

con

sta

nt(

ε')


Sorghum (CH 5)

R² = 0.9658

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

loss

(ε''

)

Frequency (f)in GHz

Sorghum CH 5


(a)

(b)

Fig 4.8: Variation of (a) dielectric constant (ε') and (b) dielectric loss (ε'') with

frequency (f in GHz) for pearl millet in powder form

R² = 0.9435

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

con

sta

nt(

ε')


Pearl Millet (HHB 62)

R² = 0.98170.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

loss

(ε'

')


Pearl Millet (HHB 62)


(a)

(b)

Fig 4.9: Variation of (a) dielectric constant (ε') and (b) dielectric loss (ε'') with

frequency (f in GHz) for wheat in powder form

R² = 0.9982

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

con

sta

nt

(ε''

)


Wheat (UP 2382)

R² = 0.9902

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

Die

lect

ric

loss

(ε'

')


Wheat (UP 2382)


4.5.2 Pulses and Legumes

The results for ε' and ε'' of chickpea, whole green gram and split green gram

for grain size 250 to 300 micrometers at the four microwave frequencies viz., 4.65

GHz (C- band), 7.00 GHz (J- band), (.35 GHz (X- band) and `14.98 GHz(Ku –band)

as obtained from the present studies are displayed in Table 4.2.

Table 4.2 : Frequency dependence of dielectric properties of pulses in powder

form at pressure (19.6 x 104

Newton / m2 ) and at room temperature (28°C)

Frequency 4.65 (GHz) 7.00 (GHz) 9.35 (GHz) 14.98 (GHz)

Food Grains ε' ε'' ε' ε'' ε' ε'' ε' ε''

Chickpea

(RSG 888)

4.52 ±

0.16

0.38 ±

0.02

3.57 ±

0.06

0.36 ±

0.01

2.78 ±

0.08

0.29 ±

0.01

1.97 ±

0.04

0.10 ±

0.004

Green Gram

(Mani)

4.04 ±

0.11

0.33 ±

0.02

2.87 ±

0.06

0.28 ±

0.01

2.50 ±

0.07

0.26 ±

0.01

1.99 ±

0.07

0.09 ±

0.003

Split Green

gram (Mani)

3.83 ±

0.11

0.31 ±

0.01

2.30 ±

0.07

0.25 ±

0.02

2.05 ±

0.05

0.20 ±

0.01

1.78 ±

0.05

0.10 ±

0.004

It is observed from Table 4.2 that the values of both dielectric constant (ε')

and dielectric loss (ε'') decrease with increase in frequency. It is also seen that the

values of dielectric constant (ε') and dielectric loss factor (ε'') for split green gram

are less than those of whole green gram at all the four frequencies. This shows that

the values of ε' and ε'' of the skin of the gram gram should be higher than the value

of dielectric constant (ε') and dielectric loss factor (ε'') of its interior. The graphical

representation of the frequency variation of ε' and ε'' of the above mentioned three

sample are shown in Figure 4.10, 4.11 and 4.12 respectively. It is seen from figures

marked (a) that the dielectric constant varies with frequency following a quadratic

pattern for all three pulses with R2

= 0.999, 0.980 and 0.948 respectively ; the ε'- f

curves being arcs of parabola in all the three cases. On the other hand from figures

marked (b) of these diagrams, it may be observed variation of ε'' with frequency is

almost linear for all the three cases, the value of the fitting coefficient R2

being

0.967,0.966 and 0,992 respectively, the slope of the line being highest for chickpea .

It is seen that the pattern of variation with frequency shown by ε' and ε'' of pulses is

similar to that observed for cereals and food grains. This may be because at

microwave frequencies, the bound water present in food grains and pulses dominates

in the dielectric response of these materials and the percentage of moisture content


in the samples of two classes of materials is also approximately the same. Therefore,

the values of dielectric constant (ε') and dielectric loss (ε'') at a particular frequency

do not show large variations.

(a)

(b)

Fig. 4.10: Variation of (a) dielectric constant (ε') and (b) dielectric loss (ε'') with

frequency (f) for chickpea in powder form.

R² = 0.9997

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

con

stan

t(ε'

)


Chickpea (RSG 888)

R² = 0.9679

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

Die

lect

ric

loss

(ε'

')


Chickpea (RSG 888)


(a)

(b)

Fig. 4.11: Variation of (a) dielectric constant (ε') and (b) dielectric loss (ε'') with

frequency (f) for whole green gram in powder form

R² = 0.9803

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

con

sta

nt(

ε')


Whole Green Gram (Mani)

R² = 0.9669

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

Die

lect

ric

loss

(ε''

)


Whole Green Gram (Mani)


(a)

(b)

Fig 4.12 : Variation of (a) dielectric constant (ε') and (b) dielectric loss (ε'') with

frequency (f) for whole green gram in powder form

R² = 0.8674

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

con

sta

nt

(ε')


Split Green gram (Mani)

R² = 0.9927

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

loss

(ε''

)


Split Green gram (Mani)


4.5.3 Oilseeds

The results obtained for mustard seeds in crushed form and soybean in

powder form on using two point method at four microwave frequencies viz., 4.65

GHz in C band,7.00 GHz in C band,9.35 GHz in X band and 14.98 GHz in are

displayed in Table 4.3.

Table 4.3: Frequency dependence of dielectric properties of oilseeds in powder

form at pressure (19.6 x 104

Newton / m2 ) and at room temperature (28°C)

Frequency 4.65 (GHz) 7.00(GHz) 9.35(GHz) 14.98(GHz)

Food Grains ε' ε'' ε' ε'' ε' ε'' ε' ε''

Mustard seeds

(Maya)

4.88 ±

0.10

0.33 ±

0.01

3.97 ±

0.08

0.31 ±

0.02

2.31 ±

0.07

0.22 ±

0.01

1.35 ±

0.04

0.05 ±

0.003

Soybean 3.82

0.12

0.36

0.06

2.63

0.13

0.24

0.05

2.42

0.19

0.15

0.07

1.15

0.08

0.04

0.003

As can be seen from the Table 4.3, the dielectric constant(ε') and dielectric

loss (ε'') both show a decrease with increase in frequency. The low values observed

for the dielectric properties of oilseeds indicate that they also have low moisture

contents. The oilseeds have high contents of fats, which show less activity for

microwave radiation. Ryynanen(1995) observed that the increase in fat content

causes the free water content in the system to decrease, which leads to reduction in

the dielectric properties of the system. This may be considered to be the possible

reason for low values of dielectric properties of oilseeds, with the difference that we

may now consider bound water rather than free water in oilseeds.

The pattern of variation of ε' with frequency is observed to be quadratic with

R2

= 0.973, as shown in Fig. 4.13(a). The ε'' of mustard seeds however varies

linearly with frequency and hence can be shown by a straight line with R2 = 0.976 in

Fig. 4.13(b). The line showing the variation for dielectric loss factor (ε'') for mustard

seeds is steeper than that for chickpea, green gram and split green gram. The

variation of dielectric properties with frequency for soybean in powder form is

depicted in Fig. 4.14. It is observed from Fig. 4.14 (a) that the dielectric constant (ε')

varies in a quadratic manner with frequency with R2

= 0.973. The dielectric loss (ε'')

shows linear variation with frequency. It can be inferred from Fig. 4.13 and Fig. 4.14

that the behavior shown by mustard seeds and soybeans is same.


(a)

(b)

Fig 4.13 : Variation of (a) dielectric constant (ε')and (b) dielectric loss (ε'') with

frequency for mustard seeds in powder form.

R² = 0.9737

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

con

sta

nt(

ε')


Mustard seeds (Maya)

R² = 0.9761

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

Die

lect

ric

loss

(ε''

)


Mustard seeds (Maya)


(a)

(b)

Fig 4.14: Variation of (a) dielectric constant( ') and (b) dielectric loss ( '') with

frequency for soybean in powder form

R² = 0.9649

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.00 5.00 10.00 15.00 20.00

Die

lect

ric

con

sta

nt

(ε')


Soyabean

R² = 0.94220.00

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

Die

lecc

tric

lo

ss (

ε'')

Frequency (f) in Gega Hertz

Soyabean


It can be concluded that dielectric properties of all food grains show a

particular kind of variation. Variation in dielectric constant is quadratic while in

dielectric loss factor is linear with frequency.

4.5.4 Wheat in Whole Grain Form

The dielectric properties of whole grains of wheat (variety UP 2382 ) were

determined by using two point method and dielectric mixture equation at the four

frequencies viz., 4.65 GHz,7.00 GHz,9.35 GHz and 14.98 GHz . First, the dielectric

properties for air – wheat mixture, represented by ε' and ε'' in Table 4.4, were

determined by employing the two point method. Then, the value of dielectric

constant (ε'2) and dielectric loss (ε''2) of wheat was computed by using dielectric

mixture equation given by equation (4.5).The values are displayed in Table 4.4

along with the values of these parameters for wheat in powder form for comparison.

Table 4.4: Comparison of Frequency variation of dielectric properties of wheat

(UP2382) in powder form and whole grain form.

Frequency

(GHz)

Dielectric

constant of

air-wheat

mixture

Dielectric

loss of air-

wheat

mixture

Volume

fraction

of air

Volume

fraction

of wheat

Dielectric

constant of

wheat in

whole form

Dielectric

loss of

wheat in

whole form

Dielectric

constant of

wheat in

powder

form

Dielectric

loss of

wheat in

powder

form

ε' ε'' v1 v2 ε'2 ε''2

4.65 4.03 ±

0.14 0.28 ±

0.01 0.38 0.62 6.79 0.58

4.28 ±

0.08 0.32 ±

0.01

7.00 2.56 ±

0.10 0.26 ±

0.01 0.35 0.65 3.67 0.48

3.97 ±

0.12 0.20±

0.02

9.35 2.28 ±

0.09 0.22 ±

0.01 0.42 0.58 3.45 0.45

3.63 ±

0.09 0.18 ±

0.01

14.98 1.31 ±

0.04 0.13 ±

0.01 0.38 0.62 1.64 0.23

1.37 ±

0.05 0.08 ±

.005

It is clear from the table 4.5 that the results obtained by two point method for

powder form and whole grain form of wheat are in agreement with each other except

for the frequency 4.65 GHz where in the values of both the dielectric constant and

dielectric loss are greater for whole grain form than the powder form. This can be

attributed to the air pockets that are formed because the size of the cell is big and

hence the sample required is also large. Both the dielectric constant and dielectric


loss decrease with increase in frequency for both the cases. This is the expected

behaviour since both have water in the bound form. The phenomenon associated

with frequency dependence of the dielectric properties is the polarization arising

from the orientation with the imposed electric field of molecules which have

permanent dipole moments. The variation of dielectric constant and loss factor of

hard red winter wheat over the frequency range 250 Hz to 12 GHz was studied by

Nelson and Stetson (1976).They also reported decrease in dielectric constant and

dielectric loss with increasing frequency in this frequency range.

4.7 Conclusion

The dielectric properties of food grains, pulses and oilseeds in powder form

can be successfully determined by using two point method. The values obtained by

using this method are found to be in agreement with the values obtained by other

methods. This method can also be used to determine the dielectric properties in

whole grain form. It can be inferred that dielectric properties of food grains, pulses

and oilseeds decrease with increase in frequency. It is observed that the grains

belonging to the same group show approximately same trend of variation of

dielectric properties with frequency. It is also concluded the values obtained for

wheat in powder form and whole grain form by two point method are in agreement

with each other.

CHAPTER - 4 DIELECTRIC PROPERTIES OF FOOD GRAINS ...

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