Effect of Frequency and Moisture Variation on Dielectric Propertiesof Pearl Millet in Powder Form

Nidhi Bhargava1, Ritu Jain1, Ila Joshi2, K. S. Sharma1

1Department of Physics, The IIS University, SFS, Gurukul M arg, Mansarovar, Jaipur..2Department of Home Science, The IIS University, SFS, Gurukul Marg, Mansarovar, Jaipur.

Abstract

Values of dielectric constant (ε’ ), dielectric loss (ε’’), relaxation time (τ), conductivity (σ) ofPearl millet (HHB 62) were measured at room temperature at four different frequencies viz 4.65 GHz,7.00 GHz, 9.35 GHz and 14.98 GHz in microwave region, using two point method and employing aspecially designed dielectric cell for powders. Also, effect of variation of moisture content on dielectricproperties of Pearl millet in powder form was studied at room temperature at 9. 35 GHz.. Both, thedielectric constant and loss factor are found to decrease with increase in frequency and to increase withincrease in moisture content. The present values of dielectric constant are in good agreement with thevalues reported by other authors.

Keywords : Conductivity, Dielectric constant, Dielectric loss, Pearl millet, Relaxation time

1.INTRODUCTION

Dielectric properties of materials are thoseelectrical characteristics of poorly conductingmaterials that determine their interaction withelectric fields. The two properties of major interestare the dielectric constant and the dielectric lossfactor of materials. These properties are importantin any processes involving radio-frequency ormicrowave dielectric heating. They determine howwell energy can be absorbed from the high-frequency alternating electric fields and thus howrapidly the materials will be heated. Since dielectricproperties of materials are highly correlated withthe amount of water in materials, sensing thedielectr ic properties can be used for rapidmeasurement of moisture content in materials suchas agricultural products and food materials.

Pearl millet (Pennisetum glaucum) is themost widely grown type of millet. It survives in

soils with high salinity, low soil fertility anddrought. India continues to be the single largestproducer of perl millet in the world. It is highinprotein as compared to other cereals .Bajra helpsmaintain cardiovascular health and helps reduceacidity problems.

Many factors, including frequency,temperature and moisture content, influence thedielectric properties of agroproducts and foodMaterials. (Venkatesh & Raghvan, 2004).Knowledge of the relationship between frequencyand dielectric properties is helpful in determiningthe optimum frequency range in which the materialin question has the desired dielectric characteristicsfor intended applications (Nelson, 2005). Themoisture-dependent dielectric properties in specificfrequency ranges can be used to develop onlinemoisture meters (Nelson et al, 1992), which maybe applied not only in drying processes but also inother unit operations in the food industry. Severalinvestigations on dielectr ic properties ofagricultural products have been reported. Dielectricproperties of chickpea flour in compressed form

Nidhi BhargavaE-mail : nidbhargava@gmail.com

J. Environ. Nanotechnol. Volume 2 (2013) 01-05 pp.

ISSN (Print) : 2279-0748 ISSN (Online) : 2319-5541

were determined by Guo et al (2008) and itwasobserved that dielectric constant and loss factorof the sample decreased with increase in frequencyfrequency at all temperatures and moisture levels .Recently, dielectric constant and loss factor of Raj-4120 variety of Indian wheat have been determinedby Sharma et al., (2010), in powder form of grainsize 125 to 150 microns at room temperature byemploying the technique proposed by Yadav andGandhi (1991) at three different frequencies lyingin C-band, X-band and Ku-band .

The purpose of this research was to studyeffect of frequency variation and moisture contentvariation on dielectric properties of pearl millet inpowder form at room temperature.

2. MATERIALS & METHODS

Pearl millet of variety HHB 62 used forthe present study were obtained from DurgapuraAgriculture Research Station of RajasthanAgriculture University, Bikaner. It is difficult tomeasure the dielectric properties of the wholegrains because of their irregular shape. Themeasurement errors are reduced by using a grindedsample of these grains ( Nelson, 1992).These grainswere grinded and converted into flour. Samplesof grain size 250 -300 microns were prepared usingsieves of mesh size 300 microns and 250 micronsrespectively.

To obtain the samples of different moisturecontents, pearl millets were first ground and keptover distilled water in covered dessicators at roomtemperature. The flours were stirred to ensure thatthe moisture absorbtion is uniform. After keepingthe flour in dessicator for a few days for desiredmoisture content, they were sealed in plastic bagsand equilibrated at room temperature. The moisturecontent was measured using moisture analyzer

Two point method (Behari, 2005) used inthe present study is a technique involvingmeasurement of reflection coefficient of a solidmaterial placed in a wave guide, backed by a short

circuiting conducting plate. In order to use thismethod for powders, the wave guide is bentthrough90° by means of a E-plane bend andterminated by a dielectric cell in which powdersample is filled up. This method is suitable for lowand medium loss dielectrics and can be adoptedfor measurement of dielectric properties of foodstuff in powder form. In this method, the set-upfor measurement of dielectric properties of powdersis shown in Fig. 1. Let for an empty short-circuitedwave guide dielectric cell , a voltage minimum isobtained with the probe located at position DR inthe slotted section. The same waveguide dielectriccell containing the sample (powder of pearl milletin the present case) of length lε will have theprobe located in the slotted section at a newposition, say D, for a voltage minima in this case.

Fig. 1: Experimental setup for determination ofdielectric properties by two point method

Then, from impedance matching at theair –powder boundary, we obtain

Rtan (D D l ) tan ll l

(1)

Where the phase factor β = ( 2π / λg), λgbeing the guide wavelength for the waveguidecontaining air .

The phase factor βε for the waveguidefilled with the dielectric is given by

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σ = ω ε 0ε ‘’ (9)

Here λ0 represents free space wavelength,λc is the cut off wavelength of the waveguide andfor the non-magnetic materials μr =1. The phasedifference φ in the waves travelling in the guidewith and without dielectric material in the cell isgiven by

φ = 2 β (“ x – lε) (3)

where “ x is the shift in minimum.

Voltage standing wave ratio(S) isdetermined for the load (food powder in this case)and then magnitude of the reflection coefficient(%Ã%) is computed by employing the relation.

( 1)( 1)SS

(4)

In the two point method, the complexdielectric constant is given by

j

j

1 e1 tan XCj l 1 e X

(5 )

where C and Ψ represent respectively themagnitude and phase of the complex quantity inthe middle of Eq. (5) and X θ represents thesolution of this transcendental equation . Thisequation provides several solutions for Xθ, whichcan be found by employing graphs and tablesprovided for solution of such equations by Hippel[1953] or alternatively the problem can be solvedby using a computer based mathematical tool likeMATLAB/ Mathematica.

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

2XY 2( 90 ) G jSl

(6)

where Gε and Sε are respectively the conductanceand susceptance of the sample. The values of Gεand Sε are obtained by separating Eq. (6) into realand imaginary parts, which providethe values of ε’and ε’’ in the following form:

2

g2

g

G ( / 2a)'

1 ( / 2a)

(7)

2g

S''1 ( / 2a)

(8)

In the present study ,a computer programin MATLAB was written to solve the transcendentalequation and obtain the values of dielectricconstant (ε’) and loss factor (ε’’ ). The conductivity(σ) and relaxation time (τ) were obtained by usingthe following relations.

Where,ω = 2π x9.35 GHz

ε o = 8.85x 10 -12 F/m .

3. RESULTS AND DISCUSSION

The values of the dielectric constant andloss factor for pearl millet are reported in Table 1for grain size 250-300 micrometers at four differentfrequencies in frequency bands C, J , X and Ku inthe microwave region. From the table we observethat both the values of dielectric constant and lossfactor decrease with increase in frequency.

Table 1. Dielectric values of pearl millet in powderform for grain size 250 – 300 micrometers

βε = ( 2π / λ0) { ε r μ r – (λ0 / λc)2}1/2 (2)

τ = ( ε’’ / ωε’) ( 10 )

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The dependence of ε’ on frequency is shown inFig. 2, which shows that variation of ε’ is almostlinear with negative slope. This isindicative ofthe fact that as frequency is increased ,the capacityof the material to store energy decreases. Animportant phenomenon contributing to thefrequency dependence of the dielectric propertiesis the polarization of molecules arising from theorientation with the imposed electric field, whichhave permanent dipole moments. Water is themajor absorber of microwave energy in the foods.When the frequency is increased, water moleculesare not able to keep up with the changes of thedirection of the electric field, because of their inertiathat is described by relaxation time τ . This is thepossible reason for a decrease in the electric fieldenergy storage and hence the value of dielectricconstant decreases. Variation of ε’’ with frequencyis shown in Fig. 3.

The value of conductivity is found todecrease with increase in frequency showing thatpolarization in the powder decreases at highfrequencies.The relaxation time is also found todecrease with increase in frequency ,being lowestin Ku band. This is apparent from low value of ε’’at this frequency showing that polarization is lessprominent as compared to other frequencies andtherefore system takes minimum time to return toequilibrium state after the fields are removed.

Fig. 2: Variation of dielectric constant withfrequency for pearl millet

Fig. 3: Variation of dielectric loss with frequencyfor pearl millet

Fig. 4: Variation of dielectric constant withmoisture content for pearl millet.

Fig. 5: Variation of dielectric loss with moisturecontent for pearl millet

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Table II gives the values of dielectricconstant and dielectric loss factor for differentmoisture contents. The presence of free moisturein a substance greatly affects its dielectric propertiessince the dielectric constant of free water is quitehigh (78 at room temperature and 2.45 GHz). Themoisture relationship is consistent in that highermoisture leads to higher values of both the dielectricconstant and the loss factor. In general, highermoisture content results in higher dielectricconstant and loss factor of the food (Komarov etal., 2005). This is clear from Fig.4 & 5 respectively.

Table 2. Dielectric values for pearl millet inpowder form for different moisture

contents

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Guo, W., Tiwari, G., Tang J. and Wang, S.,Frequency, Moisture and Temperature-Dependent Dielectric Properties of Chickpeaflour, “Biosystems Engineering”, 101, 217-224,(2008).

Hippel, A. R.V., Hippel, Dielectric Materials andApplications, The Technology Press of M.I.T.and John Wiley and Sons, Inc.,. New York,(1954).

Komarov, V., Wang, S., Tang, J., Permittivity andMeasurement. In: Chang, K.(Ed.), The WileyEncyclopedia of RF and MicrowaveEngineering, vol. 4. John Wiley Sons, Inc., NewYork, 3693–3711 (2005).

Nelson, S. O., Measurement and Applications ofDielectric Properties of Agricultural Products,IEEE Trans. Instrum. Measur., 41, 116, (1992).

Nelson, S.O., Kraszewski, A.W., Kandala, C.V.K.,and Lawrence, K.C., High-frequency andMicrowave Single-Kernel Moisture Sensors,“Trans. ASAE”, 35 (4), 1309–1314, (1992).

Nelson, S.O., Dielectric spectroscopy in agriculture,“J. Non. Cryst. Solids”, 351, 33-36, 2940–2944, (2005).

Sharma, K., Jain, R., Bhargava, N., Sharma, R., andSharma, K.S., Dielectric Studies of Wheat inPowder form at Microwave Frequencies, “IndianJ. Exp. Biol.”, 48 (10), 1002-7, (2010).

Venkatesh, M.S. and Raghavan, G.S.V., An overviewof microwave processing and dielectr icproperties of Agri-food materials ”BiosystemsEngineering”, 88(1), 1-18, (2004).

Yadav, J.S. and Gandhi, J.M., Simple MicrowaveTechnique for Measuring the DielectricParameters of Solids and their Powders, IndianJ. Pure App. Phy., 30 ,427 -431, (1992).

REFERENCES

Behari, J., Microwave Dielectric Behaviour of WetSoil , Springer. 43, (2005).