Indian Journal of Pure & Applied Physics

Vol. 39. August 200 1 . pp. 532-540

Dielectric properties of different varieties of

rapeseed-mustard oil at different temperatures

A K B ansal, P J Singh & K S Sharma*

Department of Physics, M SJ College, Bharatpur 321 00 I

and

Satyanshu Kumar & P R Kumar

National Research Centre on Rapeseed-Mustard. Sewar. Bharatpur 321 00 1

Received 5 February 200 1 : revised 1 6 May 200 1 : accepted 1 9 June 200 1

Dielectric properties of five rapeseed-mustard oil samples having different percentage of erucic acid have been studied at 1 00 kHz. 8.93 GHz and optical frequency of sodium light. The dielectric constant at optical frequency (E 00) is almos

,l the same for all the fi":7 samples. while the dielectric constant at 1 00 kHz (E O)' the dielectric constant at 8.93

GHz (E ) and the loss factor (E ) show dependence on erucic acid content. but no systematic variation with change in erucic acid is obtained which may be attributed to the presence of other fatty acids in these samples. Temperature variation of dielectric properties has �Iso be�p investigated in the range 303 to 333 K. E O and E 00 decrease regularly with increase in temperature. while E and E do not show ,a regul�r variation with temperature, except for Hyola-40 1 . which contains zero level of erucic acid. I n other samples E and E show a maximum or minimum type of behaviour at about 3 1 3 K. which may be attributed to the presence of some type of molecular resonanceiantiresonance at this temperature. The molar free energy 'of activation. (�F E ) and the macroscopic relaxation time ('tm) have also been worked out whose temperature dependence requires consideration of other fatty acids also.

1 Introduction

The dielectric properties of agri-food materials and their constituents describe their molecular interaction with electromagnetic energy, and depend on the frequency of electromagnetic field as

wel l as on the bulk and microscopic properties of the materials and their composition i . It is therefore important to know the dielectric properties of materials for the development of microwave process and control . Recently dielectric behaviour of albumin and yolk of avian egg was studied by Lokhonde et aU and for o i l seed cakes by Sharma and S ingh3 and of soyabeen oil and mustard oil by Khanna and Upadhyal at microwave frequencies. However l ittl e i nformation is available for many agri -food materials. Rapeseed-mustard oi l is one of the most important edible oils being consumed

*Directorate of College Education. Jaipur. Rajasthan

largely in India and elsewhere. Rapeseed-mustard group of oi ls are comprised of different varieties like Hyola-40 1 , T-27, NRCT-93, PCR-7 and BSH­

I etc. These samples are having different fatty acid composition. Fats and oils are triesters of glycerol and acids. These are known as triglycerides. The acids making up the triester are called fatty acids and have very long chains, most of them in the range of 1 2-22 atoms of carbon.

In the fatty acid composition of rapeseed­mustard varieties, erucic acid has got i mportance because of its applications in lubrication industries, manufacture of plastic, perfumes etc. and its harmful effects on human beings. Medical scientists find it responsible for the diseases like mycocardial fibrosis and hypocholesterolemia) . The rapeseed-mustard o i l , is characterized by high tevels of erucic acid (upto 55%). Efforts are being

BANSAL et at. : DIELECTRICS OF RAPESEED-MUSTARD OIL 533

made to reduce the level of erucic acid to less than 3% to make it fit for human consumption6.

In the present study we have determined the dielectric properties of different varieties of

rapeseed-mustard oil having different percentage of erucic acid ranging from zero to 53.33. The correlation of dielectric properties with the level of erucic acid may be useful to the agriculture scientists and help in developing varieties of rapeseed mustard whose oi l may be good for

human health. The findings may also be usefu l to medical scientists since the consumption of high

erucic acid oi l is reported to be unsafe for human beings.

The macroscopic relaxation time ('tm) and the

molar free energy of activation (M'E) have also

been worked out for these samples, which may provide valuable information for future research in this field.

2 Experimental Details

For the present investigations different samples of rapeseed-mustard of crop year 1 999 namely Hyola-40 1 , T-27, NRCT-3, PCR-7, BSH- I have been procured from National Research Centre on rapeseed-mustard located at Bharatpur (India). Fresh oil from these seeds was obtained by using a baby expeller. This oi l was then filtered by fine filters. To get the fatty acid profile one drop of oi l was taken in capped glass test tube. To it 1 ml of petroleum ether (60-80 °C) was added and mixed

with very slowly. After keeping for one hour at room temperature 1 .5 ml of 0.05 M sodium methoxide was added to it and kept at room temperature for one ·hour. Then 1 .5 ml of 8% sodium chloride solution was added to it . Layers were separated . I 111 from upper layer was injected on OLC (NUCON 5765 model) . Fatty acid profile were obtained using SP-2300 (2%) + SP-23 1 O (3%) stain less steel column using flame ionization detector (FID) . The oven temperature was maintained at 240 °C and the injector and the detector were both maintained at 290 0c.

The dielectric constant E () at 1 00 kHz was

measured using a dipole meter by directly measuring the Capacitances and Calibrating i t for standard l iquids. The dielectric constant at optical frequency was obtained by squaring the refractive index for sodium D-l ines, measured with the help of an Abbe's refractometer. The measurements of wavelength in the d ielectric and that of voltage standing wave ratio were made at 8 .93 GHz using a slotted wave-guide and a short circuited plunger. The calculation of dielectric permitti vity (E ") and dielectric loss (E ") were made fol lowing the method of Heston et aU adopted for short-circuited terminations. The values of E · and E ·' so obtained were accurate within ± 1 % and ± 5% respectively. All these measurements were made at four temperatures : 303, 3 1 3, 323 and 333K using a temperature regulating system and a constant temperature water bath . The temperature was control led electronically within ± 0.5 0c.

3 Theory

The macroscopic relaxation time is determined by using the equation

E ' = E + -----

where E * is the complex permittivity at angular

frequency 0).

* t It

On using E = (E - je ) and separating real

and imaginary parts, we get

E = E + . . . ( 1 ) 1 + oY 't 2 m

and E " = (2) 1 + oY 't 2 m

534

Variety

H yola-40 1

T-27

NRCT-93

PCR-7

BSH- I

\

\ \ I

\ \ \ \

• -

Palmitic

1 6:0

3.30

3 .99

2.30

2.26

1 .89

INDIAN J PURE & APPL PHYS, VOL 39, AUGUST 200 1

I ('or>

Tahle I - Fatty acid composition (%) in different samples

Stearic

I R :O

Tr*.

0.80

0.20

Tr*.

Tr*.

(Crop year 1 999) of rapeseed-mustard oi l

Oleic

1 8 : 1

70. 7 1

1 9.45

1 3 .75

1 2.82

33.86

Linoleic Linolenic + Eicosenoic Erucic

1 8 :2 1 8 : 3 + 20: 1 22: I

1 8.7 1 7.06 NIL

9.50 9.75 46.4R

1 4.06 1 5 .00 53.33

1 9.09 20.86 44 .94

1 4.79 1 7 .20 32.24

Substituting the value of E ll' E , E and ill in

Eq. ( I ), we may obtain the value of till (macroscopic relaxation time). Molar free energy of activation is determined by Eyring' s equationX for the dielectric relaxation

o I

J LLJ :;,: =

R e�e n t i o n ·r i ' rl e ( J\...1 i n _> -­u-, ci

!::a -!- � r9' . � R e t e n t i o n T i m e ( 1V1 i n . ) Fig. J - Chromatogram of Hyola-40 I

Fig. 2 - Chromatogram of T-27

BANSAL et al.: DIELECTRICS OF RAPESEED-MUSTARD OIL 535

'to1 = (A / T) exp(� FE / RT) where A = hlk, R = Gas constant and T = Temperature (K) and 11 FE is molar free energy of act ivation .

4 Results and Discussion

I o .-

R ete ntio n T i m e (l'vf i n . )

Fig. ) - Chromatogram of NRCT-93

Table t shows the fatty acid composItIon In different samples under study. The chromatogram of Hyola-40 1 , T-27, NRCT-93 , BSH- I and PCR-7 have been shown in Figs t to 5 respectively. Peaks corresponding to different retention times correspond to different fatty acids present in these

samples. Table 2 shows different fatty acids corresponding to different retention t ime (in min).

� o

l I

I I I

I \ I ,

o ......,

L.LJ :3 o

• � I

"¢ R e t e n t i o n T i rn c ( i'\.-1 i n . )

Fig. 4 - Chromatogram of BSH- I

536 INDIAN J PURE & APPL PHYS, VOL 39, AUGUST 200 1

Table 3 shows the values of dielectric constants at 1 00 kHz, 8.93 GHz, at optical frequency of sodium l ight, loss factor (E ") along with relaxation

t ime ('till) and molar free energy of activation (�E ) for different varieties of rapeseed-mustard oil at

303, 3 1 3, 323 and 333K. Table 4 presents the

standard deviation in the values of E o, E I , E �, E ",

I o

I ....... I

N I

r0 I

.q ,

In

R e t e n t i o n T i m e e M i n . )

Fig. 5 - Chromatogram of PCR-7

'tm and �E at 303, 3 1 3, 323 and 333K. It is seen that at each temperature Hyola-40 J which

contains zero percentage composition of erucic acid has maximum value of E () and E � which is not

correct for E ' and E ". The dielectric constant at

optical frequency (E �) is almost the same for al l

samples, while E o' E ', E " show dependence on

erucic acid content but no systematic variation with change in erucic acid is obtained which may be attributed to the presence of other fatty acids whose molecular rotations may dominate over that of erucic acid. In order to explain the complete dielectric behaviour of the oi l samples, we may be required to establ ish a correlationship between the dielectric properties of oils and the fatty acids present in the samples, as every fatty acid will contribute to dielectric properties according to its own molecular structure individually .

Table 2 - Retention time (min.) of different fatty acids.

Retention time (min.) Fatty Acid

1 .30 Palmitic acid

1 .72 Stearic acid

1 .92 Oleic acid

2. 1 5 Linoleic acid

2.49 Linolenic acid

2.70 Eicosenoic acid

3.94 Erucic acid

It is also seen that as far as E o and E � are

concerned, we are getting a particular trend with change in temperature from 303 to 333K i .e . , E o and E � both are decreasing with increase in

temperature for each sample under study. However E '

and E " do not show a regular variation with

change in temperature from 303 to 333K for al l the samples studied, except in Hyola-40 1 which contains zero level of erucic acid. Figs 6 and 7 show variation of dielectric properties E '

and E '

with change in temperature from 303 to 333K. It is observed that for Hyola-40 1 which is having zero percentage composition of erucic acid E '

regularly decreases and E "

regularly increases with increase

BANSAL et at. : DIELECTRICS OF RAPESEED-MUSTARD OIL 537

Table 3 - Experimental values of E 0 ' E ', E n , E �, 'tm and �Fe for different samples at 303, 3 1 3, 323 and 333K.

Samp1e* : Hyola-40 1 (Zero level of Erucic acid)

Temperature ( K)

303

3 1 3

323

333

3.0937

3.072 1

3 .0562

3.03 1 2

E

2.4575

2.4480

2.4232

2.4 1 09

2. 1 992

2. 1 904

2. 1 8 1 5

2. 1 726

E n

0.3398

0.3824

0.4 1 66

0.4323

'tm(PS)

27.9

27.7

28.8

28.7

�Fe (Cal/mole)

3 1 1 4.34

3233.42

3381 .33

3505.27

Sample* : BSH- 1 (32.24% composition of Erucic acid)

Temperature (K)

303

3 1 3

323

333

3 .0092

2.9937

2.9875

2.9250

E '

2.4079

2.4262

2.4201

2.4434

2. 1 9 1 8

2. 1 8 1 5

2. 1 74 1

2. 1 658

E n

0.3489

0.4 1 76

0.4550

0.4828

'tm(PS)

29.7

29.6

27.0

23.4

�FE (Cal/mole)

3 1 52.00

3274. 1 3

3341 .79

3368.76

Sample* : PCR-7 (44.94% composition of Erucic acid)

Temperature (K)

303

3 1 3

323

333

3.0325

3.007 1

3.0062

2.93 1 2

E '

2.4048

2.3898

2.39 1 3

2.39 1 3

2. 1 955

2. 1 830

2. 1 74 1

2. 1 667

E n

0.2545

0.2808

0.27 1 7

0.27 1 7

'tm(PS)

30.9

30.8

30. 1

27.7

�Fe (Cal/mole)

3 1 75.46

3298.85

3409.67

3479.94

Sample* : T-27 (46.48% composition of Erucic acid)

Temperature (K)

303

3 1 3

323

333

3.0500

3.0375

2 .98 1 2

2.9687

E '

2.4496

2.4920

2.4766

2.4766

2. 1 963

2. 1 874

2. 1 785

2. 1 697

E n

0.3949

0.4 1 65

0.4077

0.4077

tm(PS)

27.8

23.8

23.3

22.6

�FE (Cal/mole)

3 1 1 2. 1 6

3 1 39.02

3245.28

3344.86

Sample* : NRCT-93 (53.33% composition of Erucic acid)

Temperature ( K )

303

3 1 3

323

333

3.0387

2.99 1 2

2.98 1 2

2.9625

E '

2.4992

2.44 1 8

2.4734

2.4734

'" Al l the samples under study are of crop year 1 999

2. 1 948

2. 1 844

2. 1 77 1

2. 1 682

in temperature from 303 to 333K. In other samples E shows a maXImum or minImUm type of behaviour at about 3 1 3K, which may be attributed to the presence of some type of molecular resonance/anti-resonance at this temperature. Dietary fats and oi ls consist of triglyceride or esters of glycerol and high molecular or long chain

E n

0.3439

0.3322

0.3973

0.3972

'tm(PS)

23.7

26.6

23.3

22.5

�FE (Cal/mole)

30 1 6.09

3207.64

3245.2!i

3342.8 1

aliphatic acids, both saturated or unsaturated, known as fatty acids. The fatty acids of rapeseed­mustard oi l have very long chains of carbon atoms upto 22 i n their related glycerides which we can

easi ly understand from Table I . The fatty acids and their related glycerides have molecular structure characterised by paral lel arrangement of zig-zag

538 INDIAN J PURE & APPL PHYS, VOL 39, AUGUST 200 1

carbon chains perpendicular or tilted with respect to the planes at maximum spacing formed by terminal groups!). Intermolecular association in between two triglyceride molecules is possible through their resonating structures. This intermolecular correlation may break up with increasing thermal motion and there may be a switch over type of mechanism at certain temperature due to which molecular dipoles are aligned in parel le l from anti­parellel or vice-versa depending upon different chain length structure and strength of association in

2.5200 '1""1 ----------,

2.4800

2.4600

2.4400

2.4200 I

i

2.4000 it "" .

�-A 2.3800 I I , I

300 310 320 330

-+-Series1 Hyola-401 I +Series2 BSH·1

I .... SeJies3 PCR·7 I I +Series4 T·27 !

1 +"",5 tm. 340

Fig. 6 - Dielectric constants of the five samples at 8.93 GHz. as a function of temperature

different samples. Hence, molecular resonance/ anti-resonance type behaviour may be assumed at 3 1 3K in all the samples except Hyola-40 I which has comparatively short chain fatty acids attached to its triglyceride molecules as a measure constituent. As far as E " is concerned, it also shows a maximum or minimum type of behaviour at about 3 1 3K for al l the samples though less prominent in two samples, Hyola-401 and BSH- l , which may again be attributed to the presence of some type of molecular resonance/anti-resonance at this tempe­rature. Similar results were reported by Venkatesh M.S. et at. 10. These workers determined E ' and E ' for tylose (complex food material ) ethanol and ethanol/haxane mixture and reported s imi lar behaviour of E ' and E " with change in temperature. Sengwa and Kauri I have also reported simi lar results in Oligomers of ethylene glycol showing no regular variation in E ' and E ' with temperature in this frequency range, which confirms the validity of the results of present investigations.

It is also seen from Table 3 that molar free energy of activation for each sample increases as

the temperature increases from 303 to 333K. This can be explained on the basis that as the temperature increases thermal agitation increases and hence the molecules require more energy to

come to the activated state. Molar free energy of activation (I!:.Fe) increases with increase In

temperature from 303 to 333K but th is trend is not

regular. From this it can be inferred that the presence of so many fatty acid having different molecular structure show complex behaviour regarding change in molar free energy of activation with temperature.

Tabl e 4 - Standard deviation i n the value of E 0' E '. E _, E " . 'tm and �F E at temperatures 303, 3 1 3. 323 and 3 3 3 K

Temperature E o E ' E _ E " 'tm(PS) �FE (Cal/mole)

303 0.0278 0.0349 0.0025 0.0455 2.4 54.42

, u 0.0306 0.033 1 0.003 1 0.0526 2.4 55.63

, 2 , 0.0283 0.0330 0.0027 0.0620 2.7 68. 3 1

333 0.0377 0.0337 0.0023 0.0698 2.6 69.87

BANSAL et at. : DIELECTRICS OF RAPESEED-MUSTARD OIL 539

ij100ij I �,100� i

I �,���� l J�� J1� j��

Tem�lure IKJ jj�

+Seriesi �H·l

I +SeOOsJ �.l

+Se� T'Ll

i I 1 1 I ��

Fig . 7 - Loss factor of the five samples at 8.93 GHz. as a function of temperature

Tahle 3 also shows the calculated values of macroscopic rel axation time ('tm) for the samples

under study for the temperature range between 303

to 333K. It is seen that for each sample macroscopic relaxation time ('t",) is decreasing, in

general . with increase in temperature from 303 to 333K, indicating variation in the effective length of dipole i .e . more compact clustering of molecules exists at low temperatures which would take more time for reorientation process to occur, yet the trend is not systematic which can be attributed to long chain intramolecular interactions in the triglyceride molecules of the samples under investigation .

5 Conclusions

Dielectric properties of rapeseed-mustard oi I samples show dependence on erucic acid but no systematic variation with change in erucic acid is obtained. This may be attributed to the presence of other fatty acids of different chain length attached

to the triglyceride molecules of the oil . Temperature dependence of dielectric properties. molar free energy of activation and relaxation time is also influenced by the presence of other fatty acids in the triglyceride molecules of oil and hence no systematic change with temperature is observed.

Acknowledgements

The authors are thankful to the head of the

department of physics and Principal, MSJ Col lege. Bharatpur, for providing laboratory facil ities. One of them (A K B) is indebted to the University Grants Commission, New Delhi, for providing teacher fel lowship to him.

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