Microwave dielectric measurements in different varieties of rapeseed-mustard seeds in powder form

Transcription

1 Indi an Journal of Pure & Applied Phys ics Vol. 39, December 200 I. pp Microwave dielectric measurements in different varieties of rapeseed-mustard seeds in powder form A K Bansal. P J Singh & K S Sharma* Department of Physics. MSJ College. Bharatpur I *Deputy Director. Co llege Education Raj asthan. Jaipu r Received 16 May 200 I: revised 5 September 200 I: accepted 30 October 200 I Effect of packing density on dielectric parameters. relaxation time and conductivity of three different varieti es of rapeseed-mustard seeds in the form of powders has been st udied at x-band (8 1 J3 GH z) microwave frequency. Experimental res ults on powders of different relative packing fracti ons (li,.) have been used to obtain transformation to 100% solid hul k using correlation formulae of Landau-Lifshitz- Looyenga and Bottcher. In case of seed grains like rapeseed -mustard where it is not possible to define soli d bulk exactly. the authors have ass umed the fin est crushed closely packed assembl y of the seed grai ns as solid bulk for the compari son of these correlation form ul ae between powder and solid bulk. This assumption works well as there is fair agreement between the calculated va lues of dielectric parameters and the values obtai ned ex perimentall y for solid bulk. Appreciable agreement between the ex perimentall y obtained va lues and the theoretical values obtai ned by Bottcher's and Landau-Lifshitz-Looyenga's form ul ae shows large cohesion in the particles of rapeseed-mustard powders under investigation. serving as a con tinuous medium. 1 Introduction The feasibility of utilizati on of microwave energy for various industrial applicati ons has long fixed the attention of speciali sts. Hi stori call y, fi eld s such as heating, drying, sterili zation, making food and a number of other processes are based on utilization of microwave energy and its action on various food products and materi als. The latest investigations revealed new applications based on more intricate effects of electromagnetic fie ld action on biological objects including those from various branches of agricultural complex. Microwaves have been of interest in thermal treatment of agri-food material s due to the high rates of energy transfer and effic iency compared to conventi onal technologies. The dielectri c properties of agri-food materi als and their constituents describe their interactions with microwave energy, and depend on the freq uency of electromagnetic field as well as on bulk and particul ar properties of the materials (e.g. packing fraction, moisture content, density, temperature and composition of materi al). So for the development of microwave process control, it is important to know the dielectric properties of the materials. Dielectric properties of clean yellow-dent fi eld corn (Zea mays L) and hard red wi nter wheat (Triticum aesti vum L) at vari ous packing densities, and of tyl ose (a complex food material) have been determined by Venkatesh et a/ 1 Dielectric properties of albumin and yolk of avian egg were studied by Lokhande et a/. 1 and of oil seed cakes by Sharma & Singh' and of Soyabean oil and Mustard oi l by Khanna & Upadhyay" at mi crowave frequency. Recentl y Bansal et au have studied the dielectric re laxati on studies of Argemone-mexicana oil at microwave frequencies. Dielectric properties of corn and wheat kernels and soyabeans were studied by Ne lson & You ~. However, little information is avai lable for many agri-food materi als. Rapeseed-mustard is one of them. The demand for protein has recently become a dominating problem in human nutrition. For this reason, oi l-bearing pl ants are now regarded to a greater extent not onl y as a source of fats but also as a source of animal feed protein or protein suitable for direct human consumption. One important pl ant is rapeseed-mustard since its seeds constitute a source of edible oil and the meal can be utilized as a livestock feed or even for direct consumption in the form of a protein concentration or isolate. Seeds of rapeseed-mustard are spherical with mm diameter. The seed compositi on of rapeseed-mustard varies widely depending on both genetic and environmental factors. Some

2 800 INDIAN J PURE & APPL PHYS, VOL 39,DECEMBER 2001 characteristics of rapeseed-mustard seed varieties are given in Table I. Table I - Moisture contents (%), oil content (%), protein(%) and glucosinolates (micro mole/gm seed meal ) in the rapeseedmustard varieties Component BI0-902 RH30 PCR-7 M oisture content % Oi I content o/n X Protein % XO Glucos inolate (micro mole/gm seed meal ) In this study the authors have determined the dielectric properti es of rapeseed-mustard seed grains at various packing fractions. Investi gations have been reported in literature to correlate dielectric parameters of powder with solid bulk This type of correlation is useful because it makes it possible to correlate without the necessity of making big enough samples of the bulk materials for the di electric measurements. In thi s paper, the authors have correlated the dielectric parameters of rapeseed-mustard seed powders with solid bulk. For the seed grains like rapeseed-mustard it is not possible to define solid bulk exactly as the rapeseedmustard seed is a living ti ssue with oil contents from 28-43%. Here it is proposed that the finest crushed closely packed assembly of the seeds may be taken as sol id bulk for the comparison of correlation formulae between powder and bulk by defining a relative packing fraction (8,.) which is defined as the ratio of density of powder and the density of the finest crushed closely packed particl e assembly keeping similar conditions of extern ally applied pressure and temperature. The authors have compared their results with the values obtained from the correlation formulae between powder and bulk derived independently by Landau & Lifshitz 11 and Looyenga 12 The relevant correlation is:... ( I ) where E, represents the dielectric constant of the materi al in bulk and E P is the dielectric constant of the powder sample at packing fraction (8). From this equation for (E "IE') << I, we can obtain the following equation: " (E "Is::) (E 'IE ') 213 E ~ - p U s p...(2) where E," and E P" are the dielectric losses for solid and powder respectively. The experimental results have a lso been compared with the values obtained from Bottcher's 1 -' formula for powder:... (3) where E, represents the electrical permittivity of the material in bulk and Ep is th e permittivity of th e powder sample at packing fraction 8. Eq. (3) can be used for dielectric loss corre lation if E, and E r are replaced by the complex quantities E,' ~je, " and E r,' ~je r" The following two equations are obtained for the bulk permittivity E, ' and loss factor E,":. (2E~, +38-2){(38 -I )(E ~: +E;, 2 )+E,, -2E;; } Ex= (38-J) 2 (E~: +E;;) +2E ~, (38-1) + 1.., '7 " " ' ".. _ 2(38 - I)(E 1 ; +E,:E 1,)+ E 1, (38-2) +4 E fie fl E,.- o 'J " 7. (38-Jt(E 1 ; +E fi-) +2E fl (38- J)+ J 2 Exper i ment~t l Details... (4)... (5) For the present investi gations different varieti es of the rapeseed-mustard seed namely BI0-902, RH- 30 and PCR-7 have been procured from National Research Centre on Rapeseed-Mustard located at Sewar, Bharatpur (India). These samples are having almost equal % oil contents and are specially developed varieties in specific agro-c li rnatic region. Values of % moi sture content, % protei n content, % oil contents and glucosinolate (micro molelgm seed meal) for all the three samples have been evaluated usi ng Near Infrared Re fl ectance spectroscopy by ~1R product analyzer (Model Dicky John 's Jnsta Lab 600). Five samples of each variety were prepared at various particles sizes on crushing the seeds by a grinder and sieving them by using specially prepared sieves of different sizes. After sieving, the samples were transferred into g lass bottles so as to avoid any moisture intake. The bottles were labelled according to grain sizes. For each powder sample densities were measured to determine the relative packing factor (8,.) i.e. the ratio of the density of powders of various partic le sizes and the density of finest cru. hed closely packed partic"le assembly. Dielectric measurements fo r E ' and E" on these powder samples at different

3 BANSAL eta/.: DIELECTRICS OF RAPESEED-MUSTARD SEEDS ROI packing fractions were carried out at x-band (8.93 conductivity (crr) along with the values of relati ve GHz) microwave frequency at room temperature packing fraction (8,) of all the three rapeseedmustard C employing the technique proposed by seed samples used in the present study. Yadav & Gandhi 7 -x. Different components for x These dielectric measurements have been carried band microwave bench were procured from M/s out at x-band (8.93 GHz.) microwave frequency and Vidyut Yantra Udyog, Modi Nagar, India. room temperature C. The values of E,'. 1 E " 11 Microwave power is obtained from a microwave and crr li sted in Table 2 show that for all the three source which is a reflex Klystron (K-27 ill-78). The value of conductivity (crr) is obtained from samples these values systematically increase with increasing values of relative packing fraction (8,). the relation crr= oo E 11 E" where oo is the angular This is to be expected because with higher values of relative packing fraction, the inter particle hindrance frequency of measurement (8.93 GHz) and E 11 is the to dipolar motion increases, giving ri se to the higher permittivity of vacuum. The relaxation time ('t ) 11 ts values of E r' E r " and cr". The values of relaxation obtained using the equation 'tr = E "/ooe '. time are al so reported in Table 2 for al l th e three 3 Results and Discussion samples under investigation. According to the theory as developed by Debye 14 dielectric relaxation Table I contains the values of % moisture is the lag in dipole orientation behind an alternating content, % protein content, % oil contents and electric field. Under the influence of such a fi eld. glucosinolate (micro mole/gm seed meal) for all the the polar molecules of a system rotate towards an three samples before crushing them to obtain in equilibrium distribution in molecular orientation different particle sizes. with a corresponding polarization. When the polar molecules are very large or the frequency of Table 2-Dielectric parameters. conductivity and relaxati on time alternating field is very high, the rotatory moti on of of different powder samples of rapeseed-mustard as a functi on of the molecules is not sufficiently rapid for the packing fraction at frequency 8.93 GHz. attainment of equilibrium with the field. The Packing Dielectric Dielectric Conducti vity Relaxation polarization then aquires a component out of phase fraction constant loss (E p" ) (Or x I 0 2 ) mhos m 1 time (B,) (Ep') (<p)x i0'2 with the field and the displacement current acquires Experimentally observed values Extrapolated value sec a conductance component in phase with the field, resulting in thermal dissipation of energy. The dielectric loss is, thus, proportional to the ac conductivity. The rise in relaxation time with increasing value of packing fraction is due to Rapeseed-Mustard variety PCR increasing hindrance to the process of polarization The conductivity, therefore. increases suggesting that at higher compactions, no micro cracks developed in the sample due to high mechanical pressure or no additional grain boundaries Rapeseed-Mustard variety RH-30 developed which might have over-compensate the positive contribution of packing density towards !! !! 0.35 conductivity Table 3 lists the measured and computed values Rapeseed-Mustard variety of dielectric parameters for bulk from powder I measurements. The results reported at 8, = I are those measured on the finest crushed powder sample packed very closely, in a wave guide cell and pressing it under a fixed pressure and temperature so I I as to obtain minimum voids between the particles Out of powder specimens of different packing Table 2 lists the values of permittivity (E r'), loss fractions, the specimen having minimum particle factor (E r"), relaxation time (1r) and the size is defined as finest which is about 0-50 J.lrn in

4 802 INDIAN J PURE & APPL PHYS, VOL 39,DECEMBER 2001 Table 3 - Measured and computed values of dielectric parameters for bulk from powder measurements at frequenc) 8.93 GH z Relative packing E,' for solid bulk E ; for solid bulk fracti on (8,) Measured Extra- Calculated from Calcul ated Measured Ex tra- Calcul ated Calculated polatcd Boucher's from Landau, polated from from va lues formu la Lifshit z & values Boucher's Landau. Looycnga formul a Lifshitz & formula Looyenga formula Rapeseed-Mustard vari ety PCR R I (solid bulk) Rapeseed-Mustard variety RH I (soli d bulk) Rapeseed-Mustard varie6y BI I (solid bulk ) this case. The authors are assuming thi s system as solid bulk for getting correlation between powde r and solid bulk. It is to be noted that the compression applied to decrease the voids between the particles may also compress the solid particles themselves. In the present work attempts have not been made to consider thi s effect. The other experimenta ll y measured results corresponding to different values of 8, < I, have been used to obtain transformati on to I 00 per cent dense bulk using the corre lati on formulae of Landau-Lifshitz-Looye nga given by Eqs I and 2 and also using the corre lati on formul a of Bottcher given by Eqs 4 and 5. Using these correlation the bulk values obtained for E,' are very much closer to the measured values of E,' at 8, = I. This strongly supports their assumption of choosing the finest crushed close ly packed powder sample as a solid bulk. As far as the bulk va lues of E, " are concerned, the authors found that there is slight di sparity between the calculated values (using both Bottcher corre lati on as we ll as La dau-lifshitz Looyenga correlation) and the experimental values of E, ". Other workers' have also in vesti gated the disparity between the calcul ated and experimental values of E, ". They have pointed out the inaccuracy of the Bottcher's and Landau-Lifshitz-Looyenga formulae fm loss computation as one possible reason of observed disparity between the calcul ated and experimental valu es of E,". Another possib le reason for thi s disparity given by them is that the mate ri al in the form of powder is inherently less lossy than it is in the bulk form. To check the accuracy of present measurement s. extrapolated values for E P', E P" and <J" at 8, = I are obtained from curves 8, versus E p' E / and crp values for a ll three samples. From Tables 2 and 3 it is found that extrapolated values of E r' E P" and crp for all the three samples unde r in vestigation, are

5 BANSAL et (//.:DIELECTRI CS OF RAPESEED-MUSTARD SEEDS almost same as the corresponding experimental va lues for 8, = I. Bottcher's formul a has been found to hold well for higher packing fract ions and for the low permittivity va lues 11 ' (E,=.7). In the present study, thi s fact is confirmed as the sampl es under inves tigati on possess higher relati ve packing fraction and low permittivity va lue (=.7). Theoretical expressions empl oyed are app li cable to the compact pellet out of powders than to loose powder grains i.e. more suitab le to the continuous medium than to discontinuous one. In the present investigation, since we are getting appreciab le agreement between the theoretical values obta ined by Bottcher's formula and Landau-Lifshitz Looyenga's formulae and experimentall y obtained values, it may be predicted that the rapeseedmustard powders are having large cohes ion in its particles and may serve as a continuous medium. 4 Conclusion It may thu s be concluded that in case of grains like rapeseed-mustard where it is not poss ible to define the solid bulk exactly. the authors' assumpti on of the fi nest crushed closely packed assembly of the seeds grains as solid bulk for the compari son of correlation formulae of Landau Lifshitz-Looyenga and Bottcher between powder and solid bulk, works well as there is fair agreement between the calcu lated values of dielectric parameters and the va lues obtai ned experi mentall y for solid bulk. Appreciable agreement between the experimentally obtai ned values and the theoretical va lues obtained by Bottcher's and Landau-Lifshitz Looyenga's formulae shows large cohes ion in the particles of rapeseed-mustard powders under investigation, serv ing as a continuous med ium. Acknowledgements The authors are thankful to the Head of the Department of Ph ys ics and Principal, MSJ College. Bharatpur, for providing laboratory faci lities. On e of them (A KB ) is indebted to the University Grants Commission, New Delhi for providing Teacher Fellowship to him. The authors are also thankful to the Director, National Research Centre on Rapeseed-Mustard, for providing the faciliti es to work on NIR product analyzer. References Yenkatcsh M S. Deni s E St. Raghvan G S V e/ of.. Asio Pacific Micm 11 a 1'e Conference p. :IR7. 2 Lokhandc M P. Arbad B R. Lanclgc M G & Mchrotra S C. Indian.I Pure & Appl Phys. 33 ( 1996) 156. :1 Sharma K S & Singh P J..I Phwol Res. 7 ( 1994) Khanna R K & Upadhyay S K. Indian.I Pure & Af!f!/ /Jfn.l". 37 ( 1999) Bansal A K. Singh P J. Sharma K S el a/.. /ndia11.i Purl' & Appl Ph1 s. 39 (200 I ) Nelson S 0 & You T S. Am Soc A gric-engi11eers. XX-6040 ( 1988) Yadav J S & Gandhi J M. l11dian.i Pure & Appl Phrs. :1 0 (1992) 427. R Yadav J S & Gandhi J M. Indian.I Pure & Apf!l fjh\ s. :I I (1993) Dube DC..I Phvs D. 3 ( 1970) Dube DC & Parsad R..I Phys D. 3 ( 1970) 677. II Landau L D & Lifshitz E M. Elec!rodmamics oj" conlinuous media (Pergamon Press. London ) p Looyenga H. Physica.31 ( 1965 ) Bottcher C J F. The 1heorv of eleclric polari ~( lfi on (Elsevier. Amsterdam). 1952, p Debye P. Polar molecules. (Chemical Catalog. New York ) Chap Y.