CHAPTER 7 DIELECTRIC PROPERTIES OF WHEAT POWDER AND SOLID MASS : DIELECTRIC MIXTURE EQUATIONS

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1 Dielectric properties of wheat powder and solid mass CHAPTER 7 DIELECTRIC PROPERTIES OF WHEAT POWDER AND SOLID MASS : DIELECTRIC MIXTURE EQUATIONS 7.1 Introduction Dielectric properties of food grains depend on their moisture content, temperature and other properties of food grains, which in turn depend on their biochemical composition, viz., carbohydrates, proteins, fats, vitamins and minerals. Wheat is the most common food grain, which is cultivated all over the world and used to make whole flour, refined flour (Indian name - maida ), semolina (Indian name- suji ) and macaroni products. Various 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, biofuel etc. 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. Indian Wheat (whole grain) with botanical name Triticum aestivum contains in every 100 g of it, 71. g of carbohydrates, 11.8 g of proteins, 1.5 g of total fat, 1.8 g of moisture, 1. g of crude fiber, 11.4 g of dietary fiber and 1.5 g of minerals (Gopalan et al., 007). The minerals found in 100 g of Indian wheat have approximately 5.3 mg iron, 41 mg calcium, 138 mg magnesium, 17.1 mg sodium, 84 mg potassium,.7 mg zinc, 306 mg phosphorus,.9 mg manganese, 0.68 mg copper, 18 mg sulphur, 47 mg chlorine etc. Vitamins contained in 100 g of Indian wheat include 64 µg β Carotene (Vitamin A), 0.45 mg Thiamine (Vitamin B 1 ), 0.17 mg Riboflavin (Vitamin B ), 5.5 mg Niacin (Vitamin B 3 ), 36.6 µg Folic Acid, 0.57 mg total B 6 etc. The average energy outcome of the 100 grams of wheat products has been estimated to be 346 Kcal. Although wheat supplies much of the world s dietary proteins and carbohydrates, as many as one in every 00 people suffer from coeliac (also called

2 Dielectric properties of wheat powder and solid mass as celiac) disease, a condition that is caused by an adverse immune system reaction to gliadin, a gluten protein found in wheat. While the disease is caused by a reaction to wheat proteins, it is not the same as wheat allergy. The only effective treatment of this disease is life long gluten-free diet. Because of such importance of wheat in human life, we focused on the studies of dielectric properties of wheat. As water has high electrical permittivity and dielectric loss, the former decreasing and latter increasing with frequency, the wheat crop having appreciable moisture content show quantitatively measurable dielectric properties (Nelson and Datta, 001). Techniques of dielectric measurement in whole grains being more difficult, we investigate dielectric properties of wheat in powder form as the laboratory methods for powders are now well established. Several investigations of dielectric properties of agricultural products have been reported in literature. Nelson (1991, 199) observed that both the permittivity and dielectric loss factor of different varieties of food grains, like hard red winter wheat, field corn, winter barley, soybeans, rough rice etc. show appreciable dependence on moisture content, temperature and frequency. Trabelsi and Nelson (006) studied dependence of dielectric properties of wheat, soybeans and corn on their bulk densities and found that both the dielectric constant and loss factor vary appreciably with the bulk density of these species. Nelson (006) further investigated the application of microwave dielectric properties for sensing moisture content in food grains and the relationship of dielectric properties of granular and powdered materials with their bulk densities. Nelson (006) also expressed the possibility of using dielectric properties for selective heating of insects in grains and for maturity and quality sensing of fresh fruits and vegetables. Karimi et al. (009) have studied the effect of moisture on physical properties of wheat. They observed that the porosity of wheat seeds increases with moisture, whereas both the bulk density and true density decrease with moisture. The change in bulk density of food grains with moisture may be considered to cause relative changes in their dielectric properties. However, the studies made by Nelson (1991; 199; 006) were based on wide band Network Analyzer, with obvious advantage of broad spectrum measurement of dielectric properties being possible, rather directly measurable from radio frequencies

3 Dielectric properties of wheat powder and solid mass to microwave frequencies, but the accuracy of the results suffer from uncertainties as compared to laboratory techniques. In the present research, dielectric properties of wheat (RAJ 410) variety were investigated at microwave frequencies in powder form by employing Yadav Gandhi method. This method is simple and provides dielectric parameters to an accuracy of 1%. This method has been used for the first time for measurement of dielectric properties of wheat in powder form. The dielectric properties of wheat were determined in C, X and Ku band of frequency. The dielectric properties of solid mass of wheat were obtained by employing dielectric mixture equations. The results of Yadav Gandhi method based on microwave bench measurements are compared with results obtained from Nelson s method using Network analyzer for the dielectric properties of wheat in X band of microwave frequencies.the dielectric parameters of solid mass of wheat as obtained from the two methods by employing dielectric mixture equations are also compared. 7. Material and Method The sample of wheat required for the present study was obtained from Wheat Breeder, All India Co-ordinated Wheat and Barley Improvement Project (AICWBIP), Agriculture Research Station, Durgapura, Jaipur of Swami Keshvanand Rajasthan Agricultural University, Bikaner, India. The dielectric properties of Raj-410 variety of wheat were investigated in powder form by employing the technique proposed by Yadav and Gandhi (199). The method has been described in detail in chapter 3. Equations used for finding out dielectric properties of wheat powder by this method are given below. The main quantities to be measured experimentally are attenuation constant (α d ) and phase factor (β d ) for the samples, from which the values of ε' and ε'' for the sample can be determined. ' d ( 7.1) c d d " 0 d (7.) d d

4 Dielectric properties of wheat powder and solid mass The experimental arrangement is shown schematically in Figure 7.1. Fig. 7.1 : Experimental set up for measurement of dielectric properties of powders by Yadav & Gandhi (199) method In the present study the sample of Indian wheat Raj-410 was grinded in to flour by a grinder and the wheat powder of grain size between 15 and 150 microns was obtained with the help of sieves of appropriate hole sizes. Microwave power obtained from a microwave source, viz. Klystron tube, was allowed to form standing waves in the slotted waveguide section after being reflected from the short circuiting plunger in the dielectric cell, which is initially kept at its lowest position in the cell. The position of the probe in the slotted waveguide section was accurately adjusted at the node of the standing waves, as indicated by the position of the minima in the indicating meter. Now a small quantity of wheat powder was introduced in the dielectric cell and the plunger was brought over it by moving the micrometer screw till a proper contact is established. The height h of the powder in the dielectric cell is determined from the difference of readings on the scale of micrometer screw taken with and without wheat powder in the cell. On introducing wheat powder in the dielectric cell, the position of minima in the slotted section gets shifted either towards the receiver or towards the generator. When h < (λ d /4), the shift observed was towards the receiver and when (λ d /4) < h < (λ d /), the shift was towards the generator. The powder was added slowly in the dielectric cell till for a height h of the powder in the cell, the position of minima in the slotted section was the same as for the empty cell. For this position

5 Dielectric properties of wheat powder and solid mass h d or d h (7.3) The value of λ d. was directly determined by using equation (7.3) For measuring α d, the plunger was kept at the bottom of the empty cell. The probe was located at one of the maximas, i.e., at the position of a voltage antinode in the waveguide slotted section, and reading x 1 of the indicating meter was noted. The powder (wheat flour in the present case) was added slowly in the dielectric cell and position of the maxima in the slotted section was noted each time. The height of the powder column in the dielectric cell (h') was accurately adjusted so that the probe position locating the maxima in the slotted section was again at the same position as it was with the empty cell. The deflection x of the indicating meter in this state, for such height h' of the powder column, was noted. In this experiment the electromagnetic waves travel twice through the powder column in the cell before they form stationary waves in the slotted waveguide section, after being reflected from the plunger of the dielectric cell. For obtaining appreciable absorption of the wave, height of the powder column in the cell was taken equivalent to several λ d. The value of the attenuation coefficient (α d ) is then given by (Yadav and Gandhi,199) d.303 x log 1 h ' x x 1 (7.4) The values of λ d and α d for the sample of wheat powder were determined by following the procedure as described above, which when used in equations (7.1) and (7.) provide relevant values of ε' and ε'' for the wheat powder. It is pertinent to note that this experiment was performed at normal pressure on the powder in the cell, at density gms/cc. The powder was then compressed by applying a pressure of about 10 atmospheres, beyond which no significant compression was noticed. The density of the powder in the compressed state was found to be gms/cc. In the compressed state the wheat powder can be considered as forming the solid mass for this material.

6 Dielectric properties of wheat powder and solid mass Dielectric Mixture Equations Dielectric mixture equations were used to estimate the dielectric properties of the solid material from the properties of an air-particle mixture, made up of air and the pulverized particles of the solid. In the present work two component dielectric mixture equations were used, in which ε represents the complex permittivity of the mixture, ε 1 is the complex permittivity of the medium (air in the present case, for which ε 1 =1 _ j0), in which particles of the solid material having complex permittivity ε were dispersed, v 1 and v being the volume fractions of the medium (i.e., the air in this case) and the solid material (i.e. the bulk solid material of wheat in this case) respectively, such that v 1 + v = 1. The formulation of the dielectric mixture equations used in the present work are as given below (Nelson, 1991): i) Complex Refractive Index Mixture Equation: 1/ 1/ 1/ ( ) v 1( 1) v ( ) (7.5) ii) Landau and Lifshitz, Looyenga Equation: 1/3 1/3 1/3 ( ) v 1( 1) v ( ) (7.6) iii) Böttcher s Equation : v (7.7) iv) Bruggeman Hanai Equation: 1 1 1/3 1 v (7.8) v) Rayleigh s Equation: v (7.9)

7 Dielectric properties of wheat powder and solid mass Results and Discussion Indian wheat, variety Raj-410 was chosen for this study, which is an improvised variety of wheat developed by the Wheat Breeder, All India Coordinated Wheat and Barley Improvement Project, Agriculture Research Station, Dugapura, Jaipur of the Swami Keshvanand Rajasthan Agriculture University, Bikaner, India. Some of the physical properties of the grains of this variety have been shown in Table 7.1, where the properties of Shiraj Variety reported by Karimi et al.(009) have been included for the sake of comparison. In table 7.1 the symbols L, W and T respectively represent the length, width and thickness of wheat grains. Measurement of these dimensions of wheat grains were performed using a micrometer screw gauge on randomly chosen 100 grains of the sample and then finding out their average values along with the standard deviation. Other quantities included in this table are Geometrical Diameter (Dg), Equivalent Diameter (Dp) and Sphericity (Sp) of the wheat grains, defined by: g 1/3 D LWT (7.1) D p (W T) L 4 1/3 (7.13) S p D L g (7.14) Table 7.1 : Average physical properties of wheat grains Variety L (mm) W (mm) T (mm) Dg (mm) Dp (mm) Sp Raj Shiraj (Karimi et al, 009) The dielectric properties of Indian wheat in powder form (with moisture content 9.96%) have been studied at the room temperature (31.4 o C) and relative humidity 41% by using a C-band microwave set up energized by a Klystron source K at frequency GHz, an X- band set up using a Klystron tube K-7 at frequency GHz and a Ku-band microwave set up energized by a Gunn

8 Dielectric properties of wheat powder and solid mass Oscillator X-15 and PIN modulator X-451 at frequency GHz, manufactured by M/s Vidyut Yantra Udyog, Modinagar, India. The values of dielectric constant ε' and loss factor ε'' of Raj-410 Indian wheat in powder form, obtained from the present investigations at three frequencies in C-band, X-band and Ku-band respectively have been assembled in Table 7.. The values of dielectric parameters of wholekernel Scout-66 hard red winter wheat with moisture content 11.5% and density gms/cc measured by Nelson (1991) at 9.4 GHz. are included in the table for the sake of comparison. The values of dielectric constant (ε') and loss factor (ε'') of solid mass of wheat were obtained by him by using different dielectric mixture equations for volume fraction v = In the present study the density of wheat powder was gms/cc and volume fraction used was v = 0.8. Table 7. : Dielectric properties of Indian Wheat, Raj-410, in powder form and of solid mass from dielectric mixture equations Mode of Determination C-band (4.70 GHz) X-band (9.39 GHz) Ku-band (14.16 GHz) Nelsons s Values(1991) (9.4 GHz) ε' ε'' ε' ε'' ε' ε'' ε' ε'' (i) Present Experimental values for wheat powder (ii) Values for solid mass from Dielectric Mixture Equations : A). Complex Refractive Index Equation (Eq.7.5) B). Landau Lifshitz Looyenga Equation (Eq. 7.6) C). Böttcher s Equation (Eq. 7.7) D). Bruggeman-Hanai Equation (Eq.7.8) E). Rayleigh s Equation (Eq. 7.9) It may be observed from Table 7. that the present values of dielectric constant (ε') and loss factor (ε'') of the powder of Indian Wheat for X-band (ε'= 3.89, ε''= 0.8) are in good agreement with the values reported by Nelson (1991) for hard red winter wheat (ε' =.45, ε'' = 0.), the difference in the two sets of values being due to the different varieties of wheat used in the two studies. The present values of

9 Dielectric properties of wheat powder and solid mass dielectric parameters are also in good agreement with the values (ε'.61, ε'' 0.5) at 10 GHz, as observed from the graphs of frequency dependence of ε' and ε'' plotted by Nelson (199) for hard red winter wheat at 4 o C and moisture content 10.6%. The present values of the dielectric parameters of solid mass of wheat obtained from dielectric mixture equations also show trends which are similar to the trends shown by the values obtained by Nelson(1991). The present values from the Complex Refractive Index Equation (7.5) and Böttcher s Equation(7.7) are in excellent agreement with each other, whereas the values of ε' and ε'' obtained from Landau Lifshitz, Looyenga Equation (7.6), Bruggeman-Hanai Equation (7.8) and Rayleigh s Eq. (7.9) show increasing trend, which is consistent with the variation in the values of ε' and ε'' obtained by Nelson (1991) from different dielectric mixture equations. On comparison of the present X-band values with those of Nelson (1991) for X-band, it can be seen that the ε' values of the solid mass of wheat from the two investigations are in good agreement for all the mixture equations, however the present values of loss factor ε'' are comparatively lower than the values reported by Nelson for all the mixture equations. 7.4 Conclusion The present investigations provide a reliable set of dielectric data for Indian wheat variety Raj-410 at three frequencies. The results obtained by Yadav- Gandhi method (199) based on microwave bench measurements were in good agreement with the results of Nelson s method based on Network Analyzer at X band. Both,the dielectric constant(ε') and dielectric loss factor (ε'') are observed to decrease with increase in frequency.the values of dielectric parameters obtained for solid mass of wheat by using various dielectric mixture equations are in agreement with the values proposed by Nelson (1991). These studies may prove helpful in efficient design of microwave oven and microwave cooker.