ISSN Original Article Experimental Analysis of Moisture Transfer during Thin layer Drying of Black Sunflower Seeds

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1 Available online at International Journal of Agricultural and Food Science Universal Research Publications. All rights reserved ISSN Original Article Experimental Analysis of Moisture Transfer during Thin layer Drying of Black Sunflower Seeds Hosain Darvishi 1* ; Mohsen Azadbakht 2 ; Abbas Rezaei Asl 2 1 Department of Engineering, Shahre Ray Branch, Islamic Azad University, Tehran, Iran 2 Department of Agricultural Machinery Mechanics, Agricultural Sciences & Natural Resources University of Gorgan, Gorgan, Iran Corresponding Author Hosaindarvishi@gmail.com; Tel: ++98 (21) ; Fax: ++9(21) Received 05 August 2012; accepted 14 September 2012 Abstract The effect of microwave drying technique on drying kinetics of black sunflower seeds was investigated. Results indicated that drying rate increased by increasing the microwave power and decreased continuously with passing drying time and decreasing moisture content. Drying processes were completed between 3.5 and 12.5 min depending on the microwave power level. The drying process took place in the falling rate period. In this study, measured values were compared with predicted values obtained from Page s thin layer drying model. The effective moisture diffusivity increased with decrease in moisture content of carrot slices. Average effective moisture diffusivity increased from to m 2 /s with increasing the microwave power. The activation energy was calculated using an exponential expression based on Arrhenius equation Universal Research Publications. All rights reserved Keywords: Drying; moisture diffusivity; modeling; sunflower seed; microwave power. 1. Introduction Drying of moist materials is a complicated process involving simultaneous heat and mass transfer. Sunflower seeds can be safely stored at 10% moisture or less, but during warmer months the storage moisture should be at 8% or less [1, 2]. Sunflower seeds are dried usually by the use of forced air through the batch of seeds. The main disadvantages of hot air drying of products are low energy efficiency and long drying time during the falling rate period. Because of the low thermal conductivity of food materials, heat transfer to the inner sections of foods during conventional heating is limited during this period. Also, the primary problem is that small fibers rub off the sunflower hulls and float in the air. The desire to eliminate this problem, prevent significant quality loss, and achieve fast and effective thermal processing has resulted in the increasing use of microwaves for food drying. It has also been suggested that microwave energy should be applied in the falling rate period or at low moisture content for finish drying [3, 4]. When drying process is controlled by the internal mass transfer, mainly in the falling rate period, modeling of drying is carried out through diffusion equations based on Fick s second law [4, 5, 6, 7]. Molecular diffusion is the main water transport mechanism and to predict the water transfer in food materials diffusion models based on Fick s second law are used. Effective moisture diffusivity describes all possible mechanisms of moisture movement within the foods, such as liquid diffusion, vapour diffusion, surface diffusion, capillary flow and hydrodynamic flow. A knowledge of effective moisture diffusivity is necessary for designing and modeling mass-transfer processes such as dehydration, adsorption and desorption of moisture during storage. Researchers reported that effective moisture diffusivity increased with moisture content up to a limit value and then decreased and eventually became constant at high moisture contents [8, 9]. Therefore, the aim of this study was to investigate the effect of microwave output power and sample amount on the drying kinetic of black sunflower seeds, to compare the experimental data found during drying with the predicted values obtained by using Page s model, to calculate the activation energy, to calculate the effective moisture diffusivity as function of moisture content and microwave power. 2. Materials and Methods 2.1. Sample preparation Black sunflower seed samples were harvested from the experimental farm in Ilam, Iran, and were stored in the refrigerator at temperature of 4±1 C until the experiments 103

2 were carried out. The initial moisture content of the alfalfa samples was determined by using standard oven method at 103±1 C. These experiments were replicated thrice to obtain a reasonable average. Average moisture content was found to be 31±1% wet bases Experimental equipment and procedure Drying studies were carried out with a domestic digital microwave oven (M945, Samsung Electronics Ins) with the technical feature of 230V, 50 Hz and 1000 W. The oven had the dimensions of cm with a rotating glass plate having 300 mm in diameter. The oven has a fan for air flow in drying chamber and cooling of magnetron. The moisture from drying chamber was removed with this fan by passing it through the openings on the right side of the oven wall to the outer atmosphere. Furthermore, it was able to work at various microwave outputs and had a digital control facility into adjust the processing time. In order to carry out the drying experiments, 50 g of samples were weighed using a digital balance (Sartorius GP3202, Germany) with a precision of 1 g. Samples were removed from the oven periodically (every 30 s) during the drying period, and the moisture loss was determined by weighing the plate using digital balance. Four different microwave power outputs, 200, 300, 400 and 500 W were used to dry the samples and three replications were performed at each of these power outputs. Drying was carried out until the final moisture content reaches to a level less than 2% (w.b.) Modeling of drying process The moisture ratio of alfalfa samples during the thin layer drying experiments was calculated using the following equation: MR = X t X e X 0 X e (1) where MR is the moisture ratio (dimensionless), X t is the moisture content at drying time t (d.b.) and X 0 is the initial moisture content (d.b.). The values of X e, are relatively small compared to X t or X 0. Thus, Eq. (1) can be reduced to MR=X t /X 0. Among many of the theoretical, empirical and semiempirical models mentioned and examined in the literature to quantify the moisture removal behavior in food products, the empirical model proposed by Page [4] was observed to provide the best representation of the drying kinetics of many food products MR = exp (-k t n ) (2) where k is the drying rate constant (1/min) and n is equation constants model. The terms used to evaluate goodness of fit of the page s model to the experimental data were the coefficient of determination (R 2 ) and the reduced chi-square (χ 2 ) between the experimental and predicted moisture ratio values. Statistical values are defined as follows: In these equations, N is the number of observations, z is the number of constants, MR exp and MR pre are the experimental and predicted moisture ratios, respectively Moisture diffusivity The moisture and/or vapour migration during drying period is controlled by diffusion. The rate of moisture movement is described by an effective diffusivity a lumped value. Fick s second law of diffusion is used to describe a moisture diffusion process. The solution of Fick s second law in slab geometry, with the assumptions of moisture migration being by diffusion, negligible shrinkage and constant diffusion coefficients was as follows: 104 MR = 8 1 π 2 exp 2n + 1 n=o 2n + 1 π2 L 2 D eff t (5) where D eff is the effective diffusivity (m 2 /s), and L is the thickness (here half) of slab (m). The Eq. (5) can be simplified by taking the first term of Eq. (6): MR = 8 π 2 exp π2 D eff t L 2 (6) Eq. (6) is evaluated numerically for Fourier number, F 0 = D eff t/l 2, for diffusion and can be rewritten as Eq. (7) can be rewritten as: MR = 8 π 2 exp π2 F 0 (7) Thus: F 0 = 0.101ln MR 213 (8) The effective moisture diffusivity was calculated using Eq. (9) as: D eff = F 0 t (9) L Activation energy Inasmuch as temperature is not precisely measurable inside the microwave drier, the activation energy is found as modified from the revised Arehnious equation. In a first method it is assumed as related to drying kinetic constant rate (k) and the ratio of sample weight to microwave output power (m/p) instead of to air temperature. Then Eq. (10) can be effectively used as follows [10,11]: k = k 0 exp E am (10) P In the second method, the correlation between the effective diffusion coefficient and (m/p) is used for calculation of the activation energy. D eff = D 0 exp E am (11) P where k is the drying rate constant obtained by using best model (1/min), k 0 is the pre-exponential constant (1/min), E a is the activation energy (W/g), m is the mass of raw sample (g), D 0 is the pre-exponential factor (m 2 /s) and P is the microwave power (W). 3. Results and Discussion 3.1. Analysis and modeling of the drying curves The moisture content of the black sunflower seeds at each power inputs was reduced from 0.45 to 2 dry bases. It was found that the moisture content is affected by the microwave power input and drying time of the seeds was significantly reduced from 12.5 to 3.5 min as the power input increased as can be seen in Fig. 1. While the moisture content decreases gradually at 200 W, a sharp decrease occurs in moisture content with the highest microwave

3 Predicte moisture ratio Effective moisture diffusivity 10^7 (m²/s) Drying rate (kg water/kg [DM].min) Moisture ratio power level of 500 W. The results indicated that mass transfer within the sample was more rapid during the higher microwave power heating because more heat was generated within the sample Drying time(min) Fig.1. Drying curves of black sunflower seeds at different microwave powers The drying data as the moisture ratio (MR) versus drying time were fitted to the Page s thin layer drying model. Table 2 shows the fitting results of k, n, R 2 and χ 2 for the Page model. The high values of R 2 and low values of χ 2 are indicative of good fitness of Page s model to represent the variation in moisture ratio drying time of black sunflower seeds. It is clear that k and n increases as the power output increases, which implies that the drying curve becomes steeper and faster drying is obtained. The Page model constants of k and n were regressed against the microwave power using multiple regression analysis, and the Eqs. (12) and (13) were resulted: k = P R 2 = (12) n = P P P R 2 = (13) Fig. 2 compares experimental data with those predicted with the Page s model for black sunflower seeds at 200, 300, 400 and. The prediction using the Page s model showed MR values banded along the straight line, which showed the suitability of this model in describing drying characteristics of sunflower seed samples. Table 1- Estimated values of coefficients and statistical analysis for Page s model at different microwave powers P(W) Constants R 2 χ k=0.382, n= k=0.225,n= k=0.175, n= k=80, n= The drying rate, DR, is expressed as the amount of the evaporated moisture over time. Values for DR were calculated as DR= (M t+δt M t )/Δt. Fig. 3 show the drying rate versus drying time of the black sunflower seed samples at microwave powers 200, 300, 400 and 500 W. After an initial period of sample heating, the drying rate reaches its maximum value, and then the product dries itself in the falling rate period. Microwave power absorption by the product depends on its moisture content. The moisture content of the sunflower seeds was relatively high during the initial phase of drying which resulted in higher absorption of the microwave power and led to an increased product temperature. This resulted in higher drying rate due to higher moisture diffusion. As the sunflower seeds drying progressed, the loss of moisture in product decreased the absorption of the microwave power and resulted in a fall in the drying rate during the latter part of the drying. These experimental results are similar to some others published in the literature, relating to drying experiments concerning vegetables and agricultural products; for instance grape [12]; onion slices [13] and Gundelia tournefortii [7] Fig.3. Drying rates of black sunflower seeds versus drying time at different microwave powers Drying time(min) Experimental moisture ratio Fig.2. Comparison of experimental moisture ratio with predicted moisture ratio from the Page s model. Fig.4. Effective moisture diffusivity versus moisture content of black sunflower seeds at different microwave powers 3.2. Effective moisture diffusivity Variation in effective moisture diffusivity of black sunflower seed samples with moisture content at different microwave power levels is shown in Fig. 4. The effective Moisture content (d.b.)

4 Effective moisture diffusivity (m²/s) Drying rate constant, k, (1/min) ln(mr) moisture diffusivity increased with decrease in moisture content. However, the moisture diffusivity further was higher at any level of moisture content at higher microwave power level, resulting into shorter drying time. This may indicate that as moisture content decreased, the permeability to vapour increased, provided the pore structure remained open. The temperature of the product rises rapidly in the initial stages of drying, due to more absorption of microwave heat, as the product has a high loss factor at higher moisture content. This increases the water vapour pressure inside the pores and results in pressure induced opening of pores. In the first stage of drying, liquid diffusion of moisture could be the main mechanism of moisture transport. As drying progressed further, vapour diffusion could have been the dominant mode of moisture diffusion in the latter part of drying. Sharma and Prasad [8]; Sharma et al. [9] also reported similar trend in the variation in the moisture diffusivity with moisture content. A third order polynomial relationship was found to correlate the effective moisture diffusivity with corresponding moisture content of carrot slices and is given by Eq. (17) D eff = A + BX + CX 2 + DX (17) where A, B, C, D is the constants of regression, and X is moisture content (d.b.) Regression constants for microwave drying of black sunflower seeds under different powers are presented in Table 2. The high values of R 2 are indicative of good fitness of empirical relationship to represent the variation in effective moisture diffusivity with moisture content of black sunflower seeds. The variation in ln (MR) and drying time (t) for different powers have been plotted in Fig. 5 to obtain the curve slope (S=π 2 D eff /L 2 ) which can give the average effective moisture diffusivity. The average values of effective moisture diffusivity were calculated using S=π 2 D eff /L 2 and are shown in Table. 2. It was noted that average values of D eff increased greatly with increasing drying microwave power. When samples were dried at higher microwave power, increased heating energy would increase the activity of water molecules leading to higher moisture diffusivity. Table 2- Regression coefficients of effective moisture diffusivity for different microwave powers P(W) A B C D R Table 3- Result of average effective diffusivity of black sunflower seeds with different microwave power levels P(W) Average effective diffusivity (m 2 /s) Activation energy Activation energy can be calculated from the (K-m/P) curve (Fig.6) and Eq. (10). Based on statistical analysis and Page s model coefficients, k 0 and E a values were estimated as (1/min) and W/g. Fig.5. Variation in ln (MR) and drying time (in seconds) for black sunflower seeds dried at different microwave powers Fig.6. Variation of drying rate constant with sample weight/microwave operating power The activation energy can be interpreted as the minimum energy required to break solid-water or water-water interactions and to move water molecules from inside to the surface of a solid. A smaller Ea value of a sample indicates that water molecules can move more readily in the solid. Another method for calculation of activation energy is the calculation of the coefficients for Eq. (11) from (D eff ) versus (m/p) curve (Fig.7), which would yield activation energy value of W/g. The values of activation energy are comparable with the reported values of 5.54 W/g mentioned for okra [10], 13.6 W/g for pandanus leaves [14], for mint leaves [11]. Fig.7. Relationship between D eff and sample weight/microwave operating power Drying time(s) E-07 4.E-07 4.E-07 3.E-07 3.E-07 2.E-07 2.E-07 1.E-07 5.E-08 0.E+00 : ln(mr) = -051t R 2 = : ln(mr) = -114t R 2 = : ln(mr) = -138t R 2 = : ln(mr) = -174t R 2 = K = e -9.73m/P R 2 = m/p(g/w) Def f = 9E-07e m/P R 2 = m/p(g/w)

5 4. Conclusions Drying experiments of black sunflower seeds were carried out by using microwave drying. Drying of black sunflower seeds occurred in the falling rate period; no constant rate period of drying was observed. The effective moisture diffusivity increased with decrease in moisture content of samples and increases microwave power level increases. A third order polynomial relationship existed between effective moisture diffusivity and the moisture content of black sunflower seeds. The average effective moisture diffusivity varied from to m 2 /s and was significantly influenced by microwave power. References 1. Myers R. L., Sunflower: a native oilseed with growing markets, 2. Bax M.M., Gely M.C. and Santalla E.M., Prediction of crude sunflower oil deterioration after seed drying and storage process. JAOCS., 81(5), Vadivambal R. and Jayas D. S., Changes in quality of microwave-treated agricultural products - a review. Biosys Eng, 98: Wang Z., Sun J., Chen F., Liao X. and Hu X., Mathematical modelling on thin layer microwave drying of apple pomace with and without hot air predrying. J. Food Eng, 80: Midilli A. and Kucuk H., Mathematical modeling of thin layer drying of pistachio by using solar energy. Energ Convers Manag, 44: Sarimeseli A., Microwave drying characteristics of coriander (Coriandrum sativum L.) leaves. Energ Convers Manag, 52: Evin D., Thin layer drying kinetics of Gundelia tournefortii L. Food Bioprod Process, doi: /j.fbp Sharma G.P., Prasad S., Effective moisture diffusivity of garlic cloves undergoing microwaveconvective drying. J. Food Eng., 65: Sharma G.P., Verma R.C. and Pathare P.B., Thin-layer infrared radiation drying of onion slices. J. Food Eng, 67: Dadali G., Apar D.K. and Ozbek B., Microwave drying kinetics of okra. Drying Tech, 25: Ozbek B. and Dadali G., Thin-layer drying characteristics and modelling of mint leaves undergoing microwave treatment. J Food Eng, 83: Celma A.R., López-Rodríguez F. and Cuadros- Blázquez F., Experimental modelling of infrared drying of industrial grape by-products. Food Bioprod Process, 87: Pathare P.B. and Sharma G.P., Effective moisture diffusivity of onion slices undergoing infrared convective drying. J. Food Eng, 93 (3): Rayaguru K. and Routray W., Microwave drying kinetics and quality characteristics of aromatic Pandanus amaryllifolius leaves. Int Food Res J, 18(3): Source of support: Nil; Conflict of interest: None declared 107