ISSN Original Article Determine of Moisture Diffusivity as Function of Moisture content and Microwave power of Some Biomaterials

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1 Available online at International Journal of Agricultural and Food Science Universal Research Publications. All rights reserved ISSN Original Article Determine of Moisture Diffusivity as Function of Moisture content and Microwave power of Some Biomaterials Hosain Darvishi 1* ; Abbas Rezaei Asl 2 ; Mohsen Azadbakht 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 Received 07 August 2012; accepted 23 August 2012 Abstract Drying experiments of carrot slices; lemon slices; whit mulberry; black sunflower seeds and potato slices were carried out by using microwave drying. Dying processes were completed between min for carrot slices, min for black sunflower seeds, 7-17 min for whit mulberry, min for potato slices and min for lemon slices depending on the microwave power level. Moisture transfer from carrot slices was described by applying the Fick s diffusion model, and effective moisture diffusion coefficients were calculated. A third order polynomial relationship was found to correlate the effective moisture diffusivity (D eff ) with moisture content. The effective moisture diffusivity increased with decrease in moisture content of products. The average effective diffusivity values varied from to m 2 /s for carrot slices, to m 2 /s for lack sunflower seeds, to m 2 /s for lemon slices and to m 2 /s for potato slices over the microwave power range studied Universal Research Publications. All rights reserved Keywords: Drying; moisture diffusivity; moisture content; microwave power. 1. Introduction moisture diffusivity of food products are essential for Drying is an important preservation process which reduces efficient and effective design of food processing operations, water activity through the decrease of water content, including drying. Effective moisture diffusivity describes avoiding potential deterioration and contamination during all possible mechanisms of moisture movement within the long storage periods [1]. Also, food quality of is preserved, foods, such as liquid diffusion, vapour diffusion, surface the hygienic conditions are improved, and product loss is diffusion, capillary flow and hydrodynamic flow [7, 8, 9, diminished. Hot-air drying has been to date the most 10]. common drying method employed for food materials. Therefore, the aim of this study was to calculate the However, this method has many disadvantages, including effective moisture diffusivity as function of moisture poor quality of dried products, low energy efficiency and a content and microwave power for some biomaterials such long drying time. The use of microwave technology in as: carrot slices; lemon slices; potato slices; black drying agricultural products has several advantages. These sunflower seeds and white mulberry. may include decreased drying time, high energy efficiency, 2. Material and Method high quality finished products, and uniform temperature in 2.1. Samples preparation the product [2, 3]. The samples were procured from local vegetable market, Molecular diffusion is the main water transport mechanism and fresh white mulberries and black sunflower seed and to predict the water transfer in food materials diffusion samples were harvested from the experimental farm in models based on Fick s second law are used [3, 4, 5]. Tehran, Iran. The samples were stored at 4±0.5 C before Moisture diffusivity is an important physical transport they were used in experiments. The samples were removed property which is useful in the engineering analysis of from the refrigerator before experimentation and were basic food processing operations such as drying. Diffusion allowed to attain room temperature. Carrots, potatoes and phenomena are extremely complex, so reliable data are lemons were washed under running water to remove the scarce, especially for microwave drying. As a consequence, adhering impurities, and thinly sliced in thicknesses of 5 traditional food processing involving diffusion has been mm using a sharp stainless steel knife. Generally same mainly based on experimental knowledge. Accurate data on samples of uniform size (average thickness, length and 90

2 width (±1.35), 3.41 (±1.24) and (±1.60) mm, respectively) were used. The average initial moisture content of the samples were found to be 78.2±0.7% (Carrot slices); 86±0.8% (Lemon slices); 80±1.5% (White mulberry); 75±1.5% (Potato slices) and 31±1% (black sunflower seed) on wet basis, as determined by using convective oven at 103±1 C Experimental equipment and procedure A domestic microwave oven (M945, Samsung Electronics Ins) with maximum output of 1000 W at 2450MHz was used for the drying experiments. The dimensions of the microwave cavity were mm. 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. The microwave dryer was operated by a control terminal which could control both microwave power level and emission time. Experiments were performed at initial mass of 50 g for white mulberry and black sunflower seed), 30 g for carrot, potato and lemon slices at four microwave power levels. The moisture losses of samples were recorded at 30 s intervals during the drying process by a digital balance (GF-600, A & D, Japan) and an accuracy of ± g. For measuring the weight of the sample during experimentation, the tray with sample was taken out of the drying chamber, weighed on the digital top pan balance and placed back into the chamber. Drying was carried out until the final moisture content reaches to a level less than 5±1% on wet basis Moisture diffusivity The moisture ratio of 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. The effective moisture diffusivity of a food material characterizes its intrinsic mass transfer property of moisture. During drying, it can be assumed that diffusivity, explained with Fick s second law, is the only physical mechanism to transfer the water to the surface. Effective moisture diffusivity, which is affected by composition, moisture content, temperature and porosity of the material, is used due to the limited information on the mechanism of moisture movement during drying and complexity of the process. The moisture diffusivity for an infinite slab was therefore calculated by the following Eq. (2) proposed by Crank [11] considering assumptions mentioned hereunder [7, 8]: 1- Moisture is initially uniformly distributed throughout the mass of a sample. 2- Mass transfer is symmetric with respect to the centre. 3- Surface moisture content of the sample instantaneously reaches equilibrium with the condition of surrounding air. 4- Resistance to the mass transfer at the surface is negligible compared to internal resistance of the sample. 3- Mass transfer is by diffusion only. 6- Diffusion coefficient is constant and shrinkage is negligible. MR = 8 π 2 exp π2 D eff t L 2 (2) where D eff is the effective diffusivity (m 2 /s), and L is the thickness (here half) of slab (m). Eq. (2) is evaluated numerically for Fourier number, (F 0 = (D eff t)/l 2 ), for diffusion and can be rewritten as Eq. (3) can be rewritten as: MR = 8 π 2 exp π2 F 0 (3) Thus: F 0 = 0.101ln MR (4) The effective moisture diffusivity was calculated using Eq. (5) as: D eff = F 0 t L 2 (5) 3. Results and Discussion The changing of the moisture ratio versus drying time for thin layer drying of samples at various microwave powers are given in Figs A reduction in drying time occurred with increasing the microwave power level. On the other hand, mass transfer within the sample was more rapid during higher microwave power heating because more heat was generated within the sample creating a large vapor pressure difference between the centre and the surface of the product due to characteristic microwave volumetric heating. Fig.1. Variation in moisture ratio as a function of drying time for carrot slices Fig. 2: Variation of moisture ratio with drying time for the white mulberry 91

3 Fig. 3: Variation of moisture ratio with drying time for the lemon slices Fig. 7: Variation in effective moisture diffusivity with moisture content for whit mulberry at different microwave powers Fig. 4: Variation of moisture ratio with drying time for the potato slices Fig. 8: Variation in effective moisture diffusivity with moisture content for lemon slices at different microwave powers Fig. 5: Variation of moisture ratio with drying time for the black sunflower seeds Fig. 9: Variation in effective moisture diffusivity with moisture content for potato slices at different microwave powers Fig. 6: Variation in effective moisture diffusivity with moisture content for carrot slices at different microwave powers. Fig.10: Variation in effective moisture diffusivity with moisture content for black sunflower seeds at different microwave powers. 92

4 Fig. 11: Variation in ln (MR) and drying time for carrot slices dried at different microwave powers Variation in effective moisture diffusivity of samples with moisture content at different microwave power levels is shown in Figs The effective 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 [7]; Sharma et al. [8] also reported similar trend in the variation in the moisture diffusivity with moisture content. Fig. 13: Variation in ln (MR) and drying time for lemon slices dried at different microwave powers. Average diffusivities are typically determined by plotting experimental drying data in terms of ln (MR) versus drying time t in Eq. (7), because the plot gives a straight line with a slope (K) as follows: K = π2 D eff L 2 (7) Fig. 14: Variation in ln (MR) and drying time for potato slices dried at different microwave powers Fig. 12: Variation in ln (MR) and drying time for white mulberry dried at different microwave powers. A third order polynomial relationship was found to correlate the effective moisture diffusivity with corresponding moisture content of samples and is given by Eq. (6) D eff = A + BX + CX 2 + DX 3 (6) where A, B, C, D is the constants of regression, and X is moisture content (d.b.) Regression constants for microwave drying of carrot slices under different powers are presented in Table 1. The high Fig. 15: Variation in ln (MR) and drying time for black sunflower seeds dried at different microwave powers. The variation in ln (MR) and drying time (t) for samples at different microwave powers have been plotted in Figs to obtain the curve slope which can give the average effective moisture diffusivity. Average values of effective diffusivity for samples at different microwave power are values of R 2 are indicative of good fitness of empirical relationship to represent the variation in effective moisture diffusivity with moisture content of samples. 93

5 Table 1- Regression coefficients of effective moisture diffusivity for different microwave powers Product P(W) A* B* C* D* R 2 Lemon slice Carrot slice Black sunflower seed Potato slice White mulberry * 10 8 for carrot, lemon and potato slices; 10 7 for white mulberry and black sunflower seeds presented in Table 2. These values are within the general range of m 2 /s for drying of food materials [12]. The values of D eff are comparable with the reported values of to m 2 /s mentioned for apple pomace microwave drying [3], for tomato pomaco hot air drying [13], to m 2 /s for tomato pomace microwave drying at W [14], to m 2 /s for Gundelia tournefortii microwave drying at W [5]; to m 2 /s for blueberry infrared drying [15]. Table 2- Result of average effective diffusivity of samples with different microwave power levels Product P(W) Average diffusivity (m 2 /s) Carrot slices Black sunflower seeds White mulberry Potato slice Lemon slice A linear regression analysis on the average diffusion coefficient with microwave power resulted in the following relationships: For carrot slices: D eff average = P R 2 = (8) For lemon slices: D eff average = P R 2 = (9) For black sunflower seeds: D eff average = P R 2 = (10) For white mulberry: D eff average = P R 2 = (11) For potato slices: D eff average = P R 2 = (12) where P is the microwave power (W). 4. Conclusions Moisture diffusivity characteristic of carrot, lemon, and potato slices, black sunflower seeds and white mulberry have been investigated during microwave drying. The time required for drying products was considerably decreased with the increment in the drying microwave power. Effective moisture diffusivity depends on the moisture content and increases with decrease in moisture content. A third order polynomial relationship existed between effective moisture diffusivity and the moisture content of products. The average effective moisture diffusivity varied from to m 2 /s for carrot slices, to m 2 /s for lack sunflower seeds, to m 2 /s for lemon slices and to m 2 /s for potato slices and was significantly influenced by microwave power. References 1. Singh G., Arora S., Kumar S., Effect of mechanical drying air conditions on quality of turmeric powder. J Food Sci Technol, 47(3): Vadivambal R., Jayas D. S., Changes in quality of microwave-treated agricultural products - a review. Biosys Eng, 98:

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