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90 International Journal of Agriculture and Food Science 2012, 2(3): 90-95

ISSN 2249-8516

Original Article

Determine of Moisture Diffusivity as Function of Moisture content and Microwavepower of Some Biomaterials

Hosain Darvishi 1* ; Abbas Rezaei Asl 2; Mohsen Azadbakht 2 1Department of Engineering, Shahre Ray Branch, Islamic Azad University, Tehran, Iran

2Department of Agricultural Machinery Mechanics, Agricultural Sciences & Natural Resources University of Gorgan,Gorgan, Iran

Received 07 August 2012; accepted 23 August 2012AbstractDrying 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 9.5-17.5 min for carrot slices, 3.5-12.5 min for black sunflower seeds, 7-17 min for whit mulberry, 3.5-9.5 min for potato slices and 3.25-5.5 min for lemon slicesdepending on the microwave power level. Moisture transfer from carrot slices was des cribed by applying the Fick’sdiffusion model, and effective moisture diffusion coefficients were calculated. A third order polynomial relationship wasfound to correlate the effective moisture diffusivity (D eff ) with moisture content. The effective moisture diffusivityincreased with decrease in moisture content of products. The average effective diffusivity values varied from 16.33×10 -9 to1.14×10 -8 m2/s for carrot slices, 1.16×10 -7 to 3.97×10 -7 m2/s for lack sunflower seeds, 2.81×10 -8 to 5.98×10 -8 m2/s for

lemon slices and 1.05×10-8

to 4.30×10-8

m2

/s for potato slices over the microwave power range studied.© 2011 Universal Research Publications. All rights reservedKeywords: Drying; moisture diffusivity; moisture content; microwave power.

1. IntroductionDrying is an important preservation process which reduceswater activity through the decrease of water content,avoiding potential deterioration and contamination duringlong storage periods [1]. Also, food quality of is preserved,the hygienic conditions are improved, and product loss isdiminished. Hot-air drying has been to date the mostcommon drying method employed for food materials.However, this method has many disadvantages, including

poor quality of dried products, low energy efficiency and along drying time. The use of microwave technology indrying agricultural products has several advantages. Thesemay include decreased drying time, high energy efficiency,high quality finished products, and uniform temperature inthe product [2, 3].Molecular diffusion is the main water transport mechanismand to predict the water transfer in food materials diffusionmodels based on Fick’s second law are used [3, 4, 5].Moisture diffusivity is an important physical transport

property which is useful in the engineering analysis of basic food processing operations such as drying. Diffusion phenomena are extremely complex, so reliable data are

scarce, especially for microwave drying. As a consequence,traditional food processing involving diffusion has beenmainly based on experimental knowledge. Accurate data on

moisture diffusivity of food products are essential for efficient and effective design of food processing operations,including drying. Effective moisture diffusivity describesall possible mechanisms of moisture movement within thefoods, such as liquid diffusion, vapour diffusion, surfacediffusion, capillary flow and hydrodynamic flow [7, 8, 9,10].Therefore, the aim of this study was to calculate theeffective moisture diffusivity as function of moisture

content and microwave power for some biomaterials suchas: carrot slices; lemon slices; potato slices; black sunflower seeds and white mulberry.2. Material and Method2.1. Samples preparationThe samples were procured from local vegetable market,and fresh white mulberries and black sunflower seedsamples were harvested from the experimental farm inTehran, Iran. The samples were stored at 4±0.5 °C beforethey were used in experiments. The samples were removedfrom the refrigerator before experimentation and wereallowed to attain room temperature. Carrots, potatoes andlemons were washed under running water to remove the

adhering impurities, and thinly sliced in thicknesses of 5mm using a sharp stainless steel knife. Generally samesamples of uniform size (average thickness, length and

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width 13.34 (±1.35), 3.41 (±1.24) and 23.83 (±1.60) mm,respectively) were used. The average initial moisturecontent of the samples were found to be 78.2±0.7% (Carrotslices); 86±0.8% (Lemon slices); 80±1.5% (Whitemulberry); 75±1.5% (Potato slices) and 31±1% (black sunflower seed) on wet basis, as determined by using

convective oven at 103±1 °C.2.2. Experimental equipment and procedure

A domestic microwave oven (M945, SamsungElectronics Ins) with maximum output of 1000 W at2450MHz was used for the drying experiments. Thedimensions of the microwave cavity were 327×370×207mm. The oven has a fan for air flow in drying chamber andcooling of magnetron. The moisture from drying chamber was removed with this fan by passing it through theopenings on the right side of the oven wall to the outer atmosphere. The microwave dryer was operated by acontrol 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 lemonslices at four microwave power levels. The moisture lossesof samples were recorded at 30 s intervals during the drying

process by a digital balance (GF-600, A & D, Japan) and anaccuracy of ± 0.001 g. For measuring the weight of thesample during experimentation, the tray with sample wastaken out of the drying chamber, weighed on the digital top

pan balance and placed back into the chamber. Drying wascarried out until the final moisture content reaches to alevel less than 5±1% on wet basis.2.5. Moisture diffusivityThe moisture ratio of samples during the thin layer drying

experiments was calculated using the following equation:

MR =Xt −Xe

X0 −Xe(1)

where MR is the moisture ratio (dimensionless), X t is themoisture content at drying time t (d.b.) and X 0 is the initialmoisture content (d.b.). The values of X e are relativelysmall compared to X t or X 0. Thus, Eq. (1) can be reduced toMR=X t/X 0.The effective moisture diffusivity of a food materialcharacterizes its intrinsic mass transfer property of moisture. During drying, it can be assumed that diffusivity,explained with Fick’s second law, is the only physicalmechanism to transfer the water to the surface.Effective moisture diffusivity, which is affected bycomposition, moisture content, temperature and porosity of the material, is used due to the limited information on themechanism of moisture movement during drying andcomplexity of the process. The moisture diffusivity for aninfinite slab was therefore calculated by the following Eq.(2) proposed by Crank [11] considering assumptionsmentioned hereunder [7, 8]:1- Moisture is initially uniformly distributed throughout themass of a sample.2- Mass transfer is symmetric with respect to the centre.3- Surface moisture content of the sample instantaneouslyreaches equilibrium with the condition of surrounding air.4- Resistance to the mass transfer at the surface isnegligible compared to internal resistance of the sample.

3- Mass transfer is by diffusion only.6- Diffusion coefficient is constant and shrinkage isnegligible.

MR =8

π 2 exp −π 2 Deff t L2 (2)

where D eff is the effective diffusivity (m 2/s), and L is thethickness (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 −π 2 F0 (3)

Thus:F0 = −0.101ln MR −0.0213 (4) The effective moisture diffusivity was calculated using Eq.(5) as:

Deff =F0t

L2

(5)

3. Results and DiscussionThe changing of the moisture ratio versus drying time for thin layer drying of samples at various microwave powersare given in Figs. 1-5 . A reduction in drying time occurredwith increasing the microwave power level. On the other hand, mass transfer within the sample was more rapidduring higher microwave power heating because more heatwas generated within the sample creating a large vapor

pressure difference between the centre and the surface of the product due to characteristic microwave volumetricheating.

Fig.1. Variation in moisture ratio as a function of dryingtime for carrot slices

Fig. 2: Variation of moisture ratio with drying time for thewhite mulberry

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Fig. 3: Variation of moisture ratio with drying time for thelemon slices

Fig. 4: Variation of moisture ratio with drying time for the potato slices

Fig. 5: Variation of moisture ratio with drying time for the black sunflower seeds

Fig. 6: Variation in effective moisture diffusivity withmoisture content for carrot slices at different microwave powers.

Fig. 7: Variation in effective moisture diffusivity withmoisture content for whit mulberry at different microwave

powers

Fig. 8: Variation in effective moisture diffusivity withmoisture content for lemon slices at different microwave

powers

Fig. 9: Variation in effective moisture diffusivity withmoisture content for potato slices at different microwave

powers

Fig.10: Variation in effective moisture diffusivity withmoisture content for black sunflower seeds at differentmicrowave powers.

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Fig. 11: Variation in ln (MR) and drying time for carrotslices dried at different microwave powers

Variation in effective moisture diffusivity of samples withmoisture content at different microwave power levels isshown in Figs. 6-10. The effective moisture diffusivityincreased with decrease in moisture content. However, themoisture diffusivity further was higher at any level of moisture content at higher microwave power level,resulting into shorter drying time. This may indicate that asmoisture content decreased, the permeability to vapour increased, provided the pore structure remained open. Thetemperature of the product rises rapidly in the initial stagesof 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 poresand results in pressure induced opening of pores. In the firststage of drying, liquid diffusion of moisture could be themain mechanism of moisture transport. As drying

progressed further, vapour diffusion could have been thedominant mode of moisture diffusion in the latter part of drying. Sharma and Prasad [7]; Sharma et al. [8] alsoreported similar trend in the variation in the moisturediffusivity with moisture content.

Fig. 12: Variation in ln (MR) and drying time for whitemulberry dried at different microwave powers.

A third order polynomial relationship was found tocorrelate the effective moisture diffusivity withcorresponding moisture content of samples and is given byEq. (6)Deff = A + B X + C X 2 + DX 3 (6) where A, B, C, D is the constants of regression, and X ismoisture content (d.b.)Regression constants for microwave drying of carrot slicesunder different powers are presented in Table 1. The high

values of R 2

are indicative of good fitness of empiricalrelationship to represent the variation in effective moisturediffusivity with moisture content of samples.

Fig. 13: Variation in ln (MR) and drying time for lemonslices dried at different microwave powers.

Average diffusivities are typically determined by plottingexperimental drying data in terms of ln (MR) versus dryingtime t in Eq. (7), because the plot gives a straight line witha slope (K) as follows:

K = π2 Deff

L2 (7)

Fig. 14: Variation in ln (MR) and drying time for potatoslices dried at different microwave powers

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 atdifferent microwave powers have been plotted in Figs. 11-

15 to obtain the curve slope which can give the averageeffective moisture diffusivity. Average values of effectivediffusivity for samples at different microwave power are

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Table 1- Regression coefficients of effective moisture diffusivity for different microwave powersProduct P(W) A* B* C* D* R

Lemon slice 720 6.5617 -3.6559 1.3997 -0.1819 0.990540 4.7982 -1.5922 0.4346 -0.0571 0.994360 4.1508 -1.9460 0.6598 -0.0831 0.971180 2.936 -1.0394 0.2712 -0.0311 0.997

Carrot slice 500 1.4877 -1.3507 0.6745 -0.1372 0.996400 1.3081 -1.2129 0.5893 -0.1109 0.996300 1.2130 -1.2750 0.6410 -0.1174 0.986200 0.7791 -0.6854 0.3530 -0.0745 0.994

Black sunflower seed

500 3.5182 -9.2823 -2.165 -0.7627 0.998400 3.0131 -17.317 57.106 -86.443 0.999300 2.6058 -15.863 55.075 -84.287 0.998200 1.0293 -2.214 -0.7877 -2.5661 0.997

Potato slice 500 4.5678 -4.7528 2.7528 -0.6359 0.994400 3.3433 -3.8348 2.4364 -0.5639 0.968300 1.8503 -2.4272 1.8938 -0.5509 0.970200 1.3037 -1.1363 0.6558 -0.1673 0.999

White mulberry 500 2.1881 -1.4525 0.6014 -0.1123 0.999400 1.7075 -1.2053 0.5180 -0.0967 0.995300 1.3577 -1.0978 0.5613 -0.1133 0.995200 0.6427 -0.3343 0.1072 -0.0205 0.999

*×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 generalrange of 10 −6 – 10−11 m2/s for drying of food materials [12].The values of D eff are comparable with the reported valuesof 1.0465×10 -8 to 9.1537×10 -8 m2/s mentioned for apple

pomace microwave drying [3], 0.30×10 −9-2.60 ×10 −9 for tomato pomaco hot air drying [13], 1.14×10 −6 to 6.09×10 −6 m2/s for tomato pomace microwave drying at 160-800W[14], 0.55×10 −7 to 3.5×10 −7 m2/s for Gundelia tournefortiimicrowave drying at 90-800W [5]; 2.24×10 −10 to16.4×10 −10 m2/s for blueberry infrared drying [15].Table 2- Result of average effective diffusivity of sampleswith different microwave power levels

Product P(W) Average diffusivity (m 2/s)

Carrot slices

500 1.14×10 - 400 9.12×10 -9 300 7.09×10 -9 200 6.33×10 -9

Black sunflower seeds

500 3.97×10 -7

400 3.15×10-

300 2.51×10 - 200 1.16×10 -

White mulberry

500 1.90×10 - 400 1.26×10 - 300 9.98×10 - 200 4.01×10 -

Potato slice

500 4.30×10 - 400 3.46×10 -8 300 1.28×10 -8 200 1.05×10 -

Lemon slice

720 5.98×10 -8 540 5.09×10 -8 360 3.80×10 -8 180 2.81×10 -

A linear regression analysis on the average diffusioncoefficient with microwave power resulted in the followingrelationships:

For carrot slices: Deff average = 2 × 10 −11 P + 2 × 10 −9 R2 = 0.960 (8)

For lemon slices:Deff average = 6 × 10 −11 P + 2 × 10 −8 R2 = 0.995 (9)

For black sunflower seeds:

Deff average = 9 × 10 −10

P −5 × 1 0 −8

R2

= 0.974 (10) For white mulberry:Deff average = 5 × 10 −10 P −5 × 1 0 −8 R2 = 0.977 (11)

For potato slices:Deff average = 1 × 10 −10 P −2 × 1 0 −8 R2 = 0.920 (12)

where P is the microwave power (W).4. ConclusionsMoisture diffusivity characteristic of carrot, lemon, and

potato slices, black sunflower seeds and white mulberryhave been investigated during microwave drying. The timerequired for drying products was considerably decreasedwith the increment in the drying microwave power.Effective moisture diffusivity depends on the moisture

content and increases with decrease in moisture content. Athird order polynomial relationship existed betweeneffective moisture diffusivity and the moisture content of

products. The average effective moisture diffusivity variedfrom 16.33×10 -9 to1.14×10 -8 m2/s for carrot slices, 1.16×10 -

7 to 3.97×10 -7 m2/s for lack sunflower seeds, 2.81×10 -8 to5.98×10 -8 m2/s for lemon slices and 1.05×10 -8 to 4.30×10 -8 m2/s for potato slices and was significantly influenced bymicrowave power.References1. Singh G., Arora S., Kumar S., 2010. Effect of

mechanical drying air conditions on quality of turmeric powder. J Food Sci Technol, 47(3):347 – 350.

2. Vadivambal R., Jayas D. S., 2007. Changes in qualityof microwave-treated agricultural products - a review.Biosys Eng, 98: 1 – 16.

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3. Wang Z., Sun J., Chen F., Liao X., Hu X., 2007.Mathematical modelling on thin layer microwavedrying of apple pomace with and without hot air pre-drying. J. Food Eng, 80: 536-544.

4. Sarimeseli A., 2011. Microwave drying characteristicsof coriander (Coriandrum sativum L.) leaves. Energ

Convers Manag, 52: 1449 – 1453.5. Evin D., 2011. Thin layer drying kinetics of Gundeliatournefortii L. Food Bioprod Process, doi:10.1016/j.fbp. 2011.07.002

6. Sharma G.P., Prasad S., 2004. Effective moisturediffusivity of garlic cloves undergoing microwave-convective drying. J. Food Eng., 65: 609-617

7. Sharma G.P., Verma R.C., Pathare P.B., 2005. Thin-layer infrared radiation drying of onion slices. J. FoodEng, 67: 361-366

8. da Silva W.P., Precher W.J., Slive C.M.D.P.S., GomesP.J., 2010. Determination of effective diffusivity andconvective mass transfer coefficient for cylindrical

solids via analytical solution and inverse method:Application to the drying of rough rice. J. Food Eng,98: 302 – 308.

9. Da Silva C.K.F., Da Silva Z.E., Mariani V.C., 2009.

Determination of the diffusion coefficient of drymushrooms using the inverse method. J. Food Eng, 95(1): 1 – 10.

10. Crank J., 1975. The Mathematics of Diffusion. OxfordUniversity Press, Oxford, UK

11. Arumuganathan T., Manikantan M.R., Rai R.D.,

Anandakumar S., Khare V., 2009. Mathematicalmodeling of drying kinetics of milky mushroom in afluidized bed dryer. Int Agrophys, 23: 1 – 7

12. Al-Muhtaseb, A.H., Al-Harahsheh, M., Hararah, M.and Magee, T.R.A. 2010. Drying characteristics andquality change of unutilized-protein rich-tomato

pomace with and without osmotic pre-treatment. IndCrop Prod., 31: 171 – 177

13. Al-Harahsheh M., Al-Muhtaseb A.H., Magee T.R.A.,2009. Microwave drying kinetics of tomato pomace:Effect of osmotic dehydration. Chem Eng Process, 48:524 – 531

14. Shi J., Pan Z., McHugh T.H., Wood D., Hirschberg E.,

Olson D., 2008. Drying and quality characteristics of fresh and sugar-infused blueberries dries with infraredradiation heating. LWT Food Sci Technol, 41:1962 – 1972

Source of support: Nil; Conflict of interest: None declared

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