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Convective dehydration kinetics of osmotically pretreated pomegranate arils
b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 3 0 7 e3 1 6
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Research Paper
Convective dehydration kinetics of osmotically pretreated pomegranate arils Manoj Mundada a, Bahadur Singh Hathan a,*, Swati Maske b a b
Department of Food Engineering and Technology, Sant Longowal Institute of Engineering and Technology, Longowal, India College of Food Technology, Marathwada Agricultural University, Parbhani, India
article info Article history:
The freezing of the whole pomegranate at 18 C was carried out prior to convective dehydration to increase the permeability of the outer layer of the arils. The arils were osmotically
Received 15 April 2010
pretreated in 50 B of sucrose solution at 40 C for 100 min with fruit to solution ratio of 1:4 (w/
Received in revised form
w). The osmotically dehydrated arils were further dehydrated convectively at different
17 August 2010
drying air temperatures of 50, 60 and 70 C up to final moisture content of 9 1% (w.b). Among
Accepted 16 September 2010
the models investigated, the Middilli model fitted the experimental data for convective
Published online 25 October 2010
drying of natural and osmosed pomegranate arils. During convective dehydration, the average effective moisture diffusivity of natural samples and osmosed samples at drying air temperatures ranging from 50 to 70 C varied between 2.60 1010 to 4.89 1010 m2 s1, between 3.37 1010 to 5.04 3 1010 m2 s1, respectively. The activation energy was 66.12 kJ mol1 for natural samples and 42.06 kJ mol1 for osmosed samples, respectively. ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Pomegranate (Punica granatum L.) is an important fruit of tropical and subtropical regions. It is extensively cultivated in Iran, Spain, Egypt, Russia, France, Argentina, China, Japan, USA, and in India. The edible part of the fruit (arils-pulp bearing seeds) contains considerable amounts of acids, sugars, vitamins, polysaccharides, polyphenols and important minerals (Al-Maiman & Ahmad, 2002; Vardin & Fenercioglu, 2003). Pomegranate has recently been acclaimed for its health benefits, in particular, for its diseasefighting antioxidant potential viz. ellagic acid, gallic acid, and punicalagin. These three compounds are antioxidants which can promote health by destroying cell damaging free radicals (Rosenblat & Aviram, 2006). Pomegranate arils also contain other potent antioxidants such as anthocyanins and tannins. The preservation of fruits and vegetable by dehydration presents a unique challenge. Because of the structural configuration of
these products, the removal of moisture must be accomplished in a manner that is the least detrimental to the product quality. Among the different methods of food preservation, convective dehydration is the most popular and efficient way to reduce the moisture content and preserve foods. Product quality notably depends on texture, colour and flavour and they deteriorate with convective dehydration (Lenart, 1996). A well known process to achieve good quality product is freeze drying, but this is an expensive method of food preservation. Therefore, there is a need forsimple economic and technically feasible alternative drying processes, which have low capital cost and offer a method to save highly perishable products making them available to the regions away from production zones. Osmotic dehydration is one such method (Shi & Le Maguer, 2002). The cellular membrane of arils exerts high resistance to mass transfer and this slows down the rate of osmotic and convective dehydration (Erle & Schubert, 2001). Freezing has been reported to enhance mass transfer during the dehydration
* Corresponding author. Tel.: þ91 94 63 21 68 75; fax: þ91 (0) 1672 280057. E-mail address: [email protected] (B.S. Hathan). 1537-5110/$ e see front matter ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2010.09.002
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Nomenclature ANOVA B D (De)avg D0 Ea E (%) EMC M
Analysis of Variance Brix Moisture diffusvity (m2s1) Average effective moisture diffusivity (m2s1) Effective moisture diffusivity at 273 K Activation energy Mean standard deviation modulus Equilibrium moisture content Moisture content at time t on dry basis [g (water) g1(dry matter)]
of apple and African Star Apple (Saurel, Raoult-Wack, Rios, & Guilbert, 1994). Also, freezing of whole pomegranate fruit could be utilised to establish the long-term preservation of pomegranate arils prior to osmotic pretreatment. Freezing of individual arils has not been carried out to avoid problems like oozing of juice form arils during thawing and for easy handling of the raw material. Osmotic dehydration involves the partial removal of water from food, such as fruit or vegetables, by immersion in a hypertonic solution (Lenart, 1996). This appears to improve the quality of the final product because it prevents oxidative browning and the loss of volatile flavouring constituents, reducing the fruit acidity and structural collapse during subsequent air-drying (Del Valle, Cuadros, & Aguilera, 1998). Osmotic pretreatment can also minimise drying colour losses (Raoult-Wack, Guilbert, Le Maguer, & Andrios, 1991), as well as reducing nutrient losses (Shi, Le Maguer, Kakuda, Liptay, & Niekamp, 1999). However, comparing moisture diffusivities during dehydration is difficult because of variations in food composition and physical structure and also because of the different methods and models employed to estimate diffusivity. The basic equation of Fick’s unsteady state law of diffusion is of the form, vM v2 M ¼D 2 vt vr
(1)
where M is the moisture content at time t on dry basis [g (water) g1 (dry matter)] D is the moisture diffusivity (m2s1); t is the time (seconds); r is the distance (m).
MR Me Mo N
Moisture ratio Equilibrium moisture content Initial moisture content Number of data points for positive values of effective diffusivity Gas constant (8.314 kJ mol1) Reduced chi-square Distance (cm) Root mean square error Coefficient of Determination Time (sec) Temperature ( C)
R c2 R RMSE R2 t T
The different analytical solutions of Eq. (1) have been given by Crank (1975) for several geometries and boundary conditions. An analytical solution of cylindrical geometry can be made assuming that, (i) moisture is initially uniformly distributed throughout the mass of a sample (ii) mass transfer is symmetrical with respect to the centre of the cylinder (iii) the surface moisture content of the sample instantaneously reaches equilibrium with the condition of the surrounding air, (iv) the sample size and geometry remain constant during convective dehydration (Crank, 1975). The solution is of the form, MR ¼
N Mt Me X 4 ¼ Mo Me n¼1 b2n
b De t exp n 2 r
(2)
where bn are the roots of Bessel function of zero order jo(r) ¼ 0 Ade-Omowaye, Taiwo, Eshtiaghi, Angersbach, and Knorr (2003) and Shi and Le Maguer (2002) calculated effective diffusivities using the slope method but this method gives only a single value of diffusivity for the entire process and therefore does not predict the kinetics of entire osmotic dehydration process, because the value of diffusivity changes with time and with moisture content of the commodity. Some researchers calculated effective diffusivity by using only first term of the analytical solution of the Fickian model assuming that the effect of terms other than first on the value of diffusivity were non-significant (Rastogi, Eshtiaghi, & Knorr, 1999; Sharma, Prasad, & Datta, 2003). The objectives of this work are to investigate the effect of osmotic pretreatment and drying air temperature on
Table 1 e Selected convective dehydration models. Model name
Model
Page Modified Page Newton Henderson and Pebis Wang and Singh Two-term exponential model Logarithmic model Midilli et al Two-term Weibull distribution
n
MR MR MR MR MR MR MR MR MR MR
¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼
exp(Kt ) (Kt)n exp(Kt) A$exp(Kt) 1 þ At þ Bt2 A$exp(Kt)þ (1A) exp(KAt) A$exp(Kt) þ C A$exp(Ktn) þ Bt A$exp(Kt)þ(1-A) exp(KAt) AB$exp(Ktn)
Reference Page (1949) Overhults, White, Hamilton, & Ross, 1973 Rahaman (1992) Henderson and Pabis (1961) Wang and Singh (1978) Sharaf-Eldeen, Blaisdell, and Spagna (1980) Yagcioglu et al (1999) Midilli, Kucuk and Yapar (2002) Henderson (1974) Ertekin and Yaldiz (2004)
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Optimisation of osmotic dehydration process was carried out with purpose of maximising water loss, solute gain and quality of the product. The optimum conditions were 50 B osmotic solution concentration, 40 C osmotic solution temperature and 100 min process duration at fruit to solution ratio 1:4 (w/w). Following osmotic pre treatment at optimum conditions, the moisture content of the arils reduced to 74% (w.b) and the solid content increased up to 26%. To prepare a shelf-stable product, the pomegranate arils were dehydrated up to final moisture content 9 1% (w.b.) (Singh, Kingly, & Jain, 2007) at an air temperature of 50, 60, 70 C and air velocity of 1.6 m s1.
Table 2 e Total convective drying time for natural and osmosed dried pomegranate arils. Temperature ( C)
Average drying time (min) Natural
Osmosed
660 585 455
545 480 320
50 60 70
Table 3 e Analysis of variance (ANOVA) for overall effect of process variables. Source of Variation
Sum of df squares
Temperature 48100 Pretreatment 21004.17 Error 233.33 Total 69337.50
2 1 2 5
MS
F
2.2.
P-value F crit
24050.00 206.1429 21004.17 180.0357 116.66
0.0048* 0.0055*
Analysis of drying data
To study the drying behaviour at different drying air temperatures, moisture content (dry basis) and drying rate were calculated. The drying curves (moisture content vs. time) were plotted to observe the effect of process variables. Corresponding to the drying curves, drying rate curves were also plotted (Kar & Gupta, 2003). The effective moisture diffusivity (De) values of pomegranate arils during convective dehydration were calculated by considering only the first term of the Eq. (2) assuming that the effect of the terms other than the first are negligible. By considering only the first term, Eq. (2) reduces to
19.00 18.51
* Significant at 5% level of significanceWhere, variables are temperature and osmotic pretreatment
convective dehydration kinetics and determination of moisture diffusivity and activation energy.
MR ¼
2.
Materials and methods
2.1.
Experimental procedure
(3)
where, b1 is the first root of Bessel function of zero order ¼ 2.405. The average apparent diffusion coefficient was calculated by using,
Fresh, well graded, whole pomegranates var. Kandhari were procured from local market of Sangrur, Punjab, India and were washed and frozen at 18 C for a minimum duration of 24 h. Before osmotic dehydration was carried out, frozen pomegranates were thawed at room temperature. Thawed pomegranates were immediately broken into separate arils. The initial moisture content of arils varied from 80.5 to 81% (w.b).
[M.C.% (w.b)]
2 4 b1 De t exp 2 r2 b1
Pn ðDe Þavg ¼
1
De
(4)
n
where, De is the effective moisture diffusivity (m2s1); (De)avg is the average value of effective moisture diffusivity (m2s1); n are the number of data points.
-1
M.C {g [water] g [dm]}
[81.81] 4.5 [80.00] 4 [77.77] 3.5 Osmosed [75.00] 3 Natural [71.42] 2.5 [66.66] 2 [60.00] 1.5 [50.00] 1 [33.33] 0.5 [0]
0 0
100
200
300
400
500
600
700
Time (min)
Fig. 1 e Effect of drying time on moisture content of pomegranate arils at 60 C.
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2
-1
Drying rate {g [water] min }
2.5
1.5
1 Osmosed
0.5 Natural
0 0
0.5
[0]
[33.33]
1
1.5
2
2.5
3
3.5
4
4.5
[50.00]
[60.00]
[66.66]
[71.42]
[75.00]
[77.77]
[80.00]
[81.81]
-1
M.C {g [water] g [dm]} [M.C % (w.b.)]
Fig. 2 e Effect of time on drying rate of arils at 50 C air temperature.
[M.C.% (w.b)]
-1
M.C {g [water] g [dm]}
[81.81] 4.5
Natural
[80.00] 4 [77.77] 3.5
50°C 60°C
[75.00] 3
70°C
[71.42] 2.5 [66.66] 2 [60.00] 1.5 [50.00] 1 [33.33] 0.5 [0]
0 0
100
200
300
400
500
600
700
800
Time (min) M.C % (w.b) [77.77] 3.5 [80.00]
-1
M.C {g [water] g [dm]}
Osmosed
3
[71.42] 2.5
[66.66]
2
50°C 60°C 70°C
[60.00] 1.5 [50.00] 1 [33.33] 0.5 [0]
0 0
100
200
300
400
500
600
Time (min)
Fig. 3 e Effect of drying air temperature on drying behaviour of osmosed and natural pomegranate arils at different drying air temperature.
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The dependence of average moisture diffusivity on drying air temperature was obtained using the Arrhenius relationship by, ðDe Þavg ¼ Do e
long drying time as suggested by Ertekin and Yaldiz (2004) and Pokharkar and Prasad (2002). This is because the value of moisture ratio varies from 0 to 1. The value of the equilibrium moisture content is very low and it can be subtracted from both denominator and numerator formulae. Therefore, it does not greatly affect the value of moisture ratio. Also the moisture ratio is dependent upon initial moisture content at that time. In the case of long drying times, the value of equilibrium moisture content (EMC) is very much lower.
Ea RðTþ273Þ
(5)
where, T is temperature ( C), R is gas constant having constant value of 8.314 kJ mol1 K, Do is the effective diffusivity at 273 K, Ea is the activation energy. The average effective moisture 1 Þ and a straight line diffusivity (De)avg was plotted against ðTþ273 relationship with negative slope was obtained. Thermodynamically, activation energy is the ease with which the water molecules pass the energy barrier when migrating within the product.
2.3.
2.4.
In addition to R2, the various statistical parameters such as reduced chi-square c2 and root mean square error (RMSE) were also used as criteria to select the best equation. As these parameters are not a good criteria for evaluating non-linear mathematical models, therefore the percent mean relative deviation modulus (E%) was also used to select the best equation to account for variations in the drying curves of the dried samples as recommended by several authors in recent studies (Azoubel & Murr, 2004) have indicated the deviation of the observed data from the predicted line. Therefore, the best model was chosen as one with the highest coefficient of
Validity of empirical models
The validity of the commonly used empirical models for convective dehydration (Table 1) was checked by non-linear regression analysis of the experimental data. However, moisture ratio was simplified to M/Mo instead of (MMe)/(MoMe) for fitting the experimental data to the empirical models and determination of effective moisture diffusivity because of the
2.5
Adequacy of fit of empirical models
Natural
-1
Drying rate [(g) water min ]
2
1.5
1
50°C
[
60°C 70°C
0.5
0 0
0.5
1
[0]
[33.33]
[50.00]
1.5 [60.00]
2 [66.66]
2.5
3
3.5
4
4.5
[71.42]
[75.00]
[77.77]
[80.00]
[81.81]
-1
M.C {g [water] g [dm]} [M.C. % (w.b.)] 2.5
Osmosed
-1
Drying rate [(g) water min ]
2
1.5
1 50°C 60°C
0.5
70°C
0 0
0.5
[0]
[33.33]
1 [50.00]
1.5 2 [66.66] [60.00] -1 M.C {g [water] g [dm]}
2.5 [71.42]
3 [75.00]
M.C % (w.b.)
Fig. 4 e Effect of drying air temperature on drying behaviour of osmosed pomegranate arils at different drying air temperature.
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correlation, R2; and the least,c2 RMSE and mean relative deviation modulus, E. The average percent difference between the experimental and predicted values mean relative deviation modulus, E, defined by following equation: Eð%Þ ¼
n 100 X experimental value predicted value n i¼1 experimental value
preosmosed arils began at a relatively lower moisture content (74% w.b.) due to moisture removal and solute gain during osmotic dehydration. The total convective dehydration time of samples at 50, 60, 70 C when dried to final moisture content of 9 1% (w.b) are given in Table 2. The analysis of variance (ANOVA) of total convective dehydration time (Table 3) revealed that, both the osmotic pretreatment and drying air temperature had a significant effect at 5% level of confidence interval.
(8)
A value of E < 5.0 indicates an excellent fit, whilst the values > than 10 are indicative of a poor fit.
2.5.
3.1. Effect of osmotic pretreatment on convective drying kinetics
Statistical analysis
Table 2 indicates that, the total convective dehydration time at 50 C ofnatural osmosed arils was 660 min but was 545 min for samples preosmosed with the solution of sucrose. Therefore osmotic pretreatment in sucrose solution reduced by approximately 17.5 min to the convective dehydration time when compared to natural pomegranate arils at 50 C drying air temperature. This might be because leaching of some soluble components of the cellular structure of arils loosens the surface cellular structure during soaking in osmotic solution. This reduces cell wall resistance and increases the
Analysis of variance (ANOVA) was conducted to determine the effect of variable factors on drying parameter using Statistica for Windows 5.0, 1995 (Tulsa, OK, USA).
3.
Results and discussion
The drying of natural pomegranate arils started at relatively high moisture content (81% w.b) whereas the drying of 9
Effective diffusivity (10
-10
m 2s -1 )
8
Natural
7
50°C
6
70°C
5
60°C
4 3 2 1 0 0
0.5
1
1.5
[0]
[33.33]
[50.00]
[60.00]
2
2.5 [71.42]
[66.66]
-1
M.C {g [water] g [dm]} [M.C. % (w.b.)] 7
Osmosed
5
Effective diffusivity(10
-10
2 -1
ms )
6
50°C
4
60°C 70°C
3 2 1 0 0
0.5
[0]
[33.33]
1
[50.00]
1.5
2
[60.00] [66.66] -1 M.C {g [water] g [dm]}
2.5
[71.42]
3
[75.00]
3.5
[77.77]
[M.C. % (w.b.)]
Fig. 5 e Effect of air temperature on effective diffusivity during convective dehydration of osmosed and natural pomegranate arils.
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Table 4 e Average effective diffusivity of natural and osmosed dried pomegranate arils. Temperature
Natural (m2 s1)
Osmosed (m2 s1)
50 C 60 C 70 C
2.5992 3 1010 3.4264 3 1010 4.8884 3 1010
3.3723 3 1010 4.0152 3 1010 5.0432 3 1010
drying rate of arils, which agrees with the results of Riva, Campolongo, Leva, Maestrelli and Torreggiani (2004) for apricot cubes, Rodrigues and Fernandes (2007) for melons. The effect of osmotic pretreatment on convective drying time of pomegranate arils at 60 C is presented in Fig. 1. It can be seen that the moisture content of osmosed arils reduced from 2.93 to 0.10 g [water] g1 [dm] within 480 min whereas in the case of fresh arils, the moisture content reduced from 4.25 to 0.10 g [water] g1 [dm] within 585 min at 60 C of drying air temperature. Fig. 2 demonstrates increase in drying rate of preosmosed arils as compared to natural arils at 50 C of drying air temperature. The similar behaviours have also been observed at 60 C and 70 C drying air temperatures.
3.2.
rates. Drying rate later decreased with decreasing moisture for natural and osmotically pretreated samples under all the conditions of convective dehydration. The reason for the reduction of drying rate might be due to the reduction in porosity of the material and also due to shrinkage as the drying process advances. In early periods of drying, there was decline in drying rate for both osmosed and natural arils. Drying rate curves showed very low drying rates, when the average moisture content of the product approached to 1 g [water] g1 [dm]. Therefore, a considerably long drying period would be necessary to achieve final moisture content lower than 1 g [water] g1 [dm]. As indicated by drying rate curves in Fig. 4, the migration of moisture to the surface and evaporation (drying) rate from the surface decreased with decreasing moisture in the product. However, closer examination of drying rate for experimental data for values below moisture content of 1 g [water] g1 [dm] for natural arils having higher drying rate compared to osmosed arils. This might be due to the resistance offered by solute gain during osmotic pretreatment.
3.3. Effective moisture diffusivity during convective dehydration process
Effect of drying air temperature on drying kinetics
The effect of temperature on the drying curve of both osmosed and natural pomegranate arils is shown in Fig. 3. To dry the product to a final moisture content of 9 1% (w.b.), the drying time at 70 C was lower compared to drying time at 60 C and 50 C. This is due to the increase in water vapour pressure within the arils with increasing temperature, which increases moisture migration. Similar results have been obtained in case of carrot (Doymaz, 2004), garlic (Madamba, Driscoll, & Buckle, 1996) and eggplant (Ertekin & Yaldiz, 2004). It can be seen (Fig. 3) that, the osmosed and natural pomegranate arils did not have a constant rate drying period and complete drying took place during the falling rate period. The absence of a constant rate period was because the product could not provide a constant supply of water for an appreciable period of time because of rapid thinlayer drying of the product at initial stages of drying (Lahsasni, Kouhila, Mahrouz, & Jaouhari, 2004; Prakash, Jha, & Datta, 2004). Drying in the falling rate period showed that internal mass transfer occurred by diffusion. Similar results have been obtained by different authors for drying of vegetables and fruits (Doymaz, 2004; Madamba et al., 1996). Fig. 4 shows that, the drying rates were highest at the beginning of the drying process, when the moisture content was greatest, with natural arils displaying highest initial drying
Table 4 indicates that effective moisture diffusivity during convective dehydration of osmosed samples was higher than natural samples. The increase of effective moisture diffusivity (De) with osmotic pretreatment can be due to loosening of the surface cellular structure and leaching of some soluble components of the external cell layers of arils during soaking in osmotic solution of sucrose at 50 B, 40 C for 100 min of process duration, Similar results have been reported by Brambilla, Maffi, Bertolo and Torreggiani (2000) in strawberry tissue, Riva et al. (2004) in apricot cubes, and Rodrigues and Fernandes (2007) in melons. Average effective moisture diffusivity during convective dehydration for un-osmosed and osmosed pomegranate arils was 2.599 1010 to 3.3723 1010 m2 s1 at 55 C. These values are lower than the corresponding values for the experiments carried out at 65 and 75 C. Fig. 5 indicates that the effective moisture diffusivity (De) increase with drying air temperature might be because of increase in the vapour pressure inside the pomegranate arils. Comparisons with results of effective diffusivity reported in the literature are difficult because of different estimation methods and the models employed with variations in food composition and physical structure. The average effective moisture diffusivity during convective dehydration of pears by Park, Bin, and Brod (2002) was found to be 2.06e6.37x1010 m2 s1 for natural pears and
Table 5 e Various regression coefficient and statistical parameters of Page model. Temp ( C)
K
N
R2
c
E%
RMSE
Natural
50 60 70
0.0030 0.0025 0.0008
1.0979 1.1556 1.4605
0.99 0.99 0.99
9.58E 05 0.000499 0.000937
7.1354 9.0769 35.6744
4.58E 05 0.000237 0.000447
Osmosed
50 60 70
0.0026 0.0030 0.0091
1.1651 1.1789 1.0986
0.99 0.99 0.99
0.000174 0.000303 0.000545
9.9887 0.4038 26.0020
8.17E 05 0.000142 0.000252
Osmotic pretreatment
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Table 6 e Various regression coefficient and statistical parameters of modified Page model. Osmotic Pretreatment
Temp ( C)
K
R2
N
c2
Natural
50 60 70
0.0051 0.0074 0.0078
1.1001 1.1810 1.5219
0.99 0.99 0.99
9.15 10 0.0003 0.0009
Osmosed
50 60 70
0.0061 0.0057 0.0139
1.1662 1.1583 1.0990
0.99 0.99 0.99
0.0002 0.0005 0.0005
1.87e8.12 1010 m2 s1 for preosmosed pears in sucrose syrup Karathanos, Villalobos, and Saravacos (1990) also found that De was 1.60 1010 m2 s1 in apples air-dried at 55 C, while this parameter decreased to 5.0 1010 m2 s1, when samples were osmotically pretreated in 45 B sucrose solution. It is also evident from Fig. 5 that effective moisture diffusivity (De) increased with decrease in the moisture content up to a moisture level of 0.3e0.4 g [water] g1 [dm] for all the conditions of convective dehydration. This behaviour might be due to the fact, that the initial temperature of the sample being less than drying air temperature at the start of the drying process, but with reductions in moisture content, the sample heated up and subsequently the moisture diffusivity increased. Because the diffusion coefficient decreases with decreasing moisture content it is more dependent on product temperature than moisture content (Adu & Otten, 1996). Further, in the last phase of each experiment with osmosed samples below moisture contents approximately 0.5e0.6 g [water] g1 [dm], there was a sharp decrease in moisture diffusivity although product temperatures were high. This is because during drying product temperatures rise to the wet bulb temperature of the drying air. This results in increase of product temperature and effective diffusivity. However, with decrease of moisture content, the product temperature will increase and approach the dry bulb temperature of the drying air. The decrease in average effective moisture diffusivity (De)avg in later stages is due to non-availability of free water for diffusion during the finishing stage of convective dehydration. (Adu & Otten, 1996).
3.4.
Activation energy for convective dehydration
Following equations with high correlation coefficients (R2 > 0.99), were obtained for the linear plots between (De)avg 1 and ðTþ273Þ for different osmotic pretreatments.
3.4.1.
5
E%
RMSE
7.1327 16.8189 37.1217
4.57 105 0.0001 0.0004
9.9875 9.1873 26.0156
0.0001 0.0002 0.0003
For natural arils
3:493 ðDe Þavg ¼ 9:140 105 exp ðT þ 273Þ
(9)
For osmosed arils 2:225 ðDe Þavg ¼ 9:229 105 exp ðT þ 273Þ
(10)
The activation energy for the convective drying of pomegranate arils was 66.12 kJ mol1 for natural aril samples, and was 42.06 kJ mol1 in case of osmosed samples. It was observed that activation energy was higher for natural arils than for osmosed samples. The high value of activation energy for natural arils might also be due to presence of high initial moisture content of 81% (w.b.) as compared to 74% (w.b.) for preosmosed samples. Therefore, more thermal energy would be required to remove greater amounts of water from natural pomegranate arils. Similar results were reported by Reppa, Mandela, Kostaropoulos, and Saravacos (1999). Park et al. (2002) also reported that activation energy was 26.46e31.21 kJ mol1 for pears without osmotic dehydration, 24.34e28.20 kJ mol1 for pears with osmotic dehydration. These values were 39.7 kJ mol1 and 24.0 kJ mol1 for fresh and osmotically pretreated apple cubes respectively (Simal, Bauza de Mirabo, Deya, & Rossello, 1997).
3.5. Validity of empirical models for convective dehydration In all the experiments of convective dehydration of osmosed and natural samples, the general exponential model, the twoterm exponential model and the logarithmic model did not fit the experimental data. Also, the low values of R2 of the Wang and Singh and Weibull distribution models led to their
Table 7 e Various regression coefficient and statistical parameters of Newton model. Temp ( C)
K
R2
c2
E%
RMSE
Natural
50 60 70
0.0061 0.0073 0.0080
0.99 0.99 0.98
0.0008 0.0010 0.0041
10.7497 12.6943 26.9862
0.0004 0.0004 0.0019
Osmosed
50 60 70
0.0051 0.0057 0.0139
0.99 0.99 0.99
0.0003 0.1984 0.0007
13.1422 18.8802 22.4960
0.0001 0.0005 0.0003
Osmotic Pretreatment
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b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 3 0 7 e3 1 6
Table 8 e Various regression coefficient and statistical parameters of Henderson and Pabis model. Temp ( C)
A
K
R2
c2
E%
RMSE
Natural
50 60 70
1.0540 1.0311 1.1230
0.0066 0.0059 0.0089
0.99 0.99 0.98
0.0005 0.0010 0.0029
9.1007 15.6961 23.2508
0.0002 0.0005 0.0014
Osmosed
50 60 70
1.0349 1.0628 1.0368
0.0053 0.0079 0.0145
0.99 0.99 0.99
0.0001 0.0005 0.0006
2.2000 12.3116 22.8695
0.0001 0.0003 0.0003
Osmotic Pretreatment
Table 9 e Various regression coefficient and statistical parameters of Wang and Singh model. Temp ( C)
A
B
R2
c2
E%
RMSE
Natural
50 60 70
0.003672 0.004157 0.005804
0.000003 0.000004 0.000008
0.99 0.98 0.98
0.0009 0.0009 0.0022
17.1090 25.5829 49.1155
0.0004 0.0004 0.0010
Osmosed
50 60 70
0.003672 0.005488 0.009243
0.000003 0.000008 0.000021
0.99 0.99 0.97
0.0009 0.0010 0.0045
17.1090 28.5319 64.0925
0.0004 0.0004 0.0041
Osmotic Pretreatment
Table 10 e Various regression coefficient and statistical parameters of Midilli model. Temp ( C)
A
K
N
C
R2
c2
E%
RMSE
Natural
50 60 70
1.0014 0.9549 1.0020
0.0032 0.0013 0.0006
1.0848 1.2631 1.5164
0.000011 0.000020 0.000087
0.99 0.99 0.99
9.38E 05 0.0695 0.0005
8.3715 6.6154 6.3341
4.48E 05 0.000162 0.000243
Osmosed
50 60 70
0.9746 1.0010 6.6941
0.0012 0.0022 1.3026
1.3213 1.2518 0.0003
0.000096 0.000078 0.016690
0.99 0.99 0.99
0.0001 9.6E 05 0.0004
4.3090 5.0018 9.8183
4.94E 05 4.51E 05 0.000208
Osmotic Pretreatment
rejection. Table 5e10 shows that there were high values of R2 with the Page, modified Page, Newton, Henderson and Pabis, and Midilli models. For both osmosed and natural samples, the Midilli model (Table 10) had an excellent fit compared to other models due to lower value of E (%), the highest coefficient of correlation (R2), the lowest values of RMSE and c2 This indicates that this was the best model for predicting the convective dehydration behaviour of natural and osmosed pomegranate arils at drying air temperature of 50, 60 and 70 C. The adequacy offit of the Midilli et al. (2002) model was is in close agreement with results of Singh, Kumar, and Gupta (2007) for the convective dehydration of preosmosed carrot cubes.
4.
Conclusions
With the aim of preparing a shelf stable product having final moisture content of 9 1% (w.b.), osmotic pretreatment before convective dehydration of pomegranate arils results in a decrease of the convective dehydration time. Osmotic pretreatment also results in increases in drying rate and effective moisture diffusivity and decreasing activation energy during convective dehydration. Among the empirical models applied to the data, the Midilli model best described
the convective drying characteristics of natural and osmosed pomegranate arils.
references
Ade-Omowaye, B. I. O., Taiwo, K. A., Eshtiaghi, N. M., Angersbach, A., & Knorr, D. (2003). Comparative evaluation of the effects of pulsed electric field and freezing on cell membrane permeabilisation and mass transfer during dehydration of red bell peppers. Innovative Food Science and Emerging Technologies, 4, 177e188. Adu, B., & Otten, L. (1996). Microwave heating and mass transfer characteristics of white beans. Journal of Agricultural Engineering Research, 64, 71e78. Al-Maiman, S. A., & Ahmad, D. (2002). Changes in physical and chemical properties during pomegranate (Punica granatum L.) fruit maturation. Food Chemistry, 76, 437e441. Azoubel, P. M., & Murr, F. E. X. (2004). Mass transfer kinetics of osmotic dehydration of cherry tomato. Journal of Food Engineering, 61, 291e295. Brambilla, A., Maffi, D., Bertolo, G., & Torreggiani, D. (2000). Effect of osmotic dehydration time on strawberry tissue structure, In: ICEF 8 "Eight international congress on Engineering and Food" Book of abstracts (pp. 211). Puebla, Mexixco, April 9e13, 2000 Crank, J. (1975). Mathematics of diffusion (2nd ed.). London: Oxford University Press.
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Del Valle, J. M., Cuadros, T. R. M., & Aguilera, J. M. (1998). Glass transition and shrinkage during drying and storage of osmosed apple pieces. Food Research International, 31, 191e204. Doymaz, I. (2004). Convective air drying characteristics of thin layer carrots. Journal of Food Engineering, 61, 359e364. Erle, & Schubert, H. (2001). Combined osmotic and microwaveevacuum dehydration of apples and strawberries. Journal of Food Engineering, 49(2/3), 193e199. Ertekin, C., & Yaldiz, O. (2004). Drying of eggplant and selection of a suitable thin layer drying model. Journal of Food Engineering, 63, 349e359. Henderson, S. M. (1974). Progress in developing the thin-layer drying equation. Transactions of the ASAE, 17, 1167e1168, 1172. Henderson, S. M., & Pabis, S. (1961). Grain drying theory. II: temperature effects on drying coefficients. Journal of Agricultural Engineering Research, 6, 169e174. Kar, A., & Gupta, D. K. (2003). Air drying of osmosed button mushrooms. Journal of Food Science and Technology, 40, 23e27. Karathanos, V. T., Villalobos, G., & Saravacos, G. D. (1990). Comparison of two methods of estimation of the effective moisture diffisivity from drying data. Journal of Food Science, 55, 218e231. Lahsasni, S., Kouhila, M., Mahrouz, M., & Jaouhari, J. T. (2004). Drying kinetics of prickly pear fruit (Opuntia ficus indica). Journal of Food Engineering, 61, 173e179. Lenart, A. (1996). Osmotic-convective drying of fruits and vegetables: technology and application. Drying Technology, 18 (4&5), 951e966. Madamba, P. S., Driscoll, R. H., & Buckle, K. A. (1996). The thin layer drying characteristics of garlic slices. Journal of Food Engineering, 29, 75e97. Midilli, A., Kucuk, H., & Yapar, Z. (2002). A new model for single layer drying. Drying Technology, 20, 1503e1513. Overhults, D. G., White, G. M., Hamilton, H. E., & Ross, I. J. (1973). Drying soybeans with heated air. Transactions of the ASAE, 16, 112e113. Page, G. (1949). Factors influencing the maximum rates of air drying shelled corn in thin layer. MSc Thesis, Purade University. Lafayette, IN, USA. Park, K. J., Bin, A., & Brod, F. P. R. (2002). Drying of pear d’Anjou with and without osmotic dehydration. Journal of Food Engineering, 56, 97e103. Pokharkar, S. M., & Prasad, S. (2002). Air drying behaviour of osmotically dehydrated pineapple. Journal of Food Science Technology, 39, 384e387. Prakash, S., Jha, S. K., & Datta, N. (2004). Performance evaluation of blanched carrots dried by three different driers. Journal of Food Engineering, 62, 305e313. Rahaman, M. S. (1992). Osmotic dehydration kinetics of food. Indian Food Industry, 15, 20e24. Raoult-Wack, A. L., Guilbert, S., Le Maguer, M., & Andrios, G. (1991). Simultaneous water and solute transport in shrinking media: application to dewatering and impregnation soaking
process analysis (osmotic dehydration). Drying Technology, 9, 589e612. Rastogi, N. K., Eshtiaghi, M. N., & Knorr, D. (1999). Accelerated mass transfer during osmotic dehydration of high intensity electrical field pulse pretreated carrots. Journal of Food Science, 64, 1020e1023. Reppa, A., Mandela, J., Kostaropoulos, & Saravacos, G. D. (1999). Influence of solut temperature and concentration on the combined osmotic and drying. Drying Technology, 17, 1449e1458. Riva, M., Campolongo, S., Leva, A. A., Maestrelli, A., & Torreggiani, D. (2004). Structure- property relationship in osmo-air-dehydrated apricot cubes. Food Research International, 38, 533e542. Rodrigues, S., & Fernandes, F. A. N. (2007). Dehydration of melons in a ternary system followed by air drying. Journal of Food Engineering, 80, 678e687. Rosenblat, M., & Aviram, M. (2006). Antioxidative properties of pomegranate: in vitro studies. In N. P. Seeram, R. N. Schulmanand, & D. Heber (Eds.), Pomegranates ancient roots to modern medicine. Boca Raton, FL: CRC Press. Saurel, R., Raoult-Wack, A. L., Rios, G., & Guilbert, S. (1994). Mass transfer phenomena during osmotic dehydration of apple i: fresh plant tissue. International Journal of Food Science and Technology, 29, 531e542. Sharaf-Eldeen, O., Blaisdell, Y. I., & Spagna, G. (1980). A model for ear corn drying. Transactions of the ASAE, 23, 1261e1271. Sharma, G. P., Prasad, S., & Datta, A. K. (2003). Drying kinetics of garlic cloves under convective drying conditions. Journal of Food Science and Technology, 40, 45e51. Shi, J., & Le Maguer, M. (2002). Osmotic dehydration of foods: mass transfer modeling aspects. Food Reviews International, 18 (4), 305e335. Shi, J., Le Maguer, M., Kakuda, Y., Liptay, A., & Niekamp, F. (1999). Lycopene degradation and isomerization in tomato dehydration. Food Research International, 32, 15e21. Simal, S., Bauza de Mirabo, F., Deya, E., & Rossello, C. (1997). A simple model to predict mass transfers in dehydration by osmosis. Z Lebensm Unters Forsc, H A, 204, 210e214. Singh, B., Kumar, A., & Gupta, A. K. (2007). Study of mass transfer kinetics and effective diffusivity during osmotic dehydration of carrot cubes. Journal of Food Engineering, 79, 471e480. Singh, D. B., Kingly, A. R. P., & Jain, R. K. (2007). Studies on separation techniques of pomegranate arils and their effect on quality of anardana. Journal of Food Engineering, 79, 671e674. Statistica for Windows 5.0. (1995). Computer program manual. Tulsa: StatSoft, Inc. Vardin, H., & Fenercioglu, H. (2003). Study on the development of pomegranate juice processing technology: clarification of pomegranate juice. Nahrung, 47, 300e303. Wang, C. Y., & Singh, R. P. (1978). A single layer drying equation for rough rice. St. Joseph, MI: ASAE. ASAE Paper No: 78-3001.
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Research Paper
Convective dehydration kinetics of osmotically pretreated pomegranate arils Manoj Mundada a, Bahadur Singh Hathan a,*, Swati Maske b a b
Department of Food Engineering and Technology, Sant Longowal Institute of Engineering and Technology, Longowal, India College of Food Technology, Marathwada Agricultural University, Parbhani, India
article info Article history:
The freezing of the whole pomegranate at 18 C was carried out prior to convective dehydration to increase the permeability of the outer layer of the arils. The arils were osmotically
Received 15 April 2010
pretreated in 50 B of sucrose solution at 40 C for 100 min with fruit to solution ratio of 1:4 (w/
Received in revised form
w). The osmotically dehydrated arils were further dehydrated convectively at different
17 August 2010
drying air temperatures of 50, 60 and 70 C up to final moisture content of 9 1% (w.b). Among
Accepted 16 September 2010
the models investigated, the Middilli model fitted the experimental data for convective
Published online 25 October 2010
drying of natural and osmosed pomegranate arils. During convective dehydration, the average effective moisture diffusivity of natural samples and osmosed samples at drying air temperatures ranging from 50 to 70 C varied between 2.60 1010 to 4.89 1010 m2 s1, between 3.37 1010 to 5.04 3 1010 m2 s1, respectively. The activation energy was 66.12 kJ mol1 for natural samples and 42.06 kJ mol1 for osmosed samples, respectively. ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Pomegranate (Punica granatum L.) is an important fruit of tropical and subtropical regions. It is extensively cultivated in Iran, Spain, Egypt, Russia, France, Argentina, China, Japan, USA, and in India. The edible part of the fruit (arils-pulp bearing seeds) contains considerable amounts of acids, sugars, vitamins, polysaccharides, polyphenols and important minerals (Al-Maiman & Ahmad, 2002; Vardin & Fenercioglu, 2003). Pomegranate has recently been acclaimed for its health benefits, in particular, for its diseasefighting antioxidant potential viz. ellagic acid, gallic acid, and punicalagin. These three compounds are antioxidants which can promote health by destroying cell damaging free radicals (Rosenblat & Aviram, 2006). Pomegranate arils also contain other potent antioxidants such as anthocyanins and tannins. The preservation of fruits and vegetable by dehydration presents a unique challenge. Because of the structural configuration of
these products, the removal of moisture must be accomplished in a manner that is the least detrimental to the product quality. Among the different methods of food preservation, convective dehydration is the most popular and efficient way to reduce the moisture content and preserve foods. Product quality notably depends on texture, colour and flavour and they deteriorate with convective dehydration (Lenart, 1996). A well known process to achieve good quality product is freeze drying, but this is an expensive method of food preservation. Therefore, there is a need forsimple economic and technically feasible alternative drying processes, which have low capital cost and offer a method to save highly perishable products making them available to the regions away from production zones. Osmotic dehydration is one such method (Shi & Le Maguer, 2002). The cellular membrane of arils exerts high resistance to mass transfer and this slows down the rate of osmotic and convective dehydration (Erle & Schubert, 2001). Freezing has been reported to enhance mass transfer during the dehydration
* Corresponding author. Tel.: þ91 94 63 21 68 75; fax: þ91 (0) 1672 280057. E-mail address: [email protected] (B.S. Hathan). 1537-5110/$ e see front matter ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2010.09.002
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Nomenclature ANOVA B D (De)avg D0 Ea E (%) EMC M
Analysis of Variance Brix Moisture diffusvity (m2s1) Average effective moisture diffusivity (m2s1) Effective moisture diffusivity at 273 K Activation energy Mean standard deviation modulus Equilibrium moisture content Moisture content at time t on dry basis [g (water) g1(dry matter)]
of apple and African Star Apple (Saurel, Raoult-Wack, Rios, & Guilbert, 1994). Also, freezing of whole pomegranate fruit could be utilised to establish the long-term preservation of pomegranate arils prior to osmotic pretreatment. Freezing of individual arils has not been carried out to avoid problems like oozing of juice form arils during thawing and for easy handling of the raw material. Osmotic dehydration involves the partial removal of water from food, such as fruit or vegetables, by immersion in a hypertonic solution (Lenart, 1996). This appears to improve the quality of the final product because it prevents oxidative browning and the loss of volatile flavouring constituents, reducing the fruit acidity and structural collapse during subsequent air-drying (Del Valle, Cuadros, & Aguilera, 1998). Osmotic pretreatment can also minimise drying colour losses (Raoult-Wack, Guilbert, Le Maguer, & Andrios, 1991), as well as reducing nutrient losses (Shi, Le Maguer, Kakuda, Liptay, & Niekamp, 1999). However, comparing moisture diffusivities during dehydration is difficult because of variations in food composition and physical structure and also because of the different methods and models employed to estimate diffusivity. The basic equation of Fick’s unsteady state law of diffusion is of the form, vM v2 M ¼D 2 vt vr
(1)
where M is the moisture content at time t on dry basis [g (water) g1 (dry matter)] D is the moisture diffusivity (m2s1); t is the time (seconds); r is the distance (m).
MR Me Mo N
Moisture ratio Equilibrium moisture content Initial moisture content Number of data points for positive values of effective diffusivity Gas constant (8.314 kJ mol1) Reduced chi-square Distance (cm) Root mean square error Coefficient of Determination Time (sec) Temperature ( C)
R c2 R RMSE R2 t T
The different analytical solutions of Eq. (1) have been given by Crank (1975) for several geometries and boundary conditions. An analytical solution of cylindrical geometry can be made assuming that, (i) moisture is initially uniformly distributed throughout the mass of a sample (ii) mass transfer is symmetrical with respect to the centre of the cylinder (iii) the surface moisture content of the sample instantaneously reaches equilibrium with the condition of the surrounding air, (iv) the sample size and geometry remain constant during convective dehydration (Crank, 1975). The solution is of the form, MR ¼
N Mt Me X 4 ¼ Mo Me n¼1 b2n
b De t exp n 2 r
(2)
where bn are the roots of Bessel function of zero order jo(r) ¼ 0 Ade-Omowaye, Taiwo, Eshtiaghi, Angersbach, and Knorr (2003) and Shi and Le Maguer (2002) calculated effective diffusivities using the slope method but this method gives only a single value of diffusivity for the entire process and therefore does not predict the kinetics of entire osmotic dehydration process, because the value of diffusivity changes with time and with moisture content of the commodity. Some researchers calculated effective diffusivity by using only first term of the analytical solution of the Fickian model assuming that the effect of terms other than first on the value of diffusivity were non-significant (Rastogi, Eshtiaghi, & Knorr, 1999; Sharma, Prasad, & Datta, 2003). The objectives of this work are to investigate the effect of osmotic pretreatment and drying air temperature on
Table 1 e Selected convective dehydration models. Model name
Model
Page Modified Page Newton Henderson and Pebis Wang and Singh Two-term exponential model Logarithmic model Midilli et al Two-term Weibull distribution
n
MR MR MR MR MR MR MR MR MR MR
¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼
exp(Kt ) (Kt)n exp(Kt) A$exp(Kt) 1 þ At þ Bt2 A$exp(Kt)þ (1A) exp(KAt) A$exp(Kt) þ C A$exp(Ktn) þ Bt A$exp(Kt)þ(1-A) exp(KAt) AB$exp(Ktn)
Reference Page (1949) Overhults, White, Hamilton, & Ross, 1973 Rahaman (1992) Henderson and Pabis (1961) Wang and Singh (1978) Sharaf-Eldeen, Blaisdell, and Spagna (1980) Yagcioglu et al (1999) Midilli, Kucuk and Yapar (2002) Henderson (1974) Ertekin and Yaldiz (2004)
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Optimisation of osmotic dehydration process was carried out with purpose of maximising water loss, solute gain and quality of the product. The optimum conditions were 50 B osmotic solution concentration, 40 C osmotic solution temperature and 100 min process duration at fruit to solution ratio 1:4 (w/w). Following osmotic pre treatment at optimum conditions, the moisture content of the arils reduced to 74% (w.b) and the solid content increased up to 26%. To prepare a shelf-stable product, the pomegranate arils were dehydrated up to final moisture content 9 1% (w.b.) (Singh, Kingly, & Jain, 2007) at an air temperature of 50, 60, 70 C and air velocity of 1.6 m s1.
Table 2 e Total convective drying time for natural and osmosed dried pomegranate arils. Temperature ( C)
Average drying time (min) Natural
Osmosed
660 585 455
545 480 320
50 60 70
Table 3 e Analysis of variance (ANOVA) for overall effect of process variables. Source of Variation
Sum of df squares
Temperature 48100 Pretreatment 21004.17 Error 233.33 Total 69337.50
2 1 2 5
MS
F
2.2.
P-value F crit
24050.00 206.1429 21004.17 180.0357 116.66
0.0048* 0.0055*
Analysis of drying data
To study the drying behaviour at different drying air temperatures, moisture content (dry basis) and drying rate were calculated. The drying curves (moisture content vs. time) were plotted to observe the effect of process variables. Corresponding to the drying curves, drying rate curves were also plotted (Kar & Gupta, 2003). The effective moisture diffusivity (De) values of pomegranate arils during convective dehydration were calculated by considering only the first term of the Eq. (2) assuming that the effect of the terms other than the first are negligible. By considering only the first term, Eq. (2) reduces to
19.00 18.51
* Significant at 5% level of significanceWhere, variables are temperature and osmotic pretreatment
convective dehydration kinetics and determination of moisture diffusivity and activation energy.
MR ¼
2.
Materials and methods
2.1.
Experimental procedure
(3)
where, b1 is the first root of Bessel function of zero order ¼ 2.405. The average apparent diffusion coefficient was calculated by using,
Fresh, well graded, whole pomegranates var. Kandhari were procured from local market of Sangrur, Punjab, India and were washed and frozen at 18 C for a minimum duration of 24 h. Before osmotic dehydration was carried out, frozen pomegranates were thawed at room temperature. Thawed pomegranates were immediately broken into separate arils. The initial moisture content of arils varied from 80.5 to 81% (w.b).
[M.C.% (w.b)]
2 4 b1 De t exp 2 r2 b1
Pn ðDe Þavg ¼
1
De
(4)
n
where, De is the effective moisture diffusivity (m2s1); (De)avg is the average value of effective moisture diffusivity (m2s1); n are the number of data points.
-1
M.C {g [water] g [dm]}
[81.81] 4.5 [80.00] 4 [77.77] 3.5 Osmosed [75.00] 3 Natural [71.42] 2.5 [66.66] 2 [60.00] 1.5 [50.00] 1 [33.33] 0.5 [0]
0 0
100
200
300
400
500
600
700
Time (min)
Fig. 1 e Effect of drying time on moisture content of pomegranate arils at 60 C.
310
b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 3 0 7 e3 1 6
2
-1
Drying rate {g [water] min }
2.5
1.5
1 Osmosed
0.5 Natural
0 0
0.5
[0]
[33.33]
1
1.5
2
2.5
3
3.5
4
4.5
[50.00]
[60.00]
[66.66]
[71.42]
[75.00]
[77.77]
[80.00]
[81.81]
-1
M.C {g [water] g [dm]} [M.C % (w.b.)]
Fig. 2 e Effect of time on drying rate of arils at 50 C air temperature.
[M.C.% (w.b)]
-1
M.C {g [water] g [dm]}
[81.81] 4.5
Natural
[80.00] 4 [77.77] 3.5
50°C 60°C
[75.00] 3
70°C
[71.42] 2.5 [66.66] 2 [60.00] 1.5 [50.00] 1 [33.33] 0.5 [0]
0 0
100
200
300
400
500
600
700
800
Time (min) M.C % (w.b) [77.77] 3.5 [80.00]
-1
M.C {g [water] g [dm]}
Osmosed
3
[71.42] 2.5
[66.66]
2
50°C 60°C 70°C
[60.00] 1.5 [50.00] 1 [33.33] 0.5 [0]
0 0
100
200
300
400
500
600
Time (min)
Fig. 3 e Effect of drying air temperature on drying behaviour of osmosed and natural pomegranate arils at different drying air temperature.
311
b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 3 0 7 e3 1 6
The dependence of average moisture diffusivity on drying air temperature was obtained using the Arrhenius relationship by, ðDe Þavg ¼ Do e
long drying time as suggested by Ertekin and Yaldiz (2004) and Pokharkar and Prasad (2002). This is because the value of moisture ratio varies from 0 to 1. The value of the equilibrium moisture content is very low and it can be subtracted from both denominator and numerator formulae. Therefore, it does not greatly affect the value of moisture ratio. Also the moisture ratio is dependent upon initial moisture content at that time. In the case of long drying times, the value of equilibrium moisture content (EMC) is very much lower.
Ea RðTþ273Þ
(5)
where, T is temperature ( C), R is gas constant having constant value of 8.314 kJ mol1 K, Do is the effective diffusivity at 273 K, Ea is the activation energy. The average effective moisture 1 Þ and a straight line diffusivity (De)avg was plotted against ðTþ273 relationship with negative slope was obtained. Thermodynamically, activation energy is the ease with which the water molecules pass the energy barrier when migrating within the product.
2.3.
2.4.
In addition to R2, the various statistical parameters such as reduced chi-square c2 and root mean square error (RMSE) were also used as criteria to select the best equation. As these parameters are not a good criteria for evaluating non-linear mathematical models, therefore the percent mean relative deviation modulus (E%) was also used to select the best equation to account for variations in the drying curves of the dried samples as recommended by several authors in recent studies (Azoubel & Murr, 2004) have indicated the deviation of the observed data from the predicted line. Therefore, the best model was chosen as one with the highest coefficient of
Validity of empirical models
The validity of the commonly used empirical models for convective dehydration (Table 1) was checked by non-linear regression analysis of the experimental data. However, moisture ratio was simplified to M/Mo instead of (MMe)/(MoMe) for fitting the experimental data to the empirical models and determination of effective moisture diffusivity because of the
2.5
Adequacy of fit of empirical models
Natural
-1
Drying rate [(g) water min ]
2
1.5
1
50°C
[
60°C 70°C
0.5
0 0
0.5
1
[0]
[33.33]
[50.00]
1.5 [60.00]
2 [66.66]
2.5
3
3.5
4
4.5
[71.42]
[75.00]
[77.77]
[80.00]
[81.81]
-1
M.C {g [water] g [dm]} [M.C. % (w.b.)] 2.5
Osmosed
-1
Drying rate [(g) water min ]
2
1.5
1 50°C 60°C
0.5
70°C
0 0
0.5
[0]
[33.33]
1 [50.00]
1.5 2 [66.66] [60.00] -1 M.C {g [water] g [dm]}
2.5 [71.42]
3 [75.00]
M.C % (w.b.)
Fig. 4 e Effect of drying air temperature on drying behaviour of osmosed pomegranate arils at different drying air temperature.
312
b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 3 0 7 e3 1 6
correlation, R2; and the least,c2 RMSE and mean relative deviation modulus, E. The average percent difference between the experimental and predicted values mean relative deviation modulus, E, defined by following equation: Eð%Þ ¼
n 100 X experimental value predicted value n i¼1 experimental value
preosmosed arils began at a relatively lower moisture content (74% w.b.) due to moisture removal and solute gain during osmotic dehydration. The total convective dehydration time of samples at 50, 60, 70 C when dried to final moisture content of 9 1% (w.b) are given in Table 2. The analysis of variance (ANOVA) of total convective dehydration time (Table 3) revealed that, both the osmotic pretreatment and drying air temperature had a significant effect at 5% level of confidence interval.
(8)
A value of E < 5.0 indicates an excellent fit, whilst the values > than 10 are indicative of a poor fit.
2.5.
3.1. Effect of osmotic pretreatment on convective drying kinetics
Statistical analysis
Table 2 indicates that, the total convective dehydration time at 50 C ofnatural osmosed arils was 660 min but was 545 min for samples preosmosed with the solution of sucrose. Therefore osmotic pretreatment in sucrose solution reduced by approximately 17.5 min to the convective dehydration time when compared to natural pomegranate arils at 50 C drying air temperature. This might be because leaching of some soluble components of the cellular structure of arils loosens the surface cellular structure during soaking in osmotic solution. This reduces cell wall resistance and increases the
Analysis of variance (ANOVA) was conducted to determine the effect of variable factors on drying parameter using Statistica for Windows 5.0, 1995 (Tulsa, OK, USA).
3.
Results and discussion
The drying of natural pomegranate arils started at relatively high moisture content (81% w.b) whereas the drying of 9
Effective diffusivity (10
-10
m 2s -1 )
8
Natural
7
50°C
6
70°C
5
60°C
4 3 2 1 0 0
0.5
1
1.5
[0]
[33.33]
[50.00]
[60.00]
2
2.5 [71.42]
[66.66]
-1
M.C {g [water] g [dm]} [M.C. % (w.b.)] 7
Osmosed
5
Effective diffusivity(10
-10
2 -1
ms )
6
50°C
4
60°C 70°C
3 2 1 0 0
0.5
[0]
[33.33]
1
[50.00]
1.5
2
[60.00] [66.66] -1 M.C {g [water] g [dm]}
2.5
[71.42]
3
[75.00]
3.5
[77.77]
[M.C. % (w.b.)]
Fig. 5 e Effect of air temperature on effective diffusivity during convective dehydration of osmosed and natural pomegranate arils.
313
b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 3 0 7 e3 1 6
Table 4 e Average effective diffusivity of natural and osmosed dried pomegranate arils. Temperature
Natural (m2 s1)
Osmosed (m2 s1)
50 C 60 C 70 C
2.5992 3 1010 3.4264 3 1010 4.8884 3 1010
3.3723 3 1010 4.0152 3 1010 5.0432 3 1010
drying rate of arils, which agrees with the results of Riva, Campolongo, Leva, Maestrelli and Torreggiani (2004) for apricot cubes, Rodrigues and Fernandes (2007) for melons. The effect of osmotic pretreatment on convective drying time of pomegranate arils at 60 C is presented in Fig. 1. It can be seen that the moisture content of osmosed arils reduced from 2.93 to 0.10 g [water] g1 [dm] within 480 min whereas in the case of fresh arils, the moisture content reduced from 4.25 to 0.10 g [water] g1 [dm] within 585 min at 60 C of drying air temperature. Fig. 2 demonstrates increase in drying rate of preosmosed arils as compared to natural arils at 50 C of drying air temperature. The similar behaviours have also been observed at 60 C and 70 C drying air temperatures.
3.2.
rates. Drying rate later decreased with decreasing moisture for natural and osmotically pretreated samples under all the conditions of convective dehydration. The reason for the reduction of drying rate might be due to the reduction in porosity of the material and also due to shrinkage as the drying process advances. In early periods of drying, there was decline in drying rate for both osmosed and natural arils. Drying rate curves showed very low drying rates, when the average moisture content of the product approached to 1 g [water] g1 [dm]. Therefore, a considerably long drying period would be necessary to achieve final moisture content lower than 1 g [water] g1 [dm]. As indicated by drying rate curves in Fig. 4, the migration of moisture to the surface and evaporation (drying) rate from the surface decreased with decreasing moisture in the product. However, closer examination of drying rate for experimental data for values below moisture content of 1 g [water] g1 [dm] for natural arils having higher drying rate compared to osmosed arils. This might be due to the resistance offered by solute gain during osmotic pretreatment.
3.3. Effective moisture diffusivity during convective dehydration process
Effect of drying air temperature on drying kinetics
The effect of temperature on the drying curve of both osmosed and natural pomegranate arils is shown in Fig. 3. To dry the product to a final moisture content of 9 1% (w.b.), the drying time at 70 C was lower compared to drying time at 60 C and 50 C. This is due to the increase in water vapour pressure within the arils with increasing temperature, which increases moisture migration. Similar results have been obtained in case of carrot (Doymaz, 2004), garlic (Madamba, Driscoll, & Buckle, 1996) and eggplant (Ertekin & Yaldiz, 2004). It can be seen (Fig. 3) that, the osmosed and natural pomegranate arils did not have a constant rate drying period and complete drying took place during the falling rate period. The absence of a constant rate period was because the product could not provide a constant supply of water for an appreciable period of time because of rapid thinlayer drying of the product at initial stages of drying (Lahsasni, Kouhila, Mahrouz, & Jaouhari, 2004; Prakash, Jha, & Datta, 2004). Drying in the falling rate period showed that internal mass transfer occurred by diffusion. Similar results have been obtained by different authors for drying of vegetables and fruits (Doymaz, 2004; Madamba et al., 1996). Fig. 4 shows that, the drying rates were highest at the beginning of the drying process, when the moisture content was greatest, with natural arils displaying highest initial drying
Table 4 indicates that effective moisture diffusivity during convective dehydration of osmosed samples was higher than natural samples. The increase of effective moisture diffusivity (De) with osmotic pretreatment can be due to loosening of the surface cellular structure and leaching of some soluble components of the external cell layers of arils during soaking in osmotic solution of sucrose at 50 B, 40 C for 100 min of process duration, Similar results have been reported by Brambilla, Maffi, Bertolo and Torreggiani (2000) in strawberry tissue, Riva et al. (2004) in apricot cubes, and Rodrigues and Fernandes (2007) in melons. Average effective moisture diffusivity during convective dehydration for un-osmosed and osmosed pomegranate arils was 2.599 1010 to 3.3723 1010 m2 s1 at 55 C. These values are lower than the corresponding values for the experiments carried out at 65 and 75 C. Fig. 5 indicates that the effective moisture diffusivity (De) increase with drying air temperature might be because of increase in the vapour pressure inside the pomegranate arils. Comparisons with results of effective diffusivity reported in the literature are difficult because of different estimation methods and the models employed with variations in food composition and physical structure. The average effective moisture diffusivity during convective dehydration of pears by Park, Bin, and Brod (2002) was found to be 2.06e6.37x1010 m2 s1 for natural pears and
Table 5 e Various regression coefficient and statistical parameters of Page model. Temp ( C)
K
N
R2
c
E%
RMSE
Natural
50 60 70
0.0030 0.0025 0.0008
1.0979 1.1556 1.4605
0.99 0.99 0.99
9.58E 05 0.000499 0.000937
7.1354 9.0769 35.6744
4.58E 05 0.000237 0.000447
Osmosed
50 60 70
0.0026 0.0030 0.0091
1.1651 1.1789 1.0986
0.99 0.99 0.99
0.000174 0.000303 0.000545
9.9887 0.4038 26.0020
8.17E 05 0.000142 0.000252
Osmotic pretreatment
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Table 6 e Various regression coefficient and statistical parameters of modified Page model. Osmotic Pretreatment
Temp ( C)
K
R2
N
c2
Natural
50 60 70
0.0051 0.0074 0.0078
1.1001 1.1810 1.5219
0.99 0.99 0.99
9.15 10 0.0003 0.0009
Osmosed
50 60 70
0.0061 0.0057 0.0139
1.1662 1.1583 1.0990
0.99 0.99 0.99
0.0002 0.0005 0.0005
1.87e8.12 1010 m2 s1 for preosmosed pears in sucrose syrup Karathanos, Villalobos, and Saravacos (1990) also found that De was 1.60 1010 m2 s1 in apples air-dried at 55 C, while this parameter decreased to 5.0 1010 m2 s1, when samples were osmotically pretreated in 45 B sucrose solution. It is also evident from Fig. 5 that effective moisture diffusivity (De) increased with decrease in the moisture content up to a moisture level of 0.3e0.4 g [water] g1 [dm] for all the conditions of convective dehydration. This behaviour might be due to the fact, that the initial temperature of the sample being less than drying air temperature at the start of the drying process, but with reductions in moisture content, the sample heated up and subsequently the moisture diffusivity increased. Because the diffusion coefficient decreases with decreasing moisture content it is more dependent on product temperature than moisture content (Adu & Otten, 1996). Further, in the last phase of each experiment with osmosed samples below moisture contents approximately 0.5e0.6 g [water] g1 [dm], there was a sharp decrease in moisture diffusivity although product temperatures were high. This is because during drying product temperatures rise to the wet bulb temperature of the drying air. This results in increase of product temperature and effective diffusivity. However, with decrease of moisture content, the product temperature will increase and approach the dry bulb temperature of the drying air. The decrease in average effective moisture diffusivity (De)avg in later stages is due to non-availability of free water for diffusion during the finishing stage of convective dehydration. (Adu & Otten, 1996).
3.4.
Activation energy for convective dehydration
Following equations with high correlation coefficients (R2 > 0.99), were obtained for the linear plots between (De)avg 1 and ðTþ273Þ for different osmotic pretreatments.
3.4.1.
5
E%
RMSE
7.1327 16.8189 37.1217
4.57 105 0.0001 0.0004
9.9875 9.1873 26.0156
0.0001 0.0002 0.0003
For natural arils
3:493 ðDe Þavg ¼ 9:140 105 exp ðT þ 273Þ
(9)
For osmosed arils 2:225 ðDe Þavg ¼ 9:229 105 exp ðT þ 273Þ
(10)
The activation energy for the convective drying of pomegranate arils was 66.12 kJ mol1 for natural aril samples, and was 42.06 kJ mol1 in case of osmosed samples. It was observed that activation energy was higher for natural arils than for osmosed samples. The high value of activation energy for natural arils might also be due to presence of high initial moisture content of 81% (w.b.) as compared to 74% (w.b.) for preosmosed samples. Therefore, more thermal energy would be required to remove greater amounts of water from natural pomegranate arils. Similar results were reported by Reppa, Mandela, Kostaropoulos, and Saravacos (1999). Park et al. (2002) also reported that activation energy was 26.46e31.21 kJ mol1 for pears without osmotic dehydration, 24.34e28.20 kJ mol1 for pears with osmotic dehydration. These values were 39.7 kJ mol1 and 24.0 kJ mol1 for fresh and osmotically pretreated apple cubes respectively (Simal, Bauza de Mirabo, Deya, & Rossello, 1997).
3.5. Validity of empirical models for convective dehydration In all the experiments of convective dehydration of osmosed and natural samples, the general exponential model, the twoterm exponential model and the logarithmic model did not fit the experimental data. Also, the low values of R2 of the Wang and Singh and Weibull distribution models led to their
Table 7 e Various regression coefficient and statistical parameters of Newton model. Temp ( C)
K
R2
c2
E%
RMSE
Natural
50 60 70
0.0061 0.0073 0.0080
0.99 0.99 0.98
0.0008 0.0010 0.0041
10.7497 12.6943 26.9862
0.0004 0.0004 0.0019
Osmosed
50 60 70
0.0051 0.0057 0.0139
0.99 0.99 0.99
0.0003 0.1984 0.0007
13.1422 18.8802 22.4960
0.0001 0.0005 0.0003
Osmotic Pretreatment
315
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Table 8 e Various regression coefficient and statistical parameters of Henderson and Pabis model. Temp ( C)
A
K
R2
c2
E%
RMSE
Natural
50 60 70
1.0540 1.0311 1.1230
0.0066 0.0059 0.0089
0.99 0.99 0.98
0.0005 0.0010 0.0029
9.1007 15.6961 23.2508
0.0002 0.0005 0.0014
Osmosed
50 60 70
1.0349 1.0628 1.0368
0.0053 0.0079 0.0145
0.99 0.99 0.99
0.0001 0.0005 0.0006
2.2000 12.3116 22.8695
0.0001 0.0003 0.0003
Osmotic Pretreatment
Table 9 e Various regression coefficient and statistical parameters of Wang and Singh model. Temp ( C)
A
B
R2
c2
E%
RMSE
Natural
50 60 70
0.003672 0.004157 0.005804
0.000003 0.000004 0.000008
0.99 0.98 0.98
0.0009 0.0009 0.0022
17.1090 25.5829 49.1155
0.0004 0.0004 0.0010
Osmosed
50 60 70
0.003672 0.005488 0.009243
0.000003 0.000008 0.000021
0.99 0.99 0.97
0.0009 0.0010 0.0045
17.1090 28.5319 64.0925
0.0004 0.0004 0.0041
Osmotic Pretreatment
Table 10 e Various regression coefficient and statistical parameters of Midilli model. Temp ( C)
A
K
N
C
R2
c2
E%
RMSE
Natural
50 60 70
1.0014 0.9549 1.0020
0.0032 0.0013 0.0006
1.0848 1.2631 1.5164
0.000011 0.000020 0.000087
0.99 0.99 0.99
9.38E 05 0.0695 0.0005
8.3715 6.6154 6.3341
4.48E 05 0.000162 0.000243
Osmosed
50 60 70
0.9746 1.0010 6.6941
0.0012 0.0022 1.3026
1.3213 1.2518 0.0003
0.000096 0.000078 0.016690
0.99 0.99 0.99
0.0001 9.6E 05 0.0004
4.3090 5.0018 9.8183
4.94E 05 4.51E 05 0.000208
Osmotic Pretreatment
rejection. Table 5e10 shows that there were high values of R2 with the Page, modified Page, Newton, Henderson and Pabis, and Midilli models. For both osmosed and natural samples, the Midilli model (Table 10) had an excellent fit compared to other models due to lower value of E (%), the highest coefficient of correlation (R2), the lowest values of RMSE and c2 This indicates that this was the best model for predicting the convective dehydration behaviour of natural and osmosed pomegranate arils at drying air temperature of 50, 60 and 70 C. The adequacy offit of the Midilli et al. (2002) model was is in close agreement with results of Singh, Kumar, and Gupta (2007) for the convective dehydration of preosmosed carrot cubes.
4.
Conclusions
With the aim of preparing a shelf stable product having final moisture content of 9 1% (w.b.), osmotic pretreatment before convective dehydration of pomegranate arils results in a decrease of the convective dehydration time. Osmotic pretreatment also results in increases in drying rate and effective moisture diffusivity and decreasing activation energy during convective dehydration. Among the empirical models applied to the data, the Midilli model best described
the convective drying characteristics of natural and osmosed pomegranate arils.
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