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Convective drying kinetics of strawberry
Available online at www.sciencedirect.com
Chemical Engineering and Processing 47 (2008) 914–919
Convective drying kinetics of strawberry ˙Ibrahim Doymaz ∗ Department of Chemical Engineering, Yildiz Technical University, 34210 Esenler, Istanbul, Turkey Received 15 May 2006; received in revised form 14 February 2007; accepted 14 February 2007 Available online 23 February 2007
Abstract The drying kinetics of strawberry in a laboratory dryer was studied. The pre-treated with alkaline ethyl oleate solution and untreated strawberries were dried at selected temperatures of 50, 55 and 65 ◦ C with a constant air velocity of 1.2 m/s. The drying rate curves showed that drying process took place only in the falling rate period. Thin-layer drying models of Lewis, Henderson and Pabis, logarithmic, Page, Wang and Singh evaluated based on coefficient of determination (R2 ), reduced chi-square (χ2 ) and root means error (RMSE). The logarithmic model was found to be a better model for describing the characteristics of strawberry for both of the temperatures of 50 and 55 ◦ C. The values obtained from Wang and Singh were found to be more reasonable for temperature of 65 ◦ C than the other models. The transport of water during drying was described by Fick’s equation and effective diffusivity varied from 4.95 × 10−10 to 1.42 × 10−9 m2 /s. © 2007 Elsevier B.V. All rights reserved. Keywords: Strawberry; Convective drying; Alkaline ethyl oleate; Effective diffusivity; Rehydration capacity
1. Introduction Strawberry is one of the world’s largest fruit crops. According to FAO data, production quantity of strawberry was about 3,491,324 Mt in 2004 in the world. Turkey produced about 155,000 Mt [1]. Strawberry is one of the most delicate and highly perishable fruits, due to respiration, weight loss and susceptibility to fungal contamination [2]. Therefore, it can be preserved by freezing and drying processes such as freeze, osmotic, microwave, and air drying [3–6]. Besides, it could consume fresh or in many other forms such as juice, concentrate jam, and jelly and dried rehydrated with yoghurt and bakery products [7]. Drying is the most important process to preserve grains, crops and foods of all varieties. The removal of moisture prevents the growth and reproduction of microorganisms causing decay and minimises many of the moisture-mediated deterioration reactions. It brings about substantial reduction in weight and volume, minimising packing, storage and transportation costs and enables storability of the product under ambient temperatures [8]. Sun drying has long been used to dry fruits and vegetables in tropical and sub-tropical countries. While sun drying is
∗
Fax: +90 212 449 18 95. E-mail address: [email protected].
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a cheap, easy, and popular method, its application is restricted by the long drying time and the need for favourable weather. In order to improve the quality of foods, traditional sun drying techniques should be replaced with the mechanical drying, which produces a better and more consistent quality product, taking less time and minimising crop losses [9]. Drying of fruits is one of the most time and energy consuming processes in the food industry. To reduce the processing time, hence accelerating the drying process, a number of obstacles must be overcome [10]. The main problem in drying of fruits such as grape, apricot, plum, cranberry and strawberry are covered naturally with a thin-layer of wax cuticle, which controls the rate of moisture diffusion through the samples. To accelerate the drying process, chemical treatments such as ethyl or methyl ester emulsions or alkali solutions of sodium hydroxide and potassium carbonate. The effects of pretreatment solution on some fruits such as grapes, apricots, and plums are usually applied before drying so as to remove this wax cuticle and increase their permeability to water [10–14]. However, there is no information on the effects of pretreatment solution such as alkaline ethyl oleate on drying of strawberry in the literature. The objective of this study was to determine the drying and rehydration characteristics of strawberry, to investigate the effect of alkaline ethyl oleate solution on drying time, to evaluate several thin-layer drying models available in the literature and to calculate effective diffusivity.
˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I.
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2. Materials and methods
Table 1 Mathematical models applied to drying curves of strawberry
Fresh strawberry (Fragaria) were purchased from local supermarket in Istanbul (Turkey) and stored in a refrigerator at 4 ◦ C until used. The fruits were removed from the refrigerator about 2 h before experimentation and were allowed to attain room temperature. Generally, strawberries of uniform size (average berry radius, length and weight 2.43, 3.73 cm and 10.11 g, respectively) were selected for the experiment. The initial moisture content was determined by oven drying method [15]. Ethyl oleate and potassium carbonate were obtained from Merck (Darmstadt, Germany).
Model names
Mathematical expression
References
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
MR = exp(−kt) MR = a exp(−kt) MR = a exp(−kt) + c MR = exp(−ktn ) MR = 1 +at + bt2
[18,19] [20,21] [22,23] [9,24–26] [27]
2.1. Experimental procedure The drying of strawberry was investigated in cabinet dryer that is described previously by Doymaz [14] and installed in the Chemical Engineering Department of Yildiz Technical University, Istanbul, Turkey. The dryer consists of an adjustable centrifugal fan, electrical heater, air filter, drying basket and proportional temperature controller. The air velocity was measured with Testo 440 vane probe anemometer (Lutron, Taiwan) with a precision of ±0.03 m/s. The samples were dried in a perforated basket of 30 cm2 × 11 cm high. After strawberry was washed with tap water, whole shapes were dipped in alkaline ethyl oleate solution (2% EO + 5% K2 CO3 , coded AEEO) at room temperature (∼23 ◦ C). Dipping time is 1 min. The untreated samples (NAT coded) as control were just washed with tap water to take away dust and dirt prior to drying. The drying experiments of pre-treated and untreated strawberries were carried out at air temperatures of 50, 55 and 65 ◦ C and 1.2 m/s air velocity. Besides, untreated some samples cut into halves and then dried at 50 ◦ C. The dryer was run without load for 30 min to stabilise the drying conditions. Then the samples were uniformly spread on the basket as a single layer. Each sample utilised in the experiment weighed about 150 g. The initial moisture content of strawberry was about 93.2% ± 0.2 (w/w). The amount of water removed during the drying process was recorded at 30-min intervals by means of a digital balance (Mettler, model BB3000), with an accuracy of ±0.1 g. Drying continued until 20% ± 0.5 (w/w) moisture content was reached. The product was cooled and packed in low density polyethylene (LDPE) bags that were heat-sealed. The experiments were replicated three times and the average of the moisture content at each value was used. The drying data from the drying tests were then expressed as moisture ratio (MR) versus drying time.
where a, b, c, n are drying constants in models and k is drying rate constant (min−1 ).
calculated as following: rehydration capacity =
Wr Wd
(1)
where Wr is the weight after rehydration (kg) and Wd is the weight of dried material (kg). 2.3. Data analysis The moisture ratio (MR) was calculated using the following equation: MR =
M − Me M0 − M e
(2)
where M0 , M and Me are initial, after time (t) and equilibrium moisture contents, respectively. For the analysis it was assumed that the equilibrium moisture content was equal to zero [17]. Numerous mathematical models have been proposed to describe the drying characteristics of agricultural products. Five models were used to fit the drying experimental data and are presented in Table 1. The non-linear regression analysis was performed using the statistica computer program and was based on the Levenberg–Marquardt algorithm. The statistical validity of the models was evaluated and compared by means of the coefficient of determination (R2 ) the reduced chi-square (χ2 ) and root means square error (RMSE). The higher the values of the R2 , and lowest values of the χ2 and RMSE, the better the goodness of the fit [28]. These parameters can be calculated as following: N (MRexp,i − MRpre,i )2 2 χ = i=1 (3) N −z 1/2 N 1 2 RMSE = (MRpre,i − MRexp,i ) (4) N i=1
where MRexp,i is the experimental moisture ratio, MRpre,i the predicted moisture ratio, N the number of experimental data points, and z is the number of parameters in model [17,29].
2.2. Rehydration capacity 3. Results and discussion Five grams of the dried products were added to 200 ml distilled water, in a 400 ml flask beaker at 25 ◦ C for 24 h. After rehydration, samples were taken out, residual water was removed and adhering water was absorbed carefully with tissue paper and then weighed [16]. The rehydration capacity was
3.1. Influence of pre-treatment solution on drying time Curves of moisture content versus drying time for pre/untreated samples are shown in Fig. 1. Samples with dipped
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˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I.
Fig. 1. Drying behaviours of pre-treated/untreated strawberries.
Fig. 3. Drying curves of halves and whole untreated strawberries.
in ethyl oleate plus potassium carbonate solution prior to drying were found to have a shorter drying time compared to untreated samples. The drying time required to reduce the moisture from initial moisture content of about 93.2% (w.b.) to desired moisture in the final product, approximately 20% (w.b.) was 1110, 990 and 690 min for pre-treated samples, 1620, 1320 and 900 for untreated samples at drying air temperatures of 50, 55 and 65 ◦ C, respectively. These results demonstrated that drying times of pre-treated samples was about 23.3–31.5% shorter than that of untreated samples. Consequently, AEEO solution was more effective solution in strawberry drying and removed the waxy layer on the surface of berry and increased the skin permeability. So, drying time of strawberry was decreased. A similar effect of ethyl oleate has been found in drying of agricultural products such as seedless grapes and apricots [11,14]. Fig. 2 is shown the variation of drying rate with moisture content. A constant-rate period was not observed in any of the experiments of this work, so the entire drying process for strawberry occurs in the range of the falling-rate period. Moisture ratio decreases continuously with diminishing drying time. This shows that diffusion in dominant physical mechanism governing
moisture movement in the samples. The results were generally in agreement with some literature studies on drying of various fruits such as apricot and strawberry [14,28,30]. The drying rate increases with increasing the surface area exposed to heated air, strawberries dry faster if they are halved prior to drying process. The results of halves and whole dried strawberries at 50 ◦ C demonstrated that in and had drying times of 720 and 1620 min, respectively (Fig. 3). Drying time was decreased by 125% for halved strawberries dried at 50 ◦ C compared to the drying time of samples dried as whole at the same temperature. Similar result was found by Sunjka and Raghavan [10] for drying of cranberries. They reported that mechanical pre-treatment such as cutting into halves of fruits provide substantial increase in moisture ratio loss, because the surface area for mass transfer is greater.
Fig. 2. Variation of drying rate with moisture content of strawberry samples at different temperatures.
3.2. Rehydration characteristics The results for rehydration capacity are shown in Fig. 4. Rehydration of pre-treated samples was much faster than untreated samples. The pre-treatment yielded structurally a more compact product after drying process. This factor adversely
Fig. 4. Rehydration ratio of pre-treated/untreated samples.
˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I.
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Table 2 Statistical results from various thin-layer drying models for untreated strawberries T (◦ C)
Model names
R2
χ2
RMSE
50
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9968 0.9970 0.9995 0.9980 0.9894
0.000235 0.000224 0.000035 0.000146 0.000799
0.000463 0.000432 0.000067 0.000281 0.001541
55
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9892 0.9931 0.9996 0.9987 0.9975
0.000916 0.000595 0.000029 0.000112 0.000214
0.001792 0.001138 0.000054 0.000214 0.000409
65
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9787 0.9856 0.9990 0.9975 0.9996
0.001991 0.000092 0.000095 0.000240 0.000036
0.003853 0.000173 0.000173 0.000449 0.000068
influenced the rehydration of pre-treated strawberry. The rehydration tests show that the rehydration at 65 ◦ C is faster than other temperatures. 3.3. Modelling of drying curves The moisture content data at the different air temperatures were converted to moisture ratio expression (Eq. (1)) and then curve fitting computations with the drying time were done by using the five thin-layer drying models in Table 1. The statistical results from the models such as R2 , χ2 and RMSE values are shown in Tables 2 and 3. In all cases, the R2 values for the models were greater than the acceptable R2 value of 0.90, indicating a good fit [31]. Generally R2 , χ2 and RMSE values were changed between 0.9758 and 0.9998, 0.000013 and 0.001991, 0.000024 and 0.003853, respectively. As expected, the logarithmic model was found to be a better model for describing the characteristics of strawberry for both of the temperatures of 50
Fig. 5. Comparison of the experimental and predicted moisture ratios by logarithmic model for pre-treated/untreated strawberries.
and 55 ◦ C. The values obtained from Wang and Singh were found to be more reasonable for temperature of 65 ◦ C than the other models. Fig. 5 compares experimental data with those predicted with the logarithmic model for dried strawberry at 50 and 55 ◦ C. Fig. 6 compares experimental data with those predicted with the Wang and Singh model for dried strawberry at 65 ◦ C. 3.4. Effective diffusivity Fick’s second diffusion law has been widely used to describe the drying process during the falling rate period for biological materials. General series solution of Fick’s second law in spherical coordinates with the assumptions of constant moisture diffusivity and temperature, negligible shrinkage during drying is given as follows [32]: 2 2 ∞ M − Me 61 n π Deff t (5) = 2 exp − M0 − M e π n2 r2 n=1
Table 3 Statistical results from various thin-layer drying models for pre-treated strawberries T (◦ C)
Model names
R2
χ2
RMSE
50
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9956 0.9966 0.9998 0.9998 0.9922
0.000274 0.000281 0.000013 0.000095 0.000642
0.000533 0.000533 0.000024 0.000180 0.001218
55
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9937 0.9955 0.9991 0.9989 0.9936
0.000529 0.000393 0.000076 0.000089 0.000556
0.001027 0.000741 0.000138 0.000169 0.001046
65
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9758 0.9831 0.9971 0.9978 0.9997
0.002418 0.001769 0.000314 0.000227 0.000028
0.004635 0.003243 0.000550 0.000416 0.000052
Fig. 6. Comparison of the experimental and predicted moisture ratios by Wang and Singh model for pre-treated/untreated strawberries.
˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I.
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Table 4 Calculated effective moisture diffusivity at for pre-treated and untreated strawberry drying Code
Air temperature, T (◦ C)
Effective diffusivity, Deff (m2 /s)
NAT
50 55 65
4.95 × 10−10 6.64 × 10−10 1.09 × 10−9
AEEO
50 55 65
6.42 × 10−10 8.90 × 10−10 1.42 × 10−9
ing of strawberry increases the drying rate and consequently decreases drying time. Also, rehydration of pre-treated samples was much faster than untreated samples. It was found that there was no constant-rate period of drying in strawberry. Drying process took place under the falling-rate period. The logarithmic model was found to be a better model for describing the characteristics of strawberry for both of the temperatures of 50 and 55 ◦ C. The values obtained from Wang and Singh were found to be more reasonable for temperature of 65 ◦ C than the other models. The effective diffusivity varied from 4.95 × 10−10 to 1.42 × 10−9 m2 /s and increases as air temperature increases. Appendix A. Nomenclature
Fig. 7. Variation of effective diffusivity with drying temperature.
where Deff is the effective diffusivity (m2 /s), r is the radius of the sphere (m). For long drying times Eq. (5) can be simplified to a straight-line equation in the form: 2 6 π Deff t M − Me = ln − (6) ln M0 − M e π2 r2 where (M − Me )/(M0 − Me ) is the moisture ratio (MR). The effective diffusivity values were calculated by Eq. (6), using the method of slopes. It is typically determined by plotting experimental drying data in terms of ln(MR) versus time [33]. From Eq. (6), a plot of ln(MR) versus time gives a straight line with a slope of: slope =
π2 Deff r2
(7)
The results of effective diffusivity of pre-treated and untreated samples summarised in Table 4. The effective diffusivities of pre-treated samples were higher than untreated samples (Fig. 7). In addition, the values of Deff increased greatly with increasing temperature. The effective diffusivities varied from 6.42 × 10−10 to 1.42 × 10−9 m2 /s for pre-treated samples and from 4.95 × 10−10 to 1.09 × 10−9 m2 /s for untreated samples over the temperature range 50–65 ◦ C. The values of Deff lay within in general range of 10−11 to 10−9 m2 /s for food materials [31]. Based on these results, the effective diffusivity in the alkaline ethyl oleate solution treated strawberry samples was increased at all the temperatures than untreated samples. 4. Conclusions The results of this study indicate that using alkaline ethyl oleate solution and increasing air temperature during the dry-
a, b, c Deff k M Me M0 n N r RMSE R2 t T Wd Wr z
drying constants in models effective diffusivity (m2 /s) drying rate constant (min−1 ) moisture content (kg water/kg dry matter) equilibrium moisture content (kg water/kg dry matter) initial moisture content (kg water/kg dry matter) positive integer, drying constant in the models number of experimental data points radius (m) root mean square error coefficient of determination drying time (min) air temperature (◦ C) weight of dried material (kg) weight after rehydration (kg) number of parameters in model
Greek letter χ2 reduced chi-square References [1] FAO, 2005, Statistical Database, Available: http://www.fao.org. [2] B.M. Zhang, G. Xiao, V.M. Salokhe, Preservation of strawberries by modified atmosphere packages with other treatments, Packag. Technol. Sci. 19 (2006) 183–191. [3] S.D. Evans, A. Brambilla, D.M. Lane, D. Torreggiani, L.D. Hall, Magnetic resonance imaging of strawberry (Fragaria vesca) slices during osmotic dehydration and air drying, Lebensm. Wiss. U. Technol. 35 (2002) 177–184. [4] F. Shishehgarha, J. Maklouf, C. Ratti, Freeze-drying characteristics of strawberries, Dry. Technol. 20 (2002) 131–145. [5] K.A. Taiwo, M.N. Eshtiaghi, B.I.O. Ade-Omowaye, D. Knorr, Osmotic dehydration of strawberry halves: influence of osmotic agents and pretreatment methods on mass transfer and product characteristics, Inter. J. Food Sci. Technol. 38 (2003) 693–707. [6] G.R. Askari, Z. Emam-Djomeh, S.M. Mousavi, Effects of combined coating and microwave assisted hot-air drying on the texture, microstructure and rehydration characteristics of apple slices, Food Sci. Technol. Inter. 12 (2006) 39–46. [7] A. El-Beltagy, G.R. Gamea, A.H. Amer Essa, Solar drying characteristics of strawberry, J. Food Eng. 78 (2006) 456–464. [8] K.S. Jayaraman, D.K. Das Gupta, in: A.S. Mujumdar (Ed.), Handbook of Industrial Drying, vol. 1, Marcel Dekker, Inc., New York, 1995, pp. 643–649.
˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I. [9] S. Arora, S. Bharti, V.K. Sehgal, Convective drying kinetics of red chillies, Dry. Technol. 24 (2006) 189–193. [10] P.S. Sunjka, G.S.V. Raghavan, Assessment of pretreatment methods and osmotic dehydration for cranberries, Can. Biosys. Eng 46 (2004) 335– 340. [11] G.S. Raouzeos, G.D. Saravacos, Solar drying of raisins, Dry. Technol. 4 (1986) 633–649. [12] O. Masi, M. Riva, in: S. Briun (Ed.), Preconcentration and Drying of Food Materials, Elsevier, Amsterdam, 1988, pp. 203–214. [13] L. Cinquanta, M. Di Matteo, M. Esti, Physical pretreatment of plums (Prunus domestica) part 2. Effect on the quality characteristics of different prune cultivars, Food Chem. 79 (2002) 233–238. [14] I. Doymaz, Effect of pre-treatments using potassium metabisulphide and alkaline ethyl oleate on the drying kinetics of apricots, Biosys. Eng. 89 (2004) 281–287. [15] AOAC, Official Methods of Analysis, Association of Official Analytical Chemists, Arlington, VA, 1990. [16] M.K. Al-Khuseibi, S.S. Sablani, C.O. Perera, Comparison of water blanching and high hydrostatic pressure effects on drying kinetics and quality of potato, Dry. Technol. 23 (2005) 2449–2461. [17] W.A.M. McMinn, C.M. McLoughlin, T.R.A. Magee, Thin-layer modeling of microwave, microwave-convective, and microwave-vacuum drying of pharmaceutical powders, Dry. Technol. 23 (2005) 513–532. [18] D.M. Bruce, Exposed-layer barley drying, three models fitted to new data up to 150 ◦ C, J. Agric. Eng. Res. 32 (1985) 337–347. [19] A.S. Mujumdar, A.S. Menon, in: A.S. Mujumdar (Ed.), Handbook of Industrial Drying, vol. 1, Marcel Dekker, New York, 1995, pp. 1–40. [20] Q. Zhang, J.B. Litchfield, An optimisation of intermittent corn drying in a laboratory scale thin layer dryer, Dry. Technol. 9 (1991) 383– 395.
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[21] H.M. Ghodake, T.K. Goswami, A. Chakraverty, Mathematical modelling of withering characteristics of tea leaves, Dry. Technol. 24 (2006) 159–164. [22] A. Yagcioglu, A. Degirmencioglu, F. Cagatay, Drying characteristics of laurel leaves under different drying conditions, Proceedings of the Seventh International Congress on Agricultural Mechanisation and Energy, Adana, Turkey, pp. 565–569. [23] I.T. Togrul, D. Pehlivan, Modelling of drying kinetics of single apricot, J. Food Eng. 58 (2003) 23–32. [24] G. Page, Factors influencing the maximum rates of air drying shelled corn in thin layers, MS Thesis, Purdue University, USA, 1949. [25] H.O. Menges, C. Ertekin, Modelling of air drying of Hacihaliloglu-type apricots, J. Sci. Food Agric. 86 (2006) 279–291. [26] S. Simal, A. Femenia, M.C. Garau, C. Rossell´o, Use of exponential, Page’s and diffusional models to simulate the drying kinetics of kiwi fruit, J. Food Eng. 66 (2005) 323–328. [27] C.Y. Wang, R.P. Singh, Use of variable equilibrium moisture content in modelling rice drying, ASAE Paper No: 78–3001, St. Joseph, MI, USA, ASAE, 1978. [28] E.K. Akpinar, Y. Bicer, Mathematical modelling and experimental study on thin layer drying of strawberry, Inter. J. Food Eng. 2 (1) (2006), article 5. ¨ [29] K. Sacilik, A.K. Elicin, G. Unal, Drying kinetics of Uryani plum in a convective hot-air dryer, J. Food Eng. 76 (2006) 362–368. [30] G.K. Vagenas, D. Marinos-Kouris, Drying kinetics of apricots, Dry. Technol. 9 (1991) 735–752. [31] P.S. Madamba, R.H. Griscoll, K.A. Buckle, The thin-layer drying characteristics of garlic slices, J. Food Eng. 29 (1996) 75–97. [32] J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford, 1975. [33] M. Ozdemir, Y.O. Devres, The thin layer drying characteristics of hazelnuts during roasting, J. Food Eng. 42 (1999) 225–233.
Chemical Engineering and Processing 47 (2008) 914–919
Convective drying kinetics of strawberry ˙Ibrahim Doymaz ∗ Department of Chemical Engineering, Yildiz Technical University, 34210 Esenler, Istanbul, Turkey Received 15 May 2006; received in revised form 14 February 2007; accepted 14 February 2007 Available online 23 February 2007
Abstract The drying kinetics of strawberry in a laboratory dryer was studied. The pre-treated with alkaline ethyl oleate solution and untreated strawberries were dried at selected temperatures of 50, 55 and 65 ◦ C with a constant air velocity of 1.2 m/s. The drying rate curves showed that drying process took place only in the falling rate period. Thin-layer drying models of Lewis, Henderson and Pabis, logarithmic, Page, Wang and Singh evaluated based on coefficient of determination (R2 ), reduced chi-square (χ2 ) and root means error (RMSE). The logarithmic model was found to be a better model for describing the characteristics of strawberry for both of the temperatures of 50 and 55 ◦ C. The values obtained from Wang and Singh were found to be more reasonable for temperature of 65 ◦ C than the other models. The transport of water during drying was described by Fick’s equation and effective diffusivity varied from 4.95 × 10−10 to 1.42 × 10−9 m2 /s. © 2007 Elsevier B.V. All rights reserved. Keywords: Strawberry; Convective drying; Alkaline ethyl oleate; Effective diffusivity; Rehydration capacity
1. Introduction Strawberry is one of the world’s largest fruit crops. According to FAO data, production quantity of strawberry was about 3,491,324 Mt in 2004 in the world. Turkey produced about 155,000 Mt [1]. Strawberry is one of the most delicate and highly perishable fruits, due to respiration, weight loss and susceptibility to fungal contamination [2]. Therefore, it can be preserved by freezing and drying processes such as freeze, osmotic, microwave, and air drying [3–6]. Besides, it could consume fresh or in many other forms such as juice, concentrate jam, and jelly and dried rehydrated with yoghurt and bakery products [7]. Drying is the most important process to preserve grains, crops and foods of all varieties. The removal of moisture prevents the growth and reproduction of microorganisms causing decay and minimises many of the moisture-mediated deterioration reactions. It brings about substantial reduction in weight and volume, minimising packing, storage and transportation costs and enables storability of the product under ambient temperatures [8]. Sun drying has long been used to dry fruits and vegetables in tropical and sub-tropical countries. While sun drying is
∗
Fax: +90 212 449 18 95. E-mail address: [email protected].
0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.02.003
a cheap, easy, and popular method, its application is restricted by the long drying time and the need for favourable weather. In order to improve the quality of foods, traditional sun drying techniques should be replaced with the mechanical drying, which produces a better and more consistent quality product, taking less time and minimising crop losses [9]. Drying of fruits is one of the most time and energy consuming processes in the food industry. To reduce the processing time, hence accelerating the drying process, a number of obstacles must be overcome [10]. The main problem in drying of fruits such as grape, apricot, plum, cranberry and strawberry are covered naturally with a thin-layer of wax cuticle, which controls the rate of moisture diffusion through the samples. To accelerate the drying process, chemical treatments such as ethyl or methyl ester emulsions or alkali solutions of sodium hydroxide and potassium carbonate. The effects of pretreatment solution on some fruits such as grapes, apricots, and plums are usually applied before drying so as to remove this wax cuticle and increase their permeability to water [10–14]. However, there is no information on the effects of pretreatment solution such as alkaline ethyl oleate on drying of strawberry in the literature. The objective of this study was to determine the drying and rehydration characteristics of strawberry, to investigate the effect of alkaline ethyl oleate solution on drying time, to evaluate several thin-layer drying models available in the literature and to calculate effective diffusivity.
˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I.
915
2. Materials and methods
Table 1 Mathematical models applied to drying curves of strawberry
Fresh strawberry (Fragaria) were purchased from local supermarket in Istanbul (Turkey) and stored in a refrigerator at 4 ◦ C until used. The fruits were removed from the refrigerator about 2 h before experimentation and were allowed to attain room temperature. Generally, strawberries of uniform size (average berry radius, length and weight 2.43, 3.73 cm and 10.11 g, respectively) were selected for the experiment. The initial moisture content was determined by oven drying method [15]. Ethyl oleate and potassium carbonate were obtained from Merck (Darmstadt, Germany).
Model names
Mathematical expression
References
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
MR = exp(−kt) MR = a exp(−kt) MR = a exp(−kt) + c MR = exp(−ktn ) MR = 1 +at + bt2
[18,19] [20,21] [22,23] [9,24–26] [27]
2.1. Experimental procedure The drying of strawberry was investigated in cabinet dryer that is described previously by Doymaz [14] and installed in the Chemical Engineering Department of Yildiz Technical University, Istanbul, Turkey. The dryer consists of an adjustable centrifugal fan, electrical heater, air filter, drying basket and proportional temperature controller. The air velocity was measured with Testo 440 vane probe anemometer (Lutron, Taiwan) with a precision of ±0.03 m/s. The samples were dried in a perforated basket of 30 cm2 × 11 cm high. After strawberry was washed with tap water, whole shapes were dipped in alkaline ethyl oleate solution (2% EO + 5% K2 CO3 , coded AEEO) at room temperature (∼23 ◦ C). Dipping time is 1 min. The untreated samples (NAT coded) as control were just washed with tap water to take away dust and dirt prior to drying. The drying experiments of pre-treated and untreated strawberries were carried out at air temperatures of 50, 55 and 65 ◦ C and 1.2 m/s air velocity. Besides, untreated some samples cut into halves and then dried at 50 ◦ C. The dryer was run without load for 30 min to stabilise the drying conditions. Then the samples were uniformly spread on the basket as a single layer. Each sample utilised in the experiment weighed about 150 g. The initial moisture content of strawberry was about 93.2% ± 0.2 (w/w). The amount of water removed during the drying process was recorded at 30-min intervals by means of a digital balance (Mettler, model BB3000), with an accuracy of ±0.1 g. Drying continued until 20% ± 0.5 (w/w) moisture content was reached. The product was cooled and packed in low density polyethylene (LDPE) bags that were heat-sealed. The experiments were replicated three times and the average of the moisture content at each value was used. The drying data from the drying tests were then expressed as moisture ratio (MR) versus drying time.
where a, b, c, n are drying constants in models and k is drying rate constant (min−1 ).
calculated as following: rehydration capacity =
Wr Wd
(1)
where Wr is the weight after rehydration (kg) and Wd is the weight of dried material (kg). 2.3. Data analysis The moisture ratio (MR) was calculated using the following equation: MR =
M − Me M0 − M e
(2)
where M0 , M and Me are initial, after time (t) and equilibrium moisture contents, respectively. For the analysis it was assumed that the equilibrium moisture content was equal to zero [17]. Numerous mathematical models have been proposed to describe the drying characteristics of agricultural products. Five models were used to fit the drying experimental data and are presented in Table 1. The non-linear regression analysis was performed using the statistica computer program and was based on the Levenberg–Marquardt algorithm. The statistical validity of the models was evaluated and compared by means of the coefficient of determination (R2 ) the reduced chi-square (χ2 ) and root means square error (RMSE). The higher the values of the R2 , and lowest values of the χ2 and RMSE, the better the goodness of the fit [28]. These parameters can be calculated as following: N (MRexp,i − MRpre,i )2 2 χ = i=1 (3) N −z 1/2 N 1 2 RMSE = (MRpre,i − MRexp,i ) (4) N i=1
where MRexp,i is the experimental moisture ratio, MRpre,i the predicted moisture ratio, N the number of experimental data points, and z is the number of parameters in model [17,29].
2.2. Rehydration capacity 3. Results and discussion Five grams of the dried products were added to 200 ml distilled water, in a 400 ml flask beaker at 25 ◦ C for 24 h. After rehydration, samples were taken out, residual water was removed and adhering water was absorbed carefully with tissue paper and then weighed [16]. The rehydration capacity was
3.1. Influence of pre-treatment solution on drying time Curves of moisture content versus drying time for pre/untreated samples are shown in Fig. 1. Samples with dipped
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˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I.
Fig. 1. Drying behaviours of pre-treated/untreated strawberries.
Fig. 3. Drying curves of halves and whole untreated strawberries.
in ethyl oleate plus potassium carbonate solution prior to drying were found to have a shorter drying time compared to untreated samples. The drying time required to reduce the moisture from initial moisture content of about 93.2% (w.b.) to desired moisture in the final product, approximately 20% (w.b.) was 1110, 990 and 690 min for pre-treated samples, 1620, 1320 and 900 for untreated samples at drying air temperatures of 50, 55 and 65 ◦ C, respectively. These results demonstrated that drying times of pre-treated samples was about 23.3–31.5% shorter than that of untreated samples. Consequently, AEEO solution was more effective solution in strawberry drying and removed the waxy layer on the surface of berry and increased the skin permeability. So, drying time of strawberry was decreased. A similar effect of ethyl oleate has been found in drying of agricultural products such as seedless grapes and apricots [11,14]. Fig. 2 is shown the variation of drying rate with moisture content. A constant-rate period was not observed in any of the experiments of this work, so the entire drying process for strawberry occurs in the range of the falling-rate period. Moisture ratio decreases continuously with diminishing drying time. This shows that diffusion in dominant physical mechanism governing
moisture movement in the samples. The results were generally in agreement with some literature studies on drying of various fruits such as apricot and strawberry [14,28,30]. The drying rate increases with increasing the surface area exposed to heated air, strawberries dry faster if they are halved prior to drying process. The results of halves and whole dried strawberries at 50 ◦ C demonstrated that in and had drying times of 720 and 1620 min, respectively (Fig. 3). Drying time was decreased by 125% for halved strawberries dried at 50 ◦ C compared to the drying time of samples dried as whole at the same temperature. Similar result was found by Sunjka and Raghavan [10] for drying of cranberries. They reported that mechanical pre-treatment such as cutting into halves of fruits provide substantial increase in moisture ratio loss, because the surface area for mass transfer is greater.
Fig. 2. Variation of drying rate with moisture content of strawberry samples at different temperatures.
3.2. Rehydration characteristics The results for rehydration capacity are shown in Fig. 4. Rehydration of pre-treated samples was much faster than untreated samples. The pre-treatment yielded structurally a more compact product after drying process. This factor adversely
Fig. 4. Rehydration ratio of pre-treated/untreated samples.
˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I.
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Table 2 Statistical results from various thin-layer drying models for untreated strawberries T (◦ C)
Model names
R2
χ2
RMSE
50
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9968 0.9970 0.9995 0.9980 0.9894
0.000235 0.000224 0.000035 0.000146 0.000799
0.000463 0.000432 0.000067 0.000281 0.001541
55
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9892 0.9931 0.9996 0.9987 0.9975
0.000916 0.000595 0.000029 0.000112 0.000214
0.001792 0.001138 0.000054 0.000214 0.000409
65
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9787 0.9856 0.9990 0.9975 0.9996
0.001991 0.000092 0.000095 0.000240 0.000036
0.003853 0.000173 0.000173 0.000449 0.000068
influenced the rehydration of pre-treated strawberry. The rehydration tests show that the rehydration at 65 ◦ C is faster than other temperatures. 3.3. Modelling of drying curves The moisture content data at the different air temperatures were converted to moisture ratio expression (Eq. (1)) and then curve fitting computations with the drying time were done by using the five thin-layer drying models in Table 1. The statistical results from the models such as R2 , χ2 and RMSE values are shown in Tables 2 and 3. In all cases, the R2 values for the models were greater than the acceptable R2 value of 0.90, indicating a good fit [31]. Generally R2 , χ2 and RMSE values were changed between 0.9758 and 0.9998, 0.000013 and 0.001991, 0.000024 and 0.003853, respectively. As expected, the logarithmic model was found to be a better model for describing the characteristics of strawberry for both of the temperatures of 50
Fig. 5. Comparison of the experimental and predicted moisture ratios by logarithmic model for pre-treated/untreated strawberries.
and 55 ◦ C. The values obtained from Wang and Singh were found to be more reasonable for temperature of 65 ◦ C than the other models. Fig. 5 compares experimental data with those predicted with the logarithmic model for dried strawberry at 50 and 55 ◦ C. Fig. 6 compares experimental data with those predicted with the Wang and Singh model for dried strawberry at 65 ◦ C. 3.4. Effective diffusivity Fick’s second diffusion law has been widely used to describe the drying process during the falling rate period for biological materials. General series solution of Fick’s second law in spherical coordinates with the assumptions of constant moisture diffusivity and temperature, negligible shrinkage during drying is given as follows [32]: 2 2 ∞ M − Me 61 n π Deff t (5) = 2 exp − M0 − M e π n2 r2 n=1
Table 3 Statistical results from various thin-layer drying models for pre-treated strawberries T (◦ C)
Model names
R2
χ2
RMSE
50
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9956 0.9966 0.9998 0.9998 0.9922
0.000274 0.000281 0.000013 0.000095 0.000642
0.000533 0.000533 0.000024 0.000180 0.001218
55
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9937 0.9955 0.9991 0.9989 0.9936
0.000529 0.000393 0.000076 0.000089 0.000556
0.001027 0.000741 0.000138 0.000169 0.001046
65
Lewis Henderson and Pabis Logarithmic Page Wang and Singh
0.9758 0.9831 0.9971 0.9978 0.9997
0.002418 0.001769 0.000314 0.000227 0.000028
0.004635 0.003243 0.000550 0.000416 0.000052
Fig. 6. Comparison of the experimental and predicted moisture ratios by Wang and Singh model for pre-treated/untreated strawberries.
˙ Doymaz / Chemical Engineering and Processing 47 (2008) 914–919 I.
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Table 4 Calculated effective moisture diffusivity at for pre-treated and untreated strawberry drying Code
Air temperature, T (◦ C)
Effective diffusivity, Deff (m2 /s)
NAT
50 55 65
4.95 × 10−10 6.64 × 10−10 1.09 × 10−9
AEEO
50 55 65
6.42 × 10−10 8.90 × 10−10 1.42 × 10−9
ing of strawberry increases the drying rate and consequently decreases drying time. Also, rehydration of pre-treated samples was much faster than untreated samples. It was found that there was no constant-rate period of drying in strawberry. Drying process took place under the falling-rate period. The logarithmic model was found to be a better model for describing the characteristics of strawberry for both of the temperatures of 50 and 55 ◦ C. The values obtained from Wang and Singh were found to be more reasonable for temperature of 65 ◦ C than the other models. The effective diffusivity varied from 4.95 × 10−10 to 1.42 × 10−9 m2 /s and increases as air temperature increases. Appendix A. Nomenclature
Fig. 7. Variation of effective diffusivity with drying temperature.
where Deff is the effective diffusivity (m2 /s), r is the radius of the sphere (m). For long drying times Eq. (5) can be simplified to a straight-line equation in the form: 2 6 π Deff t M − Me = ln − (6) ln M0 − M e π2 r2 where (M − Me )/(M0 − Me ) is the moisture ratio (MR). The effective diffusivity values were calculated by Eq. (6), using the method of slopes. It is typically determined by plotting experimental drying data in terms of ln(MR) versus time [33]. From Eq. (6), a plot of ln(MR) versus time gives a straight line with a slope of: slope =
π2 Deff r2
(7)
The results of effective diffusivity of pre-treated and untreated samples summarised in Table 4. The effective diffusivities of pre-treated samples were higher than untreated samples (Fig. 7). In addition, the values of Deff increased greatly with increasing temperature. The effective diffusivities varied from 6.42 × 10−10 to 1.42 × 10−9 m2 /s for pre-treated samples and from 4.95 × 10−10 to 1.09 × 10−9 m2 /s for untreated samples over the temperature range 50–65 ◦ C. The values of Deff lay within in general range of 10−11 to 10−9 m2 /s for food materials [31]. Based on these results, the effective diffusivity in the alkaline ethyl oleate solution treated strawberry samples was increased at all the temperatures than untreated samples. 4. Conclusions The results of this study indicate that using alkaline ethyl oleate solution and increasing air temperature during the dry-
a, b, c Deff k M Me M0 n N r RMSE R2 t T Wd Wr z
drying constants in models effective diffusivity (m2 /s) drying rate constant (min−1 ) moisture content (kg water/kg dry matter) equilibrium moisture content (kg water/kg dry matter) initial moisture content (kg water/kg dry matter) positive integer, drying constant in the models number of experimental data points radius (m) root mean square error coefficient of determination drying time (min) air temperature (◦ C) weight of dried material (kg) weight after rehydration (kg) number of parameters in model
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