THE THIN LAYER DRYING OF TEMU PUTIH HERB

Transcription

1 The 6 th Asia-Pacific Drying Conference (ADC2009) October 19-21, 2009, Bangkok, Thailand THE THIN LAYER DRYING OF TEMU PUTIH HERB *Lamhot P. MANALU 1,2, Armansyah H. TAMBUNAN 3, Leopold O. NELWAN 3 and Agus R. HOETMAN 4 1 Graduate Student of Department of Agricultural Engineering, Bogor Agricultural University 2 Agency for Assessment and Aplication of Technology (BPPT) Gd. 2 BPPT Lt. 17 Jl. Thamrin 8 Jakarta 10340, Indonesia 3 Department of Agricultural Engineering, Bogor Agricultural University Kampus IPB Darmaga, Bogor, Indonesia 4 Ministry for Research and Technology Gd. 2 BPPT Lt. 8 Jl. Thamrin 8 Jakarta 10340, Indonesia Corresponding author: Lamhot P. MANALU. lpmanalu@yahoo.com Keywords: Drying; Herb; Temu Putih; Zedoary, Curcuma zedoaria; Thin layer drying ABSTRACT In this study, the influence of drying air temperature and drying air relative humidity on the thin layer drying of zedoary or temu putih (Curcuma zedoaria Rosc.) slices was investigated. A laboratory air dryer was designed and used for drying experiments. The system was operated in an air temperature range of o C and relative humidity range of 20 80%. Four mathematical models available in the literature were fitted to the experimental data. The performance of these models is evaluated by comparing the modeling efficiency, least root mean square error and the least reduced χ-square between the observed and predicted moisture ratio. By statistical comparison of the values for the four models, it was concluded that the Page model represents drying characteristics better than the other equations for describing single layer drying of temu putih. The effect of the drying air temperature on the drying model constants and coefficients were also determined. INTRODUCTION Indonesia is one of the herbal producing countries which produce 448 million tones and export 4.8 million tones or US$ 4.9 million in 2006 [1]. Recently, there has been an increasing demand for organic and natural vegetables due to human health benefits. Medicinal plants and herb products are of growing importance in the world. Parallel with these changes in human consumption, the products has developed rapidly. Zedoary or temu putih (Curcuma zedoaria Rosc.) is one of the important medicinal plants in Indonesia, the rhizome of plants use for traditional herbal medicine as jamu. Many small traditional medicine companies need temu putih in driedslices also called simplicia and according to national standard its moisture content should less than 10%. One of the simplest methods to reduce their moisture content to such extent that the microorganism can not grow is drying. Sun drying is a common way to dry the simplicia in Indonesia. Drying is a classical method of food preservation and it is a difficult food processing operation mainly due to undesirable changes in quality of dried product [2]. The basic objective in drying agricultural products is the removal of water in the solids up to certain level, at which microbial spoilage and deterioration chemical reactions are greatly minimized [3]. Improving the quality of the dried materials could increase this amount and thus the income. In order to improve the quality, the traditional natural sun drying technique should be replaced with modern drying methods. Drying characteristics of specific products should be determined to improve its quality. The study of the drying behavior of different products has recently been a subject of interest for various investigators. For example, apricot [4, 5], grape [6], pistachio [7], potato [8], plum [9, 10], green pepper and onion [11], pumpkin [12], onion [13], green bean and carrot [14]. For herb, black tea [15, 16], coriander leaves [17], garden mint leaves [18], dill and parsley leaves [19] and rosehip [20]. The objective of this work was to study the effect of drying air temperature and drying air relative humidity, on the drying characteristics and drying time of temu putih drying process. In addition to this, development of a mathematical model for thin layer drying of temu putih, choosing a suitable model and also investigation of the effects of drying conditions on the model coefficients which can describe the drying characteristics of temu putih. THEORETICAL CONSIDERATIONS The mechanisms of mass transfer in foods are complex. The dehydration of biological materials normally follows a falling-rate drying period. The moisture and/or vapor migration during this period is controlled by diffusion [21]. Assuming that the resistance to moisture migration is uniformly distributed throughout the interior of the homogenous isotropic material, Fick s second law can be derived as follows: 1 where: M l is the local moisture content in% d.b.; t is the drying time in min; and D is the effective diffusivity in mm 2 s -1. Assuming that the moisture is initially uniformly distributed throughout the sample, mass transfer is symmetric with respect to the centre, that the surface

2 moisture content of the sample instantaneously reaches equilibrium with the conditions of surrounding air, and that shrinkage is negligible or not taken into consideration, the solution of Eqn (1) for an infinite slab can be defined as follows [22, 23]: where MR (moisture ratio) is the unaccomplished moisture change defined as the ratio of the free water still to be removed at time t to the total free water initially available, h is the half thickness of the slab in sample in m; and n is a positive integer. For long dehydration periods (MR<), a limiting form of Eqn (2) is obtained for slab geometries by considering only the first term in their series expansion. Then, Eqn (2) can be written in logarithmic form: ln The values of the effective moisture diffusivity obtained by the method of slopes, if compared with the accurate ones obtained solving Fick s unsteady diffusion, and using iterative techniques (general implicit Euler s method of finite differences), are similar in materials where liquid diffusion is predominant [24]. The correlation between the drying conditions and the values of the effective diffusivity thus obtained can be expressed by using an Arrhenius type equation [25, 2] in the form: 4 where D 0 is pre-exponential factor (m -2 s), E a is activation energy for moisture diffusion (kj/mol) and R is ideal gas constant (kj/mol K). The coefficients of Eqn. (4) can be easily obtained by plotting on a ln(d) versus 1/T abs diagram. The E a and D 0 coefficients can be subsequently related to the drying air conditions by applying regression analysis techniques. Although transport properties are necessary to fully describe the drying kinetics of materials [26, 15], a single drying constant K, combining the effect of the various transport phenomena, is most commonly used in practice. 5 where M is a bulk value of the moisture content for the entire material and is only function of time. For this relation it is assumed that the material layer is thin enough and the air velocity is high, so that the conditions of the drying air (humidity, temperature) are kept constant throughout the material. The solution of Eq. (5) obtained by integration is [27-29]: The main advantage of Eq. (6) is the fact that the coefficients A and K introduced can be deduced by taking logarithms of both sides of the relation and the above relation can be linearized as: ln ln 7 The values of the K coefficient obtained can be related therefore to the drying air conditions by applying regression analysis techniques. From Eqn (3), the drying constant in Eqn. (7) is related to the diffusion coefficient by: 8 4 Thin-layer drying models that describe the drying behavior of biological materials fall into three categories, namely, theoretical, semi-theoretical and empirical. The semi-theoretical models are generally derived by simplifying general series solutions of Fick s second law or modification of simplified models and valid within the temperature, relative humidity, airflow rate and moisture content range for which they were developed [30]. Among the thin-layer drying models, the Lewis model the Henderson and Pabis model, the two-term model and the Page model are used frequently [31, 24]. These semiempirical models are generally used satisfactorily describing the thin layer drying behaviors [32, 2]. MATERIAL AND METHOD The Laboratory Dryer The drying experiments were carried out using the laboratory dryer in the Department of Agricultural Engineering, Faculty of Agricultural Technology, Bogor Agricultural University of Indonesia, which could be regulated to any desired drying air temperature between 30 and 80 o C and relative humidity between 20 and 90%. The air temperature and the relative humidity are controlled by AVR Atmel microprocessor controller with temperature accuracy of ±1 o C and relative humidity accuracy of ±2%. The unit is equipped with a 2000 W steam injection humidifier, a 2000 W heating and heating control unit, an electrical fan, temperature and humidity measurement sensor by SHT15 Sensirion and the 40 cm x 40 cm x 40 cm drying chamber (Fig. 1). Microprocessor Controller Electrical Fan PC Heating Unit Drying Chamber Scale Humidifier Airflow Regulator exp 6 Fig. 1. Schematic diagram of thin-layer dryer

3 The desired drying air temperature and relative humidity are maintained by PID control system. The air passed from the heating at the desired temperature and passed to the drying chamber. The lever manual-controller regulate the velocity of the drying air flowing through the drying chamber and it was measured by a hot wire digital Kanomax anemometer with the accuracy of ±0.1 m/s. The drying air temperature and relative humidity were measured directly in the drying chamber. Weighing of samples inside the drying chamber was done automatically at desired time interval using an electronic balance by GF AandD with a capacity of g and accuracy of 1 g. The dryer has equipped with data acquisition system. The Experiments The rhizome of temu putih were cut into about 3 mm slice thickness with a knife and then dipped into boiling water for 5 min [33]. Thin layers of sample slices were dried at a drying air temperature of 40, 50 and 60 o C and a drying air relative humidity of 20%, 40%, 60% and 80% using the laboratory dryer. The drying air velocity was fixed to 0.9 m/s. The weight of each drying sample used in the experiments were about 150 g. For each run the equipment was allowed at least one hours to stabilize at the specified air conditions before the test began and was maintained for the duration of the test. Then the temu putih slices were spread in a thin-layer on drying trays and placed in the drying chamber and the test started. The air temperature, air relative humidity, air velocity and sample weight were continuously monitored and recorded every 5 min during drying experiments. Drying was continued until the sample reach the fixed weight. After each drying experiment, the sample was oven-dried at 103 ± 2 o C to determine the moisture content [34]. Equilibrium Moisture Content The equilibrium moisture content (EMC) of temu putih obtained by exposed the sample to different air temperatures and relative humidities in thin-layer drying until the mass loss of the sample ceased. The EMC were determined from the final moisture content of sample in the drying experiments [35]. These values were used to calculate the moisture ratio and to fit the different models to the experimental data. Mathematical Modeling The moisture ratio (MR) and drying rate during drying experiments were calculated using the following equations: 9 10 where, M, M 0, M e, M t and M t+dt are the moisture ratio, moisture content, initial moisture content, equilibrium moisture content, moisture content at t and moisture content at t + dt (kg moisture/kg dry matter), respectively, t is drying time (min) [19]. In this study, the relationship between the constants of the best mathematical model with the drying variables of drying air temperature and humidity were also determined. The drying data were fitted to the different equations type such as the Lewis Model, Henderson and Pabis model, Page model and Wang and Singh model (Table 1). Table 1 Mathematical models given by various authors for drying curves No. Model name Equation Reference 1 Lewis exp [4,5] 2 Henderson & Pabis exp [36] 3 Page exp [2] 4 Wang & Singh 1 [37] The reduced χ-square (chi-square), root mean square error (RMSE) and modeling efficiency (EF) were used as the primary criterion to select the best equation to account for variation in the drying curves of the dried samples [38, 30-33]. Reduced χ-square is used to determine the goodness of the fit. The lower the values of the reduced χ- square, the better the goodness of the fit. The RMSE gives the deviation between the predicted and experimental values and it is required to reach zero. The EF also gives the ability of the model and its highest value is 1. These statistical values can be calculated as follows:,,,,,, 11 12,, (13) where MR exp,i is the ith experimental moisture ratio, MR pre,i is the ith predicted moisture ratio, N is the number of observations, n is the number of constants in the drying model and is the mean value of experimental moisture ratio. The relationship between the constants and coefficients of the best suited model with the drying variables of drying air temperature were determined using regression modeling [39, 6]. The drying model with highest modeling efficiency and least root mean square error and the least reduced χ-square was chosen as the best model describing the thin layer drying characteristics of temu putih. RESULTS AND DISCUSSION Drying Kinetics of Temu Putih The effect of three temperatures and relative humidity on the drying curve of temu putih slices is shown in Figs. 2, 3, 4 and 5. It can be seen that there is no constant rate drying period in the drying of temu putih. All the drying takes place in the falling rate period (Figs 6 and 7). This shows that diffusion is the dominant physical mechanism governing moisture movement in the samples. Similar

4 results were obtained by several authors for coriander leaves [17], for okra [40] and for pistachio nut [35]. It is obvious from Figs. 8 and 9 that increasing the temperature caused an important increasing in the drying rate, thus the drying time is decreased. The time required to reduce the moisture ratio to any given level was dependent on the drying condition, being highest at 60 o C and lowest at 40 o C. To reach a final moisture content, the drying time was 260 min at a drying air temperature of 60 o C and increased to 520 min at 40 o C with relative humidity of 40%. Then corresponding values were 410 min and 535 min at a relative humidity of 60%. The drying rate reached its maximum values at higher drying air temperatures. Drying rate decreases continuously with decreasing moisture content. The moisture removal inside the temu putih were higher at the beginning of drying and become slower in the end, because the migration of moisture to surface and evaporation rate from surface to air decrease with decrease of the moisture in the product, the drying rate clearly decrease. The mean drying rate was 0965 g water per g dry matter per min at a relative humidity of 40% and 1394 g water per g dry matter per min at a relative humidity of 60% at a drying air temperature of 40 o C, it increased to 3252 g water per g dry matter per min at 40% RH and 2103 g water per g dry matter per min 60% RH at a drying air temperature of 60 o C Fig. 2 Effect of relative humidity on moisture ratio of temu putih slices at 60 o C RH 80% Rh 40% Fig. 4 Effect of relative humidity on moisture ratio of temu putih slices at 40 o C Drying rate (g water/ g d.m. min) 60 C 50 C 40 C Fig. 5 Effect of temperature on moisture ratio of temu putih slices at 40% RH Fig. 6 Drying rate changes with drying time at the 60 o C and different relative humidities Fig. 3 Effect of relative humidity on moisture ratio of temu putih slices at 50 o C Drying rate (g water/g d.m.min) C 50 C 8 40 C Fig 7 Drying rate changes with drying time at the 40% RH and different temperatures

5 C 50 C 60 C % 40% 60% % Relative humidity 60% Fig. 8 Effect of temperature on drying time of temu putih slices at 40% and 60% RH Drying rate (g water/ g d.m. min) C 50 C 40 C Moisture content (% d.b.) Fig. 9 Effect of temperature on drying rate of temu putih slices at 60% RH 0 50 C Temperature 60 C Fig. 10 Effect of relative humidity on drying time of temu putih slices at 50 and 60 o C Drying rate (g water/ g d.m. min) Moisture content (% d.b.) Fig. 11 Effect of relative humidity on drying rate of temu putih slices at 50 o C The drying air relative humidity also has a significant influence on drying curves such as drying air temperature. Drying time decreases with increasing drying air relative humidity at all the drying air temperatures examined (Figs. 10 and 11). The relative humidity also caused an decrease in the drying rate, thus the drying time is increased [35]. The time required to reduce the moisture ratio to any given level was dependent on the drying condition, being highest at 80% and lowest at 20%. To reach a final moisture content, the drying time was 195 min at a drying air relative humidity of 20% and increased to 410 min at 60% RH with temperature of 60 o C. Then corresponding values were 335 min and 470 min at temperature of 50 o C. The effect of drying air temperature was most dramatic with moisture ratio decreasing rapidly with decreased humidity. Several investigators reported considerable increases in drying rates when higher temperatures were used for drying various products such as carrot [14], garlic [2], and eggplant [33]. Table 2 Equilibrium moisture content of temu putih slices in different drying conditions Temperature ( o C) RH (%) EMC (%d.b.) Drying Time (min) Equilibrium Moisture Content The equilibrium moisture content (EMC) of temu putih slices as the final moisture content of sample in the drying experiments are given in Table 2. It presents that that increasing the air temperature and decreasing relative humidity caused an decreasing in the value of the EMC. It also shows that the EMC can not reach 10% (w.b) or 11% (d.b.) at temperature of 40 o C and 50 o C (exept at 20% RH) as conditioned by national standard of Indonesia. Meanwhile, the sun drying operate in the low temperature and high humidity. Thus, it is not recommended to dry temu putih by only natural sun drying especially in humid area like Indonesia. Evaluation Of The Models The values of reduced χ-square, root mean square error (RMSE) and modeling efficiency (EF) coefficient of determination and reduced chi-square with estimated parameters for the two models are presented in Table 3. For all experiments the Lewis and Page models gave average EF values greater than The values of EF obtained for the Page model are higher than those from the Lewis model. According to the results of RMSE and chisquare values of all the thin layer drying models for all drying conditions, the Page model gave the lowest values too. The EF, reduced χ-square and RMSE values of Page model vary between and , between and 01286, and between and respectively. Thus, this model may be assumed to represent the drying behavior of temu putih. The value of drying constant k and n of Page model varied within the ranges of 010 to 199 min -1 and to , respectively, as shown in Table 4. Changes of experimental and predicted moisture ratio values with

6 drying time based on Page and Lewis Model are given in Figs. 12 and 13, respectively. Similar findings were reported for garlic slices [2]. The drying constant (k) is dependent on drying temperature because it is related to the diffusion coefficient (D), as presented by Eqn. (8). The following equation has been used to present the relationship between the drying constant and the drying temperature [41]: exp 14 where a and b are constant, and T is the drying air temperature. Fig. 14 shows that the drying constant increases exponentially with increases in the drying temperature for each level of relative humidity. The values of constant a and b were obtained by non-linear regression analyses. Table 5 presents the values a, b and r 2, where the value of r 2 were in range of 0.93 to Page Model Fig 12. Experimental and computed moisture ratio at 60 o C obtained using the Page model Lewis Model Fig 13. Experimental and computed moisture ratio at 60 o C obtained using the Lewis model Table 3 Statistical results obtained for Lewis, Henderson & Pabis, Page and Wang & Singh models Temp. ( o C) RH (%) EF χ^2 RMSE Lewis Model Average Henderson & Pabis Model Average Page Model Average Wang & Singh Model Average Constant, k (1/min) % RH 60% RH Temperature ( C) Fig. 14 Effect of temperature on drying constant (k) of temu putih Table 4 Parameter value of drying constant k and n in different drying conditions Temp. ( o C) RH (%) k (min -1 ) n

7 Table 5. The relationship between the drying constant k and drying temperature as a function of drying RH RH a b r 2 40% % CONCLUSIONS Based on this study, the following conclusions can be stated: (1) Drying air temperature and air relative humidity were significant factors in drying of temu putih slices. Higher drying air temperature and lower drying air relative humidity resulted in a shorter drying time and a lower EMC. (2) Drying of temu putih slices takes place in the falling rate period. (3) The Page model fits the thin-layer drying characteristics of the samples well. The drying constant k and n were varied from 010 to 199 min -1 and to , respectively. ACKNOWLEDGEMENT This paper was made possible by funding from the Hibah Kompetensi Research Fund of National Education Ministry of Indonesia and BPPT. The authors also would like to acknowledge the Department of Agricultural Engineering of Bogor Agricultural University and BPPT to provide facility and equipment. REFERENCES 1. Center of Agricultural Data and Information, Agriculture Ministry of Indonesia. hortikultura.deptan.go.id/index.php? option=com _content &task=view&id=134 &Itemid= , accessed on 20 April Rizvi, S.S.H. Thermodynamic properties of foods in dehydration, pp , in M.A. Rao, S.S.H. Rizvi and A.K. Datta (Eds.). Engineering Properties of Foods, 3 rd Ed., CRC Press, Boca Raton (2005) 3. Togrul, I.T. and Pehlivan, D. Mathematical modelling of solar drying of apricots in thin layers. Journal of Food Engineering, 55: (2002) 4. Togrul, I. T. and Pehlivan, D. Modelling of drying kinetics of single apricot. Journal of Food Engineering, 58(1): (2003) 5. Yaldiz, O., Ertekin, C. and Uzun, H. Mathematical modeling of thin layer solar drying of sultana grapes Energy 26: (2001) 6. Midilli, A. and Kucuk, H. Mathematical modelling of thin layer drying of pistachio by using solar energy. Energy Conversion and Management, 44(7): (2003) 7. Akpinar, E., Midilli, A. and Bicer, Y. Single layer drying behavior of potato slices in a convective cyclone dryer and mathematical modeling. Energy Conversion and Management, 44(10): (2003) 8. Menges, H.O. and Ertekin, C. Thin layer drying model for treated and untreated Stanley plums. Energy Conversion and Management 47: (2006) 9. Goyal, R.K., Kingsly, A.R.P., Manikantan, M.R. and Ilyas, S.M. Mathematical modelling of thin layer drying kinetics of plum in a tunnel dryer. Journal of Food Engineering 79: (2007) 10. Kiranoudis, C. T., Maroulis, Z. B. and Marinos- Kouris, D. Drying kinetics of onion and green pepper. Drying Technol. 10(4): (1992) 11. Doymaz, I. The kinetics of force convective air-drying of pumpkin slices. Journal of Food Engineering 79 : (2007) 12. Sharma, G.P., Verma, R.C. and Pathare, P.B. Thinlayer infrared radiation drying of onion slices. Journal of Food Engineering 67: (2005) 13. Doymaz, I. Convective air drying characteristis of thin layer carrots. Journal of Food Engineering 61: (2004) 14. Panchariya, P. C., Popovic, D. and Sharma, A. L. Thin layer modelling of black tea drying process. Journal of Food Engineering, 52, (2002) 15. Temple, S. J. and van Boxtel, A. J. B. Thin layer drying of black tea. J. agric. Engng Res., 74: (1999) 16. Ahmed, J., Shivhare, U.S. and Singh, G. Drying characteristics and product quality of coriander leaves. Trans IchemE. 79: (2001) 17. Park, K. S., Vohnikova, Z. and Brod, F. P. R. Evaluation of drying parameters and desorption isotherms of garden mint leaves (Mentha crispa L.). Journal of Food Engineering, 51, (2002) 18. Doymaz, I., Tugrul, N. and Pala, M. Drying characteristics of dill and parsley leaves. Journal of Food Engineering 77: (2006) 19. Erenturk, S., Gulaboglu, M.S. and Gultekin, S. The thin-layer drying characteristics of rosehip. Biosystems Engineering 89(2): (2004) 20. Sacilik, K. and Unal, G. Dehydration characteristics of kastamonu garlic slices. Biosystems Engineering, 92 (2), (2005) 21. Crank, J. The mathematics of diffusion, 2 nd Ed., Clarendon Press, Oxford (1975) 22. Babalis, S.J. and Belessiotis, V.G. Influence of the drying conditions on the drying constants and moisture diffusivity during the thin-layer drying of figs. Journal of Food Engineering, 65: (2004) 23. Karathanos, V. T., Villalobos, G., & Saravacos, G. D. Comparison of two methods of estimation of the effective moisture diffusivity from drying data. Journal of Food Science, 55(1), (1990) 24. Madamba, P. S., Driscoll, R. H. and Buckle, K. A. The thin layer drying characteristics of garlic slices. Journal of Food Engineering, 29, (1996) 25. Wang, Z.J.S, Liao, X., Chen, F., Zhao, G., Wu, J. and Hu, X. Mathematical modeling on hot air drying of thin layer apple pomace. Food Research International 40: (2007) 26. Karathanos, V.T. and Belessiotis, V.G. Application of a thin-layer equation to drying data of fresh and semi-dried fruits. J. agric. Engng Res., 74: (1999) 27. Palipane, K.B. and Driscoll, R.H. The thin-layer drying characteristics of macadamia in-shell nuts and kernels. Journal of Food Eng. 23: (1994) 28. Pahlavanzadeh, H., Basiri, A. and Zarrabi, M. Determination of parameters and pretreatment solution for grape drying. Drying Technology, 19(1), (2001)

8 29. Doymaz, I. and Pala, M. The thin-layer drying characteristics of corn. Journal of Food Engineering 60: (2003) 30. Özdemir, M. and Derves, O. The thin-layer drying characteristics of hazelnuts during roasting. Journal of Food Engineering, 42, (1999) 31. Akpinar, E., Bicer, Y. and Yildiz, C. Thin layer drying of red pepper. Journal of Food Engineering 59: (2003) 32. Sarsavadia, P., Sawhney, R., Pangavhane, D. R. and Singh, S. P. Drying behavior of brined onion slices. Journal of Food Engineering, 40, (1999) 33. Ertekin, C. and Yaldiz, O. Drying of eggplant and selection of suitable thin layer drying model. Journal of Food Engineering, 63: (2004) 34. Kashaninejad, M., Tabil, L. G., Mortazavi, A., & Safekordi, A. Effect of drying methods on quality of pistachio nuts. Drying Technology, 21(5), (2003) 35. Kashaninejad, M., Mortazavi, A., Safekordi, A. and Tabil, L.G. Thin-layer drying characteristics and modeling of pistachio nuts. Journal of Food Engineering, 78: (2007) 36. Pal, U. S. and Chakraverty, A. Thin layer convection drying of mushrooms. Energy Conversion and Management, 38(2), (1997) 37. Xanthopoulos, G., Oikonomou, N. and Lambrinos, G. Applicability of a single-layer drying model to predict the drying rate of whole figs. Journal of Food Engineering 81: (2007) 38. Hossain, M. A. and Bala, B. K. Thin layer drying characteristics for green chilli. Drying Technology, 20(2), (2002) 39. Menges, H.O. and Ertekin, C. Mathematical model of thin layer drying of golden apples. Journal of Food Engineering 77: (2006) 40. Doymaz, I. Drying characteristics and kinetics of okra. Journal of Food Engineering, 69: (2005) 41. Lee, G., Kang, W.S. and Hsieh, F. Thin-layer drying characteristics of chicory root slices. Trans. ASAE, 47(5): (2004)