THE THIN LAYER DRYING OF TEMU PUTIH HERB

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. E-mail: 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 40 60 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

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]: 8 1 2 1 2 1 4 2 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 8 3 4 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

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- 3000 AandD with a capacity of 0 3000 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

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. 0 50 100 150 200 250 300 350 400 450 Fig. 2 Effect of relative humidity on moisture ratio of temu putih slices at 60 o C RH 80% Rh 40% 0 100 200 300 400 500 600 700 800 900 100 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 0 50 100 150 200 250 300 350 400 450 500 550 Fig. 5 Effect of temperature on moisture ratio of temu putih slices at 40% RH 0.15 0.12 9 6 3 0 0 50 100 150 200 250 300 350 Fig. 6 Drying rate changes with drying time at the 60 o C and different relative humidities 0 50 100 150 200 250 300 350 400 450 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) 0.12 0.10 60 C 50 C 8 40 C 6 4 2 0 0 50 100 150 200 250 300 Fig 7 Drying rate changes with drying time at the 40% RH and different temperatures

600 600 500 400 300 200 40 C 50 C 60 C 500 400 300 200 20% 40% 60% 100 100 0 40% 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) 8 6 4 2 0 60 C 50 C 40 C 0 100 200 300 400 500 600 700 800 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) 0.15 0.12 9 6 3 0 0 100 200 300 400 500 600 700 800 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) 40 50 60 RH (%) EMC (%d.b.) Drying Time (min) 40 12.43 520 60 16.23 535 80 34.84 915 20 6.07 335 40 11.93 345 60 14.72 470 20 5.78 195 40 6.55 260 60 7.96 410 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 0.95. 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 0.9731 and 0.99787, between 00156 and 01286, and between 01047 and 03482 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 1.1053 to 1.2783, respectively, as shown in Table 4. Changes of experimental and predicted moisture ratio values with

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 0.96. Page Model 0 50 100 150 200 250 300 350 400 450 Fig 12. Experimental and computed moisture ratio at 60 o C obtained using the Page model Lewis Model 0 50 100 150 200 250 300 350 400 450 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 40 40 62537 06573 07873 60 0.988434 00833 02765 80 0.967804 02682 03807 50 20 0.928569 03261 06871 40 0.977142 01383 04411 60 0.982477 01313 03698 60 20 0.987240 00682 04073 40 0.981081 01309 04919 60 0.965268 02625 05588 Average 0.960061 02296 04889 Henderson & Pabis Model 40 40 0.720116 13383 11234 60 0.975334 01777 04037 80 0.985476 01210 02557 50 20 0.709634 13255 13854 40 0.727302 16494 15235 60 0.973231 02006 04570 60 20 86036 06091 12172 40 0.939613 04177 08788 60 0.967864 02428 05375 Average 76067 06758 08647 Page Model 40 40 0.973114 01286 03482 60 0.996986 00217 01411 80 0.997565 00203 01047 50 20 0.982698 00790 03382 40 0.986799 00798 03352 60 0.996806 00239 01579 60 20 0.994323 00303 02717 40 0.997745 00156 01698 60 0.997801 00166 01406 Average 0.991537 00462 02230 Wang & Singh Model 40 40 94752 14595 11732 60 0.973505 01909 04184 80 0.999323 00056 00552 50 20 0.594960 18490 16362 40 60663 08428 10890 60 0.978898 01581 04058 60 20 17496 09754 15404 40 0.999745 00018 00571 60 0.965690 02593 05554 Average 76115 06380 07701 Constant, k (1/min) 24 20 16 12 08 04 40% RH 60% RH 30 40 50 60 70 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 40 085 1.1053 40 60 045 1.1156 80 010 1.2070 20 137 1.1444 50 40 075 1.2006 60 040 1.1653 20 199 1.1239 60 40 079 1.2215 60 032 1.2783

Table 5. The relationship between the drying constant k and drying temperature as a function of drying RH RH a b r 2 40% 055 254 0.9578 60% 038 214 0.9273 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 1.1053 to 1.2783, respectively. ACKNOWLEDGEMENT This paper was made possible by funding from the Hibah Kompetensi Research Fund of National Education Ministry of Indonesia and BPPT. 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