Drying Kinetics Modelling of Basil in Microwave Dryer

Transcription

1 AGRICULTURAL COMMUNICATIONS, 2015, 3(4): Drying Kinetics Modelling of Basil in Microwave Dryer ESMAEEL SEYEDABADI Department of Agronomy, Faculty of Agriculture, University of Zabol, Zabol, Iran. *Corresponding Author: (Accepted: 14 Jun. 2015) ABSTRACT The effect of microwave power levels on drying characteristics of basil (Ocimum basilicum L.) in microwave dryer was investigated in this study. The microwave power levels of 90, 270, 450, 720 and 900W were used to dry 30 g of basil leaves. The initial moisture content of the samples was 7.25 g water per g dry base. In order to determine the kinetic parameters, experimental data were fitted to seven different models based on the ratios of differences between the initial and final moisture contents and equilibrium moisture content. The energy consumption and the effective moisture diffusivity were determined for different microwave power levels. The results showed that increasing microwave power in the range of W leads to the decrease of drying time in the range of 35 6 minutes. The comparison of proposed models showed that the logarithmic model of MR= a.exp( k.t)+b was the best fit for all microwave powers due to the highest coefficient of determination (R 2 ), the lowest chi square (χ 2 ) and the lowest root mean square error (RMSE). Therefore the mentioned model can be used to estimate the moisture in basil leaves at any time during the microwave drying process. The maximum and minimum values of consumed energy for drying in 90 and 450 W microwave powers were 52.5 and 99 W.h, respectively. The effective diffusivity of basil leaves were in the range of to m 2 s 1 for different microwave powers. Keywords: Consumption energy, effective diffusivity, logarithmic model, Ocimum basilicum, regression, thin layer drying. INTRODUCTION Basil (Ocimum basilicum L.) which is mostly cultivated in Iran is a valuable medicinal plant for the food, pharmaceutical and cosmetics applications. It is used extensively to add a distinctive aroma and flavor to food, such as salads, pizzas, meats and soups. Like other fruits and vegetables, this plant cannot be preserved fresh for a long time due to its abundant moisture content and nutrients for microbes to grow. Drying is one of the common techniques to restrain the microbial growth to inactivate enzymes and to provide the preservation of seasonal plants throughout the whole year (Lijuan et al., 2005). Moreover, drying may be useful to minimize packaging requirements and to reduce shipping weights (Maroulis and Saravacos, 2003). Drying of agricultural products with conventional methods such as hot air drying or sun drying has many disadvantages due to the inability to handle the large quantities and to achieve consistent quality standards, contamination problems and long drying times (Soysal, 2004). For instance, in hot air drying, the heat transfer to inner sections of foods is very slowly because of low thermal conductivity of food materials during the falling rate period of drying (Maskan, 2000). The alternative method, which has recently been employed widely, is the use of rapid transfer of electromagnetic energy in the form of microwave (Chen, et al. 2001). By replacing hot air by microwave energy, the drying time can be drastically reduced (Sharma and Prasad, 2004) and the quality of finished product can be ensured (Yongsawatdigul and Gunasekaran, 1996) at least in some steps of the drying process. Microwaves are the electromagnetic waves in the frequency range of 300 MHz 300 GHz with a wavelength of 1 m 1 mm. What is unique to microwaves is that as they travel through a soft medium, an increase in temperature throughout the medium can be observed. This makes it to have many applications in the food and agricultural industries and our daily life. An example is the widespread application of home microwave ovens as a food drying tool. The microwave drying reduces the drying time and prevents the food from enzymatic decomposition (Zhang et al., 2006). In addition, this

2 AGRICULTURAL COMMUNICATIONS kind of drying is more uniform and energy efficient compared to conventional hot air drying (Decareau, 1985). Doymaz et al. (2006) studied drying of dill leaves with microwave power and presented a mathematical model for it. The mathematical modeling was also studied for drying mint leaves by Ozbek and Dadali (2007). Besides that some other researchers studied the spinach drying kinetic (Ozkan et al., 2007; Karaaslan and Tuncer, 2008). The effect of different microwave power levels on banana drying has been investigated by Pereira et al. (2007). There are many other researches about drying kinetic modelling of foods and vegetables such as apple (Wang et al., 2007), carrot, chard leaves (Alibas, 2006), parsley (Soysal et al., 2006), wild cabbage (Yanyang et al., 2004), coriander leaves (Sarimeseli, 2011), garlic (Sharma and Prasad, 2004; Figiel, 2009) and black tea (Panchariya et al., 2002). In the case of basil, Taheri Garavand et al. (2011) studied the drying kinetic of basil leaves in the convective dryer. The influence of two drying method on aroma compounds of sweet basil has been evaluated by Calin Sanchez et al. (2012). Also sun drying of basil leaves has been studied by Akpinar (2006). As far as we are concerned, there has not been any reports on drying kinetic of basil leaves by microwave oven. Therefore the main objectives of this study was to investigate the drying behavior of basil leaves and to study the effect of microwave output power levels on the drying kinetics besides comparing the experimental data found during the drying process with the predicted values obtained by using some thin layer drying models. MATERIALS AND METHODS Plant Material: The fresh basil samples were purchased from a local supplier in Zabol (Sistan and Balouchestan, Iran). They were washed and stored at 4 C in a refrigerator for 24 hours in order to equilibrate the moisture content. Before drying experiments, the leaves were separated from stems and were divided into portions of 30 g each. In order to obtain the initial moisture content, four portions were dried in the oven at the temperature of 105 ºC for a day. The initial moisture content of basil leaves was obtained equal to 7.25 g water per g dry base by the following equation (Mohsenin, 1970): M d. b (1) where W0 and Wd are the initial mass and the mass of product after drying, respectively. Drying Procedure and Equipment: The digital microwave oven (Media MW F 282ELKS) with input power of 1450 W was used for drying treatments. The microwave output power level and emission time could be selected with the aid of a digital control system. The microwave oven contained a rotating glass disc driven by an electrical motor. The presence of this disc was necessary to obtain homogeneous drying and to decrease the level of the reflected microwaves on to the magnetrons. This microwave oven was used at five different output powers, i.e. 90, 270, 450, 720 and 900 W. Moisture loss during drying was periodically measured by weighing the basil leaves on the digital balance with a precision of 0.01 g. Three replications were done for each experiment. Mathematical Modelling: Usually, the changes of moisture content of agricultural products during the drying process is measured and correlated to the drying parameters in order to development of thin layer drying models ( et al., 2002). The moisture ratio of basil leaves were calculated using the following equation (Akpinar, 2006): MR (2) where MR is the moisture ratio, Mt is the moisture content at a specific time (g water per g dry base), M0 is the initial moisture content (g water per g dry base) and Me is the equilibrium moisture content (g water per g dry base) (Soysal 2004; Akpinar, 2006). The equilibrium moisture content was assumed to be zero for microwave drying as stated by (Maskan, 2000; Alibas, 2006). Energy consumption for microwave drying was obtained by Eq. (3): E P.t (3) where Et is total energy consumed for drying in every drying period (W.h), P is the microwave output power (W) and t is drying time (h) (Hebbar et al., 2004). Several models have been proposed to predict the moisture loss vs. time for different food products. In the present study seven different thinlayer drying models were selected as they represent some of the more commonly adopted. The regression analyses were done for these models by relating dimensionless moisture ratio (MR) and the drying time for 90, 270, 450, 720 and 900 W microwave powers. The names, equations and references of these models are given in Table 1. Fitting Models: The software package Matlab 2013a (Mathworks Inc., Natick, MA) was used for fitting the above models to experiments data. Comparing of models was done by use of the coefficient of determination (R 2 ), Chi square (χ 2 ) and root mean square error (RMSE). The mathematical equations of these parameters are given by Eq. (4) to Eq. (6); R 1,,,, (4) 38

3 SEYEDABADI χ,, (5) RMSE MR, MR, (6) where MRexp,i and MRpre,i are the experimental and predicted moisture ratios at time i respectively. N is the number of observations and m is the numbers of drying constants. Table 1. The mathematical thin layer models applied to the drying curves of basil leaves. Models Equations References MR=exp( (bt+at 2 )) Shi et al. (2008) MR = exp( k t n ) Mundada et al. (2010) MR = a exp( k t) Togrul and Pehlivan (2004) Lewis MR=exp( kt) Roberts et al. (2008) MR = a exp( k t n )+b.t et al. (2002) MR = a exp( k t) + b Ertekin and Yaldiz (2004) MR = 1 + a t + b t 2 Ozdemir and Devres (1999) Effective Moisture Diffusivity Determination: The drying characteristics of agricultural products in the falling rate period can be described by using Fick s diffusion equation. The solution to this equation is on the form of Eq. (7) and can be useful for samples with slab geometry by assuming uniform initial moisture distribution (Crank, 1975). MR exp π t (7) where Deff and L are the effective diffusivity (m 2 s 1 ) and the half thickness of slab (m) respectively. The mean thickness of basil leaves were equal to m. In order to obtain effective diffusivity, Eq. (7) can be written in the straight line form as following equation (Wang et al., 2007; Al Harahsheh et al., 2009): ln MR ln π t (8) The effective moisture diffusivity was determined by using the method of plotting experimental drying data in terms of Ln (MR) versus time and then calculating the slope of linear fits. RESULTS AND DISCUSSIONS The variation of moisture ratio as a function of time can be used to investigate the effect of microwave powers on drying process. In the present study, the microwave powers were 90, 270, 450, 720 and 900 W. The mass of samples were 30 g. In Fig. 1 the variation of moisture ratio (dry base) vs. time is provided. The drying process is characterized by a progressive decrease in moisture content with time. At the beginning of drying process, the initial moisture content of the product and the rate of moisture loss are high. But during drying process, the reduction of moisture content of the product leads to decrease the natural rate of drying because the microwave power absorption by the product depends on its moisture content. This finding is similar with results of several researches (Panchariya et al., 2002; Soysal, 2004; Wang et al., 2007). Fig. 1: Variation of moisture ratio vs. time during drying of basil leaves in different microwave powers. 39

4 AGRICULTURAL COMMUNICATIONS The times required to reach the basil leaves to the final moisture content were 35, 19, 14, 8 and 6 min in 90, 270, 450, 720 and 900 W microwave powers, respectively. While the moisture content decreases gradually at 90 W, a sharp decrease of moisture content occurs for the microwave power of 900 W. In fact, increase of the microwave power leads to decrease the drying time because it can increase the thermal gradient in the samples and thus increase the moisture evaporation rate. These results were in agreement with the results reported by several authors for various foods (Dadali et al., 2007; Wang et al., 2007; Al Harahsheh et al., 2009). Also it can be concluded, the drying time could be reduced up to 82% by application of 900 W instead of 90 W microwave power. Akpinar (2006) reported the suitable time for sun drying of 30 g basil leaves is about 6 hours. Therefore it can be concluded that the drying time could be shortened 5 times by application of microwave drying with 900 W power instead of sun drying. The drying energy consumption in Watt hours (W.h) for drying of 30 g basil leaves for different microwave powers are shown in Fig. 2. The energy has the minimum value for drying in 90 W. Increase of microwave power makes energy consumption faster. But it decreases for very high powers again. By assuming the difference in the quality of dried basil with different powers could be negligible, it is recommended that the drying must be done in 90 W microwave power in order to reduce drying energy consumption. Table 2 and Table 3 show the parameters of R 2, RMSE and χ 2 for models fitted in to the experimental data of drying the basil leaves with different microwave powers. The comparison of these parameters showed that, in general, all models were good approximations to predict the results of experiments, but the logarithmic models were found to have better fit than others because of higher R 2 value and lower RMSE and χ 2 values. Power 90 W 270 W Table 2. The statistical analysis of fitted models for drying of basil leaves in 90 and 270 W microwave powers. Model Lewis Lewis R RMSE Taheri Garavand et al. (2011) reported that the model could satisfactorily illustrate the drying curve of basil leaves for different convective drying conditions. Sarimeseli (2011) has also reported the validation of this model for microwave drying of coriander leaves. The logarithmic models that were fitted to the experimental results for different microwave powers are shown in Fig. 3. The horizontal axis in the figures is scaled for better vision. High values of coefficient of determination in the models show the logarithmic model can predict the drying process more accurately. Therefore these models can be useful for industrial usage. The effective moisture diffusivity values of basil leaves under the microwave power range of W are given in Table 4. These values are found to be within the general range of to 10 6 m 2 s 1 for food materials and agricultural crops (Erbay and Icier, 2010). The minimum and maximum values of the effective moisture diffusivity were and m 2 s 1 for 90 and 900 W, respectively. Therefore the moisture diffusivity increased with the increase of microwave power. This finding is in agreement with the results of other researchers such as Ozbek and Dadali (2007) and Demirhan and Ozbek (2011). Sarimeseli (2011) reported the effective moisture diffusivities 40

5 SEYEDABADI of coriander leaves were in the range of m 2 s 1 for microwave powers of W. The obtained effective moisture diffusivities in this study were higher than the results of sun drying of basil leaves that was reported to be by Akpinar (2006). Also the effective diffusivity values of foods dried in a microwave drier were higher than the values determined for convective drying (Al Harahsheh et al., 2009; Erbay and Icer, 2010). Fig. 2: The drying energy consumption (W.h) of basil leaves in different microwave powers. Table 3. The statistical analysis of fitted models for drying of basil leaves in 450, 720 and 900 W microwave powers. Power Model R 2 RMSE W 450 Lewis W 720 Lewis W 900 Lewis Table 4. The estimated effective moisture diffusivities for drying of basil leaves under different microwave powers. Microwave power levels 90 W 270 W 540 W 720 W 900 W Deff ( m 2 /s)

6 AGRICULTURAL COMMUNICATIONS Other researchers were reported the effective moisture diffusivities of coriander leaves (Sarimesli, 2001) and mint leaves (Ozbek and Dadali, 2007) for microwave drying at W were in the range of to and to m 2 s 1, respectively. These values are smaller than those of our findings for basil leaves. Fig. 3: The fitted logarithmic models for drying of basil leaves in different microwave powers. CONCLUSION In this study, microwave drying of basil leaves with various powers was investigated. The results showed that the times required to reduce moisture content of basil leaves from 7.25 to 0.1 (g water per g dry base) are 35, 19, 13, 8 and 6 min respect to applied microwave powers of 90, 270, 450, 720 and 900 W. Among the models, the logarithmic model had the best estimation to the experiments for all microwave powers. Minimum and maximum values of drying energy were 52.5 and 99 watthours respect to microwave powers of 90 and 450W. The effective moisture diffusivity were in the range of to m 2 s 1 for W microwave powers. Acknowledgements The author is grateful for the financial support of the Department of Agronomy, University of Zabol. 42

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