Modeling and Optimization of Microwave Drying of

 

Modeling and Optimization of Microwave Drying of Turmeric Slices R. Assawarachan Drying and Dehydration Technology Research Unit Faculty of Engineering and Agro-Industry Maejo University Sansai, Chiang Mai 50290 Thailand

K. Kalayanamitra and T. Keokamnerd Faculty of Engineering and Agro-Industry Maejo University Sansai, Chiang Mai 50290 Thailand

Keywords: Curcuma longa, color quality, Box-Behnken response surface methodology Abstract This study was conducted to evaluate the effects of microwave power at 164, 465 and 752 W on the quality of turmeric slices and determine the suitable drying model that can predict the product’s moisture ratio at any time of the drying process. Dried product quality was measured based on color changes using L*, a and b*. Color quality changes of dried turmeric slices were highest at 752 W quantified by L* and b* values; in contrast, a* value and total color difference (TCD) did not significantly differ with microwave power. Results were successfully determined by the Box-Behnken’s response surface methodology. The experimental data were fitted to three thin layer drying models (Newton, Page and Logarithmic). The Page and Modified Page model were found as good models to predict the drying behavior of turmeric slices. The Page model was however the most satisfactory giving the highest coefficient of determination (0.9987-0.9993) and lowest chi-square (0.7×10-42.61×10-4) and root mean square error (0.0079-0.0142). Using the model, the optimum drying conditions that gave the lowest total color difference of 3.44 were 443.93 W microwave power with 5.1% ascorbic acid and 55.6 s drying time. INTRODUCTION Curcuma longa (tumeric) is a medicinal plant widely used and cultivated in tropical regions. Recently, turmeric has been valued worldwide as a functional food because of its health promoting property (Ammon and Wahl, 1991). Drying is an important method to preserve a wide variety of products; it reduces water content and water activity to limit growth of spoilage bacteria. Some of the disadvantages of the common hot air drying are low energy efficiency and lengthy drying time. On the other hand, microwave drying technique has distinct advantages and recently has been considered as an efficient drying alternative without effects during heat transfer (Assawarachan et al., 2013). Visual color, the first priority of consumers’ perceptions, is an important characteristic of a dried quality product (Kolawole et al., 2007). The effects of drying conditions on product quality need to be understood to obtain the desired quality product. A statistical technique is an important tool to determine the optimal condition of drying. Statistical technique by Box-Behnken Design (BBD) has been applied to predict the optimization of biological, chemical, and physical processes because of its reasonable design and excellent outcomes (Narkprasom et al., 2013). Knowledge of the mathematical models of drying is required to design the drying process system. The thin-layer equations contributed to the heat and mass transfer phenomena of sweet basils required for designing new processes and improving existing commercial operations (Özbek and Dadali, 2007). However, the information related to mathematical models and microwave drying optimum conditions of color retention of dried turmeric slices was scarcely studied. The present investigation determined the most appropriate model for predicting the moisture contents change of turmeric slices during microwave drying.

Proc. IInd Southeast Asia Symp. on Quality Management in Postharvest Systems Eds.: A.L. Acedo Jr. and S. Kanlayanarat Acta Hort. 1088, ISHS 2015

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MATERIALS AND METHODS Fresh turmeric was procured from a local supplier in Thailand. The peels were separated, washed, sliced, centrifuged and stored at 4±0.5°C for one day for equilibration of moisture. Initial moisture content was determined by the AOAC (2010) method. A programmable microwave oven (Panasonic Model NN-S235WF) with a maximum output of 800 W was used. Weight loss during drying was recorded using a computer software data logger through a balance connected to a PC. A sample of 30 g of turmeric slices was spread into a single layer on the 20×20 cm2 ceramic tray. The sample was dried with three different microwave power levels of 164, 465 and 752 W from initial moisture content until the final moisture content of about 0.10±0.02% gwater/gdry matter. Moisture content data obtained from the drying experiments was converted into the dimensionless moisture ratio (MR).

MR 

Mt  Me Mi  Me

(1)

where Mi, Mt and Me are moisture content (gwater/gdry matter) at initial, specific time, equilibrium and t+dt, respectively; t is drying time (min). Due to prolonged drying, the equilibrium moisture content would be closed to zero (Me=0). The correlation coefficient (R2) was one of the primary criteria for selecting the best fitted model. In addition to R2, the chi-square (2), and root mean square error (RMSE) were used to determine the consistency of the fit. These statistical values can be calculated as follows: RMSE =

2

 =

1 N

 ( MR

 exp, i

N i 1

( MRexp,i  MR pred ,i ) 2

 MR

pred , i

)2

N  np

(2)

(3)

where MRexp,i is ith moisture content observed experimentally; MRpred,i is ith predicted moisture content; N and np are the number of observation and constants, respectively. The color of dried turmeric slices was measured in the CIELAB color system using spectrophotometer (Hunter Lab Color Flex Version 1.72, USA) which three parameters, L* (lightness), a* (redness/greenness), and b* (yellowness/blueness)”were measured in triplicates. Total color change (ΔE) was calculated using the following equation: ∆E =

( L*t  L*0 ) 2  (a*t  a*0 ) 2  (b*t  b*0 ) 2

(4)

where L*0, a*0, b*0 are the initial color measurements of fresh turmeric slices and L*t, a*t, b*t are the color measurements at a pre-specified time. Three variables of power level, ascorbic acid concentration and time were designed for the experiment according to the principle of BBD which was applied to identify and determine the optimum condition. As shown in Table 1, the three microwave drying factors were indicated as X1, X2, and X3 which prescribed into three levels, coded 1, 0, +1. Quadratic model was fitted to correlate the relationship between the independent variables and the response in order to predict the optimized condition. The following second-order expression was used in response surface analysis: Y = β0 + β1X1 + β2X2 + β11X12 + β22X22 + β12X1X2+ ε

(5)

where Y, B0, Bi, Bii, Bij are represented the estimated response, the equation parameters for the constant term, the linear terms, the quadratic terms for a single variable and the interaction terms (i =1, 2; and j=1, 2). The parameters of X1 and X2 were represented by independent variables (extraction temperature and extraction time, respectively), while ε represented the random error. The analysis of variance (ANOVA) tables were generated 600

 

and the second order model predictions were determined. The Microsoft Excel version 2010 was used to develop the response surface plots. RESULTS AND DISCUSSION Drying Characteristics The initial moisture content of turmeric slices was 8.54±0.17 gwater/gdry matter. Microwave drying turmeric slices at 164, 465 and 752 W was done until final moisture content of 0.08 gwater/gdry matter. Three well-known mathematic models, i.e. Newton, Page and Logarithmic models were used to predict the moisture ratio changes (Table 2). The drying model had the best fit to the drying data when moisture ratio (MR) and time (t) provided the highest coefficient of determination (R2), the least chi-square (2) and the lowest root mean square error (RMSE). According to these criteria, the semi-empirical Page model performed the best results. In addition, the constant parameters of the empirical model (k, n, a, b and c) for each drying temperature were described. The Page model showed an excellent fit at the most satisfactory level to predict drying behavior of the turmeric slices with the highest R2 as 0.9987 to 0.9993, the least 2 as 0.7×10-4 to 2.61×10-4, and the lowest RMSE as 0.0079 to 0.0142 during microwave drying. Previous studies revealed that the Page model was also suitable for describing drying curve in many biomaterials (Assawarachan, 2013; Dadal et al., 2007; Ozkan et al., 2007). Effects of Microwave Drying Figure 1 shows the L*, a* and b* values of fresh and dried turmeric slices. L* values decreased with drying. L* of microwave-dried samples were the highest among dried samples which were closer to L* values of fresh sample. Krokida and Maroulis (1999) showed that microwave drying prevented color damage during drying. Oven and microwave oven dried turmeric slices were darker in color. Regression coefficients for the 2nd order response surface models in terms of total color difference (TCD) are shown in Figures 2 and 3. The results demonstrated that the predicted values which were obtained from the regression model (Eq. 3) were closer to the experimental TCD values. Therefore, this model was appropriate to predict the effects of the three independent variables; microwave power levels (X1), ascorbic acid solution (X2) and time (X3) on the change of color quality of dried turmeric slices. Y = 3.45+0.2X1+0.02X2+0.04X3+0.76X1X2-0.11X1X3+0.16X2X3+1.84X12+0.43X22+0.35X32 (6)

where X1 is microwave power levels (W), X2 is ascorbic acid concentration (% w/w) and X3 is time (min). The quadratic polynomial equation (Eq. 6) fitted to response surface model was used to develop a three dimensional response surface and contour plots. The response surface and contour plots were generated to visualize the effects of the independent variables on the response (Y) of TCD of dried turmeric slicesas shown in Figures 2 and 3, respectively. By analyzing the second-order polynomial model, a minimum TCD of 3.44 was obtained under the following condition: microwave power at 443.93 W, concentration of ascorbic acid at 5.1% and 55.6 s drying time. CONCLUSIONS The experimental data were fitted to three thin layer drying models (Newton, Page and Logarithmic). The Page and Modified Page model were found as good models to predict the drying behavior of turmeric slices. The Page model was however the most satisfactory giving the highest coefficient of determination (0.9961-0.9983) and lowest chi-square (1.7×10-3- 0.04×10-3) and root mean square error (and 0.0364 to 0.0011). Dried product quality was measured based on color changes using the CIE (L*-a*-b*) color system. Color quality changes of dried turmeric slices were highest at 752 W quantified by L* and b* values; in contrast, a* value and total color difference (TCD) did not significantly differ with microwave power. The results were successfully determined by the Box-Behnken’s response surface methodology.

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Literature Cited Ammon, H.P. and Wahl, M.A. 1991. Pharmacology of Curcuma longa Plantamedica 57(1):1-7. Apintanapong, M and Maisuthisakul, P. 2011. Microwave-vacuum drying kinetics of tumeric. Agricultural Science Journal (in Thai) 42 (2):269-272. Assawarachan, R. 2013. Microwave vacuum drying kinetics of dried Spirogyra sp. Thai Society of Agricultural Engineering Journal (in Thai) 19(1):2-11. Assawarachan, R., Nookong, M., Chailungka, N and Amornlerdpison, D. 2013. Effects of microwave power on the drying characteristics, color and phenolic content of Spirogyra sp. Journal of Food, Agriculture and Environment 11(1):1-4. Dadal, G., Apar, D.K. and Özbek, B. 2007. Microwave drying kinetics of okra. Drying Technology 25(5):917-924. Kolawole, O., Joseph, C.I., Funke, A.A. 2007. Kinetics of mass transfer, and colour change during osmotic dehydration of watermelon. Journal of Food Engineering 80:979-985. Krokida, M.K. and Maroulis, Z.B. 1999. Effect of microwave drying on some quality properties of dehydrated products. Drying Technology 17:449-466. Narkprasom, N., Assavarachan, R and Wongputtisin, P. 2013. Optimization of reducing sugar production from acid hydrolysis of sugarcane bagasse by Box Behnken Design. Journal of Medical and Bioengineering 2(4):238-241. Ozkan, I.A., Akbudak, B. and Akbudak, N. 2007. Microwave drying characteristics of spinach. Journal of Food Engineering 78:577-583. Özbek, B and Dadali, G. 2007. Thin-layer drying characteristics and modelling of mint leaves undergoing microwave treatment. Journal of Food Engineering 83:541-549. Tables

Table 1. The independent variables used in the optimization study. Independent variables Microwave power (W) Ascorbic acid conc. (w/w %) Time (min)

Coded X1 X2 X3

-1 164 2 2

Coded variables 0 465 5 5

1 752 8 8

Table 2. Regression coefficients of thin layer drying models for microwave drying of turmeric slices. Drying model Newton MR = exp(-kt) Page MR = exp(-ktn) Logarithmic MR =α*exp(-kt)+c

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Microwave power (W) 335 400 545 335 400 545 335 400 545

Drying model constants

R2

 2 ×10-4

RMSE

k = 0.3295 k = 0.4192 k = 0.6277 k = 0.1359, n = 1.7350 k = 0.1760, n = 1.9154 k = 0.4055, n = 1.8127 k = 0.2215, a = 1.4036, c = -0.3116 k = 0.2371, a = 1.5784, c = -0.4856 k = 0.4526, a = 1.3207, c = -0.2408

0.9759 0.9686 0.8608 0.9993 0.9991 0.9987 0.9923 0.9897 0.9856

87.00 125.10 562.60 0.70 1.40 2.61 25.50 40.90 60.90

0.0903 0.1071 0.2236 0.0079 0.0106 0.0142 0.0455 0.0554 0.0637

 

Figures

Fig. 1. Effect of microwave power densities level on color changed of dried turmeric slices.

(a)

(b)

(c)

Fig. 2. 3D plot of total color difference as a function of (a) power level and ascorbic acid concentration, (b) power level and time, (c) ascorbic acid conc. and time.

(a)

(b)

(c)

Fig. 3. Contour plot of total color difference as a function of (a) power level and ascorbic acid conc., (b) power level and time, (c) ascorbic acid conc. and time.

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