Quality assessment and modeling of microwave-convective drying of lemon slices

Engineering in Agriculture, Environment and Food 9 (2016) 216e223

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Research paper

Quality assessment and modeling of microwave-convective drying of lemon slices Omid Mirzabeigi Kesbi, Morteza Sadeghi*, Seyed Ahmad Mireei Department of Biosystems Engineering, College of Agriculture, Isfahan University of Technology, Isfahan, 84156-83111, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 December 2014 Received in revised form 8 November 2015 Accepted 29 December 2015 Available online 31 December 2015

Juice production is the most important process used for citrus fruits. However, drying process applied in the production of powders, flakes, and slices is also of great importance. Drying behavior of lemon slices was investigated using convective (50, 55 and 60
C inlet air temperatures), microwave (specific power of 0.97 W g1), and combined microwave-convective (specific powers of 0.97 and 2.04 W g1 assisted with 50, 55 and 60
C inlet air temperatures) dehydration methods. The quality was assessed in terms of L* a* b*, total color difference (DE), chroma, Hue angle, and rehydration capacity measurements. In addition to higher drying rate, the quality parameters for microwave-convective treatments were significantly higher than those for other methods. Drying kinetic curves indicated a constant rate period followed by a falling rate period in both convective and microwave drying methods. However, for microwaveconvective treatments a falling rate period was observed with the exception of a very short accelerating period at the start. Mathematical modeling of the drying kinetics was conducted by applying the commonly used drying models to the experimental data. According to the statistical parameters, Midilli et al. model presented the best prediction for the drying kinetics. © 2016 Asian Agricultural and Biological Engineering Association. Published by Elsevier B.V. All rights reserved.

Keywords: Color Mathematical modeling Microwave-convective drying Rehydration capacity Specific power

1. Introduction Citrus fruits with an annual production of approximately 102 million tonnes worldwide rank first among other fruits (Ladaniya, 2008). They are grown commercially in more than 50 countries around the world (Ladaniya, 2008). Citrus fruits have a great nutritional potential because of their high content of vitamins, fibers, and flavonoids and terpenes (Monselise, 1986). Despite the fact that the juice production is the most important process used for citrus fruits (Kimball, 1999), in order to promote their consumption, drying process development for production of powders, flakes and slices is also noteworthy (Dıaz et al., 2003). Lemon (Citrus limon (L.) Burm. f) and lime (Citrus aurantiflia (Chrism) Swing.) are the two most familiar citrus fruits grown with excellent quality in semi-arid irrigated and coastal areas (Ladaniya, 2008). In year 2010e2011, with about 473,000 tonnes of annual production of lemon and lime, Iran ranked ninth producing country in the world (FAOSTAT, 2012). In Iran, lemon and lime are grown commonly in Hormozgan and Fars provinces in southern regions of

* Corresponding author. E-mail address: [email protected] (M. Sadeghi).

the country and have always played an important social and economical role in people's lives. The traditional method of open-sun drying is still employed in the dehydration of Iranian lime and lemon. In this method, exposure to sunlight is required so that a characteristic flavor and moisture is achieved. However, the process has hygienic problems; additionally, a very humid climate causes fluctuations in the product quality (Chen et al., 2005). Drying methods considerably affect the color, texture, aroma, porosity and rehydration characteristics of the dehydrated materials. It has been pointed out that the dehydrated products do not keep their visco-elastic behavior due to structural damages that occur during drying (Chen et al., 2005). In assessing the quality of dried lemon, special attention should be paid to the organoleptic parameters considering their sensitive nature in drying conditions. The lemon has an acidic pulp and fine texture, is pale yellow in color, and limonene being its main aroma compound (Moufida and Marzouk, 2003). Thus, rehydration capacity and color indices must be considered as two important quality factors in drying of lemon. Drying is a complicated process with simultaneous heat and mass transfer, and food drying is far more complicated because of the variation in the composition and physical structure of the food materials. Therefore, effective mathematical models along with

http://dx.doi.org/10.1016/j.eaef.2015.12.003 1881-8366/© 2016 Asian Agricultural and Biological Engineering Association. Published by Elsevier B.V. All rights reserved.

O. Mirzabeigi Kesbi et al. / Engineering in Agriculture, Environment and Food 9 (2016) 216e223

Nomenclature L* a* b* CIE color parameters a, b, c, g, n, k, k1, k2 Empirical coefficients in the drying models MR Dimensionless moisture ratio MR Average of experimental moisture ratios M Moisture content, g g1 (d.b.) Me Equilibrium moisture content, g g1 (d.b.) M0 Initial moisture content, g g1 (d.b.) MW Microwave R2 Coefficient of determination RMSE Root mean square of error t Drying time, s T Inlet hot air temperature,
C Wd Dried sample weight, g Wg Water gain percentage, % Wt Rehydrated sample weight, g DE Total color difference Subscripts exp Experimental pre Predicted

experimental studies can be of help in design, simulation, optimization, energy integration, and the control of drying process. The mathematical models fall into three categories, namely: the theoretical, semi-theoretical and empirical. The results obtained by theoretical models may be very complicated and consequently, require some assumptions that do not meet the actual drying systems. Instead, semi-theoretical and empirical models are commonly used in mathematical modeling of thin layer drying of fruits, vegetables, seafood and other agricultural products drying (Hii et al., 2009). They are practical and give sufficiently good results (Erbay and Icier, 2010). Employing microwave radiation in drying process causes internal quick heating as a result of dipoles rotation and ion movement during drying the moist body. This heating method is particularly advantageous in thermal drying, since favorable properties of water and other polar liquids generate heat in the wet parts of the drying material, and hence extracting the moisture. This method considerably reduces drying time and promotes the quality of the dried sample. When internal heating extracts moisture and brings it to the surface, the presence of a convective flow can remove it from the surface rapidly. In other words, the combined microwave-convective method could be more useful (Funebo and Ohlsson, 1998; Maskan, 2000; Tulasidas et al., 1993). Researchers have studied the drying behavior of various natural materials using microwave method, and evaluated different empirical models to describe the thin layer drying characteristics (Karaaslan and Tunçer, 2008; Kouchakzadeh and Shafeei, 2010; McMinn et al., 2005; McMinn, 2006; Wang et al., 2007). In spite of the considerable number of studies reported in the literature for drying of various agricultural products, studies performed on lemon drying are very limited and researchers have not noted this matter greatly. In a single study, a closed-type solar dryer was used to dehydrate lemon slices and study its effect on quality parameters such as L* a* b* color indices, whiteness index and water activity of the dried product (Chen et al., 2005). The present study was aimed at: 1) modeling the drying behavior of lemon slices under convective, microwave, and

217

combined microwave-convective drying methods, and 2) investigating the influence of drying methods on some quality factors such as L* a* b* values, total color difference, chroma, Hue angle and rehydration capacity of the samples. 2. Materials and methods 2.1. Material preparation and procurement Lemon samples were obtained from a local market in Isfahan (central Iran) and stored at 6
C until the experiments were conducted. Prior to the drying experiments, the samples were placed in the room, where the experiments took place, for 24 h and cut perpendicular to the fruit axis into approximately equally-sized slices (5 mm thick and 50 mm diameter). Lemon slices (with peel) had the initial moisture content of about 5.66 g g1 (d.b.). This was determined by using vacuum drying method at 70
C until a constant weight was achieved (AOAC, 1996). Biochemical analyses were conducted to provide data on chemical composition of the raw material (AOAC, 1996). Crude protein (% total nitrogen  5.70) was determined by Kjeldahl method using 5 g. Fat, ash, and fiber were determined by AOAC official methods (Numbers 922.06, 942.05 and 985.29, respectively). Total carbohydrate content was obtained by difference [100 e (moisture þ ash þ protein þ fat þ fiber)]. The analyses were conducted on the peeled samples with two replications. Water content, carbohydrates, fat, protein, fiber, and ash were 95.69 ± 0.400, 1.90 ± 0.010, 1.72 ± 0.020, 0.26 ± 0.002, 0.21 ± 0.003, and 0.22 ± 0.001 g per 100 g lemon flesh, respectively. 2.2. Experimental set-up Fig. 1 shows the schematic diagram of the drying system and instrumentations used to conduct the experiments. A 2450 MHz domestic microwave (MW) oven (LG, MC-8047; LG Electronics Inc., Seoul, South Korea) with 180, 360, 540, 720, 900 W ranges of power and cavity dimensions of 400 (W)  380 (D)  260 (H) mm was modified and developed as the MW-assisted hot air dryer. Since the ranges of power are the nominal values, the accurate power of the system for two desired levels in the experiments (180 and 360 W) was measured by the power measurement procedure test (IMPI-2 liter test) (Buffler, 1993). The power measurement was run three times and the final power was reported as the mean of three readings. In this way, the accurate powers corresponding to the nominal values of 180 and 360 W were determined as 185.5 ± 1.01 and 388.5 ± 2.97 W, respectively. Pretests were conducted to determine the required power levels according to mass of lemon slices in each experiment (about 190 g for 10 slices). No burning on the samples was observed during the drying process. In the center of the oven chamber base, a circular area of 170 mm diameter was drilled in regular 4 mm diameter holes. A cone shape pipe was fixed at the bottom of the drilled area to supply hot air into the chamber. Sample basket (200  200 mm) was suspended by nylon wires from a digital balance (Kern 572-57, ±0.1 g accuracy, KERN & Sohn GmbH, Balingen, Germany) bracket right on top of the holes in the chamber. This made possible continuous sample weight monitoring. The sample weights were next sent to the PC through a RS232 port and recorded. A rectangular area (60  100 mm) was also drilled in regular 5 mm holes in the left wall of the chamber and a duct was fixed to lead the air out. The drying air was supplied by a centrifugal fan, powered by a 2 hp 3-phase motor. The air was blown by a blower into an electrical heater through an 80 mm diameter metal duct. A 1e3-phase frequency inverter (TECO, 7300 CV, ±0.01 Hz accuracy; TECO Electric & Machinery Co. Ltd., Taipei, Taiwan) was used to adjust and control

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Fig. 1. A schematic view of the experimental set-up and instrumentations of the combined microwave-convective drying system.

the airflow rate in the range of 0e0.4 m3 s1. Ten 0.7 kW electrical coils were used in the heating chamber to supply enough thermal energy to heat up the drying air to the desired temperature. The coils were connected to the electric mains (power supply) through a temperature controller. The controller was designed to control the temperature of hot air with accuracy of ±0.1
C and acted according to increase/decrease the electrical current to the heating elements. The temperature of the inlet hot air was measured by a thermometer (PT100, 0.1
C resolution; Testo GmbH & Co., Lenzkirch, Germany). Prior to the drying tests, the air velocity was measured under the basket (top of the holes) by an anemometer (LT lutron, AM4204, ±0.1 m s1 accuracy; Lutron Electronic Enterprise Co., Ltd., Taipei, Taiwan) and adjusted through changing the coming electrical frequency to the blower by the inverter. An ultrasonic humidifier (10480 model, MEHAVARAN Co., Ltd., Tehran, Iran) equipped with a microcontroller was used to adjust and control the relative humidity of the isolated drying room wherein the experiments were conducted. The ambient relative humidity was measured by a sensor (Philips H8302 model; Philips Electronics, Eindhoven, The Netherlands) with accuracy of ±2% and linearity of ±2%, for an operating span of 20e95% RH.

In order to model the drying kinetic behavior of lemon slices, the experimental drying data were fitted to 11 commonly used mathematical models (listed in Table 1) using non-linear regression analysis. In the models equation, MR represents the dimensionless moisture ratio, namely MR ¼ (M-Me)/(M0-Me), where M is the moisture content of the product at each moment, M0 is the initial moisture content of the product, and Me is the equilibrium moisture content. The Me was measured by having the samples dry in the apparatus for an extended period of time until no significant weight loss was detected. In this method the value of Me was approximately 0.05 g g1 d.b. for drying treatments. The goodness of fits was evaluated using the determination coefficient (R2) and the root mean square of error (RMSE). The higher the value of R2 and the lower the value of RMSE, the better is the goodness of fit. All mathematical calculations including determination of models coefficients, R2, and RMSE were performed in MATLAB software (The MathWorks, Inc., Natick, MA, USA, 1998).

2.3. Drying procedures

2.5. Color measurement

For each experiment, about 190 g (10 slices) initial sample was placed in the holding basket (monolayer) floor. The experiments were carried out in three methods: 1) Convective (hot air) drying at inlet hot air temperatures of 50, 55, 60
C, 2) MW drying at power of 185.5 W (specific power of 0.97 W g1), and 3) combined MWconvective drying at inlet hot air temperatures of 50, 55, 60
C and MW output powers of 185.5 and 388.5 W (specific MW powers of 0.97 and 2.04 W g1, respectively) with three replicates. All experiments were performed by the setup shown in Fig. 1 with MW power off in convective, and blower/heater off in MW drying methods. The samples were dried to reach the final moisture content of approximately 0.15 g g1 (d.b.). The air velocity (inside the cavity; under the basket) and relative humidity were kept at constant levels of 1.5 m s1 and 25%, respectively. By continuous

The chromaticity of the samples was measured before and after drying in terms of the CIE color parameters of L* a* b*, where L* expresses lightness (from 0 to 100), a* represents color of green (-a*) to red (þa*), and b* represents color of blue (-b*) to yellow (þb*). The minimum changes in color parameters during drying process, with respect to the values for the fresh sample indicates higher quality of the dried material. A simple digital imaging method cited in the literature (AfshariJouybari and Farahnaky, 2011; Yam and Papadakis, 2004) was utilized for color measurement. The samples were placed in a specific chamber provided for taking photos by a digital camera (PREMIER, DC-4347, 4 Mega pixels quality). The system of lighting included two D65 lamps (Shenzhen 3nh Technology Co., Ltd., Guangdong, China); which is a standard illuminant commonly used in food

sample weight monitoring, the drying rate curves (drying rate versus moisture content) were obtained. 2.4. Mathematical modeling

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Table 1 Mathematical models given by various authors for thin layer drying of materials. No.

Model name

Model equation

1 2 3 4 5 6 7 8 9 10 11

Newton (Lewis) Page Modified Page Henderson and Pabis Logarithmic Two-term Two term exponential Wang and Singh Diffusion approach Midilli et al. Verma et al.

MR MR MR MR MR MR MR MR MR MR MR

¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼

research, 45.0 cm long mounted on two sides of a frame inside the chamber on either side of the lemon slices 30.5 cm above and at an angle of 45
to the samples plane (Yam and Papadakis, 2004). After taking the photos with TIFF format, they were transferred to a computer. Then, the L* a* b* values were determined in Photoshop software (Adobe Photoshop CS3). For this purpose, three slices were randomly selected from each test and one random reading point was recorded for each slice. Hence, with three replications, there were nine recorded values for each treatment. The system was calibrated against the standard color plates (RAL). From the L* a* b* values, the total color difference (DE), chroma, and Hue angle were also calculated, respectively by equations (1)e(3) and applied to describe the color changes during drying (Maskan, 2001; Esehaghbeygi et al., 2014):

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2  2  2  DE ¼ L*0  L* þ a*0  a* þ b*0  b*

(1)

where, subscript “0” refers to the color reading of fresh lemon. Fresh lemons were considered as the reference and a larger DE denotes greater color change from the reference material.

0:5  Chroma ¼ a*2 þ b*2

(2)

The chroma value indicates the degree of saturation of color and is proportional to the strength of the color (Maskan, 2001).

Hue angle ¼ tan1



b* a*

 (3)

Hue angle represents a more green color (when Hue angle > 90
) to an orange-red color (when Hue angle < 90
). 2.6. Determination of rehydration capability In order to measure the rehydration capacity for each treatment, about 10 g dried sample was allowed to rehydrate at 50
C in 500 ml distilled water for 60 min. Then, the sample was extracted and balanced after surface water reduction. The percentage of the weight gained was determined by equation (4) (Maskan, 2000):

Wg ð%Þ ¼

wt  wd  100 wd

(4)

where, wd is the sample weight before soaking (g), wt is the sample weight after soaking (g), and wg (%) is the water gain percentage. 2.7. Experimental design and statistical analysis Convective and combined experiments were analyzed using a completely randomized design in factorial layout (3  3) with three

exp (-k t) exp (-k tn) exp [-(k t)n] a exp (-k t) a exp (-k t) þ c a exp (-k1 t) þ b exp (-k2 t) a exp (-k t) þ (1-a) exp (-k a t) 1 þ at þ bt2 a exp (-k t) þ (1-a) exp (-k b t) a exp (-k tn) þ b t a exp (-k t)þ(1-a) exp (-g t)

Reference (Lewis, 1921) (Page, 1949) (White et al., 1981) (Henderson and Pabis, 1961) (Yagcioglu et al., 1999) (Henderson, 1974) (Sharaf-Eldeen et al., 1980) (Wang and Singh, 1978) (Yaldiz et al., 2001) (Midilli et al., 2002) (Verma et al., 1985)

replications. Analysis of variance (ANOVA) procedure was performed to determine significant effects of experimental factors on drying durations, color parameters values and rehydration capacities of the dried samples. The analysis was carried out in SAS statistical software (SAS Institute Inc., NC, USA). When the F-test indicated statistical significance at P ¼ 0.05 probability level, treatment means were separated by Duncan's multiple range test at confidence level of 95%. 3. Results and discussion 3.1. Drying kinetics As shown in Table 2, drying duration of lemon slices was considerably reduced about 20 and 30 times when applying specific powers of 0.97 and 2.04 W g1 compared with the corresponding values for convective drying method. This is due to the volumetric heating induced by MW application, which in turn creates an outward flux of rapidly escaping vapor, and consequently an increase in drying rate. Similar results have been reported for several natural materials such as grape (Tulasidas et al., 1993), apple and mushroom (Funebo and Ohlsson, 1998), banana (Maskan, 2000), orange slices (Dıaz et al., 2003), and apple and strawberry (Contreras et al., 2008). It is observed that increase in inlet hot air temperature had a significant effect (P < 0.05) on reducing drying time only for convective drying method. It can also be found that despite practically reducing drying duration to about half when increasing specific power from 0.97 to 2.04 W g1, the differences observed were not significant (P > 0.05). This is due to large values of drying time in the convective treatments. The standard deviations of drying time; ranging from 56 to 70 min, for convective drying method are even higher than the drying durations for the treatments dried under specific power of 2.04 W g1. Representative drying rate curves (drying rate versus moisture content) are illustrated in Fig. 2. For combined drying treatments at inlet hot air temperature of 55
C, drying rate curves had only one falling rate period with the exception of a very short accelerating period (warming-up) at the start (Fig. 2a). This is in accordance with prior microwave-assisted drying studies (Wang et al., 2007; Feng et al., 1999; Ozkan et al., 2007; Wang and Sheng, 2006). High initial moisture content of lemon slices results in higher absorption of MW, and hence higher drying rates. Moisture content reduction during drying decreases MW absorption and consequently, induces a fall in the drying rate. However, under convective and MW drying methods, respectively shown in Fig. 2b and c, the process occurred in one constant rate period followed by a falling rate period after the moisture content reached to a critical amount ́ of approximately 2 g g1 (d.b.). Drying in the falling rate period involves two processes including the movement of moisture within the material to the surface, and removal of the moisture from the

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Table 2 Drying durations and coefficients of Midilli et al. model for different dehydration conditions. Specific power (W g1)

Air temperature (
C)

Drying duration (min)

Model constants a

0 0 0 0.97 0.97 0.97 2.04 2.04 2.04 0.97 a b

50 55 60 50 55 60 50 55 60 e

Aa

1850 ±70 1150B ± 56 980C ± 61 80D ± 3 78D ± 2 73D ± 4 44D ± 1 45D ± 3 38D ± 1 145 ± 7

b

1.002 1.003 1.014 0.990 0.978 0.992 1.022 1.019 1.021 1.174

b 4.66 1.30 1.57 2.34 3.46 2.41 3.43 4.20 3.49 1.20

k          

10 10 10 10 10 10 10 10 10 10

6 5 5 5 5 5 5 5 5 4

3.78 4.57 2.22 3.20 8.98 2.17 3.32 3.54 2.45 0.1

n         

10 10 10 10 10 10 10 10 10

4 4 14 5 5 5 4 4 4

0.6470 0.1448 0 1.303 1.167 1.365 1.118 1.100 1.179 0

The means followed by the common superscript uppercase letter do not differ statistically at 5% significant level according to Duncan's multiple-range test. Table values represent mean ± one standard deviation (s.d.).

Fig. 2. Variations in drying rate with moisture content for lemon slices dehydrated under (a) combined microwave-convective drying method at inlet hot air temperature of 55
C, (b) convective drying and (c) microwave drying methods.

surface. The first process occurs poorly for treatments dehydrated under convective, and the second one occurs poorly for MW drying method. In other words, for the MW drying method alone due to lack of air flow over the material the external resistance against moisture transfer is high, while for convective drying treatments the internal resistance is high compared with the corresponding value for MW treatments.

3.2. Modeling of drying behavior The performance results of fitting the 11 mathematical models to the experimental moisture ratios indicated that all models had an acceptable goodness of fit to the experimental data (Tables 3 and

4). However, the worst and the best degree of accuracies for predicting variations in moisture ratio with time were achieved by Newton (Lewis) model and Midilli et al. model for each treatment, respectively. The lowest R2 (0.8790) and the highest RMSE (0.1052) belonged to Newton (Lewis) model under MW drying at specific power of 0.97 W g1. Henderson and Pabis model ranked second in terms of the most unsatisfactory results. Comparing Tables 3 and 4 indicates that modeling the drying behavior of lemon slices by semi-theoretical and empirical models led to the best results under combined microwave-convective drying method. Table 2 shows the coefficients of Midilli et al. model (a, b, k, and n) for all drying treatments. Better prediction of the experimental drying data by Midilli et al. model has been demonstrated for other crops such as

O. Mirzabeigi Kesbi et al. / Engineering in Agriculture, Environment and Food 9 (2016) 216e223 Table 3 Goodness of fit parameters for various mathematical models under convective and microwave drying methods. No.a

Convective drying 50
C R

1 2 3 4 5 6 7 8 9 10 11 a

Microwave drying 55
C

2

0.9654 0.9779 0.9779 0.9668 0.9976 0.9976 0.9783 0.9917 0.9938 0.9985 0.9938

2

RMSE

R

0.0507 0.0406 0.0406 0.0498 0.0134 0.0134 0.0402 0.0249 0.0216 0.0107 0.0216

0.9399 0.9687 0.9687 0.9456 0.9970 0.9970 0.9662 0.9927 0.9930 0.9976 0.9709

0.97 W g1

60
C 2

RMSE

R

0.0662 0.0478 0.0480 0.0633 0.0148 0.0149 0.0499 0.0231 0.0227 0.0135 0.0463

0.8997 0.9637 0.9645 0.9167 0.9891 0.9887 0.9528 0.9850 0.9884 0.9892 0.9877

RMSE

R2

RMSE

0.0863 0.0519 0.0516 0.0791 0.0286 0.0291 0.0595 0.0294 0.0294 0.0286 0.0304

0.8790 0.9907 0.9909 0.9214 0.9950 0.9789 0.9712 0.9948 0.9758 0.9958 0.9848

0.1052 0.0292 0.0289 0.0849 0.0213 0.0440 0.0514 0.0201 0.0471 0.0197 0.0373

221

relationships have also been reported by Karaaslan and Tunçer (Karaaslan and Tunçer, 2008) for drying spinach in combined MWconvective drying method. Fig. 3 typically shows the comparison between the predicted moisture ratio values by Midilli et al. model and the experimental data for drying lemon slices at inlet hot air temperature of 60
C and specific power of 2.04 W g1. As indicated, there was a good correspondence between the experimental and the predicted moisture ratios. Similar curves could be also drawn for other treatments. 3.3. Quality indices assessment 3.3.1. CIE color parameters The color parameters of fresh and dried samples including the L* a* b*, DE, chroma, and Hue angle values under different drying conditions, are shown in Table 6 (mean of nine readings). Low negative a* value (17.56) and high positive b* value (67.77), respectively confirm greenness and yellowness, and consequently a

The numbers devoted to the models have been specified in Table 1.

Table 4 Goodness of fit parameters for various mathematical models under combined microwave-convective drying method. No.a

0.97 W g1

2.04 W g1







50 C

1 2 3 4 5 6 7 8 9 10 11 a




55 C

50
C

60 C

55
C

60
C

R2

RMSE

R2

RMSE

R2

RMSE

R2

RMSE

R2

RMSE

R2

RMSE

0.9541 0.9963 0.9963 0.9720 0.9986 0.9951 0.9929 0.9982 0.9933 0.9998 0.9975

0.0639 0.0180 0.0180 0.0497 0.0110 0.0208 0.0251 0.0125 0.0243 0.0042 0.0149

0.9593 0.9952 0.9952 0.9748 0.9996 0.9942 0.9924 0.9994 0.9989 0.9999 0.9989

0.0591 0.0205 0.0205 0.0466 0.0059 0.0224 0.0256 0.0073 0.0097 0.0033 0.0098

0.9462 0.9968 0.9968 0.9685 0.9981 0.9948 0.9924 0.9967 0.9961 0.9998 0.9961

0.0698 0.0169 0.0169 0.0535 0.0132 0.0218 0.0263 0.0172 0.0189 0.0045 0.0189

0.9747 0.9978 0.9978 0.9874 0.9995 0.9974 0.9969 0.9987 0.9979 0.9998 0.9979

0.0465 0.0137 0.0137 0.0329 0.0063 0.0150 0.0164 0.0106 0.0135 0.0043 0.0135

0.9728 0.9967 0.9967 0.9849 0.9995 0.9962 0.9955 0.9990 0.9984 0.9997 0.9984

0.0483 0.0169 0.0169 0.0360 0.0064 0.0181 0.0197 0.0092 0.0116 0.0053 0.0116

0.9694 0.9984 0.9984 0.9856 0.9992 0.9979 0.9973 0.9978 0.9967 0.9999 0.9979

0.0517 0.0117 0.0117 0.0356 0.0083 0.0136 0.0152 0.0137 0.0171 0.0034 0.0136

The numbers devoted to the models have been specified in Table 1.

pharmaceutical powders (McMinn et al., 2005), lactose powder (McMinn, 2006), parsley (Soysal et al., 2006), and spinach (Karaaslan and Tunçer, 2008). Multiple regressions were also undertaken for accounting inlet hot air temperature (T) effect on the coefficients of Midilli et al. model. The variations in the model coefficients with T for specific MW powers of 0 (convective drying), 0.97 and 2.04 W g1 are shown in Table 5. The moisture content of lemon slices under combined MW-convective drying method could be predicted using these expressions within the range of selected conditions. The same

high (104.49
) Hue angle (>90
) indicating a more green for fresh

Table 5 Relationships between Midilli et al. model coefficients and inlet hot air temperature at various specific powers. Specific power (W g1)

Coefficients

0

a ¼ 0.0002 T2 e 0.0208 T þ 1.542 b ¼ 107 T2 e 105 T þ 0.0004 k ¼ -105 T2 þ 0.0011 T - 0.0299 n ¼ 0.0071 T2 e 0.851 T þ 25.326 a ¼ 0.0005 T2 e 0.0552 T þ 2.488 b ¼ 4  107 T2 e 5  105 T þ 0.0013 k ¼ 3  106 T2 þ 0.0003 T - 0.0075 n ¼ 0.0067 T2 e 0.7286 T þ 21.033 a ¼ 104 T2 e 0.0111 T þ 1.327 b ¼ 3  107 T2 e 3  105 T þ 0.0009 k ¼ 3  106 T2 þ 0.0003 T - 0.007 n ¼ 0.0019 T2 e 0.2073 T þ 6.633

0.97

2.04

Fig. 3. Comparison between experimental moisture ratio data and predicted values by Midilli et al. model for lemon slices dehydrated under combined microwaveconvective drying method at specific power of 2.04 W g1 and temperature of 60
C.

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Table 6 The color parameters, color difference (DE), Chroma, and Hue angle values of fresh and dried lemon slices (mean of nine readings). L*

Fresh sample Specific power (W g1) 0 0 0 0.97 0.97 0.97 2.04 2.04 2.04 0.97

Air temperature (
C) 50 55 60 50 55 60 50 55 60 e

a*

66.00 ± 3.61a Ab

45.78 ± 2.95 44.67D ± 5.85 42.89D ± 1.96 54.22A ± 2.99 54.67A ± 5.10 52.78 AB ± 2.95 50.67 BC ± 2.18 50.00 BC ± 1.87 48.89C ± 2.62 47.67 ± 4.21

b*

17.56 ± 2.68 A

DE

67.77 ± 3.11

1.00 ± 2.30 0.56A ± 2.30 0.78A ± 2.05 4.22 BC ± 2.10 6.89C ± 5.06 5.78C ± 4.05 2.00 AB ± 1.00 1.98 AB ± 3.22 1.78 AB ± 4.97 1.22 ± 3.51

BCD

50.77 ± 2.44 48.78 CD ± 4.24 D 48.44 ± 2.24 54.22 AB ± 2.28 56.56A ± 5.81 56.33A ± 4.90 53.67 AB ± 3.94 52.33 BC ± 2.96 51.78 BCD ± 4.35 52.56 ± 3.58

0 AB

32.40 ± 3.38 34.19A ± 5.39 A 35.34 ± 2.73 22.60 DE ± 2.56 19.36E ± 8.81 21.34E ± 6.06 26.28 DC ± 2.24 27.35 DC ± 3.66 28.47 BC ± 6.00 29.17 ± 5.18

Chroma

Hue angle


70.06 ± 3.10

104.49 ± 2.19

BC

50.83 ± 2.43 48.83C ± 4.24 C 48.49 ± 2.24 54.42 AB ± 2.33 57.13A ± 6.25 56.74A ± 5.17 53.71 AB ± 3.91 52.45 BC ± 3.05 52.02 BC ± 4.34 52.56 ± 3.52

88.85 CD ± 2.54 89.33D ± 2.61 89.05D ± 2.38 94.39 AB ± 2.08 96.62A ± 4.48 95.59A ± 3.77 92.16 BC ± 1.15 91.92C ± 3.38 91.57C ± 5.63 91.17 ± 3.81

L* lightness, a* greenness-redness, and b* blueness-yellowness of the samples color a Table values represent mean ± one standard deviation (s.d.). b For each column, the means followed by the common superscript uppercase letter do not differ statistically at 5% significant level according to Duncan's multiple-range test.

lemons. L* value was also approximately high (66.00) for fresh sample. The L* and b* values decreased and a* increased regardless of the drying method. However, there was a significant difference among L* values in different methods (P < 0.05), and the least change compared to the reference belonged to the treatments dehydrated under combined MW-convective method with specific power of 0.97 W g1. It has been stated that the variation in the brightness of
dried samples can be taken as a measurement of browning (Avila and Silva, 1999; Ibarz et al., 1999). Therefore, long drying duration in convective treatments and high temperatures generated by MW when applying specific power of 2.04 W g1 are probably the reasons for the samples discoloration. Funebo and Ohlsson (Funebo and Ohlsson, 1998) and Ozkan, Akbudak (Ozkan et al., 2007) reported a negative effect on color quality of dried samples (apple and mushroom, and spinach, respectively) as a result of overheating when applying high value ranges of MW power. In terms of a*, treatments dried under convective method significantly showed the highest difference with respect to the initial sample (P < 0.05) (Table 6). This parameter reached its positive values for these treatments showing that they have lost their greenness more and becoming redder when dried. Although, for all samples dehydrated under combined MW-convective and MW drying methods, a* values were negative; similar to the brightness index, the least significant damage to a* value occurred after drying with MW-convective method with specific power of 0.97 W g1. Decrease of the b* value confirms losing yellowness of the initial sample after drying by any technique (Maskan, 2001). Similar to the L* and a*, the values of b* related to MW-convective method with specific power of 0.97 W g1 were significantly higher than other treatments (P < 0.05).

changes were found in chroma between the fresh and the dried lemon slices (Table 6). This reveals the lack of yellow color stability in lemon. However, chroma values of the dried samples under MWconvective method with specific power of 0.97 W g1 were the closest ones to the reference. The values of chroma under this condition at all temperatures, and MW-convective method with the specific power of 2.04 W g1 at temperature of 50
C were significantly higher than the corresponding values for other treatments. Hue angle was less than 90
only for treatments dehydrated by convective method, and consequently it suggested reduction from a more green color (Hue angle > 90
) to an orange-red color (Hue angle < 90
) of dried samples (Waliszewski et al., 1999). It is observed that the values of this color index for convective method is significantly less than the corresponding values for other treatments. Therefore, better quality of dried lemon slices is concluded by applying MW power. This finding has been also demonstrated by several researchers (Maskan, 2000; Karaaslan and Tunçer, 2008; Soysal, 2004). Quick energy absorption by samples in combined MWconvective method causes much shorter drying times. As a result, samples do not have enough time for browning, and hence color does not change considerably. This is why; the color parameters change in the combined drying method with specific power of 0.97 W g1 is lower than the change occurred for microwave drying method. It is observed that for each MW power, changes in inlet hot air temperature had no significant effect on the color parameter values (P > 0.05) (Table 6). For example, under specific power of 0.97 W g1, the values of L* are 54.22A ± 2.99, 54.67A ± 5.10, and 52.78AB ± 2.95, respectively at air temperatures of 50, 55, 60
C which are not significantly different.

3.3.2. Total color difference, chroma, and Hue angle The total color difference (DE), which is a combination of the L* * * a b values, is a colorimetric parameter extensively used to characterize the variation of colors in foods during processing (Maskan, 2001). As shown in Table 6, for treatments dehydrated under MW and combined MW-convective drying methods; especially for MWconvective method with specific power of 0.97 W g1, the lowest color changes (the lowest DE values) were observed in comparison with the fresh sample. As presented, under this condition the least significant values of DE are 22.60DE ± 2.56, 19.36E ± 8.81, and 21.34E ± 6.06, respectively at temperatures of 50, 55 and 60
C. As mentioned, the chroma value indicates the degree of saturation of color and is proportional to the strength of the color. Large

3.3.3. Rehydration capacity Rehydration is commonly used as a quality index for dried food materials. It is a complex index revealing physical and chemical changes caused by the drying operation (Esehaghbeygi et al., 2014; Feng et al., 1999). Rehydration capacities of lemon slices dehydrated by all drying methods are shown in Table 7. According to the results, there was a significant difference between rehydration ability of the samples dried by convective method and those dried using MW application method (P < 0.05). The higher values of rehydration capacity were observed in combined and MW drying methods. This finding might be related to the possible structural collapse in the samples dried by convective drying method due to exposure to hot air for a long time.

O. Mirzabeigi Kesbi et al. / Engineering in Agriculture, Environment and Food 9 (2016) 216e223 Table 7 Rehydration capacity of dried lemon slices under various drying conditions. Specific power (W g1)

Air temperature (
C)

Weight gain percent (%)

0 0 0 0.97 0.97 0.97 2.04 2.04 2.04 0.97

50 55 60 50 55 60 50 55 60 e

55.80Ba ± 7.74b 57.36B ± 3.72 61.33B ± 8.57 92.63A ± 5.76 86.56A ± 9.26 95.77A ± 3.80 86.82A ± 9.93 92.30A ± 5.91 94.16A ± 1.44 87.20 ± 4.22

a The means followed by the common superscript uppercase letter do not differ statistically at 5% significant level according to Duncan's multiple-range test. b Table values represent mean ± one standard deviation (s.d.).

4. Conclusions Applying microwave power reduced the drying duration considerably and promoted the quality of dried lemon slices in comparison with convective drying method. However, in terms of all color parameters including L* a* b* indices, total color difference, chroma and Hue angle, the least color change compared to the reference (fresh sample) belonged to the treatments dehydrated under combined convective-MW method with specific power of 0.97 W g1. Regardless of the drying method, large changes in chroma between the fresh and the dried lemon was observed, revealing the lack of stability of yellow color in lemon. In microwave drying methods, the quick drying time might prevent high structural collapse and consequently, high water gain percentage was observed. Drying rate curves indicated two periods (a constant rate period followed by a falling rate period) in convective and microwave drying methods and only a falling rate period with the exception of a very short accelerating period (warming-up) at the start in combined dehydrated treatments. Among 11 commonly used thin layer mathematical models, Midilli et al. model could predict most satisfactorily drying kinetics behavior of lemon slices during microwave, convective and combined microwaveconvective drying. It is recommended that the effect of different drying conditions on the chemical composition and organoleptic characteristics of lemon be investigated. Also, it is suggested to study the change of lemon micro-structure under different drying methods as the further work. Acknowledgments Financial support of this research was received from Isfahan University of Technology, which is gratefully acknowledged. References Afshari-Jouybari, H., Farahnaky, A., 2011. Evaluation of photoshop software potential for food colorimetry. J. Food Eng. 106, 170e175. AOAC, 1996. Official Methods of Analysis, sixteenth ed. Association of Official Analytical Chemists, Washington, DC.
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