Abstract-- Microwave drying is a suitable technology to be practiced in different industries. In addition to the short drying time, the process tends to preserve the nutritional values of agri-food products. In the present work, mass transfer characteristics of zucchini and carrot under microwave drying were investigated. The samples with thicknesses of 3, 5, 7 and 9 mm were dehydrated by practicing different microwave power levels (100, 350, 550 and 750 W). The results showed that moisture removal from the slices occurred in a short accelerating period at the process beginning followed by a falling rate period. The moisture diffusivity increased with both increasing microwave power and the samples thickness where the average values for zucchini and carrot slices changed from 1.17×10-8 to 9.42×10-8 and from 0.73×10-8 to 5.51×10-8 m2 s-1, respectively. The average activation energy for zucchini and carrot slices varied in the range of 1.22–1.68 and 1.57–1.84 W g-1, respectively and decreased with increasing samples thickness.
Keywords-- Microwave power, Drying, Product structure, Diffusivity, Activation energy.
I. INTRODUCTION
Containing more than 80% water, fresh vegetables are prone to spoilage and classified as perishable commodities. Therefore, to improve storage stability and enhance shelf life of such products, it is necessary to apply optimal post-harvest processes and technologies (Joardder et al., 2015). As one of the main unit operations, drying of agricultural and food products is widely practiced to decrease the moisture content to a certain value (Torki-Harchegani et al., 2016). Due to reduced microbiological and enzymatic activities during the post-harvest life period, physical, chemical, and nutrient qualities of the dried products could be maintained more favorably (Vallespir et al., 2018).
To achieve better quality and optimal drying conditions, deep insight into the dehydration phenomenon is vital. Moisture removal from biological materials depends mainly on drying system and conditions as well as inherent properties of the products including structure and components (Kumar et al., 2014).
Among the different artificial drying methods commonly practiced and/or reported in the open literature, convective hot air dryers are the most popular and widely used systems to dry fruits, herbs and vegetables. However, hot air drying poses some critical shortcomings such as low energy efficiency, relatively lengthy dehydration time and environmental contamination (Darvishi et al., 2018). Microwave power has been found as an effective alternative to dry agricultural and food products (Darvishi et al., 2016). During microwave drying process, polar molecules absorb the radiated energy and the subjected product bulk is heated volumetrically (Torki-Harchegani et al., 2016). The phenomena causes internal pressure gradients and facilitates moisture removal form the material. At the recent years, microwave power has gained popularity for drying goals and some researchers have used the power to dehydrate different agricultural and food products such as onion (Arsalan and Özcan, 2010), zucchini (Cuccurullo et al., 2017), kiwi (Darvishi et al., 2018), carrot (Yan et al., 2010), okra (Dadali et al., 2007), and sweet potato (Yan et al., 2013). However, based on the open literature review, no scientific work was found on comparing moisture removal characteristics of zucchini and carrot under microwave power treatment. Therefore, the objective of this study was to investigate the microwave thin-layer drying characteristics of zucchini and carrot. The influence of microwave power level as well as drying samples thickness on moisture removal curves and moisture transfer parameters of the products were determined and compared.
II. METHODS
A. Fresh materials and drying experiments
Fresh zucchinis (Cucurbita pepo L. cv. Squash) and carrots (Daucus carota L. cv. Nantes) were purchased from a local market in Kerman, Iran. The fresh samples were stored at 4 °C before conducting the main drying experiments. The initial moisture content of the fresh samples was determined using the AOAC standard method by drying the products in an oven at temperature of 105 °C for 24 h (Saleh et al., 2020). The moisture content for the zucchinis and carrots was determined to be 15.63±0.91 and 8.87±1.25 (dry basis), respectively.
A domestic microwave oven (Panasonic, Model NN-CD997S, Japan) was used to conduct the microwave drying experiments. Before to each experiment, about 1000 g of the stored products was taken out, hand peeled and cut into slices in the direction perpendicular to the vertical axis with desired thicknesses by using a domestic cutting machine. Approximately 150 g of the sliced samples was uniformly spread as monolayer on the oven cavity.
Drying experiments were performed at four different
levels of the samples thickness (3, 5, 7 and 9 mm) and microwave power (100,
350, 500 and 750 W) in three
Table1. Mathematical thin-layer models used for modeling of microwave drying curves of the zucchini and carrot slices (Torki-Harchegani et al., 2016).
|
Model name |
Model equation |
|
Newton |
|
|
Page |
|
|
Henderson and Pabis |
|
|
Midilli–Kucuk |
|
|
Wang and Singh |
|
|
Two-term exponential |
|
|
Diffusion approach |
|
|
Logarithmic |
|
replications. During the experiments, the samples tray weigh was monitored accurately using a digital balance (ViBRA, model EG 620-3NM, Japan) at regular time intervals of 20 s, and the instantaneous moisture content of the samples was calculated. The balance was placed on the oven (outside of the oven) and, using nylon wire, the tray was connected to it. It is worthy note that, the drying experiments were continued until the samples reached a final moisture content of 0.12‒0.14 (dry basis).
During the process, drying rate (DR) was calculated as follows (Beigi, 2019):
, (1)
where, 
and 
are moisture
contents (dry basis) of the drying samples at 
and 
,
respectively. Also, represents
the drying time.
B. Description of dehydration curves
To describe the experimental drying kinetics, at first, moisture ratio (MR) was determined using the following equation:
, (2)
where, 
is initial moisture content (dry basis).
Then, variations of the obtained moisture ratio values versus process time were plotted. Applying nonlinear regression technique, the semi-theoretical thin-layer models listed in Table 1 were fitted to the curves. Goodness of the applied models was evaluated and compared by using statistical factors including root mean square error (RMSE) and coefficient of determination (R2).
C. Calculation
of effective moisture diffusivity (
) and
activation energy (
)
Diffusion of liquid and/or vapor has been introduced as the main mechanism of the moisture removal in biological materials during drying process. Generally, throughout thin-layer dehydration, the moisture content is considered to be diffused only in direction of the products thickness (one-dimensional diffusion) and can be expressed accurately by considering Fick’s second law of transient diffusion as follows:
(3)
In Eq. (3), D is effective moisture diffusivity (m2 s-1).
Supposing some simplifying assumptions including uniform distribution of moisture, negligible external resistance and shrinkage as well as constant moisture diffusivity, for different solid geometries, Eq. (3) was analytically solved by Crank (1975). The solution for an infinite slab solid is written as following form:
, (4)
where, 
is the drying samples thickness (m).
In this study, due to long process periods, Eq. (4) was simplified to only the first term of the series and written in the logarithmic form as follows:
. (5)
The experimental data in term of 
was plotted against drying time, and the effective diffusivity was computed by using Eq. (6):
, (6)
where, 
is slope of
the obtained straight-line.
Furthermore, to determine the
activation energy, the moisture diffusivity was related with microwave power (Eq.
7) and the graph of 
was plotted against
(Kumar et
al., 2014):
, (7)
where, 
is Arrhenius
constant (m2 s-1), 
is the
activation energy (W g-1), 
is the fresh
sample mass (g), and 
is the
microwave power (W).
D. Statistical analysis
To evaluate and compare the effects of microwave power level and the
drying samples thickness on the
studied parameters (
), the
experimental data for each drying treatment (with three replications) was
statistically analyzed using SPSS (19.0) computer program and the Duncan
multiple range test was practiced.
III. RESULTS
A. Drying kinetics and process time
Figures 1 and 2 represent the typical moisture removal kinetics of the zucchini and carrot slices in term moisture ratio variation against the process time at the some randomly selected dehydration conditions. It is worthy note that the values of L and P are different for carrot and zucchini curves in Fig. 1 and Fig. 2, respectively; and it is not intended to compare how the two products dry.
Figure 1 shows the influence of microwave power output
on dehydration curves of the samples. The obtained experimental results and an
analysis of variance revealed that the process time was significantly () affected by the power at the studied levels (100, 350, 550 and 750
W). Generally, because of increasing kinetic energy and higher energy absorbed
by the subjected samples, duration of drying process is shortened at higher
microwave powers. Increasing microwave output powers enhance the molecules
collisions and consequently result in more kinetic energy in the subjected
material. The phenomenon offering quicker balancing between these regions as
well as accelerated moisture evaporation from the product surface. On the other
hand, because of wider generated electromagnetic field at higher microwave
powers, energy absorbed by the material is increased resulting in more vapor
pressure inside the
Figure 1. Typical drying kinetics for zucchini and carrot slices under microwave power treatment: Influence of the power level on dehydration curves.
product and subsequently, leading to facilitated and enhanced moisture movement form the product inside to its surface. The same observation has been reported by different researchers such as Azimi-Nejadian and Hoseini (2019) for potato, Darvishi et al. (2018) for kiwi and Süfer et al. (2017) for onion.
The influence of the samples thickness on drying kinetics of the studied products is represented in Fig. 2.
According to the statistical analysis, it was evident
that the samples thickness at the practiced levels (3, 5, 7 and 9 mm) affected
significantly () the process time where increasing thickness augmented the process
duration. Generally, the thinner sliced products dry faster due to the less
distance the internal moisture needs to travel to reach the product surface.
For different drying systems and products, the same observation in term of the
effect of drying product thickness on the process duration has been reported in
the open literature by some researchers. Microwave drying of onion slices was
investigated by Süfer et al. (2017) and revealed that dehydration time
was importantly increased by increasing thickness of the drying product (from 3
to 7 mm). For infrared drying of tomato slices with thicknesses of 3, 5 and 7
mm at temperatures of 60, 70 and 80 °C, Sadin et al. (2014) found that
drying duration was significantly increased with increasing thickness of the
slices. Darvishi et al. (2016) reported that, at microwave output powers
in the range of 200–500 W, increasing thickness from 3 to 6 mm and from 6 to 9
mm resulted in significant increment in the required time for drying of kiwi
slices.
Figure 2. Typical drying kinetics for zucchini and carrot slices under microwave power treatment: Influence of the samples thickness on dehydration curves.
Average microwave drying process duration of the zucchini and carrot slices is shown in Fig. 3. Based on the obtained results, required time for microwave drying process of the samples is considerably (220–1120 s). Generally, it has been found that drying time of agricultural and food products is mainly affected by drying system and conditions as well as inherent characteristics and initial and final moisture contents of the products.
During microwave power drying, the samples are heated by energy transferred electromagnetically into the material. The mechanism enhances meaningfully the material warming rate and uniformity (Torki-Harchegani et al., 2016). Due to high moisture contents, the bulk of dielectric component for most agricultural and food products is mainly accounted by water. Therefore, as long as there is remaining moisture, the products absorb the radiated energy fast and efficiently (Azimi-Nejadian and Hoseini, 2019). In this work, high moisture contents of the samples produced high friction against the bipolar rotation and led to a quickly vapor movement and forced the moisture to be diffused towards the samples surface.
As presented in Fig. 3, in general, required time to dehydrate the zucchini slices was lower than the carrot samples. The observation is related to the products cellular structure where, in general, the structure of the epidermis and the number of lenticels and stomata govern how quickly a vegetable will lose water. For example, solanaceous fruiting vegetables do not have lenticels or stomata reducing the rate of water loss from the fruit while the thin epidermis of a zucchini presents little barrier to water loss.
Figure 3. Average drying time for microwave power drying of zucchini and carrot slices.
B. Moisture removal rate
Variations of drying rate of the samples with the moisture ratio for some randomly selected drying conditions are presented in Fig. 4. Based on the obtained results, despite of the inherent structural differences between zucchini and carrot, dehydration rate curves of the products include a short accelerating period (warming-up) in the process beginning followed by a decreasing rate period. As shown, no constant moisture removal rate was seen for microwave power drying of the samples. It is worth to note that for all of the practiced drying conditions, the same trend was observed for drying rate of the products.
When microwave drying starts, the samples temperature increases along with the process time. Therefore, the moisture removal rate increases due to increasing internal temperature and moisture vapor pressure. Furthermore, throughout microwave heating, the power absorption quantity in the subjected product is governed by moisture content. Therefore, due to containing high levels of water and relatively small moisture loss, microwave power absorption by the products has incremental trend which is leading to accelerating dehydration rate.
Reaching the samples to a temperature plateau, the process enters to a constant temperature period where the moisture removal from the products occurs by falling rate (Azimi-Nejadian and Hoseini, 2019). Generally, absorbed microwave energy depends on dielectric properties of the product. At higher moisture content of the material, dielectric constant and loss factor are higher resulting in more microwave energy absorption and consequently faster heating. As microwave drying continues, due to relatively rapid reduction in moisture content, the power absorption by the product is reduced and the process occurs in falling rate period (Lu et al., 1999). Similar finding has been reported for microwave drying of different biological products such as parsley (Soysal et al., 2006), okra (Dadali et al., 2007), and potato slices (Azimi-Nejadian and Hoseini, 2019).
C. Mathematical modeling of drying kinetics
The thin-layer models (represented in Table 1) were fitted to obtained experimental drying kinetics of the zucchini and carrot slices, and the results of statistical anal-
Figure 4. Typical dehydration rate curves for zucchini and carrot slices under microwave power treatment.
Table 2. Statistical analysis results of the mathematical models for prediction of the dehydration curves.
ysis are shown in Table 2. As the results show, all of the practiced models yielded good fit and acceptable accuracy (all of the R2 values were more than 0.93). However, among the models, the Midilli–Kucuk model was found as the best model describing the drying curves where the highest R2 values and the lowest RMSE values achieved in the case of this model.
D. Effective moisture diffusivity
Plotting the graph of 
versus
process time () and using
Eq. (5), the effective moisture diffusivity values were calculated for the
products dried at different practiced samples thicknesses and microwave powers
and the obtained average values are listed in Table 3. As shown, the
diffusivity values obtained in the present work for zucchini and carrot slices
varied in the ranges of 1.17×10-8–9.42×10-8 m2 s-1 and 0.73×10-8–5.51×10-8 m2 s-1, respectively. The obtained results showed
that moisture diffusivity for the products increased significantly () with
increasing both of the microwave power and slices thickness. Some researchers
have found the same observation in term of the effect of microwave power on the
diffusivity for different biological products such as potato slices (Azimi-Nejadian and Hoseini, 2019), intact
and brined onion slices (Süfer et al., 2017), lemon slices (Sadeghi et al.,
2013), and carrot slices (Sutar and Prasad, 2007). The observation could be
discussed by internal temperature of drying product during the process. In
general, higher microwave power enhances the samples temperature and results in
the less water viscosity. The phenomenon leads to easer water diffusion inside
the product (Azimi-Nejadian and Hoseini, 2019).
Different researchers have reported the influence of samples thickness on moisture diffusivity. For example, applying power levels of 200, 300, 400 and 500 W, Darvishi et al. (2018) studied microwave drying of kiwi slices with thicknesses of 3, 6 and 9 mm, and found that moisture diffusivity value increased significantly with the increment in the samples thickness. Azadbakht and Tajari (2015) used microwave power at levels of 200, 400 and 600 W to dry kiwifruit slices with thicknesses of 4, 6 and 8 mm. They reported the diffusivity values to be in the ranges of 3.25×10-9–6.49×10-9 m2 s-1, 3.65×10-9–1.83×10-8 m2 s-1 and 1.29×10-8–3.25×10-8 m2 s-1. Furthermore, the same observation has been reported for hot air drying of mushroom slices by Ghanbarian et al. (2016), for convective, vacuum and microwave drying of intact and brined onion slices by Süfer et al. (2017), and for infrared drying of tomato slices by Sadin et al. (2014). The hardening of drying product surface has been found as the key reason for the phenomenon (Darvishi et al., 2018). In general, because of facilitated moisture evaporation, surface hardening in thin slabs happens faster than thick slabs (Nguyen and Price, 2007) and therefore, moisture diffusivity in thinner samples is less than the value in thicker samples.
Furthermore, the statistical analyses revealed that
moisture diffusivity values for zucchini slices, at all of the practiced drying
conditions, were significantly () higher than
the values obtained for carrot slices.
E. Activation energy
The activation energy for microwave drying of the zucchini and carrot slices was calculated by the
described fitting procedure on the moisture diffusivity values, and the
obtained average values are represented in Table 4. In term of dehydration
process, the activation energy () is a
quantity of the energy required for initiating moisture diffusion inside the
drying material where, the higher energies are related to the cases in which
moisture removal is mainly controlled by the products structure (Torki-
Table 3. Average
effective moisture diffusivity (
) values of the microwave power dried products.
Table 4. Average
activation energy (
) values of the microwave power dried samples.
Harchegani et al., 2016). From the results (Table 4), for all of the practiced samples thicknesses, activation energy for the zucchini slices was less than the carrot slices. As mentioned before, the products cellular structure is the main factor impacting the activation energy for moisture diffusion. Generally, water loss rate in a vegetable is mainly controlled by the epidermis structure as well as the lenticels and stomata numbers.
From the results shown in Table 4, activation energy for moisture diffusion in the practiced products decreased with increasing samples thickness. In term of dehydration process, lower activation energy represents higher and facilitated diffusion of moisture inside the material. The observation could be described by the surface hardening phenomena. In fact, hardening of the slices in the thicker samples occurs more slowly in comparison with the thinner ones resulting in higher moisture diffusivity and consequently leads to lower energy required for activating water diffusion inside the drying samples. The same observation has been reported in the literature for microwave drying of kiwi slices (Darvishi et al., 2018) and convective drying of onion slices (Süfer et al., 2016).
IV. CONCLUSIONS
In this work, the influence of sample thickness as well as microwave
power level on drying characteristics of zucchini and carrot slices were
studied. Following an initial accelerating period, dehydration process of the products took place mostly in the decreasing rate period. According to the
statistical analysis applied to eight semi-theoretical thin-layer models, the
Midilli-Kucuk model made the best predictions. Dehydration time decreased significantly
() by
increasing microwave power and decreasing thickness. The
effective moisture diffusivity was enhanced with the increasing power and
thickness. In comparison with the thicker samples, due to faster surface
hardening, moisture diffusion in the thinner slices required more activation energy. Finally, simultaneous investigation on mass transfer
parameters and physico-mechanical properties of agri-food products is
recommended to clarify the influence of products structure on moisture removal
behavior.
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Received: April 1, 2021
Sent to Subject Editor : April 14, 2021
Accepted: September 26, 2021
Recommended by Subject Editor Maria Laura Foresti