Available on line at
Association of the Chemical Engineers of Serbia AChE
www.ache.org.rs/CICEQ
Chemical Industry & Chemical Engineering Quarterly 19 (1) 89−105 (2013)
MOHAMMAD ESMAEILI
ADABI1
ALI MOTEVALI2
ALI M. NIKBAKHT3
MOHAMMAD HADI
KHOSHTAGHAZA2
1
Department of Physics, Shahr-eQods Branch, Islamic Azad
University, Tehran, Iran
2
Department of Agricultural
Machinery Engineering, College of
Agriculture, Tarbiat Modares
University,Tehran, Iran
3
Faculty of Agriculture, Urmia
University, Urmia, Iran
SCIENTIFIC PAPER
UDC 664.854:634.38
DOI 10.2298/CICEQ111120045A
CI&CEQ
INVESTIGATION OF SOME PRETREATMENTS
ON ENERGY AND SPECIFIC ENERGY
CONSUMPTION DRYING OF BLACK
MULBERRY
The massive consumption of energy in the drying industry makes it a matter of
challenge regarding economical aspects and limited recourses. Several methods for drying, including hot air convection, vacuum, infrared and hot air convection-infrared technologies, were applied in order to estimate the consumed
energy during the drying of mulberry fruit. Moreover, microwave heating, chemical (ethyl oleate and potassium), mechanical (ultrasonic) and blanching (hot
water) pretreatments were compared. According to the results, maximum
energy consumption was recorded when no pretreatment was performed. Microwave heating in the hot air convection-infrared dryer resulted in the lowest
consumption of energy. The total energy requirement decreased with the temperature in the convection dryer res. Conversely, energy increased with air
velocity. The vacuum dryer consumed the highest amount of energy, which
was measured to be 46.95 kWh, while the lowest energy was recorded for
using infrared-convective dryers. Also, the experimental results showed that
the minimum and maximum specific energy consumption in the drying of black
mulberry were associated with microwave pretreatment in IR-hot air dryer and
control treatment in vacuum dryer, respectively. The minimum color change
(ΔE) for drying of black mulberry was found in microwave pretreated samples
dried with the vacuum dryer, yet maximum ΔE was observed in the hot air
dryer when no pretreatment was applied.
Keywords: black mulberry, drying method, pretreatments, energy consumption.
Sun drying as a conventional and traditional approach is strongly questioned due to numerous problems such as contaminations, dusts, damages caused
by insects, birds and precipitations. As a consequence, industrial dryers have been substituted [1].
Hot air convective dryers are highly applied because
of being easily facilitated and controlled. The dried
products by such dryers are commonly of high quality
compared to the sun method. Also they are faster and
easy to handle [2-3]. However, these dryers have
some drawbacks including considerable heat energy
loss, low heat efficiency [4-5] and high drying time
Correspondening author: M.E. Adabi, Department of Physics,
Shahr-e-Qods Branch, Islamic Azad University, Tehran, Islamic
Republic of Iran.
E-mail: [email protected]
Paper received: 20 November, 2011
Paper revised: 5 April, 2012
Paper accepted: 6 April, 2012
due to low heat transfer coefficient of the product [6],
which have encouraged researchers to use other
technologies such as infrared, microwave and vacuum for agricultural product drying. Infrared drying benefits from minimizing both quality and heat losses
and drying time [7]. Additionally, lower energy consumption in these dryers compared to hot air convection makes them advantageous in fruit processing
[8]. Integration of hot air convection with infrared radiation has been proven to enhance the quality of dried
product as well as reducing the drying time compared
to when they are used individually [5]. Vacuum technology in addition can be regarded as a suitable approach for upgrading the drying quality. Applying vacuum in this method can reduce temperature so that
the qualitative attributes are developed [9-10].
Several pretreatments have been used to help
moisture exit faster from the product. Microwave heat-
89
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
ing as a pretreatment, due to its volume heating characteristic, has been proven to reduce the energy and
time consumed for drying [11]. Furthermore, chemical
pretreatment (ethyl oleate) and blanching can cause a
similar consequence [12]. Also, ultrasonic energy increases the evaporation rate by producing a pressure
gradient within the fruit. High intensity acoustic waves,
on the other hand, cause cavitation phenomenon in
the molecules of water leading to larger diffusivity of
moisture [13]. Therefore, using ultrasonic energy influences the internal and external resistance of the
product and facilitates mass transfer and easier removal of water [14]. Though numerous studies have
been performed in the field of energy consumption of
drying of various agricultural products, such as cherry
fruits [15] carrot slices [16], vegetables [17], mushroom [18], mulberry [19], garlic cloves [20], pistachio
[21], longan [22], pomegranate arils [8], nettle leaves
[23], barberries fruit [24], azarole [25], carrot slices
[26], mulberry [27], red pepper [28], coroba slices
[29], potato [30] and jujube [31], few researches have
focused on the use of pretreatments and their effect
on the energy consumption of drying process.
The main objective of this research was the
comparison of using microwave, chemical, ultrasonic
CI&CEQ 19 (1) 89−105 (2013)
and blanching methodologies of pretreatment in the
drying of mulberry fruits based on the energy requirement for each. Four drying technologies of hot air
convection, infrared radiation, vacuum and convection-infrared were applied.
MATERIALS AND METHODS
Preprocessing
Fresh samples of mulberry fruit were prepared.
A thin layer of fruits was positioned on the dryer tray
and dried until the consecutive weights of samples
remained constant, i.e., the moisture content of 10%
wet basis. The ranges of temperature and air relative
humidity were measured to be 24-30 °C and 21-29%,
respectively. The final moisture content was found by
air drying representative samples of materials at 100
°C for 4 to 5 h (AOAC, 1980). The primary moisture of
mulberries was 78% w.b. The experiments were designed as detailed in Table 1. Vacuum drying was
performed using a VS-1202 v5, Korea vacuum dryer
integrated with a vacuum pump (Platinum, Emerson
model, USA). The experimental set up (infrared dryer)
is illustrated in Figure 1.
Table 1. Operational parameters of dryers
Dryer
Hot air convection
Parameter
Levels
Temperature, °C
Air velocity, m/s
2
Infrared
Radiation (W/cm )
IR-convective
Radiation, W/cm
Air velocity, m/s
Vacuum
2
40
50
60
0.3
0.7
1
0.22
0.31
0.49
0.3
0.7
1
0.22
0.31
0.49
60
Temperature, °C
40
50
Air velocity, m/s
0.3
0.7
1
Temperature, °C
40
50
60
70
80
90
Figure 1. Schematic of a laboratory infrared dryer: 1. air channel, 2. heaters, 3. infrared housing, 4. IR lamps, 5. fan, 6.air speed valve,
7. digital balance, 8. control unit.
90
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
Air parameters were adjusted by measuring
temperature and velocity using a thermometer (Lutron, Taiwan) and anemometer (Anemometer, LutronYK, Taiwan). A pressure gauge (PVR 0606A81, Italy)
was used to measure the inner pressure of the oven
and calibrated with vacuum measuring device, PVR.
A vacuum-meter indicating the vacuum pressure (50
kPa) was installed within the vacuum oven. Microwave pretreatment (SAMSUNG, Korea) was conducted with the power of 200 W at a 10 min time period. An ultrasonic device (Hielscher ultrasonic GmbH,
UP400S, POWER 400 W, Frequency 24 kHz) was
chosen to pretreat the samples with the power of 240
W lasting for 10 min. Chemical preprocessing of
samples was achieved by immersing the fruits in the
solution of ethyl oleate (2% concentration) and potassium carbonate (Merck KGaA, Germany, 5% concentration) for 1 min [12]. The samples were immersed in
hot water of 80 °C for 10 s proceeding with exposure
to water of moderated temperature to yield blanching
pretreatment [12].
Calculation of the energy consumption
The amount of energy consumed by the heater
in hot air dryer at different temperatures and air velocities is calculated using Eq. (1) [8,24,31,32]:
E t = AυρaCaΔTt
(1)
where Et is total energy consumption in each drying
cycle by the heater (kWh), A is area of the sample
container (m2), υ is air velocity (m/s), ρa density of air
(kg/m3), t total sample drying time (h), ΔT temperature
difference between air drying and ambient temperature (°C) and Ca specific heat of air (kJ/kg °C). The
amount of energy consumed by the blower and heater
should also be added. Total energy consumption in
hot air drying would be equal to the summation of
Eqs. (1) and (3).
In the IR dryer, the total energy consumption is
the sum of the energy consumed by the infrared
lamps and the centrifugal blower used to create air
flow [8]. The rate of energy expenditure by infrared
lamps is constant at any given time and is obtained as:
E1 = Kt
(2)
where E1 represents the energy consumed by the IR
lamps, K is lamp power and t is drying time.
The IR lamps used in this dryer were 250 W and
220 V coated electric fiber type made by Asram (Slovakia).
The power of all the lamps was measured by a
wattmeter and found to be equal to the nominal power
claimed by the factory. The amount of energy consumed by the blower is calculated using Eq. (3) [33]:
CI&CEQ 19 (1) 89−105 (2013)
E2 = V3/16600
(3)
where E2 is the energy consumed by the dryer in each
drying cycle (kW) and V is air velocity (m/s). The total
energy consumed (kW) by the blower can be converted to kWh by multiplying it by the working hours of
the blower. Et, representing the total energy consumption of the infrared dryer can be calculated as:
Et = E1 + E2
(4)
The amount of energy consumed in the combined IR-hot air dryer is obtained from the sum of
Eqs. (1) and (2) including the energy consumed by
hot air flow and IR lamps.
Energy consumption in microwave dryers is
equal to:
Et = Gt
(5)
where Et indicates the total energy consumed in each
drying cycle (kWh), G, microwave output power (kW)
and t drying time (h) [8,32].
Energy consumed in the vacuum dryer would be
calculated as the sum of the energy consumed by the
vacuum pump and heaters [8] as follows:
Ep = Lt
(6)
in which Ep represents energy consumed by the pump
(kWh), L nominal pump power (kW) and t drying time
(h). Subsequently, the energy consumption of heaters
is determined by Eq. (7) [35]:
E h = VIt cosθ
(7)
Eh is power consumption by the heaters (kW), V is
voltage and I intensity of current consumed by
heaters and t is operation time of heaters during drying of black mulberry. Total energy consumption in
vacuum drying is equal to:
Et = Ep + Eh
(8)
Energy consumption by the ultrasonic device is
calculated using Eq. (9):
EU = PUt
(9)
in which EU represents energy consumed (kWh), PU
ultrasonic power (kW) and t drying time (h).
Specific energy consumption
Specific energy is the amount of energy required
for drying of 1 kg of fresh product. The energy consumed in different drying methods for drying a kilogram of black mulberry is calculated using Eq. (10):
E kg =
Et
w0
(10)
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M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
where Ekg is the specific energy and W0 is the initial
weight of the sample.
Color evaluation
The appearance of both fresh and dried black
mulberry was assessed by color-difference meter
technique using ColorFlex spectrophotometer (Novasys Group Pty. Ltd.) based on Hunter L*, a*, b* color
scale. Fresh black mulberries were used as the reference base. The color difference (ΔE) was determined
using Eq. (11):
ΔE =
(L
*
0
− Lfm
) + (a
2
*
0
− a fm
) + (b
2
*
0
− b fm
)
2
(11)
where L*0, a*0 and b*0 are the color lightness, greenred and blue-yellow chromaticity of the samples, respectively; Lfm, afm, and bfm are the color lightness,
green-red and blue-yellow chromaticity of the dried
black mulberries, respectively.
RESULTS AND DISCUSSION
Hot air convection
Table 2 shows the summary of drying time and
specific energy consumption at various temperature
and velocity in hot-air convection dryer. Figure 2 presents the energy consumption for drying of samples
at different air velocities. It is clearly depicted that
drying at higher air velocities results in more energy
consumption. It can also be seen that energy consumption decreased with temperature due to the
raised temperature gradient within the fruit leading to
improved evaporation rate, which in turn reduces the
drying time and required energy. Similar results are
available in the literature [8,22,30]. The effect of ethyl
oleate on the energy consumption is also shown in
Figure 2. The energy required was significantly reduced which is highly traced back to the removal of the
waxy layer of the fruit surface and production of fine
pores affected by using potassium carbonate. In convective drying, internal mass transfer controls and
limits the diffusion of moisture from the material.
Meanwhile, heat is slowly moving from the surface
toward the core. Integration of these two phenomena
is the result of large drying time in this method. Application of solutions such as ethyl oleate can significantly increase the moisture diffusivity and reduce the
drying time through eliminating the waxy layer of the
fruit surface. The same conclusion was also achieved
by Doymaz [12,36-37]. Compared to the control drying, the energy consumption decreased 45 to 71%
(on the differing levels of velocity and temperature)
when ethyl oleate was applied. Blanching (hot water)
had an analogous effect on the drying rate in the
92
CI&CEQ 19 (1) 89−105 (2013)
sense that it caused several cracks on the outer layer
of the fruit via a temperature shock [12]. Drying rate
would be increased when the surface of samples is
damaged and subsequently lower energy will be required to exit of moisture from mulberry arils as
demonstrated in Figure 2. Using high temperature for
blanching (80 °C) intensified the reduction of energy
(85.5% on average) in comparison with the chemical
preprocessing. The specific energy consumption at
the constant air velocity in the hot-air dryer (for all the
pretreatments) decreased with temperature. Also by
increasing air velocity at the constant temperature (for
all pretreatments) the amount of specific energy increased. The minimum value of required specific
energy was found to be 56.21 kWh/kg in microwave
pretreated samples and maximum value was found to
be 424.79 kWh/kg for un-treated (control) fruits.
These results are similar to results reported by other
researchers [8,18,24,25,31]. The results of drying
time corresponding to the air velocity and temperature
in the hot air dryer, when microwave pretreatment
was applied, are detailed in Table 2.
Dipole rotations are important mechanisms of
causing temperature changes in a sample trapped in
microwave radiation field. Fruits and food products
contain significant amounts of polar molecules originated from water components of these materials.
These molecules generally have a randomized pattern, getting arranged with the incited field when microwave radiated. Since water molecules make a majority of molecules in a fruit sample at the primary
phases of the drying process, pretreating the sample
with a microwave field can heat a considerable volume of the fruit. It absorbs a great deal of energy and
water evaporates easily. Further, the outer layer of
the product is weakened to let the remaining moisture
at the ending phase of the process. This can be regarded desirable for a drying system as falling rate
periods of the drying are energy and time consuming
processes. Thus, selective heating of microwave
energy along with the volume tempering [11] are the
main reasons for the significant decrease of energy
required for drying (above 300% reduction). Motevali
et al. [8,11] evaluated the energy of microwave as a
pretreatment and found that the energy consumed
and specific energy consumption for the removal of
water from pomegranate arils decreased meaningfully
in comparison with the conventional drying methods.
The ultrasonic effect is described as producing a
pressure gradient in the sample resulting in a spongy
shaped material with no sensible change in temperature. This makes the evaporation process faster. Additionally, deformation of microscopic channels and
pores in fruit, influenced by ultrasound energy, re-
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
Table 2. The required specific energy, drying time and color change (ΔE) during the drying of black mulberry for hot air dryer
Pretreatment
Control
Air velocity, m/s
Temperature, °C
Drying time, min
ΔE
Specific energy consumption, kWh/kg
0.3
40
55.46
16.64
188.60
50
46.55
14.57
172.77
60
37.26
13.89
132.80
0.7
1
Ethyl oleate
0.3
0.7
1
Blanching
0.3
0.7
1
Microwave
0.3
0.7
1
Ultrasonic
0.3
0.7
1
40
48.87
14.17
380.61
50
37.41
13.84
341.79
60
32.54
12.08
270.43
40
42.11
13.38
424.79
50
30.32
11.24
400.64
60
25.24
11.47
338.04
40
36.25
11.47
112.14
50
27.31
10.38
101.41
60
22.14
11.18
86.92
40
30.64
10.86
214.09
50
24.22
10.41
210.33
60
18.46
9.42
169.02
40
26.05
9.11
288.85
50
20.75
8.84
250.40
60
17.21
8.12
209.26
40
32.54
11.67
101.95
50
24.37
11.04
90.14
60
20.29
11.84
77.26
40
27.47
10.99
202.20
50
22.68
11.17
175.28
60
17.39
10.68
152.11
40
22.48
12.01
237.88
50
16.09
11.28
200.32
60
14
10.19
177.07
40
15.66
9.54
70.97
50
11.35
9.07
61.31
60
10.57
8.69
56.21
40
13.91
9.01
127.04
107.64
50
10.18
8.21
60
9.54
8.21
93.24
40
12.37
7.96
147.43
50
9.68
7.17
132.68
60
7.51
7.02
116.583
40
42.84
13.27
132.53
50
32.44
11.84
120.19
60
26.29
12.98
101.41
40
37.42
12.04
261.67
50
30.73
11.64
254.15
60
22.48
11.24
208.46
40
34.66
11.67
339.83
50
23.53
10.98
287.96
60
20.21
10.02
273.65
93
Energy Consumption (kW h)
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
50
CI&CEQ 19 (1) 89−105 (2013)
Control
A
40
Ethyl Oleate
Blanching
Microwave
Ultrasonic
30
20
10
0
Energy Consumption (kW h)
40
50
60
50
B
Control
Ethyl Oleate
40
Blanching
Microwave
30
Ultrasonic
20
10
0
40
50
Energy Consumption (kW h)
50
C
60
Control
Ethyl Oleate
Blanching
40
Microwave
Ultrasonic
30
20
10
0
40
50
60
Temperature (°C)
Figure 2. Energy consumption values for hot air convective drying at the air velocity of A) 0.3 B) 0.7 and C) 1 m/s.
duced the diffusion boundary layer leading in turn to
the increased mass transfer and hence decreased
drying time. Energy of ultrasound has also proved to
be advantageous in improving the drying phenomena
through frequent and fast contractions and expansions ending in a spongy tissue. The pressure gra-
94
dient influenced by using ultrasound vibrations increases the evaporation rate and at the raised intensities of energy, cavitation in water molecules.
Consequently, drying would be carried out at a high
rate and then mass transfer would be enhanced.
However, ultrasonic energy, as reported similarly by
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
[14,38] for preprocessing the samples, was not capable enough to decline energy consumption (Figure
2), which may be due to the modest effect of ultrasound on destruction of surface layer of fruits compared to the previous pretreatments. Furthermore,
ultrasound energy does not have homogenous coverage over the sample volume influencing exclusively
vibrating elements.
Infrared radiation
Table 3 shows the summary of results for specific energy consumption and drying time at IR intensities and velocities in IR dryer. The major influencing parameters in infrared radiation drying are air
velocity and intensity of radiation. Air velocity here,
unlike in the hot air method, has an inverse effect on
the drying time (Figure 3) since higher air velocities
result in fruit cooling and increase in drying time. This
reduction in the heat gradient can cause higher
energy consumption during. Also the effect of radiation intensity is illustrated in Figure 3. As predicted,
intense radiations can raise the heat gradient leading
to faster evaporation of water and reduced drying
time. Sharma et al. [39] and Ruiz Celma et al. [40]
reported the same conclusion. According to the error
bars of the Figures 1 and 2, it can be seen that the
standard error for IR drying is higher in comparison to
other methods. This is due to the un-uniform identity
of IR radiation on the fruit resulting in fluctuated drying times during the repetitions. Nevertheless, in hot
air drying, this phenomenon is not severe. Unlike, a
uniform convection is observed and error within the
repetitions is decreased.
Figure 3b presents the influence of chemical
pretreatment on the drying of mulberries by an infrared dryer at the radiation of 31 W/cm2. A simple comparison of Figures 2b and 3b can prove that the
energy requirement of hot air dryers is rather high,
which is mainly due to lower heat transfer efficiency of
air convection in hot air convection systems [6]. Yet in
radiation systems, a part of infrared radiation is absorbed in the product resulting in high temperature
gradients and faster heating which provides better removal of moisture [6]. Blanching was also efficient
regarding energy consumption in the infrared dryer. It
reduced the energy of drying to 85% lower than the
untreated conditions (control). The maximum energy
was measured to be 9 kWh at the velocity of 0.3 m/s
and radiation of 0.22 W/cm2 (Figure 3b). Microwave
preheating dramatically reduced the energy requirement for infrared drying of mulberries.
Figure 3c shows that energy increased with air
velocity because of surface cooling effect of air [8].
Thus, increased IR intensity and decreased air velo-
CI&CEQ 19 (1) 89−105 (2013)
city reduce drying time resulting in decreased specific
energy consumption (Table 3). As a consequence,
the lowest energy was obtained to be 1.25 kWh at the
velocity of 0.3 m/s and radiation of 0.49 W/cm2. Ultrasonic pretreatment also reduced the energy consumption of infrared drying, but similarly to the hot air
dryer, this reduction was not so considerable compared to pre-mentioned treatments. The lowest value
was obtained to be 3 kWh at the velocity of 0.3 m/s
and radiation of 0.49 W/cm2, which means that drying
integrated with the ultrasound energy required 13%
less energy compared to untreated conditions. Results of analyses showed that the minimum specific
energy required for drying of black mulberry was
12.51 kWh/kg, which occurred at 0.49 W/cm2 radiation
intensity and the velocity of 0.3 m/s by using microwave pretreatment, while the maximum specific energy
requirement was 155.08 kWh/kg observed at 0.22
W/cm2 radiation intensity and 1 m/s air velocity for
untreated samples.
Convective-Infrared dryer
Table 4 illustrates the summary of results for
specific energy consumption and drying time at various temperature, velocity and IR intensity. Figures
4-6 imply that the energy consumption is reduced with
radiation intensity and temperature. Convection and
radiation, as the two main approaches for heat transfer, are simultaneously achieved in the application of
these dryers. Compared to single infrared drying, the
energy consumption during convective-infrared declines significantly as detailed in Figures 4-6. The trend
of energy with respect to temperature and radiation
intensity is predictably downward, which was formerly
proven by several researchers [38,41,42].
Also increasing air velocity increases the drying
time. This may be due to the chilling effect of the air
flow, in a way that with increasing air flow rate, the
product surface becomes cooler and the thermal gradient inside the product is decreased, resulting in an
increased drying time. A decline of 276% energy consumption was obtained to be 0.99 kWh at the temperature of 60 °C and radiation of 49 W/cm2. Blanching
effect is also presented in Figures 4-6. Decreasing
trend was steeper and much sensible when microwave energy was applied in the drying of mulberry
arils. Microwave drying can significantly shorten the
drying process by virtue of the unique advantages
such as adjustment of energy absorption level by the
wet products, possible selective heating of the interior
portions, rapid energy dissipation and more efficient
performance in the falling rate period. As depicted
Figures 7-9, the energy consumption of microwaveassisted drying was meaningfully decreased compa-
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M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
Table 3. The required specific energy, drying time and color change (ΔE) during the drying of black mulberry for IR dryer
Pretreatment
Control
IR intensity, W/cm
Drying time, min
ΔE
Specific energy consumption, kWh/kg
0.3
49
8.49
31
14.37
22
17.09
49
11.27
31
19.21
22
23.44
49
16.32
31
27.17
22
31.24
49
5.55
31
9.52
22
11.17
49
7.03
40.42
70.68
85.14
55.09
95.11
115.41
80.19
135.67
155.08
27.55
47.51
55.15
36.58
67.43
80.37
57.73
90.29
0.7
1
Ethyl oleate
0.3
0.7
1
Blanching
0.3
0.7
1
Microwave
0.3
0.7
1
Ultrasonic
0.3
0.7
1
96
2
Air velocity, m/s
31
13.5
22
16.21
49
11.27
31
19.43
12.58
13.34
11.93
13.06
12.90
11.99
12.16
10.68
11.20
8.76
8.23
8.51
7.24
7.47
8.53
8.90
6.65
22
22.1
7.72
110.44
9.35
9.21
8.68
8.72
9.48
8.43
8.05
8.82
8.91
8.05
8.91
7.16
7.53
7.11
6.86
7.28
6.48
6.66
9.89
10.12
9.62
10.51
8.76
9.47
9.02
8.51
8.99
20.00
35.07
45.57
30.64
55.39
67.51
45.13
70.08
90.09
12.51
22.43
31.02
16.62
30.74
37.52
25.09
40.11
47.59
40.31
65.28
77.54
52.25
87.75
104.07
75.24
119.63
139.12
49
4
31
7.07
22
9.61
49
6.37
31
11.08
22
13.58
49
9.31
31
14.28
22
18.86
49
2.59
31
4.37
22
6.28
49
3.29
31
6.21
22
7.43
49
4.66
31
8.2
22
9.54
49
8.27
31
13.88
22
15.53
49
10.45
31
17.55
22
20.81
49
15.84
31
23.87
22
27.83
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
Energy Consumption (kW h
20
Control
15
Ethyl Oleate
A
Blanching
Microwave
10
Ultrasonic
5
0
0.3
Energy Consumption (kW h
20
Control
15
10
0.7
1
0.7
1
0.7
1
B
Ethyl Oleate
Blanching
Microwave
Ultrasonic
5
0
0.3
20
Energy Consumption (kW h
Control
Ethyl Oleate
15
C
Blanching
Microwave
Ultrasonic
10
5
0
0.3
Air Velocity (m/s)
Figure 3. Energy consumption values for infrared drying at the radiation of A) 49 W/cm2 B) 31 W/cm2 C) 22 W/cm2.
ring to the untreated conditions. Additionally, the minimum energy of 3.46 kWh was recorded by using
ultrasound energy as a pretreatment routine at the
temperature of 60 °C and radiation of 49 W/cm2. Specific energy, at the constant air velocity, decreased
with IR intensity. This trend was consistent for all the
pretreatments which may be due to the fact that with
increasing IR intensity, surface temperature of the
samples increases and drying rate will be improved.
As a consequence, drying time is reduced and the
required specific energy decreases. Investigation of
the required specific energy in IR-hot air dryer indicated that the lowest specific energy was 8.077
kWh/kg at 0.3 m/s air velocity, IR intensity of 0.49
W/cm2, and air temperature of 60 °C using microwave
pretreatment. The highest value was obtained to be
137.01 kWh/kg at 1 m/s air velocity, illumination intensity of 0.22 W/cm2, and temperature of 40 °C in control treatment.
97
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
Table 4. The required specific energy and drying time during the drying of black mulberry for IR-hot air dryer
Pretreatment
Control
0.3
0.7
1
0.3
0.7
1
22
40
8.11
9.57
10.5
70.43
101.02
137.01
50
6.1
7.15
7.67
61.29
96.34
133.15
60
4.28
5.13
5.52
50.48
84.47
121.39
40
5.3
6.56
7.1
48.19
69.12
92.64
50
4.23
5.08
5.56
43.78
68.82
97.41
60
2.6
3.74
4.25
37.36
62.50
93.46
40
3.24
4.52
5.15
33.36
47.85
67.20
50
2.05
3.21
3.65
26.26
41.29
63.94
60
1.65
2.28
2.84
22.21
37.16
61.57
40
4.52
6.37
7.16
44.48
63.80
92.64
50
3.15
4.08
4.59
35.02
55.05
78.84
60
1.73
2.27
2.55
22.21
37.16
54.97
40
3.05
4.09
4.66
29.65
42.53
60.02
50
1.85
2.71
3.14
23.64
37.16
54.31
60
1.13
1.87
2.29
18.17
30.40
49.48
40
2.58
3.24
3.68
23.72
34.02
48.01
50
1.4
2.11
2.43
17.51
27.52
42.57
60
1.08
1.94
2.05
19.18
19.87
45.08
40
4.07
5.08
5.49
37.07
53.17
70.46
50
2.74
3.51
3.86
30.64
48.17
67.62
60
1.16
2.34
2.33
20.19
33.78
51.24
49
22
31
49
Blanching
22
31
49
Microwave
22
31
49
Ultrasonic
22
31
49
98
Specific energy consumption, kWh/kg, at different
air velocity
Temperature
°C
31
Ethyl oleate
Drying time, min, at different air
velocity
IR intensity
2
W/cm
40
2.65
3.45
3.75
25.20
36.15
48.93
50
1.65
2.16
2.56
18.38
28.90
44.85
60
0.95
1.33
1.46
13.12
21.96
32.10
40
2.1
2.86
3.05
20.76
29.77
39.79
50
0.92
1.52
1.84
13.13
20.64
32.23
60
0.65
1.04
1.2
10.09
16.89
26.38
40
3.21
3.77
4.24
27.43
39.34
54.80
50
2.05
2.67
2.95
22.76
35.78
51.68
60
1.24
1.76
1.98
17.16
28.72
43.54
40
1.85
2.22
2.61
16.31
23.39
34.05
50
1.02
1.38
1.59
11.38
17.89
27.85
60
0.62
0.85
1.27
8.58
14.36
27.92
40
1.28
1.66
1.97
11.86
17.01
25.71
50
0.81
0.95
1.41
8.31
13.07
24.70
60
0.59
0.81
1.04
8.07
13.51
22.87
40
7.24
8.55
9.41
63.39
90.92
122.7
50
5.4
6.34
7.22
55.16
86.71
126.4
60
3.94
4.48
4.98
44.42
74.33
109.51
40
5.44
6.05
6.49
44.85
64.38
84.68
50
4.01
4.35
4.81
38.08
59.87
84.27
60
2.67
3.01
3.43
30.29
50.68
75.43
40
3.52
3.85
4.02
28.54
40.94
52.45
50
2.18
2.5
2.69
21.89
34.41
47.12
60
1.83
2.05
2.29
20.69
34.63
50.36
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
14
14
12
12
10
Energy
Consumption
(kWh)
10
Energy
8
Consumption
6
(kWh)
8
6
4
4
0.22
2
40
0.49
50
0.22
2
IR Intensity
2
(W/cm )
0.31
0
0.31
0
40
60
Temperature (°C)
0.49
50
60
Temperature (°C)
(a)
(b)
14
14
12
12
10
Energy
Consumption
(kWh)
IR Intensity
2
(W/cm )
10
Energy
8
Consumption
6
(kWh)
8
6
4
4
0.22
2
0.31
0
40
0.49
50
0.22
2
IR Intensity
2
(W/cm )
0.31
0
40
60
Temperature (°C)
0.49
50
IR Intensity
(W/cm2)
60
Temperature (°C)
(c)
(d)
14
12
10
Energy
Consumption
(kWh)
8
6
4
0.22
2
0.31
0
40
0.49
50
IR Intensity
2
(W/cm )
60
Temperature (°C)
(e)
Figure 4. Energy consumption values for hot air-infrared drying at the air velocity 0.3 m/s in different temperature and IR intensity;
a) control; b) ethyl oleate; c) banching; d) microwave; e) ultrasonic treatment.
Vacuum dryer
Table 5 shows the summary of drying time and
specific energy consumption at various temperatures
in vacuum dryer. The pretreatment methods were
preceded in vacuum dryer to investigate the energy
requirement for drying of mulberry fruits. Air temperature and drying bin pressure are the main dominating features in vacuum drying. As Motevali et al.
[8] had proven in their study, the evaporation speeded
up with the air temperature (Figure 7) for untreated
samples. Moreover, after using ethyl oleate, the least
value of energy was measured to be 9.87 kWh at the
temperature of 90 °C, i.e., 65% less than untreated
drying (Figure 7). The same sequence can be traced
for blanching and ultrasound pretreatment. However,
the effect of blanching was much more sensible as
explained in the previous sections. Additionally, a
considerable decrease of energy (up to 200%) was
observed using microwave due to its surface destruction property resulting in several micro cracks
which in turn facilitates the diffusion of moisture. Re-
99
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
14
14
12
12
10
10
Energy
8
Consumption
6
(kWh)
Energy
8
Consumption
6
(kWh)
4
4
0.22
2
0.31
0
40
0.49
50
0.22
2
IR Intensity
2
(W/cm )
0.31
0
0.49
40
50
60
Temperature (°C)
IR Intensity
(W/cm2)
60
Temperature (°C)
(a)
(b)
14
14
12
12
10
10
Energy
8
Consumption
6
(kWh)
Energy
8
Consumption
6
(kWh)
4
4
0.22
2
0.31
0
40
0.49
50
0.22
2
IR Intensity
(W/cm2)
0.31
0
40
60
0.49
50
IR Intensity
(W/cm2)
60
Temperature (°C)
Temperature (°C)
(c)
(d)
14
12
Energy
Consumption
(kWh)
10
8
6
4
0.22
2
0.31
0
40
0.49
50
60
IR Intensity
2
(W/cm )
Temperature (°C)
(e)
Figure 5. Energy consumption values for hot air-infrared drying at the air velocity 0.7 m/s in different temperature and IR intensity;
a) control; b) ethyl oleate; c) blanching; d) microwave; e) ultrasonic treatment.
sults of analyses indicated that minimum specific
energy requirement was 53.54 kWh/kg for vacuum
drying of black mulberries at 90 °C by using microwave pretreatment, while the maximum value was
measured to be 469.53 kWh/kg by control treatment,
which is 8.77 times less than that at 40 °C (Table 5).
Color analysis
Color is an important quality feature in agriculture and food industry since it is closely associated
with factors such as freshness, ripeness, desirability,
100
cosmetic appearance and food safety. It is often the
primary consideration of consumers when making
purchasing decisions. ΔE is technically a feature frequently used in food industry to quantity the color variations during physical and chemical processes. The
minimum value for ΔE was observed in the vacuum
dried fruits. IR-convective, IR and hot air drying processes yielded much more color variations. This can
be compared to the results reported by Alibas [23]. In
the vacuum dryer, microwave pretreatment, in spe-
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
14
14
12
12
10
10
Energy
8
Consumption
6
(kWh)
Energy
8
Consumption
6
(kWh)
4
4
0.22
2
0
40
0.49
50
0.22
2
IR Intensity
2
(W/cm )
0.31
0.31
0
40
60
0.49
50
IR Intensity
(W/cm2)
60
Temperature (°C)
Temperature (°C)
(a)
(b)
14
14
12
12
10
10
Energy
8
Consumption
6
(kWh)
Energy
8
Consumption
6
(kWh)
4
4
0.22
2
0.31
0
40
0.49
50
0.22
2
IR Intensity
(W/cm2)
0.31
0
40
60
Temperature (°C)
0.49
50
IR Intensity
2
(W/cm )
60
Temperature (°C)
(c)
(d)
14
12
10
Energy
8
Consumption
(kWh)
6
4
0.22
2
0.31
0
40
0.49
50
IR Intensity
(W/cm2)
60
Temperature (°C)
(e)
Figure 6. Energy consumption values for hot air-infrared drying at the air velocity 1 m/s in different temperature and IR intensity;
a) control; b) ethyl oleate; c) blanching; d) microwave; e) ultrasonic treatment.
Table 5. The required specific energy, drying time and color change (ΔE) during the drying of black mulberry for vacuum dryer
Pretreatment
Control
Temperature, °C
Drying time, min
ΔE
Specific energy consumption, kWh/kg
40
61.37
9.29
469.53
50
47.81
8.77
381.01
60
38.02
8.99
313.46
70
30.61
8.21
263.15
80
20.29
8.74
207.70
90
11.47
8.86
162.03
101
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
Table 5. Continued
Pretreatment
Temperature, °C
Drying time, min
ΔE
Specific energy consumption, kWh/kg
Ethyl oleate
40
45.82
7.64
349.35
50
32.67
7.05
264.56
60
21.27
6.37
199.33
70
22.34
5.78
184.91
80
14.62
6.73
142.97
90
7.38
7.24
98.71
40
41.66
9.07
322.33
50
29.46
9.31
235.22
60
19.18
8.47
176.74
70
16.29
7.55
146.8
80
10.43
7.64
115.95
90
4.57
8.41
79.705
40
25.12
6.21
214.25
50
18.29
5.17
176.04
60
13.08
5.47
139.24
70
9.19
4.28
106.17
80
6.76
5.37
79.25
90
3.01
6.97
53.54
40
53.08
9.47
424.44
50
38.33
8.64
332.19
60
29.61
7.94
271.39
70
26.07
7.69
239.03
80
17.37
8.07
190.33
90
9.09
8.67
145.37
Blanching
Microwave
Ultrasonic
Energy Consumption (kW.h)
60
Control
Ethyl Oleate
50
Blanching
Microwave
40
Ultrasonic
30
20
10
0
40
50
60
70
80
90
Temperature (°C)
Figure 7. Energy consumption values for vacuum dryer.
cial, resulted in the lowest color change while considerable ΔEs were observed for control samples (Tables 2, 3, 5 and 6). The reduced drying time with the
application of microwave, blanching and potassium
carbonate typically decreases the exposure of the
102
samples to the dryer. This in turn reduces the color
change during drying. This was observed for all the
drying techniques used in this research. Similar results can be found in the drying of red pepper [3].
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
Table 6. The color change (ΔE) during the drying of black mulberry for IR-hot air dryer
Pretreatment
Control
IR intensity, W/cm
22
31
49
Ethyl oleate
22
31
49
Blanching
22
31
49
Microwave
22
31
49
Ultrasonic
22
31
49
2
Temperature, °C
ΔE at different air velocity
0.3
0.7
1
40
11.83
10.70
10.84
50
11.30
11.20
9.97
60
10.45
10.24
10.86
40
11.02
9.79
9.72
50
10.82
9.89
10.03
60
9.74
10.43
9.98
40
10.34
8.39
9.03
50
9.18
9.22
8.16
60
8.87
8.05
8.87
40
7.53
8.69
7.58
50
8.19
6.93
7.82
60
7.97
7.75
7.29
40
7.32
7.13
6.96
50
7.29
7.54
6.35
60
7.26
6.07
6.35
40
6.41
6.85
6.42
50
6.27
6.90
7.15
60
7.24
5.93
6.39
40
9.51
8.64
8.27
50
8.20
7.32
7.77
60
10.30
7.32
8.85
40
7.98
7.68
7.51
50
9.36
7.76
7.48
60
7.73
7.86
7.53
40
8.12
7.91
7.04
50
8.13
7.55
7.73
60
7.73
6.87
7.56
40
7.28
7.69
6.68
50
7.12
6.74
7.22
60
8.41
6.89
6.04
40
6.69
5.84
6.41
50
7.20
6.33
5.90
60
6.17
6.07
6.04
40
6.11
5.87
5.45
50
5.58
5.22
5.77
60
5.52
5.78
5.21
40
9.97
7.28
8.30
50
8.89
8.27
8.27
60
8.74
8.49
6.87
40
9.13
8.41
7.62
50
8.26
7.54
7.36
60
8.38
7.39
8.46
40
7.97
7.82
7.62
50
7.89
7.93
7.61
60
7.81
7.47
7.60
103
M.E. ADABI et al.: INVESTIGATION OF SOME PRETREATMENTS ON ENERGY…
CI&CEQ 19 (1) 89−105 (2013)
CONCLUSIONS
REFERENCES
Four types of dryers were assisted with common
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Vacuum dryers consumed the highest amount of
energy, which was measured to be 46.95 kWh as the
maximum value. The most effective pretreatment method was microwave heating, while untreated samples were dried with the highest energy consumption.
Moreover, the energy declined with temperature and
radiation intensity in the convective-infrared dryers.
Higher air velocities increased the energy of drying in
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showed that minimum specific energy was consumed
in the IR-hot air dryer with the application of microwave pretreatment. A comprehensive comparison
of the varying treatments and dryers revealed that
microwave assisted IR-hot air drying performed best
for the drying of mulberry fruits taking into consideration the energy consumption, drying time and color
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Nomenclature
Et
A
V
ρa
t
ΔT
Ca
K
G
L
Q
I
EU
PU
total energy consumption in each drying
cycle (kWh)
area of the sample container (m2)
air velocity (m/s)
density of air (kg/m3 )
total sample drying time (h)
difference of ambient and drying temperature
(°C)
specific heat of air (kJ/kg °C)
lamp power
microwave output (W)
nominal pump power (kW)
voltage (V)
intensity of current consumed by heaters (A)
represents power consumed by the ultra
sonic (kWh)
ultrasonic power (kW).
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Acknowledgment
This paper is a result of one of the research projects that has been approved by Islamic Azad University, Shahr-e-Qods, Iran. This research project has
been supported financially by Department of Research of Islamic Azad University, Shahr-e-Qods
Branch, in 2012. This is an opportunity for the authors
to extend their sincere appreciations to Department of
Research of the Islamic Azad University, Shahr-eQods Branch.
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MOHAMMAD ESMAEILI ADABI1
ALI MOTEVALI2
ALI M. NIKBAKHT3
MOHAMMAD HADI
2
KHOSHTAGHAZA
1
Department of Engineering, Islamic
Azad University Shahre Qods Branch,
Qods City, Tehran, Iran
2
Department of Engineering, Islamic
Azad University Shahre Rey Branch,
Tehran, Iran
3
Faculty of Agriculture, Urmia
University, Urmia, Iran
NAUČNI RAD
ISTRAŽIVANJE UTICAJA NEKIH PREDTRETMANA
NA POTROŠNJU ENERGIJE I SPECIFIČNE
ENERGIJE PRI SUŠENJU CRNOG DUDA
Velika potrošnja energije u industriji sušenja čini je izazovom u pogledu ekonomskih
aspekata i ograničenih resursa. Primenjeno je nekoliko metoda sušenja, kao što su:
konvektivno sušenje toplim vazduhom, vacuum sušenje, infra-crveno sušenje i kombinacija konvektivnog i infracrvenog sušenja, da bi se procenila utrošena energija za
sušenje ploova crnog duda. Pored toiga, izvršeno je poređenje nekoliko predtretmana,
kao što su: mikrotalasno zagrevanje, hemijski predtretman (etil oleat), mehanički (ultrazvučni) pretretman i blanširanje (toplom vodom). Mikrotalasno zagrevanje u sušari koja
kombinuje konvektivno i infracrveno sušenje ima najmanji utrošak energije. Ukupna potreba za energijom u konvektivnoj sušari se smanjuje sa temperaturom. Obratno, energija se povećava sa brzinom strujanja vazduha. Vakkum-sušara troši najveću količinu
energije, koja iznosi 46,95 kWh, dok je najmanja potrošnja energije nađena za konvektivno-infracrvenu sušaru. Takođe, eksperimentalni rezultati su pokazali da su minimalna,
odnosno maksimalna specifična potrošnja energije pri sušenju plodova crnog duda
vezane za mikrotalasni predtretman u konvektivno-infracrvenoj sušari, odnosno vakuum-sušaru. Minimalna promena boje je u vakuum-sušari pri sušenju plodova crnog duda
koji su prethodno tretirani mikrotalasima. Maksimalna promena boje je zapažena u konvektivnoj sušari bez prethodne obrade.
Ključne reči: crni dud, sušenje, prethodna obrada, potrošnja energije.
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