Air Blast Freezing of Lime Juice : Effect of Processing Parameters

American J. of Engineering and Applied Sciences 1 (1): 33-39, 2008
ISSN 1941-7020
© 2008 Science Publications

Air Blast Freezing of Lime Juice: Effect of Processing Parameters

1Wasan Theansuwan and 2Kitrichai Triratanasirichai
1Department of Mechanical Engineering, Mahasarakham University, Kantharawichai District,
Maha Sarakham 44150, Thailand
2Department of Mechanical Engineering, Khon Kaen University, Maung District,
Khon Kaen 4002, Thailand

Abstract: This research characterized the effects of air velocity and lime juice layer thickness on
freezing time. In the experiment, the air velocity of the freezer and the thickness of the lime juice layer
were set to be 4-12 m sec?1 and 4-10 mm, respectively. The temperatures of the lime juice were
measured from 15°C until it reached -20°C and were continuously recorded during each test. The
experimental freezing time curves showed a decreased freezing rate period. In addition, the
mathematical model of freezing time was fit to a set of the experimental sample data, which was
characterized by 6 different regression models. The results showed that the freezing times increased
when decreasing air velocity. Moreover, increasing the lime juice layer thickness would also increase
the freezing time of lime juice in which occurred mostly in S-2 stage. With the air velocity exceeding 8
m/s and the lime juice layer thickness less than 8 mm, the experiment gave the best operating condition
for the freezing time of lime juice in this freezing process. The Model (6) was found to satisfactorily
describe the curves freezing time of lime juice with R2 of 0.9656.

Key words: Air blast freezer, freezing time, lime juice

INTRODUCTION
in the last stage, the remaining water in the frozen lime

juice involves sensible sub-cooling[1]. The total freezing
Lime
or
Citrus aurantifolia is an important time (t = t1+t2+t3) is the sum of the pre-cooling time (t1),
agricultural product in Thailand. It is the smallest
the latent heat of time (t2) and sub-cooling time (t3)[2].
member of the true citrus family and native to
Several researchers have studied the effect of
Southeast Asia and India. Lime contains unique processing parameters in air blast freezing of foods.
flavonoid compounds that have antioxidant and anti-
Muftugil[14] studied the freezing time of strawberries at
cancer properties. Lime has been used to prevent
different air velocities in an air freezer at -30°C.
scurvy, a disease caused by a deficiency of vitamin C.
Leblanc et al.[9] investigated the freezing time that
Traditionally, lime has been used as a remedy for
required to decrease the temperature of a french fry in
indigestion, heartburn and nausea. It also has cooling
air blast freezer. Chevalier et al.[4] studied that the
effects on fevers and can help ease coughs and various
cylindrical gelatin gels that were frozen at atmospheric
respiratory disorders[13].
pressure with different operating conditions (air-blast

Freezing and ultimately freeze drying, is one of the
freezing at different air temperatures and brine
important processes which widely applied in food
freezing). Boonsumrej et al.[3] studied the changes of
preservation. Air blast freezer is a common type of
quality of tiger shrimp which frozen by air blast
freezer, which can be used for a variety of irregular
freezing on different air velocity. Martins et al.[10]
shapes including small sized products[1-2]. There are two
investigated the quality of frozen strawberries which
major considerations in an air freezer system: an energy
were influenced by the super-cooling capacity during
input which required moving the air past the food
air blast freezing on operational variables: initial
product and the air velocity distribution in the freezer
temperature, air temperature, air velocity and
chamber[7]. In the freezing Process, the temperature of
strawberry maximum diameter.
the product falls in a manner of consisting of three

In this study, an air blast freezer, which is a part of
stages; the first stage involves sensible pre-cooling. The
the freeze dryer[17], was designed to freeze lime juice.
second stage involves the extraction of latent heat and
The aim of the study was to investigate the effects of
Corresponding Author: Wasan Theansuwan, Department of Mechanical Engineering, Mahasarakham University,
Kantharawichai District, Maha Sarakham 44150 Thailand
33

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Am. J. Engg. & Applied Sci., 1 (1): 33-39, 2008

the freezer’s air velocity and the layer thickness of the
Table 1: Properties of lime juice treated under air blast freezing
lime juice on freezing time of lime juice. In addition,
Description Result Unit
the freezing time mathematical model was fit to a set of
Vitamin C
330.20
mg 100 g?1 (by dry weight)
the experimental sample data, which was characterized
Citric acid
28.14
g 100 g?1 (by dry weight)
pH 2.45
pH-range
by a regression model.
Moisture content
92.22
g 100 g?1

Water activity
0.99
-
MATERIALS AND METHODS


Sample preparation: Lime (Citrus aurantifolia) was
purchased from a local market in Khon Kaen, Thailand.
It was unsorted, washed and squeezed into lime juice
(60 units/1 liters of juice) using a stainless steel juice
squeezer. The average amount of lime juice per one
kilogram of lime is 582.6 mL or 0.5532 kg. The lime
juice was collected and examined the properties by the
Laboratory Center for Food and Agricultural Products
Co., Ltd, Thailand (LCFA), indicated in Table 1.



Experimental apparatus preparation: Figure 1 Description Particular
1. Compressor
3.5 kW of cooling capacity on -40°C
illustrates the schematic diagram of air blast freezer,
evaporator unit and 40°C condenser unit,
which was designed by researcher[17]. Briefly, it
motor 2 hp, 3 phase (Open reciprocating)
consists of refrigeration system, cooling fan, freezing
2. Condenser
5.25 kW of heat rejection (air cooled)
chamber and measurement instruments. The rectangular
3.
Expansion
valve
3.5 kW (Sporlan, Model CG-032)
Thermostatic 3.5 kW, thermostatic
trays were made from 0.8 mm-stainless steel thick. The
charges available -18°C to -40°C
trays were divided into 8 blocks. Each tray had a small
4. Evaporator
Cooling capacity 3.75 kW, Ø 15 mm of
hole at the bottom of the block to allow lime juice to
tubing (50×50 mm aligned parallel), 10
mm of the fin spacing, 10 m2 (heat
flow from one block to the other. The cooling unit was
transfer surface)
designed to be 3.75 kW of cooling capacity using R-22
5. Cooling fan
1/3 hp, single phase, 1450 rpm
refrigerant. The fin spacing of the evaporator is 10 mm
6. Tray
250×400×20 mm (SS-304)
and area surface is 10 m2. The air velocity ranged
7. Receiver tank
3.5 kW, Ø 10 mm of tubing
8. Flow meter
Testo GmbH and Co, Model testo 645
between 0-12 m sec?1 in order to determine the suitable
9.
Inverter
T-VERTER 2N-Series-220*1.5 kW,
conditions for freezing process in each tray. The
Model N2-202-M
mechanical devices in the freezing chamber were used
10. Data logger
Yokogawa, Model: DAQSTATION X200
11. Thermocouple
Type-S
to control the direction of air flow into each tray in
12. Insulation
Polystyrene 50 mm of thickness
order to equalize the velocity. The wall was covered

with 0.005 m polystyrene sheets to improve thermal
Fig. 1: Experimental freezing apparatus and sample tray
insulation. Two thermocouple (type-T) wires were
immersed in the lime juice in each tray. They were
with the thickness of 4, 6, 8 and 10 mm at the velocity
placed 133.33 mm. apart at the middle of the tray. The
of freezing air of 4, 6, 8, 10 and 12 m sec?1. Freezing
speed of the cooling fan was controlled via an inverter
process started with an initial temperature of 15±1°C
(T-VERTER 2N-Series-220*1.5kW; Model N2-202-
and continued until final temperature of -20±1°C.
M). The electrical signals of the samples were collected

During the experiments, temperature of lime juice,
by a data logger (YOKOGAWA; Model inlet and outlet temperature of compressor, inlet and
DAQSTATION DX200) and the data were stored on a
outlet temperature of evaporator, inlet and outlet
floppy disk.
temperature of condenser, inlet and outlet temperature

of expansion valve and inlet and outlet pressure of
Experimental method: The fresh lime was used in the
compressor were recorded. The operating conditions of
experiment. Before freezing, lime was washed, cut into
the freezer are shown in Table 2.
two pieces and squeezed to get lime juice (60 units/1 L

of juice) using a stainless steel juice squeezer. The juice
Empirical model: In this experiment, the temperature
was poured into six trays and placed on three shelves in
of the lime juice was recorded by the data
the freezing chamber. Lime juice was frozen as layer
logger every 4 min, which was started with an initial

34

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Am. J. Engg. & Applied Sci., 1 (1): 33-39, 2008

Table 2: Operating conditions of air blast freezer of the experiments

Where regression coefficients are ?i = 1, 2, …, 10
Operating
condition
and e
------------------------------
ijk is the errors or residuals (Montgomery et al.,
Description Mean
SD
S.E.M.
2001; Myers, 1990).
Inlet temperature of compressor (°C)
-9.9
4.53
1.43

The regression analysis was performed via SPSS
Outlet temperature of compressor (°C)
100.0
3.80
1.20
computer program. The coefficient of determination
Inlet temperature of evaporator unit (°C)
-30.0
3.65
1.15
(R2) was primary criterion for selecting the best
Outlet temperature of evaporator unit (°C) -20.0
4.11
1.30
equation to describe the curve equation. In addition to
Inlet temperature of condenser unit (°C)
98.0
3.37
1.06
R2, the mean square of the deviations between the
Outlet temperature of condenser unit (°C) 30.0
3.37
1.06
Inlet temperature of expansion valve (°C) 28.0
3.37
1.06
experimental and calculated values for the models and
Outlet temperature of expansion valve (°C) -32.0
3.62
1.15
the root mean square error analysis were used to
Inlet Pressure of compressor (psi)
5.0
3.23
1.02
determine the goodness of the fit[6,8].
Outlet Pressure of compressor (psi)
175.0
3.09
0.98


RESULTS AND DISCUSSIONS
temperature of 15±1°C until it reached -20±1°C. The

boundary conditions were; u = air velocity = 4, 6, 8, 10
Figure 2 and Table 3 show the relationship
and 12 m sec?1 and ?x = lime juice layer thickness = 4,
between temperature of lime juice and freezing time.
6, 8 and 10 mm. The freezing process experiments have
On the first stage (S-1), the initial temperature of the
3 repetitions.
lime juice (15°C) was decreased rapidly until it reached

The freezing time model was fit to a set of
the freezing point of water (0°C). The freezing rate on
experimental sample data, which was characterized by a
S-1 at different lime juice layer thickness and air
regression model. The freezing time, ? is a dependent
velocity varied in range of 0.27-1.22 min/°C. In the
variable, which was assumed to be function of air
second stage S-2, the temperature slightly changed from
velocity (u) and limejuice layer thickness (?x) as:
0°C to -2°C. The freezing rate on S-2 at different lime

juice layer thickness and air velocity varied in range of




? = f (u, ?x) (1)
0.96-15.67 min/°C. In the last stage (S-3) the

temperature decreased again and reached the set point

According to Plank’s model (Nagaoka at el., 1955;
temperature of -20°C. The freezing rate on S-3 at
Muftugil, 1986; Mannapperuma at el., 1994), the fitting
different lime juice layer thickness and air velocity
models were considered as the second-order in two
varied in range of 0.76-2.76 min/°C. The interesting
variables which is the basic method for estimating the
point was in S-2, it was noticed that the increased
freezing times of the foods. Therefore, the models were
freezing rate was changed proportional to the increase
selected and experimentally obtained:
of lime juice layer thickness. In S-1 and S-3, the

freezing rate were also increased when increasing the
2
Model 1: ? = ? ( x
? )(u) + ? ( x
? )(u )
lime juice layer thickness but had no significance. In S-

1
2
(2)
2
2
2
+? (?x )(u) + ? ( x
? )(u ) + ? + e
2, at every air velocities, the freezing rate was increased
3
4
5
ij
radically when increased the lime juice layer thickness

from 4-6 mm., from 6-8 mm., from 8-10 mm. and from

1/ 2
2
1/ 2
Model 2: ? = ? ( x
? )(u ) + ? ( x
? )(u ) + ? + e (4)
1
2
3
ijk
8-10 mm. In S-1 and S-3, at the same condition, the

freezing rate was slightly increased.

1/ 2
1/ 2
2
Model 3: ? = ? ( x
?
)(u) + ? ( x
?
)(u ) + ? + e (5)

The relationship between ratio of freezing time and
1
2
3
ijk

lime juice layer thickness and air velocity at different
lime juice layer thickness was shown in Fig. 3. The
1/ 2
1/ 2
1/ 2
2
Model 4: ? = ? ( x
?
)(u ) + ? ( x
?
)(u )

1
2
(6)
ratio of freezing time and lime juice layer thickness
2
1/ 2
2
2
+? ( x
? )(u ) + ? ( x
? )(u ) + ? + e
3
4
5
ijk
at different lime juice layer thickness of 4, 6, 8,

10 and 12 mm. varied in range of 10.0-11.28(u = 4 m
2
Model 5: ? = ? (?x)(u) + ? ( x
? )(u )
sec?1), 7.58-8.63 (u = 6 m sec?1), 6.0-7.93 (u = 8 m

1
2
(7)
sec?1), 5.78-7.47 (u = 10 m sec?1) and 5.75-7.37 (u = 12
1/ 2
1/ 2
2
+? ( x
?
)(u) + ? ( x
?
)(u ) + ? + e
3
4
5
ijk
m sec?1) min/mm, respectively. In addition, when


comparing the ratio of freezing time and layer
1/ 2
1/ 2
1/ 2
Model 6: ? = ? ( x
?
)(u ) + ? ( x
?
)(u)
thickness of lime juice at air velocity of 8 with 10
1
2
m/s and 8 with 12 m sec?1, the decreasing percentage
1/ 2
2
1/ 2
+? ( x
?
)(u ) + ? (?x)(u ) + ? (?x)(u)

3
4
5
(8)
in ratio of freezing times and layer thickness of
2
2
1/ 2
2
+? ( x
? )(u )+ ? (?x )(u ) + ? (?x )(u)
6
7
8
lime juice was varied in range of
2
2
+? ( x
? )(u ) + ? + e
9
10
ijk

35

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Am. J. Engg. & Applied Sci., 1 (1): 33-39, 2008

20
)
20
4 mm
4 mm
C)
6 mm
°
C
6 mm
8 mm
10
10
r
e (°
8 mm
u
10 mm
10 mm
erat
0
p
e
r
a
t
u
r
e
(
0
p
e
m
e
t
e
m
-10
ic -10
i
ce t
e
ju
e

j
u
-20
m -20
i
m
Li
L
-30
-30
0
20
40
60
80
100 120
0
20
40
60
80
100 120

FreezingTime (min)

Freezing Ti me (min)


(a)


(b)


20
20
4 mm
)
4 mm
)
C
6 mm
°
C
6 mm
8 mm
e
(
10
8 mm
r
e (°
10
10 mm
u
10 mm
e
r
a
t
ur
0
erat
p
0
m
t
e
mp
e
te -10
-10
i
ce
u
e
juic -20
-20
i
me j
Lim
L
-30
-30
0
20
40
60
80
100 120
0
20
40
60
80
100 120

Freezing Ti me (min)

Freezing Ti me (min)


(c)

(d)




C) 20
4 mm
6 mm
u
r
e (° 10
8 mm
10 mm
e
r
at
p
0
e
m
-10
u
i
ce t
-20
i
me j
L
-30
0
20
40
60
80
100 120
Freezing Ti me (min)

(e)

Fig. 2: The relationship between temperature of lime juice and time during freezing process at different lime juice
layer thickness and air velocity of: a) 4 m sec?1, (b) 6 m sec?1, (c) 8 m sec?1, (d) 10m sec?1 and (e) 12 m sec?1

1.95-8.0% at 4, 6, 8 and 10 mm of lime juice layer
freezing time and layer thickness of lime juice was
thickness. At the same lime juice layer thicknesses, the
slightly changed at the air velocity of more than to 8 m
increasing percentage in ratio of freezing time and layer
sec?1 at every lime juice layer thickness.
thickness of lime juice was varied in range of 17.0-

Figure 4 shows the relationship between ratio of
46.81% when comparing the air velocity of 4 with 6 m
freezing time and layer thickness of lime juice and lime
sec?1 and 4 with 8 m sec?1. Therefore, the ratio of
juice layer thickness at different air velocity. The ratio

36

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Am. J. Engg. & Applied Sci., 1 (1): 33-39, 2008

Table 3: Experimental freezing times at different air velocity and
12
lime juice layer thickness
)
m


Freezing times (min)
d
m 11
n
------------------------------------------------------------------
e
a
in/
Air
Layer thickness of lime juice (mm)
10
velocity ------------------------------------------------------------------
tim
s
s
(
m
(m/s)

4 6 8 10
9
kne
4 S-1 8.62±0.18 13.25±0.16 16.56±0.19 18.36±0.26
ic
6 8.43±0.31 10.20±0.33 19.59±0.17 16.75±0.28
f
r
e
e
z
ing
8
r
th
8 7.66±0.26 10.37±0.29 10.41±0.26 15.67±0.35
o of
a
ye
10
4.34±0.17 9.22±0.23 9.48±0.26 13.13±0.26
7
e
l
4 m s?1
12
3.99±0.13 8.37±0.16 9.39±0.34 12.95±0.29
r
a
ti
ic
6 m s?1
8 m s?1
4 S-2 7.31±0.06 3.72±0.14*
27.33±0.58* 31.34±0.58*
6
The
e
ju
10 m s?1
6 3.72±0.15 9.79±0.17*
19.59±0.17* 23.40±0.22*
lim
12 m s?1
8 2.46±0.12 8.49±0.33*
15.88±0.14* 20.03±0.39*
5
4
6
8
10
10
2.04±0.05 7.68±0.20*
14.84±0.29* 16.87±0.30*
Lime juice layer thickness (mm)
12
1.92±0.07 7.63±0.25*
13.55±0.25* 16.39±0.36*

4 S-3 21.83±0.24 35.9±0.39 43.63±0.41 50.11±0.80
6 17.70±0.5 24.14±0.19 34.07±0.16 45.37±0.33
Fig. 4: Influence of lime juice layer thickness on
8 13.79±0.14 20.68±0.33 33.56±0.39 40.37±0.71
freezing times at different lime air velocity

10
13.6±0.24 20.67±0.42 33.53±0.36 37.64±0.42
12
13.7±0.27 20.69±0.25 32.64±0.38 36.65±0.52

S-1 = freezing time on the first stage, S-2 = freezing time on the
2.32-4.00 ? for the limejuice layer thickness of 4 and 6
second stage, S-3 = freezing time on the final stage. Values are mean
mm. Therefore, the freezing time was extremely
± S.E.M, The significance of the freezing time was evaluated by the
changed at the limejuice layer thickness of more than 6
analysis of variance (ANOVA). *Significantly difference (p<0.05 as
mm and the air velocity of more than 8 m sec?1.
compare to the layer thickness of lime juice at 4 mm)


The results of statistical analysis undertaken on
12
sum of squares and R2. These curve fitting criteria for
these models were shown in Table 4. Generally, sum of
m 11
4 mm
d
squares (regression and residual) and R2 values were
i
n/m
6 mm
varied between 196460.07-215510.35, 1096.65-
10
8 mm
i
me an
20146.93 and 0.3685-0.9656, respectively. The Model
t
s
s
,
m
10 mm
g
9
n
kne
(6) gave better prediction on the freezing time of lime
ic
juice than other models with R2 of 0.9656.
8
r
th
The fitting curves procedure showed that the
f
freezi
ye
o
7
results of the Model (6) could be used to model the
i
o
e

la
freezing time behavior of examined lime juice sample,
e rat
6
h
e
juic
but it could not indicate the effect of freezing air
T
lim
5
velocity and layer thickness. To account for the effect
4
6
8
10
12
of the freezing variable on the models regression
Air velocity, m s?1

coefficient (?1-?10), the values of regression coefficient

were shown in Table 4.
Fig. 3: Influence of air velocity on freezing times at

The accuracy of the established Model (6) was
different lime juice layer thickness
evaluated by comparing the computed freezing time

with the experimental freezing time in sets of freezing
of freezing times and layer thickness of lime juice at
condition. The performance of the Model (6) at the
different air velocity of 4, 6, 8, 10 and 12 m sec?1 varied
freezing air velocity and lime juice layer thickness has
in range of 10.0-5.75 (?x = 4 mm), 11.28-5.78 (?x =
been illustrated in Fig. 5 (a) with plot of residuals (eijk)
6mm), 10.29-7.29 (?x = 8 mm) and 10.43-7.37 (?x =
versus predicted ( ˆy ) ( e = y ? ˆy , y
ijk
ijk
ijk
ijk
ijk
is
10 mm) min/mm, respectively. When comparing the
experimental data value), which indicates that a mild
lime juice layer thickness of 6 with 8 mm and 6 with 10
tendency for the variance of the residuals increased as
mm., the increasing percentage in ratio of freezing time
the predicted freezing times increased. Figure 5 (b) is a
and limejuice layer thickness was varied in range of
plot of experimental freezing time versus predicted
23.87-32.22 ? at the air velocity of 8, 10 and 12 m sec?1.
freezing time of lime juice. The predicted data
At the same air velocities of 8, 10 and 12 m sec?1, the
generally banded around the straight line, which
increasing percentage in ratio of freezing time
showed the suitability of the Model (6) in describing
and limejuice layer thickness was varied in range of
freezing time behavior of lime juice.

37

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Am. J. Engg. & Applied Sci., 1 (1): 33-39, 2008

Table 4: Predicted model with nonlinear regression summary statistics and the values of the regression coefficients of the models determined
through regression method for lime juice sample
Description Model
No.

-------------------------------------------------------------------------------------------------------------------------------------------

1 2 3 4 5 6
R2 (Asymptotic 95%)
0.8346
0.7134 0.3685 0.8249 0.9080 0.9656
Sum of squares
Regression 211328.9
207463.55
196460.07 211020.39 213672.07 215510.35
Residual 5278.10
9143.45
20146.93 5586.61 2934.93 1096.65
Regression coefficient
?1
-9.3298 0.9853 7.4032 -15.0081 7.1121 -463.6767
?2
0.4893 0.0768 -0.4946 0.0913 -0.4125
133.8524
?3
0.2951 80.9615
-6.1552 0.5228 -24.0419
-3.0819
?4
-0.0170

-0.0051 1.3472 168.7904
?5
97.0105

88.5459 113.9094
-52.8776
?6
1.3316
?7
-4.6321
?8
1.5544
?9
-0.0415
?10
394.7945

10
Residual analysis
Coordinate of freezinf data
100
5
)
80
e ijk
0
l (
l
u
e
60
-5
e
sidua
R
-10
40
P
r
e
d
i
c
t
e
d va
-15
20
20
40
60
80
100
120
20
40
60
80
100
120
Predicted
Experimental value


(a)
(b)

Fig. 5: Comparison of experimental and predicted freezing time by the Model (6) for different air velocity and lime
juice layer thickness: (a) Plot of residuals (eijk) versus predicted value ( ˆy ), (b) Plot of predicted value
ijk
versus Experiment value

CONCLUSION
becomes uncertain. In addition, the freezing time at the

second stage (S-2) was increased distinctly when the

This study investigated the effect of air velocity of
lime juice layer thickness was increased. Moreover, the
4-12 m sec?1 in range and lime juice layer thickness of
freezing time was slightly decreased when the air
4-10 mm in range on freezing time of lime juice. The
velocity was increased more than 8 m/s at any given
mathematical model was determined as the freezing
lime juice layer thickness.
time model.The freezing time model (Model (6)) was

In order to explain the freezing time behavior of
fit to a set of the experimental sample data, which was
characterized by a regression model.
lime juice, six regression models were compared to

The freezing time of lime juice depended on air
their coefficient of determination (R2). According to the
velocity and lime juice layer thickness. The freezing
results, the Model (6) could adequately describe the
time was increased when decreased air velocity or
freezing time behavior of lime juice. The effects of the
increased lime juice layer thickness. Lime juice layer
air velocity and lime juice layer thickness to the
thicknesses of 8 mm caused non-linearity on the freezing time of lime juice were examined in the
freezing time at the air velocity of 8, 10 and 12 m sec?1,
experiments at different conditions. The Model (6) gave
so the practical condition of the freezing process
the predicted results with an R2 of 0.9656.

38

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Am. J. Engg. & Applied Sci., 1 (1): 33-39, 2008


The future work of this study would be using the
8. Gunhan, T., V. Demir, E. Hancioglu and A.
results to obtain the best operating condition for the air
Hepbasli, 2005. Mathematical modeling of drying
blast freezer in order to get the sensible freezing time of
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ACKNOWLEDGMENT
supercooling in frozen strawberries: Experimental

analysis, cellular automation and inverse problem
This research is financially supported by
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Mahasarakham University and National Research of
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Thailand, Thailand.
1994. Effective surface heat transfer coefficients

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