During Drying of Carrot Samples
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Transcript of During Drying of Carrot Samples
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ORIGINAL PAPER
Experimental Evaluation of Quality Parameters
During Drying of Carrot Samples
Maria Aversa & Stefano Curcio & Vincenza Calabr &
Gabriele Iorio
Received: 24 April 2009 /Accepted: 5 October 2009# Springer Science + Business Media, LLC 2009
Abstract The aim of this work was to evaluate the effect of
operating conditions on the quality of dried foods,monitoring shrinkage, rehydration capacity, and color
changes. Carrot samples of different shapes and dimensions
were dried in a convective oven by air whose temperature
and velocity were chosen in a range of physical significance.
Two values of air velocity, i.e., 2.8 and 2.2 m/s, were
considered; air temperature was chosen equal to 50 C,
70 C, and 85 C. It was observed that more drastic drying
conditions indeed improved the drying rate, but were also
responsible for a decrease of food rehydration capacity.
Food behavior was observed comparing some characteristic
parameters as obtained upon fitting the experimental data by
simple modeling equations. In particular, the drying curves
were fitted by the Newtons model, whereas, shrinkage was
modeled by a linear relationship between volume variation
and moisture content. Rehydration capacity was estimated by
water regain percentage, which was evaluated after 5 h.
Keywords Drying . Carrots . Shrinkage . Color.
Rehydration
Introduction
Thermal processing affects food quality, so that a substantial
degradation in some essential attributes, such as color, flavor,
texture, nutrients, and rehydration capacity is generally
observed (Maskan 2001a, b; Khraisheh et al. 2004).
As drying progresses, the initial food sample structural
equilibrium, which determines its size and shape, may fail
due to the water transport from the solid matrix. The loss of
water causes a change in the mechanical and structural
properties of food, resulting in reduced structure mobility
due to both low water content and increased solid fraction.
During drying, a field of contraction stresses develops in
the food structure, thus, promoting a change in food shape
and size. This phenomenon is known as shrinkage (Mayor
and Sereno 2004; Ratti 1994; Mrquez and De Michelis
2009). The optimization of drying process is strictly related
to the identification of the operating conditions, which in
the cheapest way, allow attaining high quality dried foods,
i.e., products exhibiting fast rehydration capacity, small
shrinkage, and attractive color (Maskan 2001a, b, Ratti and
Crapiste 2009).
Dried foods are susceptible to color deterioration
(Romano et al. 2008; Martins et al. 2008; Seth et al.
2008). Color is considered as the key quality attribute due
to its relation with flavor and aroma (Krokida et al. 1998).
Food discoloration is the consequence of various reactions
including pigment degradation, which involves, browning
especially carotenoids and chlorophyll such as Maillard
reaction, condensation of both hexoses, and amino compo-
nents and oxidation of ascorbic acid (Lozano and Ibarz
1997; Krokida et al. 1998). All the above undesired
reactions depend on the operating conditions chosen to
perform drying process. In order to minimize color
deterioration, both a suitable design of manufacturing and
M. Aversa (*):
S. Curcio:
V. Calabr:
G. IorioDepartment of Engineering Modeling, University of Calabria,
Ponte P.Bucci, Cubo 39/c,
Rende, Cosenza, Italy
e-mail: [email protected]
S. Curcio
e-mail: [email protected]
V. Calabr
e-mail: [email protected]
G. Iorio
e-mail: [email protected]
Food Bioprocess Technol
DOI 10.1007/s11947-009-0280-1
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treatment equipments and a proper choice of process
conditions are needed.
Rehydration process, usually used to restore the properties
typical of a fresh product, takes place when dried food comes
in contact with water. Rehydration of dried plant tissues
consists of three simultaneous processes: imbibition of water
into dried material, swelling, and extraction of soluble
components (Moreira et al. 2007). Generally, dried foodsare not so prone to uptake water; in the literature, several
studies showed that the rate and the degree of rehydration are
strictly correlated to duration and severity of drying
(Saravacos 2006; Pappas et al. 1999). The degree of
rehydration is dependent on the extent of cellular and
structural disruption and can be considered as a measure of
the damage caused by drying process (Shittu and Raji 2008).
A minimization of shrinkage and therefore, the presence of
well-defined intercellular voids may increase rehydration rate
(Haas et al. 1974; Krokida et al. 1999).
Shrinkage, rehydration, and color change were adopted
by several authors to evaluate the damages induced by processing on food matter. Sanjan et al. (2004) analyzed
the effect of storage temperature on the quality of
dehydrated broccoli florets. Maskan (2001a) studied drying,
shrinkage, and rehydration characteristics of kiwi fruits
during drying by hot air and by microwaves. Color was
utilized as a quality parameter for orange juice (Tiwari et al.
2008) for dried chempedak (Chong et al. 2008), dried bay
leaves (Demir et al. 2004), dried coconut (Niamnuy and
Devahastin 2005), and for dried potatoes, carrots, and
bananas (Chua et al. 2004). Prachayawarakorn et al. (2008)
considered shrinkage and color as quality parameters to
analyze drying of banana samples. Rehydration and color
were used as quality parameters for dried peppers by Vega-
Glvez et al. (2008). Shrinkage, rehydration capacity, and
color were all utilized during drying of cooked rice to assess
the quality of the final product (Luangmalawata et al. 2008),
whereas, shrinkage and rehydratability were chosen for potato
drying by Khraisheh et al. (2004) as quality parameters.
Drying process is controlled by external or internal mass
transfer, respectively, either in the constant or in the falling
rate period (Aversa et al. 2007). In convective drying
process, external resistance to mass transfer is strongly
affected by air velocity. As far as food dimension is
concerned, some authors found that in a thicker sample,
the formation of channels and pores, representing a
preferential path for water transport, facilitated an overall
increase in the water diffusion coefficient (Saravacos and
Maroulis 2001). Conversely, it should be noted that lower
resistance to mass transfer was observed when water
transport occurred along a shorter path (thinner sample),
even though, as far as food dimension is concerned, it was
pointed out that a lower limit of practical significance does
actually exist (Senadeera et al. 2003).
It seemed, therefore, crucial to assess the influence of
both air velocity and food dimensions on drying rate of
carrots. The aim of this work was to evaluate, from an
experimental point of view, the effect of operating
conditions and of sample dimensions on the quality of
dried carrots, by analyzing shrinkage rehydration capacity
and color changes.
Experimental
Carrot samples of different shapes and dimensions were
dried by air in a lab-scale convection oven (Memmert
Universal Oven model UFP 400). The carrots that were
bought in a local market were cut in cylindrical and slab
shapes (Fig. 1).
Both cylinder length and slab side (L) had an initial
value of 30 mm. Three different values of initial cylinder
diameter and slab thickness, i.e., 5, 10, and 15 mm, were
chosen to perform the present experimental analysis, thusleading to a total number of six kinds of food samples.
For each sample, the following food parameters were
monitored, with respect to time:
& weight, by using a precision balance (Mettler AE 160)
with an accuracy of 0.0001 g;
& color, by using a Minolta color reader (CR-300, Japan),
which allowed estimating the amounts of three primary
colors in a three-component colors model. The estimation
was performed by numerical integration of some charac-
teristic colors matching functions, which were expressed
by ICI coordinates (L*, a*, b*) system. L*, a*, and b*indicated the whiteness/darkness, redness/greenness, and
blueness/yellowness values, respectively. A standard
white color was used as a reference. Three replicate
readings were carried out; the average value and
corresponding standard deviation were reported; and
& dimensions, by using a vernier caliper with an accuracy
of 0.02 cm.
The lab-scale oven allowed monitoring air temperature
by a Dostmann electronic Precision Measuring Instrument
(P 655), its humidity (by rh 071073 probe), and the inlet
velocity by H 113828 probe. The convective flow of drying
air was obtained by a line of fans placed along the edge of
Fig. 1 Carrot samples shape and drying air flow
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oven internal tray. Two values of air velocities, i.e., 2.8 and
2.2 m/s, were chosen; air absolute humidity, AH, was kept
constant throughout all the experiments and equal to
10.32 g water/m3
dry air; air temperature, Ta, was equal to50 C, 70 C, and 85 C. The food samples were placed on
a wide mesh perforated tray. Oven dimensions allowed
analyzing six samples at the same time.
Food weight was periodically measured during each drying
experiment, which had a 5-h duration. Each carrot sample
previously dehydrated was eventually rehydrated by immer-
sion in water at room temperature for 5 h. Carrots weight was
periodically measured during rehydration. Before each mea-
surement, the carrot samples were taken out and blotted with
paper towel to eliminate the excess water on their surface.
Each of the tests (both drying and rehydration) was
repeated twice to ascertain its reproducibility.The main dimensions of the samples were experimen-
tally measured during drying and on the basis of these
measurements, the time evolution of food volume was
calculated. In particular, in the case of cylinder, length and
diameter at half of the length were recorded; in the case of
the slab, the values of the two sides and four different
values of the thickness, taken along the perimeter, were
measured.
Results and Discussion
Drying Curves
In the following, the effect of both air velocity and air
temperature on convective drying of carrots is presented.
Figures 2 and 3 show the time evolution of normalized food
moisture contentXXe
XoXe in the case of a slab-shaped and
cylindrical sample, respectively.
Where X (Kg water/Kg dry solid) is the moisture content ona dry basis, X0 is the corresponding initial value, Xe is the
moisture content in equilibrium with drying air, and t is the
drying time. The reported standard deviations indicate a
good reproducibility of experimental tests, since most of the
Fig. 3 Drying curves obtained
for a cylindrical carrot under
different conditions
(AH=10.32 g water/m3 dry air,
diameter=5 mm)
Fig. 2 Drying curves obtained
for a slab-shaped carrot sample
under different conditions
(AH=10.32 g water/m3 dry air,
thickness=5 mm)
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Table1
Resultsofdryingcurvesf
ittingprocedureusingNewtonmodel(slab
samples)
T=5mm
T=10mm
T=15mm
Airconditions
(u,
Ta
)
r2
k (1/min
)
k (standarderror)
95%
confidence
limits
r2
k (1/min)
k (standarderror)
95%
confidence
limits
r2
k (1/min)
k (standarderror)
95%
confidence
limits
2.8
m/s,
85
C
0.9
89
0.0
34
0.0
017
0.0
30
0.9
89
0.0
18
0.0
006
0.0
17
0.9
98
0.0
13
0.0
001
0.0
13
0.0
38
0.0
19
0.0
13
2.8
m/s,
70
C
0.9
81
0.0
23
0.0
02
0.0
20
0.9
96
0.0
12
0.0
002
0.0
12
0.9
97
0.0
08
0.0
00
0.0
08
0.0
27
0.0
13
0.0
08
2.8
m/s,
50
C
0.9
92
0.0
13
0.0
005
0.0
12
0.9
91
0.0
08
0.0
002
0.0
07
0.9
97
0.0
06
6e
05
0.0
06
0.0
15
0.0
08
0.0
06
2.2
m/s,
85
C
0.9
88
0.0
27
0.0
01
0.0
24
0.9
93
0.0
13
0.0
003
0.0
126
0.9
99
0.0
09
4e
05
0.0
09
0.0
3
0.0
14
0.0
09
2.2
m/s,
70
C
0.9
85
0.0
18
0.0
01
0.0
16
0.9
94
0.0
1
0.0
002
0.0
09
0.9
99
0.0
07
5e
05
0.0
07
0.0
21
0.0
104
0.0
07
2.2
m/s,
50
C
0.9
85
0.0
1
0.0
005
0.0
09
0.9
79
0.0
06
0.0
002
0.0
06
0.9
98
0.0
04
3e
05
0.0
044
0.0
11
0.0
066
0.0
045
Table2
Resultsofdryingcurvesf
ittingprocedureusingNewtonmodel(cylin
dricalsamples)
D=5mm
D=10mm
D=15mm
Airconditions
(u,
Ta
)
r2
k (1/min
)
k (standarderror)
95%
confidence
limits
r2
k (1/min)
k (standarderror)
95%
confidence
limits
r2
k (1/min)
k (standarderror)
95%
confidence
limits
2.8
m/s,
85
C
0.9
70
0.0
51
0.0
05
0.0
38
0.9
92
0.0
30
0.0
011
0.0
27
0.9
95
0.0
18
0.0
003
0.0
17
0.0
64
0.0
32
0.0
18
2.8
m/s,
70
C
0.9
81
0.0
48
0.0
04
0.0
38
0.9
91
0.0
2
0.0
008
0.0
2
0.9
98
0.0
11
0.0
001
0.0
11
0.0
59
0.0
22
0.0
11
2.8
m/s,
50
C
0.9
80
0.0
25
0.0
02
0.0
20
0.9
93
0.0
14
0.0
004
0.0
13
0.9
96
0.0
07
8e
05
0.0
07
0.0
30
0.0
15
0.0
07
2.2
m/s,
85
C
0.9
85
0.0
34
0.0
02
0.0
29
0.9
91
0.0
22
0.0
007
0.0
20
0.9
97
0.0
11
0.0
001
0.0
106
0.0
38
0.0
24
0.0
112
2.2
m/s,
70
C
0.9
91
0.0
35
0.0
02
0.0
31
0.9
93
0.0
15
0.0
0055
0.0
14
0.9
99
0.0
09
5e
05
0.0
09
0.0
39
0.0
16
0.0
09
2.2
m/s,
50
C
0.9
98
0.0
23
0.0
009
0.0
20
0.9
87
0.0
09
0.0
003
0.0
08
0.9
99
0.0
05
7e
06
0.0
052
0.0
25
0.0
1
0.0
05
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error bars are so small to be hidden by the corresponding
point marker.
The so-called Newtons model was widely adopted to
describe the time evolution of drying process. It is based on
an exponential equation according to a pseudo first order
reaction kinetics (Ertekin and Yaldiz 2004; Senadeera et al.
2003):
X Xe
Xo Xe exp k t 1
The parameter k is the so-called drying constant and
represents a measure of drying process rate. The values of
Xe, as obtained from the psychrometric chart, were equal to
0.029, 0.052, and 0.124 at a temperature of 85 C, 70 C,
and 50 C, respectively.
The experimental data collected in all the tested
conditions were fitted by the Newtons model so as to
evaluate the effect of operating conditions on k parameter
and therefore, on the drying rate. The results of the fitting
procedure are synthetically shown in Tables 1 and 2 for
slab-shaped and cylindrical samples, respectively. The high
values of linear regression coefficient, r2, as well as the
reported 95% confidence limits indicate a very good
agreement between the predictions of the Newtons model
and the experimental drying curves. For all the experi-
ments, an increase in drying constant value is observed as
both air temperature and its velocity increase and food
characteristic dimension decreases. This result is similar to
that obtained by Torul (2006) who found akcoefficient, in
the case of carrots, increasing with air temperature and
ranging between 0.011985 and 0.026821 1/min through
Fig. 4 Rehydration curves
(S slab-shaped sample and C
cylindrical sample) obtained
upon the same drying conditions
(u=2.8 m/s, Ta=85 C)
Fig. 5 Rehydration curves for a
cylindrical carrot previously
dried under different conditions
(D=5 mm)
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Air conditions during drying (u, Ta) Cylinders (D=5mm) Cylinders (D=5mm) Slabs (T=5mm)
2.8 m/s, 85 C 54% 45% 47%
2.8 m/s, 70 C 53% 50% 64%
2.8 m/s, 50 C 77% 53% 70%
2.2 m/s, 85 C
2.2 m/s, 70 C 58%
2.2 m/s, 50 C 71%
Table 3 Waterregain of different
samples after rehydration
Fig. 6 Volume variation as a
function of food moisture
content (on wet basis) at
different drying conditions
(slab-shaped carrot having an
initial thickness of 15 mm)
Fig. 7 Normalized volume ver-
sus normalized moisture content
at different drying conditions
for a slab-shaped sample
(T=15 mm)
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Table4
Resultsofshrinkagecurvesfittingprocedureusinglinearmodel(cylindricalsamples)
Cylinders
10mm
15mm
r2
k1
k1
standard
error
k1
95%
confidence
limits
k2
k
2
standard
error
k2
95%
confidence
limits
r2
k1
k1standard
error
k1
95%
confidence
limits
k2
k2
standard
error
k2
95%
confidence
limits
2.8
m/s,
85
C
0.9
78
0.9
19
0.0
30
0.8
56
0.0
59
0.0
11
0.0
36
0.9
58
0.8
23
0.0
35
0.7
51
0.0
72
0.0
14
0.0
44
0.9
83
0.0
82
0.8
94
0.0
10
2.8
m/s,
70
C
0.9
16
0.8
88
0.0
77
0.7
19
0.2
39
0.0
34
0.1
65
0.9
86
0.7
42
0.0
19
0.7
02
0.2
60
0.0
08
0.2
44
1.0
57
0.3
13
0.7
83
0.2
77
2.8
m/s,
50
C
0.9
45
0.7
84
0.0
47
0.6
83
0.1
43
0.0
18
0.1
06
0.9
84
0.7
07
0.0
19
0.6
68
0.2
40
0.0
07
0.2
25
0.8
84
0.1
81
0.7
45
0.2
56
2.2
m/s,
85
C
0.9
76
0.8
13
0.0
34
0.7
41
0.1
11
0.0
15
0.0
79
0.9
75
0.7
44
0.0
22
0.7
00
0.1
36
0.0
08
0.1
20
0.8
85
0.1
43
0.7
89
0.1
53
2.2
m/s,
70
C
0.9
86
0.8
30
0.0
26
0.7
74
0.1
80
0.0
11
0.1
56
0.9
94
0.8
64
0.0
15
0.8
34
0.1
65
0.0
06
0.1
52
0.8
86
0.2
05
0.8
95
0.1
78
2.2
m/s,
50
C
0.9
70
0.7
72
0.0
34
0.7
01
0.2
78
0.0
13
0.2
50
0.9
87
0.8
31
0.0
19
0.7
92
0.1
82
0.0
08
0.1
6
0.8
43
0.3
06
0.8
71
0.1
98
Table5
Resultsofshrinkagecurvesfittingprocedureusinglinearmodel(slab
samples)
Slabs
5mm
10m
m
15mm
r2
k1
k1
standard
error
k1
95%
confidence
limits
k2
k2
standard
error
k2
95%
confidence
limits
r2
k1
k1
standard
error
k1
95%
confidence
limits
k2
k2
standard
error
k2
95%
confidence
limits
r2
k1
k1
standard
error
k1
95%
confidence
limits
k2
k2
standard
error
k2
95%
confidence
limits
2.8
m/s,
85
C
0.9
84
0.8
40
0.0
24
0.7
910.8
90
0.1
23
0.0
08
0.1
060.1
40
0.99
5
0.8
58
0.0
15
0.8
270.8
88
0.1
10
0.0
06
0.0
970.1
23
0.9
94
0.8
29
0.0
13
0.8
030.85
5
0.1
24
0.0
05
0.1
140.1
33
2.8
m/s,
70
C
0.9
92
0.7
81
0.0
30
0.7
100.8
51
0.2
39
0.0
16
0.2
000.2
78
0.99
6
0.7
79
0.0
11
0.7
560.8
01
0.2
47
0.0
04
0.2
380.2
57
0.9
97
0.8
13
0.0
1
0.7
920.83
4
0.1
94
0.0
04
0.1
850.2
03
2.8
m/s,
50
C
0.9
98
0.7
21
0.0
11
0.6
980.7
46
0.2
67
0.0
05
0.2
560.2
79
0.99
6
0.8
36
0.0
11
0.8
130.8
59
0.1
95
0.0
05
0.1
860.2
04
0.9
99
0.8
21
0.0
03
0.8
140.82
7
0.1
72
0.0
01
0.1
690.1
75
2.2
m/s,
85
C
0.9
92
0.7
64
0.0
19
0.7
240.8
04
0.2
07
0.0
07
0.1
910.2
23
0.99
6
0.8
54
0.0
13
0.8
270.8
81
0.1
70
0.0
05
0.1
590.1
82
0.9
98
0.8
52
0.0
07
0.8
380.86
6
0.1
36
0.0
03
0.1
310.1
42
2.2
m/s,
70
C
0.9
99
0.7
44
0.0
03
0.7
370.7
51
0.2
53
0.0
015
0.2
500.2
57
0.99
4
0.7
62
0.0
14
0.7
330.7
90
0.2
06
0.0
06
0.1
940.2
19
0.9
98
0.8
09
0.0
08
0.7
920.82
6
0.1
97
0.0
04
0.1
890.2
05
2.2
m/s,
50
C
0.9
99
0.7
28
0.0
04
0.7
190.7
38
0.2
70
0.0
012
0.2
650.2
74
0.99
6
0.8
28
0.0
11
0.8
050.8
52
0.1
99
0.0
05
0.1
890.2
08
0.9
99
0.8
37
0.0
04
0.8
300.84
5
0.1
67
0.0
02
0.1
630.1
70
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infrared drying, which was performed at the temperature
range of 5080 C. At the same value of air temperature, k
increases as air velocity rises for all the tested values of
food thickness (Table 1). The same effect is observed in the
case of cylindrical samples (Table 2).
To compare the effect of operating conditions on the
time evolution of drying rate, the percentage water content
reduction (WR%) was defined as the percentage ratio
between the evaporated water, calculated at a definite time
t, and the initial water content:
Kgwater t 0 Kgwatert
Kgwater t 0 *100 WR% 2
After 1 h and in the case of carrot slabs having a
thickness of 5 mm, an increase in drying temperature from
50 C to 85 C resulted in an increase of WR% from 71.3%
to 89.8%, in the case of 2.8 m/s in air velocity and from
54.8% to 88.2% in the case of 2.2 m/s. After 1 hour, an
increase of air velocity from 2.2 to 2.8 m/s resulted in an
increase in WR from 88.2% to 89.8% and from 54.8% to
71% when operating temperature was equal to 85 C and
50 C, respectively.
The results shown in Tables 1 and 2 confirm that drying
rate increases as food dimension (thickness or diameter)
decreases.
After 1 hour and when drying was performed by air at 85 C
and 2.8 m/s, a decrease in food characteristic dimension by
66% (from 15 to 5 mm) resulted in an increase of WR% from
70.2% to 89.8% and from 82.7% to 89.9% for a slab-shaped
and for a cylindrical sample, respectively.
Fig. 8 L* variations for slab-
shaped carrots (T=5 mm) during
drying at different conditions
Fig. 9 a* variations for slab-
shaped carrots (T=15 mm)
during drying at different
conditions
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Rehydration Curves
Food rehydration capacity can provide useful knowledge of
the damage that occurred during drying. The results,
presented as moisture content on a dry basis, X, versus
rehydration time show that food dimensions strongly affect
the process rate since a thinner sample reached a stationary
value earlier than a thicker one (Fig. 4).
Figure 5 shows the influence of drying conditions on
rehydration curves of cylindrical samples; actually, the
effect of air temperature and inlet velocity is not immediately
evident, since the curves are very close to each other. A more
informative comprehension of process behavior can instead be
obtained by analyzing the so-called water regain, which is
defined as the percentage ratio between the water regained atthe end of rehydration process (after 5 h) and water loss during
drying. In order to perform a significant comparison, only the
samples characterized by the same value of initial and final
moisture contents, as measured at the end of drying process,
are considered. The obtained results and the corresponding
operating conditions are reported in Table 3. The initial
value of moisture content (X0) ranged between 7 and 9 Kg
water/Kg dry solid and the final value of moisture content
(Xe) ranged between 0.02 and 0.07 Kg water/Kg dry solid.
As expected, the samples that have the same initial
dimension, generally, exhibit higher water regain if the
Table 6 Variations of color parameters of carrot slab samples (initial thickness 5 mm) after 4 h of drying
L* L* standard deviation L* variation a* a* standard deviation a* variation b* b* standard deviation b* variation
85 C, 2.8 m/s
Initial 58.67 2.786 8% 16.04 1.15966 28% 33.78 0.43841 16%
Final 54.2 5.04874 11.51 0.65054 39.335 3.59917
85 C, 2.2 m/s
Initial 48.885 1.6617 2% 16.795 0.09192 4% 28.45 3.71938 12%
Final 47.92 2.06475 16.04 1.85262 31.805 5.02753
50 C, 2.8 m/s
Initial 48.895 0.53033 7% 20.955 2.8355 30% 30.71 2.23446 12%
Final 45.57 1.38593 27.16 1.66877 34.365 0.88388
50 C, 2.2 m/s
Initial 50.93 5.14774 4% 24.145 0.92631 18% 33.98 1.7112 9%
Final 49.095 3.2739 28.465 0.16263 36.96 0.39598
70 C, 2.8 m/s
Initial 51.825 0.37477 3% 1.15258 36% 32.00 2.44659 24%
Final 53.565 5.39522 26.04 4.03051 39.68 1.48492
70 C, 2.2 m/s
Initial 53.045 5.63564 0% 17.885 1.22329 31% 30.51 2.786 28%
Final 53.215 0.30406 23.46 5.98212 38.97 2.27688
Fig. 10 b* variations for slab-
shaped carrots (T=10 mm)
during drying at different
conditions
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operating conditions chosen to perform the preceding
drying process were milder. This behavior is similar to that
reported in the literature by Kowalski and Rybicki (2007)
and Lewicki et al. (1997).
Table 3 confirms, as it was already shown in Fig. 4, that
a thinner sample is characterized by larger water regain as
compared to thicker one.
Shrinkage
Figure 6 shows, in a typical case, the experimental trends
expressing food total volume versus its moisture content on
a wet basis. A significant change of total volume, V, is
observed during the process in all the tested condition.
Figure 6 shows, in addition, that the effect of air
Table 7 Variations of color parameters of carrot slab samples (initial thickness 10 mm) after 4 h of drying
L* L* standard deviation L* variation a* a* standard deviation a* variation b* b* standard deviation b* variation
85 C, 2.8 m/s
Initial 53.935 1.03945 2% 15.77 1.37179 8% 32.445 0.95459 25%
final 52.8 1.25865 16.965 0.02121 40.54 0.52326
85 C, 2.2 m/s
Initial 44.155 0.88388 20% 15.445 1.18087 10% 24.39 1.68291 37%Final 52.82 2.29103 16.975 0.0495 33.34 2.20617
50 C, 2.8 m/s
Initial 49.415 4.36285 3% 18.90 2.02233 33% 30.04 3.23855 16%
Final 51.055 4.56084 25.14 0.5374 34.83 0.79196
50 C, 2.2 m/s
Initial 49.54 3.3234 9% 19.745 3.3729 26% 31.655 3.79716 12%
Final 53.945 2.87792 24.835 1.59099 35.59 2.5173
70 C, 2.8 m/s
Initial 52.985 0.16263 5% 18.82 1.78191 35% 29.25 3.12541 24%
Final 55.41 1.93747 25.42 1.0748 36.16 0.73539
70 C, 2.2 m/s
Initial 51.875 0.77075 10% 17.49 0.9051 48% 27.525 1.09602 40%
Final 56.85 0.19799 25.945 0.71418 38.665 2.67993
Table 8 Variations of color parameters of carrot slab samples (initial thickness 15 mm) after 4 h of drying
L* L* standard deviation L* variation a* a* standard deviation a* variation b* b* standard deviation b* variation
85 C, 2.8 m/s
Initial 53.045 0.50205 2% 15.835 1.20915 20% 30.94 0.87681 32%
Final 54.16 5.74171 18.94 0.74953 40.95 2.63044
85 C, 2.2 m/s
Initial 45.76 1.78191 5% 15.75 5.28916 45% 27.045 8.49235 42%
Final 43.595 1.09602 22.89 2.10718 38.41 2.74357
50 C, 2.8 m/s
Initial 54.265 10.6137 7% 20.28 0.70711 13% 32.205 0.40305 8%
Final 57.94 6.50538 22.99 0.46669 34.855 0.44548
50 C, 2.2 m/s
Initial 54.23 10.7056 17% 19.23 3.57796 12% 30.5 4.15779 5%
Final 63.235 2.26981 16.86 5.96798 29.08 5.17602
70 C, 2.8 m/s
Initial 51.84 6.06698 6% 16.055 4.278 46% 29.03 8.48528 29%
Final 55.155 0.10607 23.465 0.81317 37.445 1.77484
70 C, 2.2 m/s
Initial 48.81 0.41012 6% 19.66 2.00818 24% 34.665 0.95459 12%
Final 51.605 2.62337 24.335 2.10011 38.72 0.5374
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temperature and its velocity is rather insignificant on volume
variation trend since the points, obtained at the same value of
moisture content on a wet basis, tend to overlap even if the
drying conditions are changed. Similar results were obtained
for all the carrot samples taken in consideration.
In order to verify the actual effect of temperature and air
velocity from a quantitative point of view, it was decided to
fit the experimental data by a linear model (Mayor andSereno 2004) that was found to provide the best agreement
between normalized volume (V/V0) and normalized moisture
content on a dry basis (X/X0).
The subscript 0 refers to the initial values of both volume
and moisture content.
Figure 7 shows, a plot of V/V0 versus X/X0 that confirms
a good linear correlation between volume and moisture
content. Equation 3 was, therefore, tested to fit the
experimental data as functions of the operating conditions
and sample dimensions:
V
V0 k1
X
X0 k2 3
The parameter k1 can be regarded as an index of the drying
rate and k2 is the normalized volume corresponding to a
completely dried material (X 0).
The results of fitting procedure are summarized in
Tables 4 and 5. Both the high value of correlation
coefficient, r2, and the relatively low standard error, confirm
that the proposed linear model exhibits a remarkable
agreement with the experimental data at different drying
conditions. The thinnest cylindrical samples (data not shown),
however, were not used for shrinkage characterization due to
the difficulty to correctly measure the geometrical character-
istics of the samples, which did not maintain their initial
cylindrical shape during drying experiments. This effect, due
to anisotropy of the carrot structure, causes a curvature of
sample that is mainly evident for the smaller samples. Only in
some cases, a slight increase of k1 was observed if, for the
same shape and the same initial size, both air temperature
and air velocity were raised. However, it is not possible to
identify a dependence of k1 upon air velocity or air
temperature variations, since the calculated 95% confidence
limits tend to overlap in most of the tested conditions.
Color Parameters
Figures 8, 9, and 10 present time evolutions of L*, a*, and
b* during the drying of carrots at different operating
conditions. Actually, the experimental data do not exhibit
a regular trend. The values, however, are very close, at least
at the beginning of the process, to those obtained by
Krokida et al. (1998) who found that in the case of carrots
drying at 70 C, L*=45, a*=15, and b*= 22. In the present
case, the initial values of L*, a*, and b* were 51.24.6,
18.2 2.8, and 30.53.7, respectively. At the end of the
drying process, i.e., after 4 h, the variation of color
parameters was found depending on the drying condition
(Tables 6, 7, and 8). It should be remarked that slab-shaped
samples, due to the larger exposed surface, exhibited a
higher accuracy than the cylindrical ones as far as color
measurement was concerned. For this reason, only theresults related to the slab-shaped samples were reported. As
showed in Tables 6, 7, and 8 in the case of slab thickness of
5, 10, and 15 mm, respectively, the experimental tests did
not show a monotonous trend of L* (Krokida et al. 1998);
whereas, the majority of the tests showed, as drying process
proceeded, an increase of both a* and b* due to the
presence of carotenes (Sumnu et al. 2005). An increase in b*
value was also observed by Krokida et al. (1998, 2001a, b).
In the range of the operating conditions tested in this work,
carotenes seemed to be well preserved. However, a depen-
dence of color parameters upon air temperature or velocity
variation could not be identified since a monotonous trendwas not observed.
Conclusions
The effects of operating conditions on carrots drying
process were presented. Three important quality parameters,
i.e., shrinkage, color changes occurring as drying proceeds,
and rehydration capacity after drying were considered. It has
been observed that the utilization of milder conditions allows
achieving a better rehydration capacity of the product. In the
range of operating conditions tested in this study, it is
suggested to use an operating temperature of 50 C and an
inlet velocity of 2.2 m/s; this minimizes food damage since
rehydration capacity is enhanced. As far as color variation and
shrinkage effect were concerned, it was observed that the
experimental data did not exhibit a unique trend.
Acknowledgments One of the authors (M.A.) would like to address
her special thanks to Wouter de Heij* and Bert Tournois*, for the
enlightening discussions with them and their invaluable advices, and
Aart-Jan van der Voort*, for his technical support (* TOP b.v., Food
Technology and Life Science, Wageningen, The Netherlands).
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