Hydration kinetics of dried apple as affected by drying conditions, Kinetyka suszenia - artykuły
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Journal of Food Engineering 68 (2005) 369–376
www.elsevier.com/locate/jfoodeng
Hydration kinetics of dried apple as affected by drying conditions
Cristina Bilbao-S´ inz
*
, Ana Andr´s, Pedro Fito
Departamento de Tecnolog´ a de Alimentos, Universidad Polit´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain
Received 4 December 2003; accepted 9 June 2004
Abstract
Weight gain during soaking of apple cylinders dried at different levels of microwave incident power and different air temperatures
was modeled according to two different approaches. The first one based on Ficks equation of diffusion was not suitable to model the
sorption data. The second proposed model was Pelegs equation, which gave a good fit to experimental data. The weight gain and
volume recovery were higher at higher levels of microwave power; however, higher levels of microwave incident power caused a
lower water holding capacity.
2004 Elsevier Ltd. All rights reserved.
Keywords: Apple; Microwaves; Re-hydration; Volume; Peleg
1. Introduction
Although not derived from any physical laws and
hence empirical, its application has been demon-
strated for some food materials (
Hung, Liu, Black, &
Trewhella, 1993
;
Sopade & Obekpa, 1990
;
Sopade, Aji-
segiri, & Badau, 1992
;
Singh & Kulshrestha, 1987
;
Garc
´
a-Pascual, Sanju´n, Benedito, Mateos, & Mulet,
2000
).
In the present study two models are discussed in
order to explain the observed data during re-hydration
of dried apple and to predict behavior depending on
the different drying conditions. It is showed that for this
kind of samples, re-hydration process is not a diffusion-
controlled process. The present work also studied the
hydration in terms of mass and volume recovery as a
way to analyze the injuries of the apple tissue caused
by the different drying conditions.
Re-hydration is a complex process aimed at the resto-
ration of raw material properties when dried material is
in contact with a liquid phase. Predrying treatments,
subsequent drying and re-hydration per se induce many
changes in structure and composition of plant tissue
(
Lewicki, Witrowa-Rajchert, & Mariak, 1997
), which re-
sult in impaired reconstitution properties. Hence, re-
hydration can be considered as a measure of the injuries
to the material caused by drying and treatments preced-
ing dehydration (
Lewicki, 1998
).
Different authors have modeled the re-hydration
process, most of them considering a diffusion controlled
mechanism and analyzing the experimental data based
on Ficks laws of diffusion with their appropriate
equations (
Cranks, 1975
). However, a relatively simple
equation was proposed (
Peleg, 1988
) to fit data sorption.
2. Materials and methods
*
Corresponding author. Address: Unilever R&D Colworth, Food
Research, Sharnbrook, Bedfordshire MK441LQ, UK. Fax: +44-1234-
222-259.
E-mail address:
C. Bilbao-
S´inz).
Apples (var. Granny Smith) with moisture content
varying between 84% and 88% (w/w) were used in this
work. Samples for drying experiments were obtained
by cutting the apples into cylindrical pieces with a
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2004.06.012
370
C. Bilbao-S
´
inz et al. / Journal of Food Engineering 68 (2005) 369–376
diameter of 20 mm and 20 mm height. The axis of each
cylinder was parallel to the main axis of the apple fruit.
Some of these cylinders were vacuum impregnated (VI)
prior to drying. This pretreatment was carried out in or-
der to exchange the product internal gas by the impreg-
nation solution. The adequate control of VI prior to
drying may be used as a tool both to improve mass
transfer and to develop engineered products (
Fito
et al., 2001
).
Vacuum impregnated samples were prepared by
immersion in an isotonic solution of apple juice (11.7
Brix) at 50 mbar pressure for 10 min and 10 min more
at atmospheric pressure (
Fito & Pastor, 1994
). The solu-
tion–fruit ratio was large enough (50:1) to avoid signif-
icant changes in the solution concentration throughout
the process. At the end of the impregnation treatment
apple cylinders were removed from the solution, mois-
ture content was determined gravimetrically and sugar
content with a refractometer. The obtained values 86%
±2 and 12 Brix ±1 respectively, showed no significant
differences in the concentrations due to the impregna-
tion treatment.
Pretreated and non-pretreated samples were dried in
a combined air-microwave oven. The drier consisted
essentially of an air-flow section and a microwave sec-
tion, both of them assembled to a microwave oven cav-
ity functioning as a drying chamber. The air conditions
were set at a range of temperatures (25, 30, 40, and 50
C) and microwave powers (0, 3, 5, 7 and 10 W/g of ini-
tial sample weight). The air velocity and the air relative
humidity were fixed at 1 m/s ±0.08 and 60 ± 5% respec-
tively. During drying sample weight was continuously
monitored. Samples were dried to a final humidity con-
tent of 10%.
After drying, apple cylinders were re-hydrated by
immersion in 50 ml of distilled water at 50 C for 7 h.
Sample weight changes were controlled throughout the
hydration process. Before weighting, the samples were
gently blotted with tissue paper in order to remove the
superficial water. In order to evaluate the swelling of
the samples, they were photographed at certain time
intervals throughout the soaking process. The diameter
and the radius were estimated from the photographs
using the adobe photoshop programme. The water hold-
ing capacity was determined by centrifuging the re-hy-
drated samples at 4000 rpm in specially designed tubes
for 10 min. The tubes had a supporting net in the middle
so that the liquid could drip.
tent of the samples was approximately 30% (
Andres,
Bilbao, & Fito, 2004
).
The re-hydration ability was determined by the re-
hydration ratio R(t) calculated according to Eq.
(1)
and the volume recovery calculated according to Eq.
(2)
:
R
ð
t
Þ¼
X
ð
t
Þ
X
0
ð
1
Þ
R
v
ð
t
Þ¼
V
ð
t
Þ
V
0
ð
2
Þ
X(t) and X
0
being the instantaneous and initial moisture
contents on dry basis respectively, V(t) the instantane-
ous volume and V
0
the initial volume before drying.
Figs. 2 and 3
show the re-hydration curves for non-
impregnated and impregnated apple cylinders in terms
of re-hydration ratio and volume recovery respectively.
The curves exhibit the characteristic re-hydration behav-
ior whereby an initial high rate of water absorption is
followed by slower absorption in later stages. The rapid
initial water uptake was probably due to the filling of
capillaries on the surface of the sample. As water
absorption proceeds, the soaking rate starts to decline
due to the filling of free capillaries and of the intercellu-
lar spaces with water. The cells highly shrunken during
drying, when re-hydrated absorbed water in response
to the relaxation of the cell walls. Such relaxation vac-
uum effect could promote not only the cell filling
through the permeable cell wall but also the filling of
intercellular spaces (
Barat, Chiralt, & Fito, 1998
). Sub-
sequently, amounts of water absorbed with further
soaking became minimal until equilibrium was attained,
signaling the maximum water capacity of the apple sam-
ple (
Abu-Ghannam & McKenna, 1997
). It is worth to
mention that after 7 h of re-hydration, samples dried
at the highest microwave power showed bigger volumes
than the corresponding volumes of the fresh cylinders.
Different models are proposed to quantify the effect
of the drying conditions on the re-hydration ratio. First,
it was assumed that liquid water sorption by apple cyl-
inders was a diffusion controlled process. Therefore,
an effective diffusion coecient could be calculated from
the second Ficks law. The transport equations used cor-
responded to a finite cylinder.
o
X
ð
t
Þ
ot
o
2
X
ð
t
Þ
ol
2
þ
o
X
ð
t
Þ
or
2
¼
D
e
ð
3
Þ
To solve this differential equation the following assump-
tions were made: the initial moisture content is uniform
in the cylinder and the cylinder surface attains satura-
tion moisture instantly at soaking. The boundary condi-
tion considered was in relation to the symmetry of the
solid. The sample moisture content at the beginning of
the process sets the initial condition. Assuming these
hypotheses, Eq.
(3)
became Eqs.
(5) and (6)
, these are
the solutions for small times and large values of time,
3. Results and discussion
Fig. 1
shows the experimental drying curves for all
experimental conditions; rapid drying was achieved for
high levels of microwave power while air temperature
did not greatly facilitate drying until the humidity con-
2
C. Bilbao-S
´
inz et al. / Journal of Food Engineering 68 (2005) 369–376
371
Fig. 1. Drying curves of apple cylinders dehydrated combining air at a range of temperatures and microwaves powers.
respectively, of the second Ficks law using the first term
of the series solution.
Y is the dimensionless moisture given by Eq.
(4)
.
Y
¼
X
ð
t
Þ
X
eq
X
0
X
eq
where X(t) is the dry basis moisture of the cylinder at
any time, X
0
the original moisture and X
eq
the equilib-
rium moisture content.
ð
4
Þ
Y
¼
1
8D
e
t
p
lr
ð
5
Þ
372
C. Bilbao-S
´
inz et al. / Journal of Food Engineering 68 (2005) 369–376
1.0
No VI;T = 25
°
C
1.0
VI;T = 25
°
C
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
t (min)
0.1
t (min)
0.0
0.0
0
60 120 180 240 300 360 420
0
60 120 180 240 300 360 420
0W/g
3W/g
5W/g
7W/g
10 W/g
0W/g
3W/g
5W/g
7W/g
10 W/g
1.0
No VI;T = 30
°
C
1.0
VI;T = 30
°
C
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
t (min)
0.1
t (min)
0.0
0.0
0
60 120 180 240 300 360 420
0
60 120 180 240 300 360 420
0W/g
3W/g
5W/g
7W/g
10 W/g
0W/g
3W/g
5W/g
7W/g
10 W/g
1.0
No VI;T = 40
°
C
1.0
VI;T = 40
°
C
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
t (min)
0.1
t (min)
0.0
0.0
0
60 120 180 240 300 360 420
0
60 120 180 240 300 360 420
0W/g
3W/g
5W/g
7W/g
10 W/g
0W/g
3W/g
5W/g
7W/g
10 W/g
1.0
No VI;T = 50
°
C
1.0
VI;T = 50
°
C
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
t (min)
0.1
t (min)
0.0
0.0
000180 240 300 360 420
0
60 120 180 240 300 360 420
0W/g
3W/g
5W/g
7W/g
10 W/g
0W/g
3W/g
5W/g
7W/g
10 W/g
Fig. 2. Water absorption curves of apple samples dried combining air at a range of temperatures and microwaves powers.
Y
¼
a
1
exp
D
e
t
ð
6
Þ
spreadsheet was applied. The sum of squares of differ-
ences between experimental and predicted dimensionless
moisture contents was minimized in order to identify the
value.
Fig. 4
shows the calculated dimensionless moisture
plotted against the experimental dimensionless moisture,
for short and long times respectively, of dried samples at
r
2
where, D
e
is the effective diffusion coecient, l and r are
the height and diameter of the cylinder at any time.
In order to determine the effective diffusion coecient
(D
e
), Newtons method, available in Solver for EXCEL
a
2
l
2
a
3
C. Bilbao-S
´
inz et al. / Journal of Food Engineering 68 (2005) 369–376
373
120
No VI;T = 25
°
C
120
VI; T = 25
°
C
100
100
80
80
60
60
40
40
20
t(min)
20
t(min)
0
0
0 60 120 180 240 300 360 420
0 60 120 180 240 300 360 420
0W/g
3W/g
5W/g
7W/g
10 W/g
0W/g
3W/g
5W/g
7W/g
10 W/g
No VI;T = 30
°
C
VI;T = 30
°
C
120
120
100
100
80
80
60
60
40
40
20
t(min)
20
t(min)
0
0
0 60 120 180 240 300 360 420
0 60 120 180 240 300 360 420
0W/g
3W/g
5W/g
7W/g
10 W/g
0W/g
3W/g
5W/g
7W/g
10 W/g
No VI;T = 40
°
C
VI;T = 40
°
C
120
120
100
100
80
80
60
60
40
40
20
t(min)
20
t(min)
0
0
0 60 120 180 240 300 360 420
0 60 120 180 240 300 360 420
0W/g
3W/g
5W/g
7W/g
10 W/g
0W/g
3W/g
5W/g
7W/g
10 W/g
No VI;T = 50
°
C
VI;T = 50
°
C
120
120
100
100
80
80
60
60
40
40
20
t(min)
20
t(min)
0
0
0 60 120 180 240 300 360 420
0 60 120 180 240 300 360 420
0W/g
3W/g
5W/g
7W/g
10 W/g
0W/g
3W/g
5W/g
7W
/g
10
W/g
Fig. 3. Volume recovery curves of apple samples dried combining air at a range of temperatures and microwaves powers.
40 C and different levels of incident microwave power.
The results obtained for this temperature have been cho-
sen as representative of those obtained for other drying
temperatures. It is observed in these figures that the
model was not appropriate for this kind of material,
which seems to indicate that diffusion was not the rate
controlling mechanism for sorption of liquid water.
Hydration of dried apples was also modeled using the
empirical Peleg model (
Peleg, 1988
), which can describe
the characteristic shape of the sorption curves shown in
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