Journal of Food Engineering 21(1994)
355-363
Physical and Transport Properties of Peas During Warm
Air Drying
Gon@o
L. Medeiros
& Albert0
M. Serene”
University of Porto, Faculty of Engineering, Department of Chemical Engineering,
Rua dos Bragas, 4099 Port0 Codex, Portugal
(Received 16 June 1992; accepted 29 January 1993)
ABSTRACT
Warm air drying of peas was studied to assess the effect of drying conditions on some relevant physical properties of the product. Experimental
drying data was well correlated by a previously described model where an
implicit variationof moisture d@sivity during the dehydration is considered. Using that model, the experimental data could be reproduced within
6.1%. Drying conditions have influenced shrinkage of the product, as
expressed by apparent density, and moisture di#usivitybut not sorption
isotherm. Variation of moisture di@sivity during the process was calculated using the method of slopes. The results were shown to be consistent
withassumptions included in the drying model considered.
NOTATION
Water activity
Specific heat @J/kg per%)
cP
C
Parameter for GAB equation (eqn (2))
C, - C, Parameters for Lozano equation (eqn ( 1))
D
Effective diffusivity of water in solids (m*/s)
Fo
Fourier number ( = Dt/R2)
h
Surface heat transfer coefficient (W/m* per “C)
Absolute
humidity of air (kg/kg dry basis (db))
H,
k
Parameter for GAB equation (eqn (2))
K
Thermal conductivity of solid (W/m per”(Z)
%
*To whom correspondence
should be addressed.
355
Journal
of Food
Engineering
0260-8774/94/$07.00
Ltd. England. Printed in Great Britain
-
0
1994
Elsevier
Science
356
R
t
T
G. L Medeiros, A. M Sereno
Kll
Radius of solid particles (m)
Time (s)
Temperature (“C)
Air temperature (“C)
Air velocity (m/s)
Moisture content (kg/kg dry basis (db))
Parameter of GAB equation (eqn (2)) (kg/kg dry basis (db))
P
PS
Apparent density of solid ( kg/m3)
Apparent density of dry solid matrix (kg/m3)
T,
>
Subscripts
0
e
Initial state
Equilibrium
INTRODUCTION
Warm air drying is still an important means of reducing moisture
content of fruits and vegetables and achieves an enhanced resistance to
degradation due to the corresponding decrease in water activity.
In order to model adequately the behaviour of such products during
drying, different physical and transport properties are needed. Several
researchers have studied this subject and in most cases presented models
for the estimation of such properties. Effective moisture diffusivities are
usually estimated from drying experiments as described by, among
others, Saravacos (1967) and Karathanos et al. (1990). Crapiste et al.
(1988) have also developed a model to predict moisture diffusivities.
This subject has been reviewed by Saravacos (1986). Solid shrinkage
during drying and corresponding density variation have been studied by
Suzuki et al. (1976), Lozano et al. (1983) and Mattea et al. (1989). The
model proposed by Lozano et al. ( 1983) was used in this study. Thermal
and other thermodynamic properties have also received the attention of
several researchers. Rizvi ( 1986) and Sweat ( 1986) give detailed reviews
of such work.
In this study the behaviour of peas during drying by warm air was
studied. The major objective was to test the ability of a mathematical
model recently proposed by Sereno and Medeiros ( 1990) to describe the
drying behaviour and to identify the effect of air temperature on relevant
physical and transport properties of the product.
Properties of peas during warm air drying
MATERIALS
357
AND METHODS
Deep-frozen peas of the ‘Agrilusa’ brand were used. The peas had
been stored at - 18°C for about one month, and were thawed at 8°C for
12 h. Peas were air dried in a pilot dryer, using constant temperature,
humidity and velocity of the air during each run. Several sample sets of
20 peas each were individually positioned over a stainless steel 5 mm
square mesh net and were dried by a vertical upwards flow of air at 2.2
m/s.
At selected times during the drying operation, each set of 20 peas was
withdrawn from the dryer. Each sample was analyzed for water activity,
density and moisture content. Water activity was measured with an
electric hygrometer (Thermoconstanter
from DEFENSOR -Novasina
AG, Switzerland) after a stable reading was reached. Density was
measured by means of a 50 ml picnometer using n-heptane; after introducing the peas, the picnometer was de-aerated under the vacuum of a
common laboratory water jet. Two independent density values were
obtained for each sample and averaged. Moisture content was evaluated
from the weight loss of the sample after 24 h in an electric oven at 50°C
and atmospheric pressure, followed by another 24 h period at 70°C and
60 mmHg absolute pressure.
For the numerical fitting of experimental data a non-linear leastsquares regression prograrmne based on the Levenberg-Marquardt
algorithm and proposed by Press et al. ( 1988) was used.
RESULTS AND DISCUSSION
Apparent density
Figure 1 shows experimental results obtained for apparent density of
the peas dried with air at 30, 50 and 65°C. The model proposed by
Lozano et al. (1983) was fitted to this data:
p = C, + C,X+ C,exp( - C,X)
The calculated parameters
(1)
C, - C, are presented in Table 1.
Water sorption isotherm
Water sorption isotherm data based on measured water activities at
25°C are shown in Fig. 2. Experimental data, which refer to desorption
G. L Medeiros, A. M Sereno
358
1
\
*.
*.
\a
.
65°C
2.
1OOD J
0.0
0.5
1.0
1.5
2.0
2.5
Moisture Content (kglkg db)
Fig. 1. Apparent density vs moisture content.
TABLE 1
Parameters Calculated for Lozano Equation [eqn ( 1 )]
C,
C2
C,
C,
30°C
50°C
65°C
1148
- 27.1
65.6
3.43
1141
- 24.2
159
3.37
1167
-40.1
247
3.22
1.2
Average relative
deviation (%)
1.3
of water, are well correlated by the GAB model (Anderson,
Boer, 1953; Guggenheim, 1966):
XmCkv
x=(l
- ka,)(l-
ka,+ Cku,)
0.75
1946; de
(2)
The calculated parameters are presented in Table 2.
In the same Fig. 2 two adsorption isotherms obtained by Lafuente and
Pifiaga ( 1966) and Pixton and Henderson ( 1979) are also included.
Properties of peas during warm air drying
359
0.8
0.6
E
.
30%
.
50°C
.
65%
-
GAB equation(eqn2)
- -- - --
-----
0.0
adsorptlon, 250~ (btuente
and Pifiaga, 1966)
adsorption, 25°C (Pidon
and Anderson, 1979)
J
1
0.70
0.75
0.80
0.85
0.90
0.95
1.oo
Water Activity
Fig. 2. Sorption isotherm of peas at 25°C.
TABLE 2
Parameters Calculated for GAB Equation [eqn (2)]
&
0.0516
1.41
0.954
C
k
Average relative
deviation (%)
5.2
Prediction of drying curves
Prediction of drying curves was made using the model proposed by
Sereno and Medeiros ( 1990). This model assumes the combined modification of apparent density of dry solid matrix and water diffusivity, with
moisture content of the solid.
,osD =psoDo = constant
(3)
For the computer simulation, values of Tables 1 and 2 were used
together with the operational parameters included in Table 3. The experimental data (Fig. 3) was adequately described by the model being
360
G.L Medeiros, A. M Serene
TABLE 3
Values for the Parameters Used for the Simulation of the Drying Curves
O-0049
2.26
20
0,315 (Qashou et&., 1972)
3.3 (Polley et al., 1980)
2.2
71.2
R,,
&
TI
K
5
L’,
11
2.5
1
0
2
Time (h
Fig. 3. Drying curves d peas. Experimental
.
30%
.
50%
.
65%
3
4
)
values and simulation results.
tested. The best agreement of the predicted curves to the experimental
data was obtained for the values of water diffusivity shown in Table 4.
The average relative difference between the experimental and predicted
values is 6.1%.
Water diffusivity
The method of slopes (Perry & Chilton, 1973; Marousis et al., 1989;
Karathanos et al., 1990) was used to calculate water diffusivity as a
function of moisture content for the peas dried at 30,50 and 65°C. This
method is based on the solution of Fick’s equation for idealized initial
361
Properties of peas during warm air drying
TABLE 4
Absolute Humidity and ,oSD Values Used for each Temperature
(2,
4
(kg/kg :I
basis)
(m’/S)
30
50
0.006 5
0.0070
1.03.10-1”
153.10-1”
3.1.10- 10
4.6.10-10
65
0.0075
2.18.10-1”
6.6. lo- I(’
p, - ratio of dry matter to apparent total volume [ = ,o/( If X)].
and boundary
icles:
conditions
3
o-
as given by Crank (1975).
e
=-$_ 3 exp( n
For spherical part-
nZn2Fo)
1
According to that method effective moisture diffusivity can be estimated by comparing the slopes of the theoretical (eqn (4)) and experimental drying curves (Fig. 3), at each moisture content:
D=
R2
dX
dFo
l-1
(5)
iheo
The slopes of the drying curves
of Fig. 3 were calculated using a
.
smoothing programme based in a running-spline algorithm (Christian &
Tucker, 1984).
Plots of effective moisture diffusivity vs moisture content for the three
temperatures studied are presented in Fig. 4. In the same figure, the
dotted lines represent the values of moisture diffusivities implicit in the
drying model used, showing a reasonable agreement during most of the
final part of the drying process.
CONCLUSIONS
The results obtained indicate that at low moisture content, apparent
density seems to increase with drying air temperature. Values of parameters for the Lozano et al. (1983) model were calculated. Drying temperature has not shown any significant influence on the desorption
G. L Medeiros, A. M Sereno
362
7,
1
P
1 5 -r
?= 4 --
0.0
a
30°C
.
65%
.’
-
smooth approximation
-----
eq” 3
0.5
1.0
/’
1.5
2.0
2.5
Moisture Content (kglkg db)
Fig. 4. Effective diffusivity vs moisture content.
isotherm at 25°C. The experimental isotherm lies well within the range of
previous works. Values of the parameters for the GAB equation were
calculated.
A good agreement was obtained between experimental drying data
and the curves predicted by the model of Sereno and Medeiros (1990).
The average relative deviation encountered was 6.1%. The experiments
allowed the estimation of water diffusivities in the peas during drying for
different values of average moisture content. These values, calculated by
the method of slopes, proved to be consistent with the estimate implied
in Sereno and Medeiros’ drying model.
ACKNOWLEDGMENTS
The authors acknowledge the financial support of JNICT - Junta
National de Investigacao Cientifica e Tecnologica, and of NATO’s
Scientific Affairs Division through project NATO-SfS-PO-PORTOFOOD.
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Properties of peas during warm air drying
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