Thin Layer Drying Process of Some Leafy Vegetables under
Open Sun
O.P. Sobukola,1,* O.U. Dairo,2 L.O. Sanni,3 A.V. Odunewu1 and B.O. Fafiolu1
1
Department of Food Science and Technology, University of Agriculture, P.M.B. 2240, Abeokuta, Nigeria
2
Department of Agricultural Engineering, University of Agriculture, P.M.B 2240, Abeokuta, Nigeria
3
International Institute of Tropical Agriculture (IITA) Ibadan, Nigeria (Present Address)
Open sun drying experiments in thin layers of crain-crain (CC), fever (FV) and bitter (BT) leaves grown
in Abeokuta, Nigeria were conducted. The drying process took place in the falling rate period and no
constant rate period was observed from the drying curves. Eight thin layer mathematical drying models
were compared using the multiple determination coefficients (R2), reduced chi-square (2) and root
mean square error (RMSE) between the observed and predicted moisture ratios. Accordingly, Midilli et
al. model satisfactorily described the drying curves of the three leaves with R2 of 0.9980, 2 of 2.0 104
and RMSE of 1.09 102 for CC leaves; R2 of 0.9999, 2 of 2 106 and RMSE of 1.11 103 for FV
leaves; and R2 of 0.9998, 2 of 1.9 105 and RMSE of 3.3 103 for BT leaves. The effective diffusivity
was found to be 52.91 1010, 48.72 1010 and 43.42 1010 m2/s for CC, BT and FV leaves, respectively.
Key Words: leafy vegetables, open sun drying, thin layer models, diffusivity
INTRODUCTION
Bitter leaves (Vernonia anyadalina), crain-crain
leaves (Corchorus olitorus), and fever leaves (Ocimum
viride) are important green leafy vegetables widely
consumed in Nigeria and some parts of West Africa.
Alongside their culinary usefulness, bitter and fever
leaves are being used for medicinal purposes because
they possess anti-hypertensive properties (Fayemi,
1999). These leaves are also a source of essential amino
acids and are rich in vitamins A, B1, B2 and C and minerals. Leafy vegetables are rapidly perishable commodities, and they start deteriorating immediately
within a day after harvest, hence, they have to be
processed to extend their shelf-life for off-season use.
Drying of fruits and vegetables is one of the oldest
procedures for food preservation known to man and is
*To whom correspondence should be sent
(e-mail: [email protected]).
Received 6 March 2005; revised 3 June 2006.
Food Sci Tech Int
2007; 13(1):35–40
© 2007 SAGE Publications
ISSN: 1082-0132
DOI: 10.1177/1082013207075953
the most important process for preserving food since it
has a great effect on the quality of the dried products.
The main objective in drying agricultural products is
the reduction of the moisture content to a level which
allows safe storage over an extended period. Also, it
brings about substantial reduction in weight and
volume, minimising packaging, storage and transportation costs (Okos et al., 1992). Sun drying is the conventional method used to obtain dried leafy vegetables,
requiring only low capital, simple equipment and low
energy input. Open sun drying is a well known food
preservation technique that reduces the moisture
content of agricultural products and thereby prevents
deterioration within a period of time regarded as the
safe storage period (Sacilik et al., 2006). In spite of its
many disadvantages, sun drying is still practised in
many places throughout the world such as tropical and
sub-tropical countries. Solar energy is an important
alternative source of energy and preferred to other
energy sources because it is abundant, inexhaustible
and non-pollutant. Also, it is renewable, cheap, and
environmentally friendly (Basunia and Abe, 2001).
Vegetable yields usually shoot up following the rainy
season, thus, forcing prices to dramatically fall, hence,
drying these leafy vegetables at this time of plenty will
help to reduce price fluctuations while increasing
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36
O.P. SOBUKOLA ET AL.
income for the producers. Fresh produce losses that
can at times be as high as 70% can also be avoided
(Waithaka, 1992).
Thin layer drying (TLD) means to dry as one layer
of sample particle or slice (Akpinar, 2006). It normally
forms the basis of understanding the drying characteristics of food materials since every material is unique
(Mwithiga and Olwal, 2005). Drying of moist materials
is a complicated process involving simultaneous,
coupled heat and mass transfer phenomena, which
occur inside the material being dried (Yilbas et al.,
2003). TLD has been used to estimate drying times of
several products and to generalise drying curves. In the
development of TLD models for agricultural products,
generally moisture content of the material at anytime
after it has been subjected to a constant relative
humidity and temperature is measured and correlated
to the drying parameters (Midilli et al., 2002; Togrul
and Pehlivan, 2004).
There have been several studies on the mathematical modelling and experimental studies on the
TLD processes of various vegetables, fruits and agro
based products such as bay leaves (Demir et al., 2004),
laurel leaves (Yagcioglu et al., 1999), green pepper,
green bean and squash (Yaldiz and Ertekin, 2001),
apricot (Togrul and Pehlivan, 2003), egg plant (Ertekin
and Yaldiz, 2004), carrot (Doymaz, 2004a) and fig
(Doymaz, 2005). However, information about the sun
drying kinetics of CC, BT and FV leaves are not available in the literature. Therefore, the main objectives of
this work were: to study the drying kinetics of FV, BT
and CC leaves under open sun; to fit the drying curves
with eight thin layer mathematical models available in
literature; and to determine the diffusivity coefficients
of FV, BT and CC leaves.
dried samples were packed in low-density polyethylene
(LDPE) bags and sealed thermally to prevent moisture
adsorption. The experiments were repeated three
times to obtain accurate results thus average values
were used. Moisture content of the samples was
obtained using oven drying methods (AOAC, 1990)
while the ambient air temperature was measured using
a digital infrared thermometer (model KM814).
Mathematical Modelling
Moisture ratio (MR) was calculated using
MR Mt/Mo as stated by several researchers (Midilli
and Kucuk, 2003; Togrul and Pehlivan, 2003; Yaldiz
and Ertekin, 2001) due to the continuous fluctuation of
the relative humidity of the drying air during the open
sun drying process.
Eight TLD models in Table 1 were tested to select
the best model for describing the drying curve equation of the leaves during open sun drying. DataFit software, version 6.1 (Oakdale engineering, 1999) was
used to fit experimental data. The best equation
describing the curve for the leaves was selected using
R2 as the primary criterion. The 2 as the mean square
of the deviations between the experimental and calculated values for the models and RMSE analysis were
used to determine the goodness of fit (Akpinar et al.,
2003a; Akpinar et al., 2003b; Akpinar et al., 2003c;
Demir et al., 2004; Midilli and Kucuk, 2003; Yaldiz and
Ertekin, 2001).
They were calculated as:
l
n
(MRi MRpre,i) · (MRi MRexp,i)
i1
i1
R2 (1)
n
n
2
2
(MRi MRpre,i) · (MRi MRexp,i)
i1
MATERIALS AND METHODS
The sun drying process was carried out in October
2005 in Abeokuta, Nigeria. Fresh samples of CC, BT
and FV leaves were obtained from Kuto market in
Abeokuta. The leaves were separated from the stalk
manually and then rinsed with clean water. The
samples were distributed uniformly in a single layer on
a sample tray and then exposed immediately in the sun
after weighing them to an accuracy of 0.001 g (Mettler
model AE 240). The leaves placed in three different
trays were sun exposed at the same time. Each experiment started at about 08:00 and then continued until
20:00. Moisture losses in the leaves were monitored by
determining the weight changes at 3-h intervals during
drying. No measurement was made after 12 h. The
drying process was continued until the samples reached
the desired moisture level (11–13%, wet basis). The
(MR
MRpre,i)2
Nn
exp,i
2
Experimental Study
i1
n
i1
n
(2)
1
RMSE (MRpre,i MRexp,i)2
N i1
1/2
.
(3)
RESULTS AND DISCUSSION
Figure 1 shows the variation of air temperature
during open sun drying of CC, BT and FV leaves on a
typical day in October 2005 at Abeokuta, Nigeria.
During the drying experiments, the daily mean values
of ambient air temperature ranged between 26.0 to
43.7 0.1°C. The highest ambient air temperature was
reached between 13.00 and 15.00 hours.
Moisture ratio decreased continuously with drying
time in all leaves (Figure 2), hence, no constant rate
period was observed. The whole drying process was
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Thin Layer Drying of Some Leafy Vegetables
Figure 1. Variation of ambient temperature during
open sun drying of leafy vegetables in October 2005
at Abeokuta, Nigeria.
observed to have taken place in the falling rate period
starting with the initial moisture content (81 0.5% for
CC; 82 0.5% for FV, and 83 0.6% for BT, wet
basis) to final moisture contents (11 0.5% for CC;
12 0.8% for FV; and 13 0.1% for BT, wet basis).
However, in the falling rate period, the material
surface is no longer saturated with water, thus, drying
rate is controlled by diffusion of moisture from the
interior of materials to the surface. This observation
has been made earlier (Doymaz, 2005; Lahsasni S. et
al., 2004; Togrul and Pehlivan, 2004). Since the migration to surface of moisture evaporation rate from
surface to air decreased by diminishing the moisture in
the products, the drying rate clearly decreases. This
may be due to the lower heat transfer potential
between ambient air and the leafy vegetables, that
does not favour the evaporation of water from the
leaves. This can be attributed to a drop in temperature
of ambient air after reaching a peak of about 43.7 °C
(Figure 1). During the falling rate period, the drying
rate decreases continuously with decreasing moisture
content and increasing drying time (Karathanos and
Belessiotis, 1997; Yaldiz et al., 2001; and Lahsasni et
al., 2004).
The rate of drying in BT leaves was slower than in
the other leaves (Figure 3) likely because of the
smaller surface area which allows for less water to be
removed per time compared to the other two leaves.
The broad nature of the BT leaves creates a smaller
surface area for moisture removal. Consequently, a
longer period of drying was observed compared with
CC and FV leaves.
37
Figure 2. Comparison of experimental and predicted
moisture ratios of leafy vegetables versus drying time.
() Experimental CC, () experimental FV, () experimental BT, (—) predicted values.
Figure 3. Drying rate of leafy vegetables versus
drying time. () CC leaf, () FV leaf, () BT leaf.
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38
O.P. SOBUKOLA ET AL.
1.09 102 and 7.13 102, respectively. It was
observed that the Midilli et al. (2002), page and logarithmic models had the highest values of R2 for all the
leafy vegetables. However, the results also revealed
that the RMSE and 2 values of the Midilli et al. and
logarithmic models were lower than the values
obtained by the page model for BT and FV leaves.
Hence, both the Midilli et al. and logarithmic models
could be selected to represent the TLD behaviour of
Drying Curves Modelling
The drying data as the MR versus the drying time
were fitted to eight TLD models evaluated by previous
researchers (Table 1). The TLD model constants and
goodness of fit coefficients for CC, FV and BT leaves
are represented in Tables 2, 3 and 4, respectively. For
CC leaves, R2, 2 and RMSE values were between
0.91544 and 0.99801; 2.0 104 and 26.30 102; and
Table 1. TLD curve models for the variation of MR with time, t, for leafy vegetables.
Model Number
Mode Name
Model Equation
References
1
2
3
4
5
6
7
8
Page
Newton
Henderson and Pabis
Two-term
Wang and Singh
Midilli et al.
Logarithmic
Approximation of Diffusion
MR exp(ktn)
MR exp(kt)
MR a exp(kt)
MR a exp(gt) b exp(kt)
MR 1 at bt2
MR a exp(ktn) bt
MR a exp(kt) c
MR a exp(kt) (1 a) exp(kbt)
Diamante and Munro (1993)
Mujumdar and Menon (1995)
Zhang and Litchfield (1991)
Sharaf-Eldeen et al. (1980)
Wang and Singh (1978)
Midilli et al. (2002)
Yagcioglu et al. (1999)
Yaldiz and Ertekin (2001)
Table 2. Estimated parameters and comparison criteria of MR for open sun drying of CC leaves.
Model Number
Model Coefficients Constants
R2
RMSE
2
1
2
3
5
6
7
8
k 0.80410; n 0.46843
k 0.37258
a 0.98574; k 0.36768
a 0.22544; b 0.01274
a 0.99998; b 0.00705; k 0.94111; n 0.28403
a 0.90348; c 0.09471; k 0.54565
a 2217.56379; b 0.99999; k 0.37262
0.99746
0.96944
0.96977
0.91544
0.99801
0.99265
0.96944
0.01230
0.04290
0.04260
0.07130
0.01090
0.02100
0.04290
0.000190
0.002040
0.002270
0.006360
0.000200
0.000632
0.002630
Table 3. Estimated parameters and comparison criteria of MR for open sun drying of FV leaves.
Model Number
Model Coefficients Constants
R2
RMSE
2
1
2
3
4
5
6
7
8
k 1.02219; n 0.33743
k 0.40401
a 0.98864; k 0.39977
a 1.02928; b 0.04065; g 0.39977; k 0.39977
a 0.23659; b 0.01397
a 1.00001; b 0.00689; k 0.78469; n 0.61207
a 0.88785; c 0.11204; k 0.65251
a 3.22947; b 1.00000 k 0.40333
0.99887
0.95959
0.95981
0.95981
0.91267
0.99997
0.99993
0.95959
0.00816
0.04885
0.04872
0.04885
0.46360
0.00111
0.00204
0.00204
0.000080
0.002600
0.002970
0.003970
0.268700
0.000002
0.000006
0.007370
Table 4. Estimated parameters and comparison criteria of MR for open sun drying of BT leaves.
Model Number
Model Coefficients Constants
R2
RMSE
2
1
2
3
4
5
6
7
8
k 0.84782; n 0.47501
k 0.41418
a 0.99367; k 0.41204
a 1.03507; b 0.04139; g 0.41201; k 0.41204
a 0.24036; b 0.01414
a 1.00003; b 0.00779; k 0.49683; n 0.98443
a 0.91712; c 0.08333; k 0.56871
a 1.00196; b 0.99999; k 0.415325
0.99578
0.97867
0.97874
0.97874
0.93622
0.99982
0.99945
0.97867
0.01615
0.03633
0.03628
0.03628
0.06284
0.00331
0.00581
0.03633
0.000326
0.001467
0.001645
0.002193
0.004935
0.000019
0.000048
0.001886
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Thin Layer Drying of Some Leafy Vegetables
BT and FV leaves under open sun drying. But, the
RMSE value of the Midilli et al. model was slightly
lower than that of the logarithmic model and thus was
selected to represent the TLD behaviour of BT and FV
leaves under open sun according to the highest value of
R2 and the lowest values of RMSE and 2.
For CC leaves, the Midilli et al. page models may be
selected to represent the TLD behaviour under open
sun. However, due to its slightly lower values of RMSE
and 2, the Midilli et al. model was selected as a model
describing the TLD behaviour of CC leaves under
open sun. However, for CC leaves, the two terms
model (model number 4) could not describe the drying
curve since the model coefficients and constants, R2, 2
and RMSE were observed to be zero. The accuracy of
the established model was evaluated by comparing the
experimental and predicted moisture ratio values for
CC, BT and FV leafy vegetables (Figure 4). The closeness of the plotted data to the straight line representing
equality between the experimental and predicted
values illustrates the suitability of the Midilli et al.
model for describing the drying behaviour of CC, BT
and FV leaves.
Effective Diffusivity Determination
Fick’s second model can be used to describe the
drying behaviour of the leafy vegetables. The analytical solution of one-dimensional Fick’s law of diffusion
with the assumptions of moisture migration being by
diffusion, negligible shrinkage and constant diffusion
coefficients for slab like materials was as follows
(Crank, 1975).
Mt
8 1
(2n 1)22Defft
. (4)
MR 2 2 exp Mo n1 (2n 1)
4H2
For long drying periods, Equation (4) can be simpli-
39
fied to only the first term (n 1) with a small error
(Doymaz, 2005; Riva and Peri, 1986):
8
2Defft
MR ln 2 4H2
(5)
where MR, Mt, Mo are the moisture ratio, moisture
content at time, t and the initial moisture content,
respectively, H is the half thickness of the samples and
Deff is the effective diffusivity in m2/s.
The diffusion coefficients are typically calculated by
plotting experimental drying data in terms of ln(MR)
versus drying time. A plot of ln(MR) versus drying
time gives a straight line with a slope of
2Deff
Slope .
4H2
(6)
During the open sun drying process of CC, BT, and
FV leafy vegetables, the effective diffusivity was
52.91 1010, 48.72 1010 and 43.42 1010 m2/s,
respectively. These values are within the general
range of 109 to 1011 m2/s for food materials and agricultural crops (Madamba et al., 1996; Doulia et al.,
2000). Similar results have been reported for bay
leaves and grasses (Demir et al., 2004), laurel leaves
(Yagcioglu et al., 1999), kale leaves (Mwithiga and
Olwal, 2005) and mint leaves (Doymaz, 2006).
However, the differences in values for Deff of the
leaves may be attributed to the differences in nature
and structure of the materials.
NOMENCLATURE
Deff
H
MR
MRexp
MRpre
Mt
Mo
R2
t
T
effective diffusivity (m2/s)
half thickness of sample (mm)
moisture ratio
experimental moisture ratio
predicted moisture ratio
moisture content at time t (kg water/kg dry
matter)
initial moisture content (kg water/kg dry
matter)
coefficient of determination
drying time (h)
temperature (°C)
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