A Modeling Study for Moisture Diffusivities and Moisture Transfer Coefficients in Drying of “ Violet de Galmi ” Onion Drying

Abstract

In the present work, the mass transfer characteristics, namely moisture diffusivity and moisture transfer coefficient of “Violet de Galmi” variety of onions were evaluated using the analytical model. Onions were dried in a single layer at different temperatures (40 ̊C, 50 ̊C, 60 ̊C, and 70 ̊C) and for a relative humidity of drying air of 20%. The results showed a reasonably good agreement between the values predicted by the correlation and the experimental observations. This model computed the Biot number, effective moisture diffusivity, and mass transfer coefficient. Effective diffusion coefficient values are obtained between 0.2578 × 10−9 m2·s−1 and 0.5460 × 10−9 m 2·s−1. Mass transfer coefficients of “Violet de Galmi” onion drying vary between 3.37 × 10−7 m·s−1 and 13.38 × 10−7 m·s−1. Numbers of mass transfer Biot are found between 0.9797 and 2.9397. The activation energy Ea is 31.73 kJ·mol −1

Full text

Advances in Chemical Engineering and Science, 2022, 12, 172-196
https://www.scirp.org/journal/aces
ISSN Online: 2160-0406
ISSN Print: 2160-0392
DOI:
10.4236/aces.2022.123013 Jul. 28, 2022 172 Advances in Chemical Engineering and Science
A Modeling Study for Moisture Diffusivities and
Moisture Transfer Coefficients in Drying of
“Violet de Galmi” Onion Drying
Aboubakar Compaoré1,2*, Samuel Ouoba1*, Kondia Honoré Ouoba3*, Merlin Simo-Tagne2,
Yann Rogaume2, Clément Ahouannou4, Alfa Oumar Dissa1, Antoine Béré1, Jean Koulidiati1
1Laboratoire de Physique et de Chimie de l’Environnement (LPCE), Ecole Doctorale Sciences et Technologie (ED-ST), Université
Joseph KI-ZERBO, Ouagadougou, Burkina Faso
2Laboratoire d’Etudes et de Recherche sur le Matériau Bois (LERMAB), Nancy-Université, ENSTIB, Epinal, France
3Laboratoire des matériaux et Environnement (LA.M.E.), Ecole Doctorale Sciences et Technologie (ED-ST), Université Joseph
KI-ZERBO, Ouagadougou, Burkina Faso
4Laboratory of Energetics and AppliedMechanics (LEMA), Polytechnic College of Abomey-Calavi, Abomey-Calavi University,
Cotonou, Bénin
Abstract
In the present work, the mass transfer characteristics, namely moisture diffu-
sivity and moisture transfer coefficient of “
Violet de Galmi
”
variety of onions
were evaluated using the analytical model. Onions were dried in a single layer
at different temperatures (40˚C, 50˚C, 60˚C, and 70˚C) and for a relative hu-
midity of drying air of 20%. The results showed a reasonably good agreement
between the values predicted by the correlation and the experimental obser-
vations. This model computed the Biot number, effective moisture diffusivit
y,
and mass transfer coefficient. Effective diffusion coefficient values are ob
tained
between 0.2578 × 10−9 m2·s−1 and 0.5460 × 10−9 m2·s−1. Mass transfer coeffi-
cients of “
Violet de Galmi
” onion drying vary between 3.37 × 10−7 m·s−1 and
13.38 × 10−7 m·s−1. Numbers of mass transfer Biot are found between 0.9797
and 2.9397. The activation energy
Ea
is 31.73 kJ·mol−1.
Keywords
“
Violet de Galmi
”
Onion, Diffusion Coefficient, Drying Coefficient,
Lag Factor
1. Introduction
Onion is one of the most important crops and one of common consumer prod-
How to cite this paper:
Compaoré, A.,
Ouoba, S
., Ouoba, K.H., Simo-Tagne, M.,
Rogaume, Y
., Ahouannou, C., Dissa, A.O.,
Béré, A
. and Koulidiati, J. (2022) A Model-
ing Study for Moisture Diffusivities and
Moisture Transfer Coefficients in Drying of
“
Violet de Galmi
” Onion Drying.
Advances
in Chemical Engineering and
Science
,
12,
172
-196.
https://doi.org/10.4236/
aces.2022.123013
Received:
June 27, 2022
Accepted:
July 25, 2022
Published:
July 28, 2022
Copyright © 20
22 by author(s) and
Scientific
Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution International
License
(CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access

A. Compaoré et al.
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10.4236/aces.2022.123013 173 Advances in Chemical Engineering and Science
ucts in World. In 2012, world production was estimated to 82,851,732 tons of
onion. Among these global data, production in West Africa was estimated at
2,131,600 tons [1]. Local variety of improved onion “
Violet de Galmi
”
consti-
tuted 75% of the production in this African region because this is an insect and
disease resistant and high-yielding cultivar [2]. “
Violet de Galmi
” variety was in
an elliptical form with purple coloration. This round bulb consisted of external
dark-violet color flakes and light purple trend at the level of internal flakes. Onions
“
Violet de Galmi
” are a seasonal vegetable being harvested from the market gar-
den during February-April in West Africa. Fresh onions, having relatively high
moisture contents, are very sensitive to microbial spoilage during storage, even
under refrigerated conditions. Therefore, within a few weeks following harvest
they must either be consumed or processed into various products. Drying is the
most common form for onion processing during its period of abundance. Onions
can be dried for longer shelf-life by reducing the moisture content to a low level
that prevents microbial spoilage and moisture-related deteriorative reactions.
Since drying is an energy intensive operation, and due to the sharp increase in
energy cost over the last few years, it has become the prime concern of the re-
searchers to find the means of attaining optimum process conditions for good
quality products, which leads to energy savings. Towards this goal, accurate de-
termination of moisture transfer parameters for the drying operation of onion is
essential. Moreover, decreasing the energy consumption in the drying process
will decrease the environment impact in terms of pollutants and hence protect
the environment.
To achieve these goals, in the literature, many experimental and theoretical
studies have been conducted on the determination of onion drying profiles by
many researchers [3]-[13]. Indeed, in a comprehensive review of thin-layer dry-
ing curve models, Kucuk
et al
. [3] evaluated the empirical models of onion dry-
ing considering the pretreatment of product, the drying parameters and drying
methods employed on onion. In this review, Midilli-Kucuk model was found
one of the best models employed for onion drying applications. Mota
et al
. [4]
were obtained three empirical models (Newton, Modified Page and Logarithmic)
for describing relatively well the air convective drying kinetics which was eva-
luated at 30˚C, 50˚C and 60˚C. For predicting moisture distribution in each layer
of onion bulb, a model of multi layers onion drying was investigated in base of
one dimensional partial equation. This model was useful to estimate the drying
time of outer layer to the desired level [13]. In a laboratory scale infrared-con-
vective dryer, the infrared convective drying of onion slices (6 mm thickness)
was observed in a falling rate process with different values of average effective
moisture diffusivity. Its values increased for air constant temperature and air con-
stant velocity as applied radiation intensity was increased. For an air constant
temperature and fixed radiation intensity, its values decreased with increase in
air velocity. The drying time increased with this increase in air velocity because
of the increased cooling effect at the surface of the product [5] [7] [8]. In the two
convective (50˚C, 60˚C and 70˚C) and microwave (328 W, 447 W and 557 W)

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drying techniques of onion slices, the constant activation energies of the convec-
tive or microwave onion drying were evaluated using an exponential expression
based on Arrhenius equation [9]. During the convective, vacuum, and micro-
wave drying techniques of onion slices either dipped into 8% NaCl solution for
40 min or intact, the activation energies of pre-treated onions were lower than
slices contained no salt solution under vacuum and microwave drying condi-
tions. Brine solution had a negative effect on convective drying time but had a
positive effect on both vacuum and microwave drying times [10]. The use of
(60˚C, 70˚C and 80˚C for 1, 3, 5 and 10 min) blanching as a pre-treatment be-
fore drying of onions resulted in enhanced phyto-chemical content and drying
efficiency [11]. For improving this drying efficiency of onion slices
i.e.
the dry-
ing time reduced and effective diffusivity increased, Albitar and al. [12] adopted
the instant controlled pressure drop technique as a blanching-steaming pre-
treatment of fresh onion slices before their drying. So, sensorial and nutritional
attributes of dried onions could be preserved with a perfect decontaminated end
product. Arslan and Özcan [6] carried out experiments of the sun, oven (50˚C
and 70˚C) and microwave oven (210 and 700 W) drying of onion slices to mon-
itor quality degradation of dried onions. The highest values of the K, Ca, Na, Mg
and P mineral were measured in oven dried onions. Sun and microwave oven
drying revealed better color values in the dried onions. The phenolic contents of
microwave oven dried onions were the best. In addition, the thin-layer infrared
radiative and convective drying modeling was carried on different varieties of
onion. In this drying technique, the coefficients of drying models obtained by
fitting of experimental data were expressed according drying conditions of onion
[7] [14] [15] [16]. For determining the moisture transfer parameters, onion slic-
es in thin layer were dried using different drying techniques such as infrared,
convective, microwave and vacuum drying. The different transfer modes of mois-
ture were defined in term of effective moisture coefficient. For quantifying these
moisture transfers, the effective diffusion coefficients for each variety of onion
was obtained by the conventional solution of Fick’s second law of diffusion de-
veloped by Crank [8] [9] [10].
Although there is a large amount of studies available in the literature to de-
termine and calculate the moisture transfer parameters such as moisture diffu-
sivities and drying constants for the onions subjected to drying, no studies have
been carried out to determine moisture diffusivities and moisture transfer coef-
ficients using the drying process parameters in terms of
lag factor
and
drying
coefficient
. This approach developed by [17] [18] is a simple but efficient tool to
determine the moisture diffusivities and moisture transfer coefficients of solid
objects exposed to drying. In this regard, in practice design engineers and work-
ers prefer to use it for design and optimization of the process, which not only
saves their valuable time but also helps in evaluating these parameters in a sim-
ple and accurate way without getting into complicated analysis [19]. In this
model, the transient moisture diffusion process observed in the drying of solid
foods is similar in form to the process of transient heat conduction in these ob-

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jects. The governing Fickian equation is exactly in the form of the Fourier equa-
tion of heat transfer, in which temperature and thermal diffusivity are replaced
by concentration and moisture diffusivity. The mass (moisture) transfer in dry-
ing of solid objects being in the transient form, Biot number and Fourier num-
ber of moisture transfer are defined after establishing a similarity between tran-
sient heat transfer and transient moisture transfer [20]. To show the goodness
and the simplicity of this transient model, in the literature, several studies have
used it to determine/estimate drying process parameters and drying moisture
transfer parameters for wet solids drying. The model was shown to adequately
determine the drying process parameters (e.g., drying coefficient, lag factor, and
half-drying time) and moisture transfer parameters (e.g., moisture diffusivity and
moisture transfer coefficient), and to calculate moisture content for drying spic-
es (e.g. saffron stigmas [21]), fruits (e.g. lemon, bananas, passion fruit and kiwi-
fruit [22] [23] [24] [25] [26]), vegetables (e.g. yam, broccoli, eggplant and potato
[26] [27] [28] [29] [30]) and powders (e.g. lactose powder [31]). Otherwise, the
graphical model for solid slabs has been developed and validated with some illu-
strative examples. Using this graphical methodology determination of the drying
process parameters did not require solution of transcendental equations nor us-
ing an iterative solution technique [32]. An analytical technique of model was
developed and verified by these illustrative examples to determine drying times
of geometrically (slab, cylinder, and sphere) and irregularly shaped multi-di-
mensional objects by use of the geometric shape factors. The shape factors are
employed and based on the reference drying time for infinite slab geometry. The
irregular objects considered were approximated by representative elliptical cy-
linder and ellipsoid for two and three-dimensional shapes, respectively [33] [34].
In an extension of this model, a number of drying correlations, namely Bi-S,
Bi-Re, Bi-G and Bi-Di were proposed to determine the moisture transfer para-
meters for solids drying processes. These correlations were found to adequately
predict the moisture distribution profiles of some illustrative examples in its
works, and were considered to be suitable for use in practical drying applications
[20] [35] [36] [37].
It is in this logical approach that, using the experimental data, the aim of this
study is to determine the moisture diffusivity and moisture transfer coefficients
for “
Violet de Galmi
” onion subjected to convective drying using the analytical
model developed by Dincer and Dost [19]. In addition, the effective moisture
diffusivity as a function of air-drying temperature is also determined.
2. Materials and Methods
2.1. Materials
Fresh onion bulbs of the variety “
Violet de Galmi
” were purchased on stalls of
fruits and vegetables from the central market, Ouagadougou, Burkina Faso. Fresh
onions were sorted, cleaned and peeled by hand from their external layer. These
onions were then stored at 4˚C in a refrigerator prior to the experiments [14].

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After being stabilized at room temperature for 2 h, some homogeneous samples
of onion were sliced 1 - 3 mm thick perpendiculars to the length of the largest
semi-spindle of onion using an electric chain saw (model Secotom-10, Struers
with precision of 1 - 3 mm). We obtained sliced samples from 30 mm to 50 mm
in diameter and 1 - 3 mm thick. The sliced onions were then wrapped by using
aluminum foil in an environment having average temperature of 24˚C to pre-
vent all contact of the sample with ambient humidity. This permitted to avoid
discrepancy in the composition of onion due to evaporation or to prevent the
decay caused by microorganisms [38]. The mass of dry matter was determined
by drying the onions at the end of the experiments in an electric oven at 70˚C for
24 h [39]. Experiments were repeated three times to get a reasonable average.
2.2. Experimental Set-Up
Thin-layer drying experiments were carried out in a wet laboratory dryer with
control electronics Mincon 32 (VC0018, otsch Industrie technik Gmbh, Ger-
man) (Figure 1). Dryer included, among other things, a console, a drying cham-
ber, a cabinet. The relative humidity and the temperature of the drying air were
controlled by the control panel and could be varied from 10% to 96% and from
10˚C to 90˚C. The speed of the air was regulated according to temperature and
relative humidity determined by the panel. The drying chamber with a capacity
of 190 liters was made in polished stainless steel. The maximum load of the
Figure 1. Description of the laboratory Dryer. 1: Drying room, 2: door, 3: command desk,
4: mechanic compartment, 5: support, 6: passage, 7: electric cabinet, 8: electric switch, 9:
connexion, 10: humidity and temperature probe, 11: hood, 12: tank of water.

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products by tray was 30 kg when the dryer operated in an environment of 25˚C.
The fuse of the unit as well as all electrical components and control were inte-
grated in the electrical cabinet. Measurement errors in the temperature accord-
ing to time and to space in the drying chamber were respectively ±0.1 to ±0.5˚K
and ±0.5 to ±1˚K. Measurement errors in the time relative humidity were ±1%
to ±3% (Figure 1).
2.3. Experimental Protocol
A sliced sample was placed on a tray hanging from an electric balance (Model
S4002, DENVER Instrument with precision of 1 - 3 g) which was located at the
top of the dryer (Figure 2). The tray was made of nylon mesh allowing the
crossing of the air on the surfaces of the sample. Balance is positioned in the
center of the circular slot performed above the dryer. Minor vibrations caused by
the driving forces of the fan ensuring the forced convection are eliminated by
pressing tare of the balance before starting each one of the experiment. The ex-
periments are conducted at temperatures of 40˚C, 50˚C, 60˚C and 70˚C and at
20% RH.
For each drying condition, averages of three repetitions were taken as data. At
the end of each experiment, the sample was heated in an oven temperature of
70˚C during 24 h of drying in order to obtain the mass of dry onion skeletal
structure [39]. Before performing experiments, dryer was turned on without a
loading in order to stabilize at the instruction given by the command board.
Heating and cooling rates are 0.6˚C/min and 0.3˚C/min, respectively. The dryer
is therefore run for a sufficient period of time to reach the set point. The acquisi-
tion system is composed of a data logger (NI USB-6210, National Instruments),
a LABview program installed in a computer.
Figure 2. A schematic of the drying system. 1: Onion slices, 2: Balance, 3: Heater, 4: Fan,
5: Tray, 6: Water and electric material, 7: Panel, 8: Data logger.

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2.4. Data Analysis
2.4.1. Mathematical Formulation of Drying
The transient moisture diffusion process observed in drying of solid objects is
similar in form to the process of heat conduction in these objects. The governing
Fickian equation is exactly in the same form of the Fourier equation of heat trans-
fer, in which temperature and thermal diffusivity are replaced with moisture and
moisture diffusivity, respectively, as following [40]:
( )
0 infinite plate
,
1with 1 infinite cylinder
2 sphere
n
eff n
X yt
XDy n
t yy
y
→

∂



∂∂ 
= = →

  

∂ ∂∂

 
→

. (1)
Consider an onion layer as an infinite slab being dried in a medium. Assume
constant thermo-physical properties for the onion and for the drying medium,
and the effect of heat transfer on the moisture loss is negligible. The moisture
diffusion is assumed to occur in the thickness direction in the onion slab. Under
these conditions, the one-dimensional transient moisture diffusion equation in
rectangular coordinate can be written in the following form:
2
2
eff
XX
D
ty

∂∂
=
∂∂

(2)
This equation can be non-dimensionalized by the ratio of water content as
follows
2
2
eff
D
ty

∂∂
=
∂∂

φφ
(3)
0
eq
eq
XX
XX
−
=−
φ
(4)
The equilibrium moisture content of onion was obtained from the Modified
Henderson model of onion sorption on the respective ranges of 30˚C - 50˚C and
15% - 85%. This model was written as [41]:
( )
( )
1
2.48
5
ln 1
1
100 3.6 10 10.87
w
eq
a
XT
−

−
=
−× +


(5)
Equation (3) has the following boundary and initial conditions.
( )
,0 1y=
φ
(6)
( )
0, 0
t
y
∂=
∂
φ
(7)
()()
,, ;0.1 100
eff m i
Lt
D h Lt B
y
∂
− = ≤≤
∂
φφ
(8)
2.4.2. Analytical Solution
The solution of the governing Equation (3) and the conditions in Equations (6)-(8)
with
y
= 0 yields dimensionless center moisture distributions for the correspond-

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ing objects in the following forms [19]:
for 0.1 100 and 100
nn i i
n
AB B B
∞
= << >
∑
φ
(9)
The above solution can be simplified if the values of
2
10
F
µ
> 1:2 are negligi-
bly small. Thus, the infinite sum in Equation (9) is well approximated by the first
term only,
i.e.
[32]:
11
AB≈
φ
(10)
1
0.2533
exp 1.3
i
i
B
AB

=
+

(11)
( )
2
1 10
expBF= −
µ
(12)
with
m
i
eff
hL
BD
=
and
02
eff
D
Ft
L
=
.
Drying profile of onion was defined by the form similar to what has been done
for the cooling profile for an object in transient heat transfer during the least
squares method. This following model was proposed for the drying of onion us-
ing a dimensionless lag factor
G
and coefficient of drying
S
(s−1) [36].
( )
expG St= −
φ
(13)
The coefficient of drying was expressed the drying capacity of the product per
unit of time and lag factor was an indication of internal resistance to mass trans-
fer in the product during the drying process. These settings were useful for the
evaluation and representation of the drying process. The values of the dimen-
sionless moisture content can be found using the experimental moisture content
measurements from Equation (4). The Equation (10) and Equation (13) permit-
ted to put that
G
=
A
1. Thus, the Biot number of moisture transfer for the onion
slices was obtained as following:
1.3ln
0.2533 ln
i
G
BG
=−
(14)
The moisture diffusivity for the onion slices is given by the following equa-
tion:
2
1
eff
L
DS

=

µ
(15)
where the corresponding characteristic root
μ
1
for an infinite slab object is given
as [42]:
( )
1
atan 0.640443 0.380397
i
B= +
µ
(16)
Equation (15) can easily be used to determine the moisture diffusivity values
for the slab onions. The equations determining the moisture transfer coefficients
are given in the following form:
i eff
m
BD
hL
=
(17)

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2.4.3. Activation Energy
The activation energy
Ea
was identified taking into account the variation of the
effective diffusion coefficient of water in respect with the temperature following
the Arrhenius relation [10].
()
3
0
exp 10 273.15
a
eff
E
DD RT

= −


+

(18)
2.4.4. Fitting Parameters
The precision of data modeling was assessed using the following statistical crite-
ria:
● The coefficient of determination (
R
2)
R
2 is used in the context of statistical models whose main purpose is the pre-
diction of future outcomes on the basis of other related information. It is the
proportion of variability in a data set that is accounted for by the statistical mod-
el. It provides a measure of how well future outcomes are likely to be predicted
by the model. The coefficient of determination is not likely to be 0 or 1, but ra-
ther somewhere in between these limits. The closer it is to 1, the greater rela-
tionship exists between experimental and predicted values. This value is used for
the quantative comparison criteria and shows the level of agreement between
measured and predicted values [43]. It was one of the first criteria used to select
the appropriate model to describe the behavior of drying of fruits and vegetables
[4].
( )
( )
2
,,
1
2
2
,
1
1
N
exp i pre i
i
N
exp exp i
i
PP
R
PP
=
=
−
= − −
∑
∑
(19)
● Root Mean Square Error (
RMSE
)
Root-mean-square deviation,
RMSD
, or root-mean-square error,
RMSE
, is a
frequently used measure of the differences between values predicted by a model
or an estimator and the values actually observed from the thing being modeled
or estimated.
RMSD
is a good measure of accuracy and serves to aggregate the
residuals into a single measure of predictive power. It is required to reach zero
and can be calculated as [39]:
( )
12
2
,,
1
N
exp i p re i
i
PP
RMSE N
=

−

=

∑
(20)
● Chi-square reduced (
χ
2)
It is the mean square of the deviations between experimental and predicted
values for the models and used to evaluate the fitting agreement of each model.
The lower the values of
χ
2, the better the goodness of the fit and could be calcu-
lated as follows [43]:
( )
2
,,
1
2
N
exp i pre i
i
Pp
Nz
=
−
=−
∑
χ
(21)

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● Sum of Squared Errors,
SSE
SSE
is the measure of the deviation of a sample from its “theoretical value”.
This parameter is defined as the difference between the experimental and pre-
dicted data and defined as [44]:
()
2
,,
1exp i p r
N
ei
i
SSE P P
=
= −
∑
(22)
where
P
is the setting for drying given,
Pexp
,
I
is the experimental value of the pa-
rameter,
Ppre
,
I
is the value predicted by the parameter
P
,
exp,i
P
, is the average
value of the parameter
P
,
N
is the number of observations and
z
is the number of
constants in each regression. The goodness of fit was found where
R
2 was highest
and lowest values of
RMSE
,
χ
2 and
SSE
were found [39].
3. Results and Discussions
3.1. Drying Kinetics
Figure 3 shows the curves of drying for “
Violet de Galmi
” onions drying at four
temperatures (40˚C, 50˚C; 60˚C and 70˚C) and at relative humidity of 20%. On
Figure 3, all drying kinetics follows a drying falling-rate period. The absence of a
constant drying rate period may be due to the thin layer of product that did not
provide a constant supply of water for an applied period of time. Also, some re-
sistance to water movement may exist due to shrinkage of the product on the
surface, which reduces the drying rate considerably. The moisture content de-
creases gradually while the drying time elapsed, exhibiting a smooth downward
curve. The dominating mechanism of moisture transfer during this period was
moisture diffusion similar to that for most fruits and vegetables as such kiwi-fruit,
Figure 3. Curves of the onion drying in conditions of relative humidity of 20% and tem-
peratures of 40˚C, 50˚C, 60˚C and 70˚C.
Time(h)
0 0.5 1 1.5 2 2.5 3
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Experiment Data
40°C(RH=20%)
50°C(RH=20%)
60°C(RH=20%)
70°C(RH=20%)

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avocado, mango, cassava and banana [45] [46] [47] [48]. The drying times onions
were obtained at 1.5, 2.0, 2.8 and 3.0 h at 70˚C, 60˚C, 50˚C and 40˚C, respec-
tively. The air temperatures have a major effect on the drying kinetics. The air
temperature increase from 40˚C to 70˚C leads to an increase in slope of curve
describing the onion drying kinetics. By reducing the drying time of 40˚C to 70˚C,
represents nearly 50% of the time of drying at air temperature of 40˚C. The dry-
ing air temperature influence water diffusion transport in onion convective dry-
ing when the air temperature increment between the different drying conditions
is by at least 10˚C. The air drying running at higher air temperature and higher
energy power leads to higher temperatures of onion slices implying a larger
driving force for mass transfer. The mass transfer rates were higher in the begin-
ning of the drying processes and gradually reduced through the end of the
process. Thus more energy was absorbed by the water at the onion surface in-
itially, resulting in faster drying and with the onion surface drying out subse-
quently, heat penetration through the dried layer decreased thus retarding the
drying rates. Also the differences in drying rates caused by different air temper-
atures levels are higher at the beginning of drying; while by the end of the
processes all drying kinetics are closer to each other. During the processes to-
wards the end of drying, the influence of temperature on the drying kinetics is
lower than at the beginning and that moisture transport is hindered more and
more by internal mass resistances of onion bulb [28]. Because this temperature
effect on drying, stepwise changes in temperature from one drying period to
another period could be employed so as to avoid a degradation of heat-sensitive
natural compounds (antioxidants, color, vitamins) of onion slices and so to save
much energy of drying. Similar observations were obtained in a review from Sa-
gar and Kumar [49] on the recent advances in the drying and the dehydration of
fruits and vegetables. In literature, our results were generally in agreement with
some previous findings on onion drying. It was indicated in a convective drying
of
yellow
onion under drying air temperature of 30˚C to 60˚C. It was visible how
the moisture (expressed in dry basis) follows an exponential decay, and how the
increase in temperature accelerated this drying process. In this, at 30˚C the dry-
ing stabilized after approximately 7 hours whereas at 60˚C 2 hours were suffi-
cient, representing a very important reduction in the drying time (more than
70%) [4]. Three levels of airflow temperature (40˚C - 50˚C - 60˚C) were applied
to dry onion by a custom designed fluidized bed dryer equipped with a heat
pump dehumidifier, predicting its drying behavior by regression, fuzzy logic and
artificial neural network techniques [50]. Drying temperatures exerted a pro-
nounced effect on the quality changes of sliced onion. The drying temperatures
up 60˚C did not show any influence on quality changes. However, drying tem-
peratures above 60˚C significantly influenced the color of the dried onion [51].
By using oven drying method (50˚C and 70˚C), the onion drying time up to the
moisture content of approximately
Xf
= 0.818 (
dry basis
) could be shortened by
11.76% and 79.41%, when compared to sun drying, respectively. For microwave
oven drying at 210 W and 700 W, the onion drying times to reach the moisture

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content of
Xf
= 0.960 (
dry basis
) were 10.5 min and 8.33 min, respectively. This
was due to the fact that drying at higher temperature and higher energy power
which led to higher temperatures implied a larger driving force for heat transfer
[6]. The effect of drying temperature on drying kinetics of onion (
Allium cepa
L.
)
slices with convective and microwave drying showed that the drying rate in-
creased with increasing in drying air temperature and consequently decreased
the required drying time. It is a fact that the higher temperature difference be-
tween the drying air and onion slices increases the heat transfer coefficient,
which influences the heat and mass transfer rate. Drying of onion slices general-
ly occurred in falling rate period because of quick removal of moisture [52].
3.2. Drying Process Parameters
The experimental data of onion drying curves obtained at air temperatures of
drying (40˚C, 50˚C, 60˚Cand 70˚C) and at relative humidity of 20%, in the form
of moisture content ratio, are fitted to Equation (13). The statistical results of fit-
ting are performed by using the MATLAB.7.12.0 (R2011a) installed on the net-
work of the University of Lorraine (France). Table 1 summarizes the statistics
and drying parameters for modeling convective onion drying at air temperatures
of 40˚C to 70˚C.
Among these established statistical results, the values of
SSE
,
R
2 and
RMSE
obtained for modeling drying have been varied between 0.0288 and 0.0468, be-
tween 0.9824 and 0.9931 and between 0.0235 and 0.0370 respectively. The aver-
age values of
SSE
,
R
2
and
RMSE
are 0.0415, 0.9883 and 0.0301, respectively. This
average value of
R
2 is very close to 1 while the average values of
SSE
and
RMSE
are close to 0. These statistic parameters of model have been satisfied for model-
ing drying of this onion variety. To illustrate the quality of model fitting, Figure
4 shows the experimental data compared to those of the model obtained at dry-
ing temperatures. In this figure, the results showed a reasonably good agreement
between the values predicted from the correlation and the experimental observa-
tions. Therefore, the drying parameters obtained from the model reflect the
physical and thermal behavior of the process of convective drying of the “
Violet
de Galmi
”
onion in thin layers. Due to the fact that drying has an exponentially
decreasing trend, two drying parameters like lag factor (
G
, dimensionless) and
drying coefficient (
S
,
s−1) could be introduced for describing this drying. Drying
Table 1. Drying process and fitting parameters for drying the onion with drying air tem-
perature.
Model 40˚C 50˚C 60˚C 70˚C
G
1.111 1.136 1.150 1.162
S
(s−1) × 104
2.856 3.322 4.794 5.056
SSE
0.0344 0.02883 0.05614 0.04681
R
2 0.9919 0.9931 0.9824 0.9858
RMSE
0.02572 0.02355 0.037 0.03421

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Figure 4. Modeling of onion drying curves at air temperatures of 40˚C, 50˚C, 60˚C and
70˚C.
coefficient shows the drying capability of onion slice and has a direct effect on
the moisture diffusivity. In Table 1, the obtained drying coefficient values,
S
,
were 2.856 × 10−4, 3.322 × 10−4, 4.794 × 10−4 and 5.056 × 10−4 s−1 for drying tem-
peratures of 40˚C, 50˚C, 60˚C and 70˚C, respectively, providing an accurate in-
dication of the drying behavior (Figure 3). This
S
value range was in concor-
dance with that of other food products such as carrot (1.038 × 10−4 s−1 at 50˚C),
prune (0.70 - 3 × 10−4 s−1 at 60˚C), potato (0.70 - 9 × 10−4 s−1 at 40˚C - 80˚C),
starch (6 × 10−4 s−1 at 59˚C), okra (9 × 10−4 s−1 at 40˚C), broccoli (13 - 24 × 10−4
s−1 at 50˚C - 75˚C), passion fruit peel (1.83 - 2.74 × 10−4 s−1 at 50˚C - 70˚C)
(Table 2 and Table 3). Kaya
et al.
[53] reported the drying coefficient for some
regularly shape agricultural products such as slab carrot (0.20 - 0.41 × 10−4 s−1),
slab pumpkin (0.16 - 0.33 × 10−4 s−1) and cylindrical carrot (0.19 - 0.39 × 10−4 s−1)
dried in a convective hot air dryer at temperatures of 30 - 60˚C and constant air
flow rate of 1 m·s−1 [53]. Torki-Harchegani
et al.
[22] were obtained drying coef-
ficient values range 0.027 - 0.24 × 10− 4 s−1 for the whole lemons. The whole lem-
ons were dried in a convective hot air dryer at drying temperatures of 50˚C -
75˚C and a constant air velocity (1 m·s−1) [22]. In infrared thin layer drying of
saffron stigmas, Torki-Harchegani
et al.
[21] found the drying coefficient varied
from 4.75 - 16.11 × 10−4 s−1 at the temperatures from 60˚C to 110˚C [21]. From
the obtained results, the drying coefficient increased with increasing drying air
temperature. This is due to the fact that an increment in drying temperature in-
creases the heat and mass transfer between the heating media and solid object
and consequently, leads to a higher drying capability of the object. Babalis
et al.
[54] reported the same results for convective hot air drying of figs in the temper-
Time(h)
00.5 1 1.5 2 2.5 3
=(X
t
-X
eq
)/(X
0
-X
eq
)
0
0.2
0.4
0.6
0.8
1
1.2
Experience-Model
40°C(RH=20%)
50°C(RH=20%)
60°C(RH=20%)
70°C(RH=20%)
Model

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ature range of 55˚C - 85˚C and the air flow rate in the range of 0.5 - 3 m·s−1 [54].
The lag factor (
G
) is an indicator of the magnitude of both the internal and
external resistance to moisture transfer from the product and has a direct effect
on the moisture transfer coefficient as a function of the Biot number [36] [55].
The values of lag factor (1.111, 1.136, 1.150 and 1.162 for air temperatures of
40˚C, 50˚C, 60˚C and 70˚C, respectively) were more than 1 indicating moisture
diffusion within the samples is controlled by both internal and external resis-
tance. The variation in calculated values, between 1.111 and 1.162, reflects lumped
system specific mass transfer properties. The obtained lag factor values in this
study are consistent with reported results for the foods products (Table 4) such
as prune (1.0016 - 1.0086) [20] [32] [35] [37], potato (1.0074 - 1.255) [20] [35]
[56], broccoli (1.000 - 1.006) [28], passion fruit peel (1.020 - 1.051) [24], eggplant
(1.032 - 1.117) [29], lactose powder (1.086 - 1.370) [31], lemons (1.118 - 1.158)
[22], saffron (1.096 - 1.104) [21] and apples (1.108 - 1.209) [57].
3.3. Mass Transfer Biot Number
The lag factor is an indicator of magnitude of both internal and external re-
sistances of a solid object to the heat and/or moisture transfer during drying
process as a function of the Biot number (Equation (14)). There are three cases
of the Biot number:
Bi
< 0.1, 0.1 <
Bi
< 100 and
Bi
> 100. The case of
Bi
< 0.1 in-
dicates that minor internal resistance and major surface resistance across the
boundary layer (external resistance) to moisture transfer and moisture gradient
inside the product are so small while,
Bi
> 100 means that internal resistance is
much more than external resistance. The case of 0.1 <
Bi
< 100 indicates the ex-
istence of both finite internal and surface resistances and is known as the most
common case for the drying applications [36] [55]. The calculated Biot number
values were in the range of 0.9797 - 2.9397 at drying temperature range 40˚C -
70˚C (Table 2), indicating the presence of the both internal and external resis-
tances to moisture transfer, which is consistent with reported results for foods
on literature (Table 4) such as potato (0.0851 - 1.1280, at 40˚C - 80˚C) [20] [35]
[56], broccoli (0.2299 - 0.3228, at 50 - 75˚C) [28], passion fruit peel (0.1018 -
0.3199, at 50˚C - 70˚C) [24], lemon (0.361 - 2.894, at 50˚C - 75˚C) [22] and lac-
tose powder (0.185 - 1.370, at 20˚C - 40˚C) [31]. The variation in calculated Bi
numbers suggests that it was a function of the product and drying process para-
meters. During onion convective drying, the
Bi
number was found to increase
Table 2. Biot numbers and moisture transfer parameters calculated for onion drying with
air temperature.
Parameters 40˚C 50˚C 60˚C 70˚C
Bi
0.9797 1.4323 1.6003 2.9397
μ
1
0.7893 0.9142 0.9523 1.1547
Deff
× 109 (m2·s−1) 0.2578 0.3975 0.5287 0.5460
hm
× 107 (m·s−1) 3.37 5.69 8.46 13.38

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with increasing temperature indicating internal mass transfer control (Table 2).
The results are in agreement with those reported for the convective drying of
lemon slices [22], broccoli [28], eggplant slices [29] [58], potato [56] [59], pas-
sion fruit peel [24], saffron [21] and banana [60]. McMinn
et al.
[56] found that
the microwave and combined microwave-convective drying of potato slices and
cylinders were controlled by power levers. Thus the Bi number increased with
power level (slab, 0.521 and 0.650 for 90 and 650 W, respectively) [56]. This is
why a number of Biot number correlations for mass transfer, namely Bi-S, Bi-Re,
Bi-G and Bi-Di were proposed according the medium and product parameters
for use in practical drying applications [20] [35] [36] [37].
3.4. Effective Diffusion Coefficient
Analysis of moisture transfer mechanism in the falling rate period, based on the
procedure described by [61], confirmed the diffusive nature of moisture move-
ment. The non-linear pattern of moisture ratio and time on a semi-log plot also
indicated the diffusive nature of moisture transport rather than capillary. It may
be attributed to shrinkage in the product, non-uniform distribution of initial
moisture and temperature coupled with variation of moisture diffusivity with
moisture content [62]. The mass diffusivity in the diffusion model is a funda-
mental property based on molecular interactions and on the physical structure of
the material [63]. All of the various phenomena of mass diffusion are represented
by the effective diffusion coefficient. Using the values of
μ
1 and
S
, effective diffu-
sion coefficient
Deff
of “
Violet de Galmi
” onion drying is calculated from Equa-
tion (15). The calculated values of
Deff
at onion drying conditions are found be-
tween 0.2798 × 10−9 m2·s−1 and 0.8121 × 10−9 m2·s−1 (Table 2). The magnitude
range of these
Deff
values is close to those proposed for air convective drying of
onion obtained by the conventional solution of Fick’s second law of diffusion
developed by Crank (Table 3) such as the range 3.33 - 8.559 × 10−9 m2·s−1, at
30˚C - 60˚C obtained by [4], the range 0.025 - 0.032 × 10−9 m2·s−1, at 30˚C - 60˚C
obtained by [5], the range 0.747 - 1.554 × 10−9 m2·s−1, at 50˚C - 70˚C obtained by
[6], the range 34.9 - 94.4 × 10−9 m2·s−1, at 50˚C - 70˚C obtained by [9], the range
1.96 - 14 × 10−9 m2·s−1, at 50˚C - 70˚C obtained by [10], the range 2.084 - 2.271 ×
10−9 m2·s−1, at 40˚C - 60˚C obtained by [13]. The small difference range of
Deff
for
onion convective drying are due to the diffusivities dependence of the drying
characteristics (air velocities, combination of Infrared, vacuum, microwave dry-
ing techniques with convective drying) and product characteristics (thickness,
pretreatment namely water blanching and blanching-steaming). Dincer and
Hussain [35] explained that the variation of moisture diffusivity data of the foods
was often due by structure complexity of foods [35]. Pathare and Sharma [5]
noted that the diffusivities are negatively correlated with air velocity. The in-
crease in air velocity accelerated the cooling effect, which reduced the tempera-
ture of the product and water vapor pressure [5]. Concerning the temperature
parameter, the temperature level had a considerable influence on the magnitude

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Table 3. The drying process parameters and moisture transfer parameters for drying of food products in literature.
Product
Drying conditions Model coefficients Drying coefficients
Shape
Ta
(˚C)
RH
(%)
νa
(m/s)
CD
× 10
3
(m)
Bi
(−)
S
× 104
(1/s)
G
(−)
Deff
× 108
(m2/s)
hm
× 107
(m·s−1)
Carrot [32] Slab 50 — 2.5 5 0.637 1.038 1.08 0.5189 6.6084
Prune [32] Slab 60 15 3 - 5 5 0.05 0.79 1.0086 3.854 4.0261
Prune [20] Slab 60 — 3.5 2.5 0.4054 3 1.0037 6.6905 108.49
Potato [20] Cylinder 60 — 1.0 13.5 0.3139 0.7 1.032 6.7172 15.618
Potato [20] Sphere 40 — 1.0 9 0.3736 9 1.0074 94.198 391.02
Starch [36] Cylinder 59 — 2.0 5 0.0929 6 1.0181 12.991 2.4137
Yam [36] Sphere 105 11 — 30 47.9471 46 1.2864 151.1 2.4151
Prune [35] Slab 60 — 3.0 7.5 0.0745 0.7 1.0016 19.889 19.756
Potato [35] Cylinder 80 — 1.2 3 0.0851 1 1.1981 0.0567 0.1610
Okra [35] Sphere 40 — 1.0 9 0.3119 9 1.0074 94.259 326.65
Broccoli [28]
Slab 50 — 1.2 10 0.2716 13.7 1.006 357.8 1921.5
Slab 50 — 1.75 10 0.2299 12.8 1.002 601.12 2731.9
Slab 50 — 2.25 10 0.2060 12.3 1.003 488.86 1991.9
Slab 60 — 1.2 10 0.2901 17.5 1.004 597.47 3517.2
Slab 60 — 1.75 10 0.2595 17.6 1.002 826.54 4232.8
Slab 60 — 2.25 10 0.2392 16.8 1.002 788.97 3613.4
Slab 75 — 1.2 10 0.3228 18.9 1.001 1067.6 6469.1
Slab 75 — 1.75 10 0.3000 22.7 1.001 1282.3 7224.3
Slab 75 — 2.25 10 0.2629 24.0 1.000 1666.7 8725.6
Passion fruit
peel [24]
Slab 50 — 2.0 6.7 0.1447 1.83 1.026 1.049 4.530
Slab 60 — 2.0 6.7 0.1103 2.25 1.020 1.403 4.619
Slab 70 — 2.0 6.7 0.1018 3.12 1.019 1.994 6.062
Slab 50 — 3.5 6.7 0.3199 1.58 1.051 0.632 6.039
Slab 60 — 3.5 6.7 0.2253 2.20 1.038 1.057 7.111
Slab 70 — 3.5 6.7 0.2178 2.74 1.037 1.339 8.702
—: not measured, CD: characteristic dimensions.
of the diffusivity during convective drying (0.2798 × 10−9 m2·s−1 for 40˚C and
0.8121 × 10−9 m2·s−1 for 70˚C, respectively). Such results from this analysis are in
accordance with the experimental observations (Figure 3). They are also similar
with the results for onion convective drying reported by Mota
et al.
[4], by De-
miray
et al.
[9], by Asiah and Djaeni [13] and with the results for the food con-
vective drying such as potato [56], lemon [22], spirulina [39], blackwood passion
fruit [24] and banana [64]. Torki-Harchegani
et al.
[21] obtained in saffron dry-
ing that any increment in the drying temperature leaded to an increment in the

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diffusivity. In fact, an increase in temperature caused a decrease in water viscos-
ity and increased the activity of water molecules. These phenomena facilitated
diffusion of water molecules in object capillaries and consequently, increased the
moisture diffusivity [21]. Sharma
et al.
[8] reported that the moisture diffusivity
of food material was affected by its moisture content, temperature as well as its
composition and porosity. This may indicate that as the moisture content de-
creased, the permeability to water vapor increased as it provided more pore
structure open. The temperature of the product would have risen rapidly in the
initial stages of drying due to more absorption of convective heat. This increased
the water vapour pressure inside the pores resulted into pressure induced open-
ing of the pores. Furthermore, Süfer
et al.
[10] concluded in their study that slice
thickness and pretreatment application affected Deff adversely, whereas temper-
ature increase enhanced the moisture diffusivities at high temperatures, any
augmentation in energy of heating causes an increase in the molecular activity of
water, thus generating higher diffusivity. In addition, high salt concentrations in
vegetable cells may reduce or prevent moisture removal from food system [10]
[65].
3.5. Mass Transfer Coefficient
Knowing both the moisture diffusivities and mass transfer coefficients for the
various systems is essential, as more complex mathematical models and correla-
tions which can provide a more in depth understanding of the drying operations
require data on specific mass transfer parameters. This is an important drying
parameter that depends on mass diffusivity, viscosity, velocity of the fluid, and
geometry of the transfer system [63]. The mass transfer coefficient values of “
Vio-
let de Galmi
” onion drying in thin layers are obtained from Equation (17) using
the Biot number and effective moisture diffusivities. Mass transfer coefficients
hm are presented in Table 2. The values of hm obtained for drying conditions of
“
Violet de Galmi
” onion slice vary between 3.37 × 10−7 m·s−1 and 13.38 × 10−7
m·s−1. These results were found in the range (0.16 - 8725.60 × 10−7 m·s−1) of those
available in the existing literature for different foods and drying conditions
(Table 4), such as the convective drying of potato (0.16 - 391.02 × 10−7 m·s−1, at
40˚C - 80˚C) [20] [35] [56], broccoli (1921.5 - 8725.6 × 10−7 m·s−1, at 50˚C -
75˚C) [28], passion fruit peel (4.530 - 8.702 × 10−7 m·s−1, at 50˚C - 70˚C) [24],
lemon (0.163 - 9.008 × 10−7 m·s−1, at 50˚C - 75˚C) [22], saffron (2.6433 - 8.7203 ×
10−7 m·s−1, at 60˚C - 110˚C) [21] and eggplant (6.478 - 2.190 × 10−7 m·s−1, at 50˚C
- 70˚C) [29]. Ours results were also in accordance with those from Markowski
[66] who determinates an average mass transfer coefficient value of 1.371 × 10−7
m·s−1 during drying of fresh carrot slices, those by Elbert
et al.
[67] with a value
of 4.81 × 10−7 m·s−1 during parboiled rice drying, those by Tsami and Katsioti
[68] with 4.026 × 10−7 m·s−1 during drying of prune slices, and those by Ruiz-
Cabrera
et al.
[69] with 6.608 × 10−7 m·s−1 during carrot drying. Examination of
the relative magnitude of the mass transfer coefficient for onion showed a variation

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Table 4. The moisture diffusivities obtained from the conventional solution of Fick’s second law of diffusion and the activation
energy for onion drying.
Thickness
(mm)
Deff
× 109
(m2/s)
Ea
(kJ·mol−1 or W·g−1)
Drying Conditions Ref
3 - 4 3.33 - 8.55 26.4
Convective (
Ta
= 30˚C - 60˚C,
Va
= 0.35 m/s) [4]
6 ± 0.01 0.025 - 0.032 5.06 - 10.63
Infrared Convective (
IR
= 26.5 - 44.2 kW/m2,
Ta
= 30˚C -
60˚C,
Va
= 0 - 1.5 m/s) [5]
6 ± 0.1 0.021 - 0.157
Infrared radiation (
IR
= 0.3 - 0.5 kW,
Ta
= 35˚C - 45˚C,
V
a
= 1 - 1.5 m/s) [7] [8]
10 ± 0.1
0.834
Sun (
-)
[6] 0.747 - 1.554
Air oven (
Ta
= 50˚C - 70˚C)
40 - 48.69
Microwave oven (210
- 700 W)
7 ± 1 34.9 - 94.4 45.6
Convective (
Ta
= 50˚C - 70˚C) [9]
259 - 508 7.897 - 5.461
Microwave (
IR
= 328 - 557 W)
3 - 7
1.96 - 14 3.28 - 34.13
Convective (
Ta
= 50˚C - 70˚C,
Va
= 0.5 m/s)
[10] 9.757 - 17.23
Vacuum (
Ta
= 50˚C - 70˚C,
Va
= 1.5 m/s)
32 - 914 2.25 - 6.08
Microwave (80
- 400 W)
10 ± 0.1 0.0474 - 0.0527 2.367 - 9.779
Water blanching (60
˚C - 80˚C) + convective (
Ta
= 60˚C) [11]
2 0.102 - 0.209
Blanching
–steaming (
Pv
= 0.2 -
0.5 MPa and vacuum of 5 kPa)
+ convective (
Ta
= 40˚C,
Va
= 1 m/s,
Pa
= 267 Pa) [12]
10 ± 0.1 2.084 - 2.271
Convective (
Ta
= 40˚C - 60˚C) [13]
with the temperature parameters under the drying operations providing further
comprehension of the mass transfer characteristics. For our onion drying expe-
rimentation, the value of
hm
(3.37 × 10−7 m·s−1 at 40 - 5.69 × 10−7 m·s−1 at 50 -
8.46 × 10−7 m·s−1 at 60 - 13.38 × 10−7 m·s−1 at 70˚C) varied positively with drying
temperature. It showed that the value of hm increased with drying temperature
of onion. This is a confirmation of results obtained for drying constant
S
and
diffusivity
Deff
. This dependence result of temperature was also similar to that
from McMinn [31] for dying lactose powder, form Torki-Harchegani
et al.
[22] for drying lemon, from Mrkić
et al.
[28] for drying broccoli, from Tor-
ki-Harchegani
et al.
[21] for drying saffron and Bezerra
et al.
[24] for drying
passion fruit peel.
3.6. Average Activation Energy
The values of the diffusivity for “
Violet de Galmi
” onion drying at drying tem-
perature are used to estimate the values of the diffusivity for an infinite temper-
ature and the average activation energy
Ea
by Equation (18). Thus, the natural
logarithm of the diffusion coefficient is represented according to the absolute
temperature reciprocal (Equation (17)) [24]. The values obtained for the water
diffusion coefficient at infinite temperature D0 and the average activation energy
Ea
for “
Violet de Galmi
” onion drying are respectively 1.372 m2·s−1 and 31.73

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kJ·mol−1 with
R
2 = 0.99,
SSE
= 2.666 × 10−6 and
RMSE
= 0.001633. The values
obtained for the average activation energy are close to those obtained in the lite-
rature of convective drying of onion (Table 3) such as
Ea
= 26.4 kJ·mol−1 from
Mota
et al.
[4],
Ea
= 10.93 - 34.13 kJ·mol−1 from Süfer
et al.
[10] and
Ea
= 45.6
kJ·mol−1 from Demiray
et al.
[9]. The range of energy activation of 5.06 kJ·mol−1
to 10.63 kJ·mol−1 is reported for infrared convective drying of onion slices under
air temperature conditions of 35˚C - 45˚C [5]. Ours
Ea
values are in the range of
Ea
= 12 - 110 kJ·mol−1 of food and agricultural products were, hence findings
were in agreement with literature [70]. The differences between the values re-
ported in the present work and the values highlighted in the literature are related
with the dependence of the diffusion coefficient of a food with the conditions
within the material on drying [24]. For Baia Periforme variety, the activation
energy for convective drying of onion was
Ea
= 34.081 kJ·mol−1 at temperature
range of 40˚C - 60˚C [71]. Ours Ea of onion slices were also close to green beans
(
Ea
= 35.43 kJ·mol−1, convective drying; [72]), radish (
Ea
= 15 - 40 kJ·mol−1, va-
cuum drying; [73]), carrot (
Ea
= 22.43 kJ·mol−1, infrared drying; [74]) and mul-
berry (
Ea
= 21.2 kJ·mol−1, convective drying; [75]). These values can be inter-
preted as an energy barrier that must overcome the drying mechanism to re-
move moisture. This vision of the drying energy barrier is to advantage devel-
oped by other approach theory for drying modeling [76] [77] [78].
4. Conclusion
The results of this study indicate that the model developed by Dincer and Dost
[19] can be used with reasonable accuracy and confidence to calculate the mois-
ture diffusivity and mass transfer coefficient values for convective drying of “
Vio-
let de Galmi
” onion for temperature range of 40˚C - 70˚C and a relative humid-
ity of 20%. Through this model, effective diffusion coefficient values are ob-
tained between 0.2578 × 10−9 m2·s−1and 0.5460 × 10−9 m2·s−1. Onion mass transfer
coefficients vary between 3.37 × 10−7 m·s−1 and 13.38 × 10−7 m·s−1. Mass transfers
Biot Numbers of are found between 0.9797 and 2.9397. Activation energy is de-
termined for using the Arrhenius Type equation on the drying temperature
range. The activation energy
Ea
is 31.73 kJ·mol−1. This study contributes to esti-
mate the “
Violet de Galmi
” onion of mass transfer and drying process parame-
ters, an onion variety consumed largely in West Africa. This information can be
useful in designing and simulating drying equipment and in using this onion as
a drying product. However, other parameters such as the fleshy leaves structure
of “
Violet de Galmi
”
onion could have effects on the water migration during
drying. This could be the subject of other research on “
Violet de Galmi
”
onion as
the effect of drying on the nutritional composition of this variety.
Acknowledgements
The authors are thankful to Project “2ie Développement durable et énergie re-
nouvelable” of the Fondation Institut International d’Ingénierie de l’Eauet de

A. Compaoré et al.
DOI:
10.4236/aces.2022.123013 191 Advances in Chemical Engineering and Science
l’Environnement (2iE) with the collaboration of Université Joseph KI-ZERBO
(UJKZ) and Embassy of France in Burkina Faso for financial support.
Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this pa-
per.
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