ABSTRACT stream, vacuum drying, or microwave method. Cocoa

Researches on drying by using microwaves energy, along with traditional methods of heating, are widely
reported. In this paper, we analyze the experimental results of microwave drying of cocoa bean. Drying experiments were
by using a domestic microwave oven which operated at three power levels. Microwave drying is based on a unique
volumetric heating mode with electromagnetic radiation at 2,450 MHz. The responses of the agriculture product to
dielectric heating result in rapid energy coupling into the moisture and lead to fast drying. A significant reduction in drying
time in microwave drying is often accompanied by an improvement in product quality, making it a promising food
dehydration technology. Preliminary theoretical analysis by using an analytical approach by using Dincer and Dost model
for the drying of cocoa bean is presented. The study gives a brief description of efforts made to obtain basic drying
parameters under different microwave drying conditions from experimental results.
Keywords: drying, microwave, cocoa bean, dincer and dost model.
Agricultural products are dried to inhibit
microbial development and quality decay. However, the
extent of drying depends on product end-use. Products are
dried after harvest to the moisture content that allows
microbial stability during storage 1-2. For examples,
vegetables are blanched before drying to avoid rapid
darkening, and drying is not only carried out to inhibit
microbial growth, but also to avoid browning during
storage. For fruits, the reduction of moisture acts in
combination with its acid and sugar contents to provide
protection against microbial growth. Other products as
crackers are dried beyond the microbial growth threshold
to confer a crispy texture, which is liked by consumers.
Most farmers use conventional drying method (electric
furnace or sun-drying method). Other possibilities are by
using gas stream, vacuum drying, or microwave method.
Cocoa (Theobroma cacao L.) is a perennial cash
crop and its natural habitat is the humid tropics. In most
tropical countries, agricultural products like cocoa are
harvested all the year round and the beans must be dried
immediately after fermentation to reduce mass losses and
prevent spoilage. The end products from cocoa bean
especially chocolate and beverages are considered among
the basic food in many countries of the world; however the
quality of these end products is a function of how they are
processed. The fermentation and drying of this crop are
the major critical steps in the sequence of its processing.
Drying can be achieved naturally by making use of solar
energy or artificially by using heated cocoa bean dryers.
However farmers are weary of the problem of excessive
drying and quick drying of cocoa beans by heated dryers;
because cocoa is sold by weight, excessive drying will not
be economical in terms of amount of money received by
Generally, the moisture content of the cocoa bean
is approximately 55% at the end of fermentation and this
must be reduced to less than 8% before the cocoa can be
stored sold or transported 3. However if the moisture is
reduced too much, the shells become too brittle and break
but if moisture is too high, mould growth occurs during
storage. The rate of drying is critical to final quality. Too
slow or too rapid a drying rates result in excessively acid
bean with case shrivelling. Most farmers use conventional
drying method. Drying by using heat energy, the heating
elements supply heat to the materials and energy absorbed
only at surface and be transferred into the other part of
materials by conduction which taking amount of time.
The use of microwave energy has been reported
widely for sintering ceramics 4-10 as well as for drying
food 11-12. In a microwave furnace, the material will
absorb microwave energy convert the energy into heat.
Heat is generated internally within the materials resulting
homogenous heating. In general, the time-averaged power
dissipated per unit volume in a material can be expressed
as 13.
Pv =
Where ?“ is the imaginary part of the complex dielectric
constant of the material, ? is the angular frequency of the
electric field E, and ?o is the permittivity of free space.
An experiment of drying characteristics of cocoa
bean by using a microwave furnace was successfully
performed and reported previously. In this paper, analysis
of experiment results of drying characteristics of cocoa
bean by using a microwave furnace will be presented.
2.1 Sample preparation
Cocoa beans with initial moisture content of 54%
in average were obtained from a local farmer (Ladongi,
VOL. 11, NO. 19, OCTOBER 2016 ISSN 1819-6608
ARPN Journal of Engineering and Applied Sciences
©2006-2016 Asian Research Publishing Network (ARPN). All rights reserved.
South East Sulawesi). Prior to drying, samples were
selected then washed. All samples then were pellet and cut
into slab shape with desired size of 10 mm in diameter and
3 mm in thickness.
2.2 Drying experiment
The microwave drying experiments were
performed in a 2.45 GHz microwave oven (Panasonic NEC236,
Japan) 12. The maximum automatic output
microwave power is 800 W. The power consumption is
1.43 kW. Cocoa bean drying experiments were performed
by using three power levels: high power was calculated as
600 W, medium power as 300 W and low power as 150
W. Two hundred grams of samples were placed in the
oven. The microwave oven equipped by a temperature
controlling system. Microwave powers were varied with
Figure-1. Schematic picture of a microwave drying
system: (1) control; (2) lamp; (3) temperature
control; (4) samples on turntable plat.
2.3 Analysis of drying experiment
The moisture content value was determined as:
? = ??? ? ???/?? (2)
where M is moisture content, Wt is the weight of sample
(g) at any time and Wd is the weight of the dried sample.
The analysis of microwave drying models can be
classified into three categories, i.e. heat-mass transfer
model, empirical model and diffusion model 14-15. In
this paper we use diffusion model models.
3.1 Drying experiment
Drying curves of cocoa beans dried with different
methods are presented in Figure-2. It shows that the
moisture content and drying rate decreased continuously
with drying. There are no constant rates drying because
most crops as well as cocoa exhibit the constant rate
drying characteristics at their critical moisture content.
Cocoa exhibits a constant rate behaviour during drying,
from moisture content of 70-100% 3. However the initial
moisture content in this experiment is not up to this range.
At the falling rate period the movement of moisture within
the cocoa to the surface is governed by diffusion since the
material is no longer saturated with water. The graph
shows that the moisture ratio decreased as the drying time
increased. Figure-2 also exhibits a faster drying in the
microwave compared than in the conventional furnace on
all microwave powers. Microwave heating is mainly due
to polarization and ionic conduction of water molecules in
cocoa. Simplify can be described that the ionic conduction
losses and due to dipolar rotation towards microwave
frequencies with temperature. The absorption of
microwave energy and conversion to heat is due to
polarization and conduction would result in a rise in
temperature, and this is given by the following equation
“ E?m?
where, ??
“ is the permittivity of free space (8.85×10-12
V/m3), ????
“ is the relative effective dielectric loss due to
ionic conduction and dipolar reorientation, f is the
frequency (Hz), Erms is the root mean square of the
electric field within the material (V/m), ? is the bulk
density of dielectric material (kg/m3
) and Cp is the heat
capacity of the material at constant pressure (J/kgoC). The
water dipole attempts to continuously reorient in
microwave’s oscillating field. The dipole lags between the
dipole and the field leads to an energy loss by heating. The
ease of the movement depends on the strength and extent
of the hydrogen bonded network.
Figure-2. Drying curves of cocoa bean slabs (diameter of
10 mm) in a microwave with different microwave power
and in an electric furnace (diameter of 10 mm).
3.2 Model of drying experiment
According to Fick’s second law for diffusion.
The governing Fickian equation is in the form of Fourier
equation of heat transfer in which temperature (T) and
VOL. 11, NO. 19, OCTOBER 2016 ISSN 1819-6608
ARPN Journal of Engineering and Applied Sciences
©2006-2016 Asian Research Publishing Network (ARPN). All rights reserved.
thermal diffusivity are replaced with moisture content (M)
and moisture diffusivity, respectively. The time
dependency of heat and moisture transfer equations can be
written as following equation 16-18:
??? ? ?
??? ?? ?
??? = ??
? ???
??? (4)
??? ? ?
??? ?? ?
?? ? = ??
? ???
?? ? (5)
Where m= depend on shape, 0, for infinite slab, 1
for infinite cylinder, and 2 for sphere; y= radius for sphere
and cylinder; ?=thermal diffusivity; D= moisture
diffusivity, and t= time.
By using dimensionless temperature and moisture
content parameter, we can develop an unsteady state
diffusion of moisture content in agriculture product from
Fick’s second law:
?? ? = ?
?? ?
?? ? (6)

? = ??
?? (7)
From Figure-2 we can assume that drying has an
exponentially trend and should be following equation,
?= C exp(-kt) (8)
Where C = lag factor and k= drying coefficient
For one-dimensional transient moisture diffusion
in an infinite slab, we can use Dincer and Dost Model 17-
? = k?2
where Y= half thickness of slab, and ?=coefficient of
characteristic of drying equation
? = ?????0.??????? ?? + 0.??????? (10)
Lag factor and drying coefficient are determined
by drying experimental graphs then Biot number then can
be calculated. By using those parameters, ? and D were
then determined.
Preliminary calculation of the parameters is
shown in Table-1. Detail model calculation results with
various sample sizes will be published in another paper.
power (Watt)
Cocoa bean samples
G K(1/s) ? Bi D (m2
Thickness /s)
150 3 10 1.3823 0.03166 0.563 0.327 0.96
300 3 10 1.4782 0.05626 0.549 0.564 1.30
600 3 10 1.4973 0.08212 0.5791 0.756 2.77
Experiments of application microwave energy for
drying cocoa beans have been performed. Slab cocoa bean
samples with diameter of 10 mm and thickness of 3 mm
were prepared. A domestic microwave oven operated at
three power levels with temperature control was applied
for drying experiment system. Conventional drying was
used as a comparison by applied electric furnace. The
microwave showed a faster drying than conventional ones
at all microwave powers. Analysis of the experimental
data was performed by using available theoretical models.
Drying parameters determined by using the model and
experimental drying graphs. This model could be used as a
tool for microwave drying of cacao bean more efficiency.
Thanks to all members of Material Physics
Laboratory and Theoretical and Computational Physics
Laboratory, Department of Physics, Faculty of
Mathematics and Natural Sciences, Halu Oleo University
for all facility supports.
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