2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
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essential because high moisture content of soaked rice (>
35% dry-basis) is easy to degrade by many effects such
as infection by microorganism, yellowing by non-
enzymatic reaction etc. Drying process is an important
process affecting product quality and there are many
methods to reduce moisture content such as hot air
drying, infrared (IR) drying [7, 8], microwave (MW)
drying [18]. The safe moisture content of grain kernel for
long shelf life is about 16±1.0% dry-basis [16, 19]. One
method of high performance heat and mass transfer is
electromagnetic irradiation for examples: microwave,
infrared and radio frequency. Infrared irradiation
technique is an effective method which provides the high
heat and mass transfer rates but it enhances color of
product.
The objectives of this research were to investigate
effect of drying strategy: Hot air convection, IR and
combined HA+IR of Leb Nok Pattani parboiled paddy on
product qualities in terms of yellowness, white belly,
morphological structure, physicochemical and texture
analysis.
III. Materials and Method
A. Raw Material
A local medium-grain rough rice Leb Nok Pattani was
provided by the Rice Research Institute at Patthalung
province, Thailand. The drying of paddy was carried out
in a tray dryer with a dimension of 0.55
u
0.55
u
0.50 m
3
.
The dryer basically consisted of the electrical heater unit
of 10 kW, a centrifugal fan driven by a 1.5 hp motor and
a temperature controlling unit.
B. Experimental procedure
Fresh medium-grain paddy was cleaned and soaked in
warm water at 70±1
q
C for 3 hour in order to obtain
saturated paddy kernels. The soaked rough rice was then
steamed at 100±1
q
C for 30 min to obtain parboiled rice
without white belly before drying. Finally, the parboiled
rough rice was dried with HA convection, IR and
combine HA+IR. The experiments were carried at drying
temperature ranges of 60 to 100
q
C, inlet air flow rate of
1.0+0.2 m/s with the infrared powers of 1,000 and 1,500
W. The average initial moisture content of sample was in
the range of 54±1%d.b. During drying process mass of
the sample was recorded at every 3 min interval until it
reached the final moisture content to 22±1%d.b. It was
then ventilated with ambient air until the moisture content
of paddy was about 16% dry-basis [16].
C. Equilibrium moisture content (EMC) and mathematic
drying modeling
In this study the EMC values were determined at the
controlled air temperature ranging of 40-60
q
C
corresponding to relative humidity of 11-87% using the
static gravimetric method. The parboiled paddy was put
in the seal glass bottle with five different saturated salt
solutions
comprising
LiCl,
KNO
3
,
NaCl,
Mg(NO
3
)
2
•6H
2
O, and MgCl
2
•6H
2
O. After a few day,
sample was weighed and was taken around 10-14 days
for getting the equilibrium state with saturated salt
solutions. This state was acknowledged when consecutive
weight measurements difference lower than 0.001 g.
Then the moisture content of parboiled rice samples was
determined following the AOAC standard method [1].
The EMC value of sample was shown by means of
triplication.
TABLE
I
M
ATHEMATICAL MODEL FOR PREDICT EQUILIBRIUM MOISTURE
CONTENT OF RICE
Model
Model equation
Oswin [12]
[
]
B
eq
)) RH(-1/() RH(A M
=
Halsey [9]
B/1
eq
]
C)
A/R(T -
) RH ln(
[ M
+
=
BET [3]
(RH)]
-
C(RH)
-
(RH)][1
-1[
) RH(CM
M
m
eq
=
GAB [5]
C.k.(RH)
k.(RH)
-
k.(RH))(1
-1(
) RH .(k.C.M
M
m
eq
+
=
Henderson [5]
C/1
eq
]
)BT(A
) RH(-1 ln(
[ M
+
=
Five EMC models were selected for the fitting of the
experimental data. These models are Halsey, BET,
Oswin, Henderson and GAB models (shown in Table 1).
The EMC was shown as a function of temperature and
relative humidity. The parameters of these models were
solved using the non-linear regression analysis. The
suitability of the equations was evaluated and compared
using correlation coefficient (R
2
) and root mean square
error (RMSE) value.
D. Effective diffusion coefficient (D
eff
)
The Fick’s law of diffusion was used to describe the
transport of water inside the sample surface in terms of
diffusivity with the assumption that the parboiled rice is
of spherical shape, as shown in the following equation
(Crank’s equation)
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2
0r
Dt 29
π
exp
9
1
2
0r
Dt 24
π
exp
4
1
2
0r
Dt 2
π
exp
2
π
6
eqM inM
eqMM
MR
(1)
E. Rice quality
1. Head rice yield (HRY)
The determination of head rice yield was performed
according to the procedure set by the Rice Research
Institute, Phatthalung province, Thailand. Head rice yield
was calculated by dividing the head rice weight by the
initial rough rice weight. This value was determined in
duplicate.
2. Yellowness of parboiled rice (b*)
The surface color of dried parboiled rice was
measured using a C.I.E. lab colorimeter (JUKI, model
JP7100, Tokyo, Japan.) in L*, a* and b* scales. The color
of the product was expressed in terms of yellowness
value (b* value of CIE lab).
3. Whiteness of parboiled rice
