EFFECT OF SOAKING TIME AND DRYING MEDIA ON QUALITIES OF PADDY DRIED BY FLUIDIZATION TECHNIQUE

© ADC’03
EFFECT OF SOAKING TIME AND
DRYING MEDIA ON QUALITIES OF
PADDY DRIED BY FLUIDIZATION
TECHNIQUE
Wathanyoo Rordprapat1, Adisak Nathakaranakule2, Warunee Tia2 and
Somchart Soponronnarit2
2School of Energy and Materials, King Mongkut’s University of
Technology Thonburi, Suksawat 48 Road, Tungkru, Bangkok 10140,
Thailand
The effect of paddy soaking time and drying media, i.e.
hot air and superheated steam, on qualities of paddy dried
by fluidized bed dryer were studied. The studied qualities
were emphasized on head rice yield, whiteness and
percentage of white belly. Paddy was prepared by soaking
in water with initial temperature of 80oC for 2, 3 and 4
hours. After that it was dried in conditions (for both
drying media) of temperature of 150o C, bed height of 10
cm and velocity of 1.5 times of minimum fluidized
velocity. The experimental results showed that there was
condensation occurred during early drying stage of
superheated steam. This condensation made paddy
temperature increased rapidly and made moisture content
of paddy dried by this method higher than those dried by
hot air. Paddy dried by superheated steam had higher head
rice yield than that dried by hot air in all soaking
conditions, while its whiteness was vice versa. Head rice
yield of paddy soaked for 2 hours dropped faster than
those soaked for 3 and 4 hours. Dried paddy initially
soaked for 3 and 4 hours had 1-5 percentages of white
belly, which are acceptable levels in commercial parboiled
rice. The percentages of white belly were much less than
those of paddy soaked for 2 hours.
Keywords: condensation, quality, soaking, superheated
steam.

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Introduction
The parboiling process of consists of pre-soaking paddy in water until
saturation, with subsequent steaming followed by drying The penetration of
water into the rice kernel is important as the process results in less breakage
during milling, thus increasing head rice yield (Bhattachaya, 1985). Soaking
of paddy forms an important operation since the duration directly effects
parboiling. Theoretically, soaking of paddy can be done at gelatinization
temperature or below gelatinization temperature, In the traditional
parboiling process, paddy is steeped in water until attaining the moisture
content between 42-55%d.b. This stage is taken overnight or longer at
ambient temperature. Steaming at 100oC to gelatinize the starch, resulting in
the grain swelling follows it. In India, at present, the parboiling is usually
carried out by soaking paddy at 70 to 75oC for about 4 hours by initially
heating the water to 80-85oC, drained off the water, followed this by open
steaming for 10-15 minutes and dried before storing and milling (Gariboldi.,
1972). For paddy drying by fluidized bed technique using hot air found
that head rice yield was increased whilst the whiteness of rice was
decreased, cooking and eating qualities found that similar parboiled rice
(Taweerattanapanish et al., 1999, Inprasit et al., 2001). At present, paddy
drying by fluidized bed technique using superheated steam (Taechapairoj et
al., 2003, Rordprapat et al., 2002) found that the head rice yield of paddy
dried by superheated steam was higher than that dried by hot air.
This research study effects of soaking time and drying media, i.e. hot
air superheated steam, on qualities of paddy dried by fluidized bed dryer.
The qualitative indicators used for evaluating the paddy are head rice yield,
whiteness, and percentage of white belly.
Materials and Methods
The schematic diagrams of superheated steam and hot air fluidized bed
dryers are illustrated in Figure 1(a) and (b) respectively.
The superheated steam fluidized bed dryer comprised five
components, i.e. a cylindrical chamber with an inner diameter of 15 cm and
a height of 100 cm, a 13.5 kW electrical heater, a backward-curved blade
centrifugal fan driven by a 2.2 kW motor, a reverse flow cyclone, and a
small boiler capable of generating steam at a rate of 31 kg/h. Superheated
steam temperatures were controlled by PID-controller with an accuracy of
± 1oC.




694

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Steam outlet
Air outlet
Damper 1
Cyclone
Paddy inlet
Recycle tube
Recycle tube
Paddy inlet
Drying
Drying
chamber
chamber
Air inlet
Paddy
Paddy
Boiler
outlet
outlet
Distributor
Distributor
Dust
Heater
Damper 2
Superheater
Blower
Blower
a) Superheated steam fluidized bed dryer (b) Hot air fluidized bed dryer
Figure 1 schematic diagrams of fluidized bed dryer

The hot air fluidized bed dryer, comprised three components: a
cylindrical chamber with an inner diameter of 20 cm and a height of 100
cm, a 12 kW electrical heater, and a backward-curved blade centrifugal fan
driven by a 1.5 kW motor.
Long grain rough rice (Supanburi 1 variety) from Pathum Thani Rice
Research Center in Pathumthani Province, Thailand was used in
experiments. The rice was cleaned and soaked in water with initial
temperature of 80oC for 2, 3 and 4 hours, before being tempered for another
one hour. The experimental conditions were set up at a bed depth of paddy
10 cm, superheated steam and air temperatures of 150oC, superficial
velocities of 1.5Umf (Umf of hot air = 1.65 m/s and Umf of superheated steam
= 2.55 m/s). Grain temperature and drying air temperature were measured
by Type-K thermocouples connected to a data logger giving an accuracy of
± 1oC. The moisture content of paddy was determined by hot air oven at a
temperature 103oC for 72 hours. Paddy kernels after drying were slowly
cooled down to ambient temperature and kept in a polypropylene bag. They,
then, were gently ventilated with ambient air until their moisture contents
reached 16 % d.b. Finally, two samples were taken from the dried kernels.
First 300g sample was kept in a seal plastic bag for two weeks before
testing head rice yield, whiteness and white belly. Another 250g sample was
shelled by Rubber Roll Husk, polished by Satake Rice Polisher, and graded
by Rice Grader, before its color was measured by Kett degital whiteness
meter (Model C-300), which was calibrated with a white reference color.



695

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Results and Discussion
Experimental results for on conditions of superheated steam and hot air
drying, i.e. drying rate, grain temperature changing rate, head rice yield,
whiteness, and percentage of white belly, are discussed as follow:
Drying and Grain Temperature Changing Rates
The effects of soaking time on changing of moisture contents and grain
temperatures of paddy (dried by superheated steam and hot air with drying
times at a drying temperature 150oC, a bed depth of paddy 10 cm, and
superficial velocities 1.5Umf) were shown in Figure 2. Initial moisture
contents of paddy after soaking for 2, 3 and 4 hours were in a range of 42-
45% d.b.
As shown in Figures 2a, 2b and 2c, the moisture contents of 2-hours
soaked paddy dried by hot air were decreased much faster than those of 3
and 4-hours soaking paddy during the first minutes of drying (lower than
30% d.b). This was because, in the case of soaking paddy for 2 hours, the
time for moisture to adsorb into paddy kernel was shorter than of 3 and 4
hours. Moisture content of the paddy in this case, therefore, was mostly at
the surface of paddy kernels and was easily removed by hot air. When
compared between hot air and superheated steam drying, moisture content
of paddy dried by superheated steam decreased slower than that dried by hot
air. This was because the moisture content of paddy dried by superheated
steam increased due to condensation, which noticeably occurred during the
first minute of this drying method. This early-stage condensation was also
reported in researches of Iyota et al. (2002), Taechapairoj et al. (2002) and
Rordprapat et al. (2002). After 3-4 minutes, the drying rates of both drying
methods were almost the same because of closer paddy temperature, leading
to the equality of moisture diffusion inside the paddy. The temperature of
paddy dried by the superheated steam increased faster than that dried by hot
air during the first minute of drying. This phenomenon was due to latent
heat from steam condensation added to paddy during the early-stage of
drying, and due to superior heat transfer properties of superheated steam
compared with hot air.


696

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160

90
140
120

80
d.b.)
100
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Drying time(min)


a) soaking time of 2

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.
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Drying time(min)
Drying time(min)

b) soaking time

of 3

c) soaking time of 4


F
igure 2. Drying and temperature changing rates (Superheated steam
and hot air at superficial velocities 1.5Umf, drying

Head Rice Yield
The relationship between the changing rates of moisture content and
head rice yield of 2, 3 and 4-hour soaked paddy dried by superheated steam
and hot air drying methods were presented with head rice yields of
reference paddy of each method which obtained from paddy dried by
ambient air, as shown in Figures 3a, 3b and 3c.


697

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90
80

80
70
)
b.
60
)

70
d.
head rice yield of reference
50

60
d
(
%
40
i
el

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n
t
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%
50
30
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nt 40
20
i
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30
10
Hot air

Superheated steam 0
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20
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i
s
t
u
r
e
c
-10
M 10
-20

0
-30

0
1
2
3
4
5
6
7

Drying time(min)


a) soaking time of 2

90

80
90
80
80

70
80
70
)
b.)
60
)
b.
60
70

70
)
d.
50
%
d.
50
60
head r ice yield of reference paddy
60
d(%
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(
%
40
40
50
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n

30
yield(
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n
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(
%
50
head rice yield of reference paddy
nt
30
nt
Hot air
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o 40

20
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40
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Superheated steam
c
Hot air
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yiel
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30
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ad r
20
Head r
i
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t
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-10
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-20
10
-20
0

-30
0
-30
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7

Drying time(min)
Drying time(min)


b) soaking time of 3
c) soaking time of 4












Figure 3. The relationship the changing rate of moisture content and head
rice yield of paddy dried by superheated steam and hot air at
superficial velocities 1.5Umf, drying temperature 150oC before
drying soaked paddy at initial temperature 80oC.

The head rice yields of paddy dried by superheated steam were higher
than those dried by hot air in all experiments. Head rice yields of both
drying methods increased beyond those of their references after first minute
of drying. These results could be explained by the gelatinization process of
starch, which helped joining cracks inside paddy kernels (Ranghavendra
698

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Rao & Juliano., 1970 and Taechapairoj, C., 2002). For paddy, the suitable
temperature and moisture content for forming gel were at 73-86oCand 24-
25%w.b respectively (Zhout al et., 2001, Taweerattanapanish et al., 1999
and Pillaiyar et al., 1993). From this suitable condition, temperature was the
main factor causing the differences of head rice yield between paddy dried
by both drying methods and reference paddy. After one minute of drying
time head rice of 2-hour soaked paddy dried by hot air rapidly decreased
(see Figure 3a) because fast during in this conditioned paddy cauesd stress
develop inside paddy kernels, as a result raising cracked and broken grains.
Head rice yields of paddy soaked for 3 and 4 hours (see Figures 3b and 3c)
were higher than those of paddy soaked for 2 hours. This was because the
range of gelatinization process of paddy dried in these cases were more
suitable more than that in case of paddy soaked for 2 hours. Superheated
steam drying made paddy temperature increase rapidly to gelatinization
temperature in the early stage of drying, while the moisture content of
paddy during this stage was still in the suitable range of the gelatinization
process (see Figures 2). This was the reason why the head rice yield of
paddy dried by superheated steam was higher than that dried by hot air,
which had slower temperature increasing rate and faster drying rate, thus
less suitable condition for gelatinization. The head rice yield of both drying
method started to decrease after 2-3 minutes of drying, because paddy
kernels had high temperature and moisture gradient, causing stress develop
inside the kernels, as a result cracked or broken rice after milling.
Whiteness
Figures 4a, 4b and 4c indicated whiteness of rice at the first minute of
superheated steam drying decreased much faster than that of hot air drying,
The fast drop of whiteness in the superheated steam drying was due to rapid
increase in paddy temperature which accelerated mallard reaction and
transition of color substances from rice husk and rice bran into endosperm
[Khan., (1974), Yap et al., (1988) and Inprasit et al., (2001)]. Longer drying
time increased paddy temperature in both drying methods, which induced
mallard reaction and caused whiteness of rice slightly drops. Figure 4a and
4c is shown whiteness slightly increased after 2 minutes of superheated
steam drying, This phenomenon, confirmed by several repetitions, may be
resulted from high moisture diffusion rate, caused by high paddy
temperature and moisture content at this drying time. The high diffusion rate
may move color substances out of paddy kernels.








699

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45
260

40
240

220
35
200
C)

s 30
o
es
180
r
e
(

25
160
en
it
Hotair
140
t
u

20
h
Superheated steam
120
e
r
a
W
p

15
100
80
m

10
60
Te
5

40
0
20

0
1
2
3
4
5
6
7

Drying time(min)


a) soaking time of

55
260

55
260
50
240
50
240
45
220
45
220
200
40
C)
200
o
40
C)
o
35
180
180

r
e
(
35
Hot air
160
r
e
(
30
iteness
t
u
160
30
Hot air
t
u
h
Superheated steam
140
25
i
t
eness
140
Superheated steam
W
120
e
r
a
p
25
h
120
e
r
a
20
p
100
m
W 20
100
m
15
80
Te
15
80
Te
10
60
10

60
5
40
5
40
0
20
0
20
0
1 2 3 4 5 6 7
0
1
2
3
4
5
6
7
Drying time(min)
Drying time(min)





b) soaking time of 3
c) soaking time of 4


Figure 4. The changing rates of whiteness and temperature of paddy
.
dried by superheated steam and hot air at superficial
velocities 1.5Umf, drying temperature of 150oC before



700

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White Belly
Figure 5 showed the relationships between white belly and drying time
for superheated steam and hot air drying at temperature of 150oC, and the
superficial velocities of 1.5Umf. The kernel which has an opaque white area
higher than 50% of its total area is considered as the white belly category,
according to Thai Standard Rice (Ministry of Commerce, Thailand, 1997).
White belly of paddy soaked for 2 hours (Figure 5a) was more than 20% for
all drying time because gelatinization of rice starch may be occurs only on
surrounding surface of rice kernel. White belly of paddy soaked for 3 and 4
hours (Figures 5b and 5c) decreased with drying time and were much lower
than that of paddy soaked for 2 hours. The reduction of the white belly was
resulted from gelatinization process, as described in “head rice yield”
section. The effect of soaking time for 3 and 4 hours on white belly of
paddy dried by superheated steam and hot air were clearly seen almost in
both drying methods, especially during the first two minutes of drying.
Decreasing rates of white belly of paddy dried by superheated steam had
similar trend to those dried by hot air. At the end of drying (5 or 6 minutes),
white belly was less than 2% for both drying conditions.

701

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45
40
35

)
% 30
25
e
lly(

20
ite b
h 15
Hot air
W 10
Superheated steam

5
0
0
1
2
3
4
5
6
Drying time(min)

a) soaking time of 2

45
20
18
40
)
16
35
) 14
%
30
12
Hot air
25
e
ll
y
(
%

10
Superheated steam
20
e
belly(
t
e
b
it
8
h
15
Hot air
6
W
Whi 10
Superheated steam
4

5
2
0
0
0
1
2
3
4
5
6
0
1
2
3
4
5
6

Drying time(min)
Drying time(min)
b) soaking time of 3
c) soaking time of 4


Figure 5. The changing rates of white belly of paddy dried by
superheated steam and hot air. at superficial velocities 1.5Umf,
drying temperature 150oC before drying paddy soaked at initial
temperature 80oC.


702

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