SECTION 2 HIGH TEMPERATURE FLUIDISED BED DRYING

(Experimental study on high moisture paddy- Vietnamese rice varieties)
75
Effect of high temperature fluidized drying and tempering on head rice yield
and mechanical strength of Vietnamese rice varieties


C was used for the high
moisture content (32.5% db) paddy. This was probably contributed by the occurrence of partial
gelatinization of starch at high temperature. Some researchers suggested the need for tempering
the grain for 25-30 minutes if high temperature drying is used (Poomsa-ad et al. 2005;
Prachayawarakorn et al. 2005). Although there are a few research works being reported on high
temperature fluidized bed drying, no real application has been reported. Therefore, more
understanding on this subject is required.

76
This study aims to enhance the knowledge on the effect of high temperature drying and tempering
on head rice yield, fissured kernels, mechanical strength and quality changes of Vietnamese rice
varieties.

The specific objectives of this study are:
 To study the feasibility of high temperature fluidized bed drying of high moisture content
Vietnamese rice varieties (two rice varieties were considered in this study).
 To study the optimum drying temperature and tempering duration as reflected by the
lowest number of fissured kernel after the completion of drying followed by tempering.
 To study the mechanical strength of the rice kernels under different drying and tempering
regimes.
 To study the effect of high temperature drying and tempering on the whiteness, pasting
properties and crystallinity of rice starch. These properties can reflect the cooking quality
of rice.

Materials and Methods

Fluidized bed drier:
A batch lab-scale drier (HPFD150) developed at Chemical Engineering Department, Nong Lam
University, Vietnam was used in this experiment (Figure 1). This dryer consists of three main
components, namely (i) a cylindrical shaped drying chamber (40 cm in height x 15 cm diameter)


After tempering, the samples were thin layer dried at 35
0
C to safe storage level of moisture content
(below 14% wb). The dried rice was sealed in plastic bags and kept at room temperature for 3 days
before determining the head rice yield (HRY), cracked grain ratio and mechanical strength of rice
kernels. 200 gram of paddy was also dried in the thin layer dryer at 35
0
C for 16 h down to 14% wb
and used as control sample. All treatments were undertaken in triplicate.

Moisture content determination
The moisture contents before and after tempering and final moisture content after thin layer drying
of each drying run were determined by drying 5-10 gram of rough rice (in duplicate) in an oven at
130
0
C for 16h.

Head rice yield
100 gram of paddy was dehusked/dehulled and milled by laboratory milling system and whole
kernel was separated from broken kernel to determine the head rice yield, which is defined as the
ratio of the mass of unbroken kernel to the mass of paddy.

Cracked grain ratio
Fissure enumeration was carried out in duplicate on 50 manually dehulled brown rice kernels per
each measurement by visual observation with the assistance of a light box. 79Fluidized bed dryer (developed at Chemical
Engineering Dept., Nong Lam University,
Vietnam)
Tempering of rice in sealed glass jars Thin layer dryer used in this study Dried rice of replicate 1 (27 treatments)

Figure 1: Equipment used in drying and tempering of the rice samples

80
Fresh paddies Figure 2. Schematic diagram of experimental and measured parameters.

determination
Moisture
determination
Moisture
determination
Head rice yield (%)
Fissured kernels (%)
Breaking force (N)

81
Results and Discussion

The evolution of moisture content during fluidized bed drying and subsequent tempering for both
rice varieties is presented in Figure 3. The percentage of moisture content removal of the paddy
was found to be in the range of 7.7 to 12.0%. As can be seen in Figure 3, increasing drying
temperature led to larger percentage of moisture removal.

Tempering period not only helped moisture redistribution inside the rice kernel but also caused
slight loss of moisture during prolong tempering duration (40 min as shown in Figure 3). Within
2.5 min of fluidized bed drying, the moisture content for A10 variety dropped by 8.7-9.4% at 80
0
C
and 11.0-12.0% at 90
0
C, while it dropped by 7.7-8.6% at 80
0
C and 9.7-11% at 90
0
C for OM2717
variety. Extending drying time to 3.0 min did remove further 1% moisture content in both drying

30
40
0 10203040506070
Operation time, min
Moisture content, %wb
80C, 2.5 min
80C, 3.0 min
90C, 2.5 min
90C, 3.0 min
80C, 2.5 min
80C, 3.0 min
90C, 2.5 min
90C, 3.0 min
A
10 OM271
7

Drying duration
Tempering duration
Figure 3. The reduction of moisture contents during fluidized bed drying and tempering at high
temperatures for two Vietnamese rice varieties (A10 and OM2717).

Head rice yield

Drying time effect: Increased drying time reduced the head rice yield significantly (Figure 4). In
our other experiments we found that a longer than 3 minutes drying time caused a remarkable
reduction in moisture content below 17.5% wb but caused higher fissures in the grain (results not
presented here). As mentioned earlier, a longer exposure of grain at higher temperature causes
strain on the grain due to the rapid removal of moisture from the surface while it takes time to
diffuse water from the interior. The large differential moisture will cause more fissures in the

Tempering time, min
80oC, 2.5 min 80oC, 3.0 min
90oC, 2.5 min 90oC, 3.0 min
Reference
A10
0
10
20
30
40
50
60
70
010203040506070
Tempering time, min
Head Rice Yield, %
80oC, 2.5 min
80oC, 3.0 min
90oC, 2.5 min
90oC, 3.0 min
Reference
a) A10 variety b) OM2717 variety Figure 4. Effect of tempering time on HRY at drying temperature of 80
0
C and 90
0
C for 2.5
and 3.0 min. Reference HRYs of A10 and OM2717 were 54.5% and 43.26%, respectively

OM2717

DryingTemp
0.000 0.000 0.000
DryingTime
0.000 0.001 0.000
TemperingTime
0.000 0.000 0.029
DryingTemp*DryingTime 0.960 0.524 0.091
DryingTempt*TemperingTime
0.000
0.486
0.014
DryingTime*TemperingTime 0.471 0.967 0.597
DryingTemp*DryingTime*TemepringTime 0.961 0.295 0.213 Fissured kernels:
Drying temperature and time effect: Drying temperature, drying time and tempering time had a
very significant effect on fissured grain percentage. Without tempering, the percentage of fissured
kernels was 20-40% for A10 variety and more than half of OM2717 white rice kernels were
broken (60-86%). Increase in drying time also increased the amount of damaged rice kernels
(Figure 5).

Tempering time effect: Figure 5 indicates that the tempering of the rice significantly decreased
the amount of fissured kernels. This demonstrates how important it is to temper the paddy for an
optimal time if a high drying temperature is used. There was a continuous decrease in the fissured
kernels as the tempering duration was increased particularly for OM2717 rice variety, however,
based on the A10 variety we can see that a time of 30-40 minutes is required to achieve a low
fissured kernels. The tempering has two simultaneous effects, one is to allow moisture to

20
30
40
50
60
70
80
90
0 20406080
Tempering time, min
Fissured kernels, %
80oC, 2.5 min
80oC, 3.0 min
90oC, 2.5 min
90oC, 3.0 min
Reference
OM2717
0
10
20
30
40
50
60
70
80
90
0 20406080
Tempering time, min
Fissured kernels, %

80
0 20406080100
Fissured kernels, %
HRY, %
OM2717 A10 Reference-A10 Reference-OM2717Figure 6. Relationship between head rice yield (HRY) and fissured kernels for A10 and
OM2717 varieties.
87 (a) (b) (c) 88

Table 2: Mechanical breaking strength of low temperature (35
o
C) and high temperature
dried rice (dried for 2.5 and 3 minutes)

Average mechanical strength of rice kernels (N)
80
o
C 90
o
C
Rice variety
Reference drying
(35
o
C)
2.5 min 3 min 2.5 min 3 min
A10 41.2 42.1 41.5 44.5 49.3
OM2717 39.7 33.7 35.9 47.2 52.6

A10
0
10
20
30
40


(a) rice variety A10 (b) rice variety OM2717 Figure 8. Effect of tempering time and drying temperature on breaking force (mechanical
strength) of rice kernels.

Colour of milled rice
The color of the milled rice was measured expecting that the high temperature drying and
tempering can influence on the yellowness of the rice. This is due to the browning reaction at
higher temperature. The results indicated that the yellowness value was higher for the samples
dried at 90
o
C than those dried at 80
o
C. This value was also increased by the tempering time. The

89
total difference which is the combination of all principal colour parameters also changed as the
tempering time is increased. The actual observation by an eye showed some differences on the
untreated and treated samples, but all these changes were still at acceptable level as a colour of
commercial rice. This will be further investigated.

Some literatures suggest that the high temperature drying has a similar effect as an ageing of rice.
In our initial work we have found some changes on the cooking property (instrumental analysis) of
rice due to tempering. The samples are being analyzed now at UQ (Australia) since no facility is
available at Nong Lam University. This will be reported later.

80oC, 2.5 min
80C, 3.0 min
90oC, 2.5 min
90oC, 3.0 min
ReferenceOM2717
28
30
32
34
36
38
40
0 10203040506070
Tempering time, min
Total color difference, Delta E*
80oC, 2.5 min
80oC, 3.0 min
90oC, 2.5 min
90oC, 3.0 min

OM2717
8
9
10
11
12
13

many factors should be considered such as variability of moisture of the paddy, effect
of impurities, investment cost, farmers’ adoptability, scale-up problem etc. We will
consider some of these aspects in future experiments.

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