This paper presents technique of rice drying.

1. Agricultural Sci. J. 40(3)(Suppl.): 281-284 (2009) ว. วิทย. กษ. 40(3)(พิเศษ): 281-284 (2552)
ผลของฟลูอิไดเซชันอุณหภูมสูงที่มตอคุณภาพของขาวเหนียว
ิ ี
Effects of High-Temperatures Fluidization on Qualities of Waxy Rice
เพชรรัตน ใจบุญ1 สมเกียรติ ปรัชญาวรากร2 สักกมน เทพหัสดิน ณ อยุธยา2 และ สมชาติ โสภณรณฤทธิ1 ์
1 2 2 1
Jaiboon, P. , Prachayawarakorn, S. , Devahastin, S. and Soponronnarit, S.
Abstract
Waxy rice is served as daily meals in some South East Asian countries, including Thailand. The quality of waxy
rice can be deteriorated during processing after harvesting. Fluidization technique is recommended to dry waxy
rice, due to its higher capacity and required shorter drying time. However, high-temperature fluidized bed drying
causes the change of morphology of rice starch and this may affect the quality of waxy rice. In this study, the
changes of microstructure, thermal properties, starch digestibility and X-ray diffraction (XRD) of waxy rice (RD 6)
after hot air fluidized bed drying were investigated. The re-moistened waxy rice with an initial moisture content of
approximately 28% dry basis (d.b.) was dried at 90-150 C. Then, it was tempered and ventilated at ambient
condition until the moisture content of waxy rice reached approximately 16% (d.b). The experimental results
showed that the degrees of gelatinization increased with increasing drying temperature. Higher drying
temperature fused the starch granules of waxy rice and then the degrees of crystallinity decreased as measured
by XRD. In addition, for the starch digestibility, dried waxy rice by high-temperature fluidized bed dryer could
digest rapidly than the reference waxy rice, which obtained from shade drying.
Keywords: fluidized bed, gelatinization, starch hydrolysis, waxy rice
บทคัดยอ
ขาวเหนียวนิยมบริโภคเปนอาหารหลักสําหรับบางประเทศในทวีปเอเชียตะวันออกเฉียงใตรวมทั้งประเทศไทย คุณภาพของ
ขาวเหนียวถูกทําลายไดโดยกระบวนการตางๆ การนําเทคนิคฟลูอิไดเซชันมาใชสําหรับอบแหงขาวเหนียวสามารถทําได
เนื่องจากมีประสิทธิภาพการถายเทความรอนและมวลสูงทําใหใชเวลาในการอบแหงสั้น อยางไรก็ตามการอบแหงดวยเครื่อง
อบแหงแบบฟลูอิไดซเบดที่อุณหภูมิสูงทําใหโครงสรางของสตารชเปลี่ยนแปลงและอาจสงผลตอคุณภาพตางๆ ของขาวเหนียว
งานวิจัยนี้มีวัตถุประสงคเพื่อศึกษาผลของอุณหภูมิอากาศอบแหงที่มีตอคุณภาพของขาวเหนียวไดแก สมบัติดานอุณหศาสตร
โครงสรางระดับจุลภาค ระดับของความเปนผลึก และสมบัติดานการยอย เปนตน ขาวเปลือกเหนียวมีความชื้นเริ่มตนประมาณ
28% (d.b.) อบแหงดวยเครื่องอบแหงแบบฟลูอิไดซเบดที่อุณหภูมิอากาศอบแหง 90-150 C จากนั้นนําไปเก็บในที่อับอากาศ
และนํามาเปาลมเย็นจนกระทั่งความชื้นสุดทายประมาณ 16% (d.b.) จากผลการทดลองพบวา การเพิ่มอุณหภูมิของอากาศ
อบแหงทําใหเกิดเจลาทิไนเซชันมากขึ้น อุณหภูมิอบแหงที่สูงขึ้นทําใหเม็ดสตารชถูกทําลายสงผลใหคาระดับของความเปนผลึก
ของสตารชลดลงซึ่งวัดโดย XRD นอกจากนี้ ขาวเหนียวที่ผานการอบแหงดวยเครื่องอบแหงแบบฟลูอิไดเซชันที่อุณหภูมิสูง
สามารถยอยไดเร็วกวาขาวอางอิงที่อบแหงแบบคอยเปนคอยไป
คําสําคัญ: ฟลูอิไดซเบด เจลาทิไนเซชัน การยอยแปง ขาวเหนียว
Introduction
Waxy rice (Oryza sativa L.) is consumed as staple food grain in some South East Asian countries including
Thailand (Keeratipibul และคณะ, 2008). In Thailand, waxy rice is mainly consumed in the northern and northeastern
1
สายวิชาเทคโนโลยีพลังงาน คณะพลังงานสิ่งแวดลอมและวัสดุ มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี ถนนประชาอุทิศ ทุงครุ กรุงเทพฯ 10140
1
Energy Technology Division, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, Pracha u-tid Rd., Tungkru,
Bangkok 10140
2
ภาควิชาวิศวกรรมเคมี คณะวิศวกรรมศาสตร มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี ถนนประชาอุทิศ ทุงครุ กรุงเทพฯ 10140
2
Department of Chemical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Pracha u-tid Rd., Tungkru, Bangkok 10140
3
ภาควิชาวิศวกรรมอาหาร คณะวิศวกรรมศาสตร มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี ถนนประชาอุทิศ ทุงครุ กรุงเทพฯ 10140
3
Department of Food Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Pracha u-tid Rd., Tungkru, Bangkok 10140

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parts. Waxy rice after harvesting is typically dried by sun drying on concrete pad or by an LSU dryer. However,
such low-temperature drying may take a long time and consequently leads to the low drying capacity. To obtain
shorter drying time and higher drying capacity, fluidized bed drying is an alternative method. Fluidized bed drying
is not only a faster drying technique but has also proved to, in some cases, yield dried rice with higher quality. For
example, it was shown that rough rice dried at higher temperatures in a fluidized bed dryer (> 100 C) had higher
head rice yield than the rice dried in shade (Prachayawarakorn et al., 2005; Tirawanichakul et al., 2004; Jaiboon
et al., 2009) Such change in the property is indeed the characteristics of rice containing high amylose, which can
form a network during gelatinization. On the other hand, in the case of waxy rice, which mainly contains
amylopectin, formation of gel network is more difficult.
The aim of this investigation was to study the effect of high-temperature drying in a fluidized bed dryer on the
microstructure, thermal properties, starch digestibility and X-ray diffraction of waxy rice.
Meterials and Methods
Figure 1 shows a schematic diagram of a hot air fluidized bed dryer and its accessories. The system consists
of three major components: a cylindrical drying chamber with an inner diameter of 20 cm and a height of 140 cm;
12 kW electrical heaters with a temperature controller; and a backward-curved-blade centrifugal fan, which was
driven by a 1.5 kW motor. Exhaust air could be recycled, if needed, by means of two butterfly valves.
Figure 1 A schematic diagram of a batch hot air fluidized bed dryer.
Dried long grain rough waxy rice (RD 6) was re-moistened, homogenized and kept in a cold storage at 4-6 C
for 7 days prior to an experiment. The initial moisture content of the re-moistened waxy rice was 28.8% (d.b.).
Before starting of each experiment, the waxy rice was placed in ambient environment until its temperature was
close to ambient temperature. A batch of 1.9 kg of re-moistened sample was dried in the fluidized bed dryer. The
experiments were carried out at temperatures of 90-150 oC at a superficial air velocity of 2.5 m/s. The desired
moisture content after fluidized bed drying was 22-24% (d.b.). The semi-dried waxy rice was then tempered for
either 30 or 120 min. After that, the sample was ventilated with ambient air in a thin-bed ventilator at a superficial
air velocity of 0.15 m/s until the moisture content of the sample reached 16% (d.b.). The sample was kept in a
sealed plastic bag at 4-6 oC for 2 weeks before quality analysis. For quality tests, the microstructure of each
sample was observed by scanning electron microscope (SEM), thermal properties of waxy rice flour was
performed using a differential scanning calorimeter (DSC), starch digestibility (GI) was analyzed according to the
method proposed by Goni et al. (1997) and the degree of crystallinity was measured by X-ray diffraction (XRD).
Results and Discussion
Microstructure and thermal analysis of waxy rice
The results of SEM observations on the morphological changes of starch granules of waxy rice dried at
different temperatures are shown in Figure 2. The starch granules of reference waxy rice exhibited the
characteristically irregular polygons, with diameters in the range of 2-9 m. On the other hand, as can be seen in

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Figure 2, at drying temperature of 150 oC, starch granules swelled, fused themselves and were closely connected
due to starch gelatinization. In Figure 2b-c, however, although partial gelatinization occurred, the morphology of
starch granules was not different from that of a reference waxy rice sample.
(a) Reference waxy rice (b) Waxy rice dried at 90 C (c) Waxy rice dried at 110 C (d) Waxy rice dried at 150 C
and tempered for 30 min and tempered for 30 min and tempered for 30 min
Figure 2 Scanning electron micrographs of reference rice and of samples dried at different temperatures.
The above-mentioned starch granule morphologies agreed well with the DSC results (see Table 1). Higher
degrees of starch gelatinization were observed at higher drying temperatures. The degrees of starch gelatinization
at drying temperatures of 90-150 oC were in the range of 4.8-35.4%.
Table 1 Thermal analysis results of waxy rice flour at various conditions.
90
Transition 80
H DG
Processing condition temperature (C)
Total starch hydrolysis (%)
70
(J/g) (%)
To Tp Tc 60
50
Reference 61.1 68.5 76.8 6.8 0 40 Reference
T = 90 °C, t = 30 min 62.0 70.0 78.4 6.5 4.8 30 T = 90 °C, tempering 120 min
T = 90 °C, t = 120 min 62.8 70.6 78.4 6.1 10.4 20
T = 130 °C, tempering 30 min
T = 150 °C, tempering 30 min
T = 110 °C, t = 30 min 62.3 69.7 78.2 5.4 20.8 10
T = 130 °C, t = 30 min 61.9 69.5 78.4 5.0 26.4 0
0 30 60 90 120 150 180 210
T = 150 °C, t = 30 min 62.9 70.9 79.2 4.4 35.4 Digestion time (min)
T = temperature (oC), t = tempering time (min) Figure 3 In vitro starch hydrolysis rate of reference and
dried of waxy rice at drying temperature of 90,
130 and 150 oC and various tempering time.
Starch hydrolysis of waxy rice
The total starch hydrolysis versus various digestion times of reference waxy rice and dried samples are shown
in Figure 3. The minimum hydrolysis was found with the reference waxy rice while the starch digestion increased
for the dried samples. When the drying temperature employed was higher, the starch hydrolysis was faster, due to
more disruption of crystalline region (Chung, et al., 2006).
Table 2 Model parameters, hydrolysis index (HI) and average glycemic index (GI) of waxy rice starch samples.
Processing condition C (%) K (min-1) HI GI
Reference 70.2 0.83 125.6 108.6
T = 90 °C, t = 120 min 71.8 0.83 128.3 110.1
T = 130 °C, t = 30 min 73.8 0.92 132.1 112.2
T = 150 °C, t = 30 min 78.4 0.95 136.2 116.7
T = temperature (oC), t = tempering time (min)
Table 2 shows the results of in vitro starch digestion, including the estimated parameters C and k in the
starch hydrolysis model, the hydrolysis index HI and the glycemic index GI of the reference waxy rice and dried
samples. All hydrolysis parameter of dried waxy rice samples at various temperatures were higher than that of
reference waxy rice. The GI value of reference waxy rice sample was 108 referring to more rapid starch

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digestibility than standard white bread (GI = 100). The GI value of waxy rice found in this study was increased by
1% at drying temperature of 90 oC and 8% at 150 oC.
X-ray diffraction patterns
Figure 4 shows the X-ray diffraction patterns of waxy rice sample at different drying conditions. The native
waxy rice flour showed A type pattern with reflection peak at 17 and 18 Å, together with individual peaks at 15 and
23 Å. The XRD patterns of dried waxy rice was still the same as that of reference sample but their degree of
crystallinity was smaller. Table 3 shows the degree of crystallinity for the waxy rice dried at different temperatures.
The degree of crystallinity was lower at higher drying temperature. The lower degree of crystalline represented
more disruption of crystalline region, thereby allowing more rapid starch digestion and higher GI value.
Table 3 Degree of crystallinity of waxy rice
flour at different drying temperatures
Degree of
Processing condition
Relative intensity
crystallinity (%)
(d) Reference 14.54
(c) T = 90 °C, t = 120 min 13.85
(b) T = 130 °C, t = 30 min 13.11
(a) T = 150 °C, t = 30 min 11.79
5 10 15 20 25 30 35
Diffraction angle (2)
40 45 50
T = temperature (C), t = tempering time (min)
Figure 4 X-ray diffraction patterns of waxy rice flour (a) reference, (b) T = 90 °C, t = 120 min,
(c) T = 130 °C, t = 30 min, (d) T = 150 °C, t = 30 min (T = temperature, t = tempering).
Summary
High-temperature fluidized-bed drying caused a starch to be gelatinized. The higher degree of waxy rice
starch gelatinization was found at higher drying temperature. During gelatinization of waxy rice starch, the
crystalline region was disrupted and this disruption affected the change of morphology and the starch digestion.
GI value increased with increasing drying temperature.
Acknowledgements
The authors express their sincere appreciation to the Commission on Higher Education for supporting the
study financially. Author Jaiboon thanks the Commission on Higher Education for supporting her doctoral study
through a grant fund under the Strategic Scholarships for Frontier Research Network Program.
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