Saiyavit Varavinita,
Sujin Shobsngobb,
Warunee Varanyanondc
Pavinee Chinachotid,
Onanong Naivikule
a
Department of Biotechnology, Faculty of Science, Mahidol University, Bangkok, Thailand
b
Department of Chemistry, Faculty of Science, Mahidol University, Bangkok, Thailand
c
Institute of Food Research and Product Development, Kasetsart University, Chatuchak, Bangkok,
Thailand
d
Department of Food Science, College of Food and Natural Resources, University of Massachusetts, Amherst, MA, USA
e
Department of Food Science , Faculty of Agro-Industry, Kasetsart University, Chatuchak,
Bangkok 10900, Thailand
Effect of Amylose Content on Gelatinization, Retrogradation and Pasting Properties of Flours
from Different Cultivars of Thai Rice
Presently, rice cultivars are categorized according to amylose content into three groups: low,
medium and high amylose content cultivars. The correlation of amylose content with gelatinization
properties, retrogradation, and pasting properties of eleven cultivars of Thai rice were investigated.
Rice flour was prepared from milled rice by the wet grinding process. Onset (To), peak (TP) and
conclusion (Tc) temperatures of gelatinization, (determined by DSC) were found to be highly
positively correlated with amylose levels. This correlation could be used for prediction of amylose
content of rice flour. Low amylose starch could also be characterized by low degree of
retrogradation (%R). The data obtained from RVA-viscograms (peak viscosity, breakdown, setback,
and pasting temperature) can be used only for characterization of the group of low amylose starches
(waxy rice). It was demonstrated that low amylose rice starch provided the highest peak viscosity
and breakdown and the lowest setback and pasting temperature among the groups investigated.
Keywords: Thai rice cultivars; Amylose content; Gelatinization temperature; Pasting properties
1 Introduction
The physicochemical properties of a certain starch are influenced by the ratio of amylose and
amylopectin in this particular starch. The amylose/ amylopectin ratio was thought to affect both
gelatinization and retrogradation of starch from various botanical sources [1-3]. During gelatinization starch granules swell and form gel particles. In general, starch granules are rich in
amylopectin, the layers being interlaced with strands of open amylose chains. Upon swelling linear
molecules of amylose diffuse out of the swollen granules making up the continuous phase (network)
outside the granules [4]. Waxy starches usually swell to a greater extent than their non-waxy
counterparts [5]. Amylose has been proposed to act as a restraint to swelling [4]. Internal lipids in
native cereal starches have been shown to have effects on the swelling and gelatinization properties
of the granules [6]. The term retrogradation is used to describe changes that occur upon
Correspondence: Saiyavit Varavinit, Department of Biotechnology, Faculty of Science, Mahidol
University, Rama 6 Road, Bangkok 10400, Thailand. Phone: +66-2-245-5650, Fax: +66-2-2463026, e-mail: scsva@mahidol.ac.th.
cooling and storage of gelatinized starch. Initial, short-term development of crystallinity in starch
gels (occurring upon cooling) is attributed to molecular organization and crystallization of the
amylose fraction [7, 8]. A study on the influence of amylose and amylopectin contents on gelatinization and retrogradation properties of different starches (except for rice starch) have shown that
amylose was negatively correlated to the onset and the peak temperatures of gelatinization [2].
These results were later confirmed by another study in wheat starch [9]. The objective of this work
was to correlate amylose contents of rice from various cultivars in Thailand with gelatinization,
retrogradation and pasting properties in order to further understand the role of amylose on
functionality of Thai rice.
2 Materials and Methods 2.1 Materials
Different cultivars of rice were kindly donated by Kasetsart University, Bangkok, Thailand. The
cultivars employed in this study are Horn-Mali (H), Luang 11 (L), Pathumthani 1 (P), Chainat (C),
Plaingam-Prajeenburi (PP), Kordeaw (KD), Kor-Khor 10 (K10), Kor-Khor 6 (K6),
San-Patong (S), Kheaw-Prajeenburi (KP), and Khaw-Tahang (KT).
All chemicals used in this study were of analytical grade and purchased from Merck (Darmstadt,
Germany).
2.2 Preparation of rice flour
Rice (1 kg) was soaked in distilled water for 1 h and ground with a hammer mill under continuous
addition of distilled water to obtain a rice slurry. The slurry was filtered through a press filter to
obtain a rice cake with a moisture content of about 40%. The cake was ground into powder by a
disc mill and dried in a hot air oven (Mem-mert, Schwabach, Germany) at 50 °C overnight. The
dried rice flour was ground and sieved through a 100 mesh sifter to obtain rice flour.
2.3 Determination of some physicochemical properties of rice flour
Proximate analysis: Moisture, protein, fat and ash contents were determined by methods described
in AOAC [10-12].
Apparent amylose contents: Apparent amylose contents of rice flour from different cultivars of
rice (base on weight free of moisture and protein) were measured by the iodine affinity method
[13].
Gelatinization properties: Gelatinization properties of rice flours were examined by a Differential
Scanning Calorimeter (DSC Pyris, Perkin Elmer, Belerica, MA, USA). Starch from each of the
flour samples (based on weight free from moisture and protein) was dispersed in water to give a
starch/ water ratio of 1:2. Each starch suspension was then transferred to an aluminum pan (50 μL)
and hermetically sealed. After equilibration at room temperature for 1 h, the sample was heated
from 20 to 120 °C at a rate of 10 °C/min. An empty pan was used as reference and the DSC was
calibrated with indium. All measurements were done at least in duplicate. The onset (T0), peak (Tp)
and conclusion (Tc) temperatures of gelatinization, and the melting enthalpy (ΔH in J/g of dried
starch) were recorded. After gelatinization, each pan was immediately cooled and stored at 5 °C for
three days, re-scanned at 10 °C/min from 5 to 120 °C and the melting enthalpy was again recorded
(ΔHr). % Retrogradation was calculated by the following equation:
%R = (ΔHr/ΔH)x100
Pasting properties: A Rapid Visco Analyser (RVA) (Series 4V, Newport Scientific Pty. Ltd,
Warriewood, Australia) was employed to investigate the pasting properties of flour samples. In this
assay, 1.5 g (dry basis) of flour was dispersed in 25 mL of distilled water. The heating and cooling
cycles were programmed as follows: The sample was held at 50 °C for 1 min , heated to 95 °C
within 3 min, held at 95 °C for 2 min and subsequently cooled to 50 °C within 2 min.
3 Results and Discussion
Proximate analyses of rice flours produced from different cultivars of Thai rice are presented in
Tab. 1. All of the values used were calculated as the quantity of starch for the analysis of thermal
properties by DSC. The results of amylose contents of starch in the different rice varieties analyzed
by the iodine affinity method are presented in Tab. 2. The gelatinization properties of the starches
from different rice cultivars as studied by DSC are listed in Tab. 3. The onset, peak and conclusion
temperatures (Tab. 3) were plotted against amylose contents and the relationTab. 1. Proximate analysis of flour from different cultivars of Thai ricea.
Tab. 2. Amylose contents [g/100g starch] in different cultivars of rice flour determined by the
iodine affinity methoda.
ship are shown in Figs. 1A-1C. The positive correlation (r) between amylose content and onset,
peak and conclusion temperatures was 0.84, 0.88, and 0.85, respectively. However, no correlation
was observed between amylose content and melting enthalpy. Due to this good relation between
amylose content and gelatinization temperature of the investigated Thai rice starches, the amylose
content could be estimated from the gelatinization temperatures. It was demonstrated that our
investigation gave similar results as a study on maize starches with varying amylose content (066.1%), which indicated that the gelatinization temperatures increased with increasing amounts of
amylose [14].
However, with wheat starch [9], the authors found a decrease in gelatinization temperature with
increasing amylose content. They explained that the starch with higher amylose content contains
more amorphous and less crystalline regions, leading to a lower gelatinization temperature and
enthalpy. In contrast, Obannietal. [15] explained
Fig. 1A. Relationship between amylose content and onset temperature of gelatinization for starch
from different rice cultivars.
Fig. 1B. Relationship between amylose content and peak temperature of gelatinization for starch
from different rice cultivars.
Fig. 1C. Relationship between amylose content and conclusion temperature of gelatinization for
starch from different rice cultivars.
that starch granule crystallites that required less energy to melt would melt first [14].
The values of RVA pasting properties of different cultivars of rice starch are presented in Tab. 4
and a typical RVA curve that points out the various headings listed in Tab. 4 is shown in Fig. 2.
Rices are classified according to their amylose content into three groups, i.e., low amylose content
rice or waxy rices (K10 = 4.87%, K6 = 4.47%, and S = 5.28%), medium amylose content rices (H =
14.63%, and P = 15.45%) and high amylose content rices (L= 21.95%, C = 23.98%, PP = 26.02%,
KD = 24.39%, KP = 26.42%, and KT = 22.76%). The low amylose or waxy rice starch provides
high breakdown (K10 = 20.50, K6 = 20.42, and S = 21.00 RVU), while the medium and high
amylose rice starches provide low breakdown (highest breakdown in these groups is found for PP,
PP = 13.50 RVU). Rice starches with higher breakdown provide higher peak viscosity (K10 =
68.33, K6 = 47.83, and S = 70.83 RVU) except for PP with a peak viscosity of 48.33 RVU.
However, the breakdown for PP is not as high as that of the waxy rice starch (breakdown for PP =
13.50 RVU). It can be demonstrated that the breakdown value can be used for differentiation of low
amylose starches from those of medium and high amylose rices. However, medium and high
amylose rice starches cannot be differentiated. The starch granules of rice with high breakdown
demonstrated the ease of starch granules to be broken upon heating after the maximum swelling at
the peak viscosity. Waxy rice possesses this breakdown which results in the extra stickiness of the
paste.
Pasting temperature of waxy rice starches are among the lowest (K10 = 72.90 °C, K6 = 69.40 °C,
and S = 73.80 °C),
Fig. 2. Typical Rapid Visco Analyser curve.
whereas the other groups of rice starches possess pasting temperatures of more than 90 CC. The
setback value is reflecting the degree of retrogradation of a starch paste. Waxy rice starch provides
the lowest setback (value for K10 = 11.82, for K6 = 6.50, and for S = 10.00 RVU), while other
types of rice starch possess setback values over 25 RVU. In waxy rice starch paste retrogradation
occurs to a lesser extent than in other rice starches. This is in agreement with the degree of
retrogradation (%R) obtained from the DSC. The degree of retrogradation of waxy rice starch is
lower than that of other groups (%R for K10 = 18.24, K6 = 38.24, and S = 30.05). Other groups of
rice starch have a %R of more than 45. It can be concluded that data obtained from RVA
viscograms cannot be used for differentiating between medium and high amy-lose rice starches.
However, the correlation between amylose contents and gelatinization temperatures analyzed by
DSC can be categorized and used for differentiation between low, medium and high amylose rice
starches.
4 Conclusion
Amylose content provided a highly positive correlation with the gelatinization temperatures (onset,
peak and conclusion temperatures) of starch isolated from Thai rice cultivars. The amylose content
in Thai rice starch could be predicted from the gelatinization temperature. The data obtained from
the RVA viscogram (peak viscosity, breakdown, setback and pasting temperature) can be used only
for categorization of the group of low amylose starch (waxy rice). It was demonstrated that low
amylose rice starch provided the highest peak viscosity and breakdown and the lowest setback and
pasting temperature among the groups.
Acknowledgement
We thank the RBD-Program of Thailand's National Science and Technology Development Agency
for financial support.
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(Received: November 16, 2002)
(Revised: March 3, 2003/April 1, 2003)
(Accepted: April 2, 2003)