Optimization of instant jasmine rice process and its physicochemical properties
, Prisana Suwannaporn
Institute of Food Research and Product Development, Kasetsart University, Bangkok 10900, Thailand
Department of Food Science and Technology, Kasetsart University, Bangkok 10900, Thailand
Received 4 February 2009
Received in revised form 7 April 2009
Accepted 14 April 2009
Available online 23 April 2009
Quick cooking rice
Instant, or quick-cooking, rice is becoming more popular nowadays. However, it still poses problems with
respect to rehydration time and quality. This study investigated the effects of processing factors: mois-
ture content, pressure and drying temperature on physical and physicochemical properties of instant rice
and its eating quality using response surface methodology (RSM). Hardness, chewiness and the whiteness
index (WI) were used as responses due to their high R
(0.927, 0.633 and 0.836, respectively) and lack-of-
ﬁt. The hardness and chewiness of rice decreased as moisture content and pressure increased. Higher dry-
ing temperatures caused increases in hardness and chewiness. Only pressure and moisture content
affected density, rehydration ratio, and increase in the volume of instant rice, which was due to the
porosity of the kernels. Rehydration ratio had a negative correlation with density (r=0.886) but a posi-
tive correlation with volume increase (r= 0.637). Pressure was the main factor inﬂuencing the pasting
properties of instant rice. All pasting properties of instant rice were far lower than those of milled rice,
but instant rice had higher cold paste viscosity, which is typical of pregelatinized ﬂour. This indicated
rapid water absorption and shorter cooking time. Instant rice processing also caused development of
amylose–lipid complexes observed as the V-type pattern in an X-ray diffractometer.
Ó2009 Elsevier Ltd. All rights reserved.
In modern lifestyles, instant food is becoming more popular.
However, instant rice is still beset by the problems of long rehydra-
tion time and inferior quality compared to cooked milled rice. The
texture of cooked rice is related to its amylose content and the ﬁne
structure of amylopectin. The intra- and/or intermolecular interac-
tions of starch with other components in rice such as protein, lipid
and non-starch polysaccharides results in a harder texture (Ong
and Blanshard, 1995). Moreover, processing conditions also affect
the texture of cooked rice in a way similar to the parboiling pro-
cess. The basic processes in preparing instant rice and parboiled
rice are similar, consisting of soaking, steaming and drying. These
processes have marked impacts on the organoleptic properties of
cooked rice. Derycke et al. (2005a) found that the heat-moisture
conditions during parboiling, cooling and drying had impact on
cooked parboiled rice. They observed that the texture of cooked
parboiled rice was usually ﬁrmer and less sticky than that of
non-parboiled rice. This ﬁrmer texture was related to the level of
crystalline amylose–lipid complexes formed during parboiling
which were stable during the cooking process.
According to Robert (1972), dry grains of instant rice should be
separate and should resemble milled rice in shape. Its bulk density
should be 0.4–0.42 g/cm
with a low percentage of broken kernels.
After rehydration, the volume of instant rice should increase to
1.5–3 times that of dry grains, and its color, ﬂavor and texture
should be similar to cooked rice (Smith et al., 1985) with no hard
core or ungelatinized center (Luh et al., 1980). Previous investiga-
tors tried to propose instant rice processes which were mainly con-
cerned with three main factors: (1) the initial moisture content, (2)
the degree of gelatinization and (3) the drying or pufﬁng method.
The initial moisture content could be manipulated by the temper-
ature of the water used in soaking and/or time. The initial moisture
content has been reported to affect the product’s homogeneity (Baz
et al., 1992), degree of gelatinization, percentage of broken kernels
(Ahromrit et al., 2006) and degree of starch leaching (Bello et al.,
2006). Degree of gelatinization was related to cooking method,
cooking time and/or temperature. Partial gelatinization (around
80%) (Smith et al., 1985) or complete gelatinization either by boil-
ing or steaming have been proposed as necessary in the instant rice
preparation process. High pressure cooking process resulted in
more homogeneous gelatinization and reduced the percentage of
broken kernels (Bhattacharya, 1985; Baz et al., 1992). Drying pro-
cesses varied from single step drying at low temperature (70 °C)
for a long time (2–3 h) to multi-step drying at high temperature
for a short time to induce case hardening followed by low temper-
ature drying for a long time to reduce moisture content (Robert,
1972; Ozai-Durrani, 1948). Other drying methods included the
use of tray dryers or centrifugal ﬂuidized bed dryers (Baz et al.,
1992; Ramesh and Rao, 1996; Carlson et al., 1979), drum dryers
(Robert, 1972; Lewis and Lewis, 1991; Ando et al., 1980), a
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*Corresponding author. Tel.: +662 5625038; fax: +662 5625021.
E-mail address: firstname.lastname@example.org (P. Suwannaporn).
Journal of Food Engineering 95 (2009) 54–61
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freezethaw process (Robert, 1972) and high pressure cooking
(Leelayuthsoontorn and Thipayarat, 2006; Bhattacharya, 1985;
Baz et al., 1992).
In this study, response surface methodology (RSM), an empirical
modeling technique used to estimate the relationship between a
set of controllable experimental factors (Myers and Montgomery,
2002), was applied to optimize the instant rice preparation pro-
cess. The process factors with the greatest effect on instant rice
product quality were investigated together with their interactions.
Moreover, the changes in physical and physicochemical properties
during the instant rice preparation process were also studied in or-
der to understand their relationships with the product and its eat-
2. Materials and methods
2.1. Raw materials
A sample of the inbred KDML 105 rice variety was obtained as
paddy from the Thailand Rice Research Institute. The paddy sam-
ples were dehusked using a McGill sample sheller (dehusker),
and the rice bran was removed using a McGill No. 2 mill. Samples
were milled to a constant degree of milling (DOM = 90). The DOM
was measured using a Satake Milling Meter MM-1B. The white rice
was packed in plastic bags and kept at 4 °C.
2.2. Experimental design
A ﬁve-level, three-variable central composite design (CCD) was
applied to estimate the relationship between variables concerning
texture and the whiteness index (WI) of instant rice. CCD consisted
of eight factorial points, six axial points (two axial points on the
axis of each design variable at a distance of 1.68 from the design
center) and six center points, leading to 20 sets of experiments.
The experiments were run in random order to minimize the effects
of unexpected variability in the observed responses due to extrane-
Milled KDML 105 rice was soaked in water for various soaking
times to obtain moisture contents of 35–60% wet basis
;1.68 to +1.68 level). After soaking, the rice grains were
cooked under pressures of from 11.6 to 28.4 lb/in
(or 8.00 10
to 19.58 10
;1.68 to +1.68 level) for 5 min. Then, the
cooked rice was dried using a tray dryer at temperatures from
166.4 to 233.6 °C(X
;1.68 to +1.68 level) to obtain a product
with a moisture content of less than 12%. The variables and their
process levels are shown in Tables 1 and 2.
Regression analysis was performed based on the experimental
data and was ﬁtted to an empirical second order polynomial model
as shown in the following equation:
where Ywas the response variable, B
were the regression
coefﬁcients of variables for intercept, linear, quadratic and interac-
tion terms, respectively, and x
were independent variables.
For optimization and validation; cooked rice prepared from
electric cooker was used as control sample. The model with no sig-
niﬁcant lack-of-ﬁt and high R
was selected. The optimum condi-
tions were those which gave no signiﬁcant differences from
control sample. Validation was obtained by comparison the attri-
butes that gave no signiﬁcant lack-of-ﬁt and high R
value and prediction value using t-test at 95% level of conﬁdence.
2.3. Density, rehydration ration and volume increase
Dry instant rice was put in 100-ml cylinder and tapped 25–30
times to allow uniform compacking of grain, then recorded the vol-
ume and weighted the instant rice in the cylinder. Density was cal-
Density ¼weight of instant rice ðgÞ
volume of instant rice ðmlÞ:
2.3.2. Rehydration ratio
Rehydration ratio was determined using 10 g of dry instant rice
added with 100 ml water heated by microwave for 6 min, drain the
excess water for 5 min, and then weighed. The rehydration ratio
was calculated as weight of rice before and after cooking:
Rehydration ratio ¼weight of instant rice after cooking ðgÞ
weight of instant rice before cooking ðgÞ:
2.3.3. Volume increase
Volume increase was determined by measuring the volume of
20 g of instant rice before and after cooking by microwave 6 min
using graduated cylinders tapped 25–30 times to allow uniform
compacking of grain. The volume increase calculated as the volume
of rice before and after cooking as:
Volume increase ¼volume of rice after cooking ðmlÞ
volume of rice before cooking ðmlÞ:
2.4. Scanning electron micrographs (SEM)
SEMs of instant rice samples were taken with a Hitachi Table-
Top/Tischmikroskop model TM-1000 at magnitude x 80. The sam-
ples were prepared by breaking a rice kernel at the center and
sticking it on the stub without gold ﬁlm coating.
2.5. Texture proﬁle analysis (TPA)
Thirty grams of instant Jasmine rice was rehydrated with
110 ml water and microwaved for 6 min. TPA was performed using
a texture analyzer (TA-XT.plus, Stable Micro System, UK). Follow-
ing Park et al. (2001), 10 g of rehydrated rice was molded into a
block using a cylindrical container. It was compressed to 60% with
a rod-type probe (2.5 diameters) at a speed of 1.7 mm/s. Hardness,
adhesiveness, springiness, cohesiveness, gumminess and chewi-
ness were determined.
2.6. Whiteness index (WI)
The whiteness of rehydrated rice was measured using a color-
imeter (Chromameter model CR-300, Japan). Measurement was
based on the Hunter system with color values of L,aand b. The
measurements were performed in two replications and were re-
peated three times per replicate. The whiteness index (WI) was
calculated as follows:
Coded levels for independent variables used in developing experimental data.
Factor Code Level
(1.68) 1 0 +1 +
Moisture Content (% wb) X
34.90 40 47.5 55 60.10
11.60 15 20 25 28.40
Drying Temperature (°C) X
166.36 180 200 220 233.64
W. Prasert, P. Suwannaporn /Journal of Food Engineering 95 (2009) 54–61 55
WI ¼100 ð100 LÞ
2.7. Pasting properties
Pasting properties of instant rice were determined using a Rapid
Visco Analyzer (Rapid Visco Analyzer model RVA3D Newport Sci-
entiﬁc Instruments and Engineering, Australia) according to AACC
standard method No. 61–02 (AACC, 1995). A programmed heating
and cooling cycle was used at constant shear rate. The sample was
equilibrated at 50 °C for 1 min, heated to 95 °C for 3.48 min, held at
95 °C for 2.70 min, cooled to 50 °C for 3.88 min, and ﬁnally, held at
50 °C for 1.24 min. Total time was 12.30 min. These tests were
done in triplicate. Paste viscosity plotted in arbitrary RVA units
(RVU) versus time was used to determine the peak viscosity (PV),
trough viscosity, ﬁnal viscosity (FV), breakdown viscosity
(BKD = PV trough), and setback viscosity (SB = FV trough).
2.8. X-ray diffraction measurements
One gram of milled rice ﬂour and 1 g of instant rice ﬂour were
packed tightly in rectangular silicon cells. Samples were spread
evenly to obtain a smooth surface. The sample cell was then placed
in sample holder. X-ray diffraction patterns were obtained with a
diffractometer (JEOL, model JDX-3530, Japan) using monochro-
radiation 1.542 ÅA
. The diffractometer was operated
at 40 kV, 45 mA and the spectra scanned over a diffraction angle
(2h) range of 5–40°at a step size of 0.02°2hper second. Percentage
of crystallinity was calculated as the percentage of peak area to the
total diffraction area using this equation (Cheetam and Tao, 1998):
%Relative crystallinity ¼area above the smooth curve 100
total diffraction area above the baseline :
3. Result and discussion
3.1. Process optimization
The responses modeled as linear, quadratic and cubic functions
of the three independent variables were tested for adequacy and
model ﬁtness using ANOVA. The selections of adequate models
were determined using model analysis, lack-of-ﬁt test and R-
square analysis (Chen et al., 2005). The ‘‘lack-of-ﬁt” test compared
the residual error to the pure error from replicated design points.
The model with no signiﬁcant lack-of-ﬁt and high R
The results showed that hardness, chewiness, and whiteness in-
dex had high coefﬁcient of determination (R
) which equaled
0.927, 0.633 and 0.836, respectively, and no signiﬁcant lack-of-ﬁt
(Table 3). Park et al. (2001) reported a high correlation between
the instrumental texture parameters hardness and chewiness and
the sensory attributes of cooked rice. Likewise, Prakash et al.
(2005) found positive correlation between instrumental hardness
and sensory hardness and chewiness in thermal processed rice.
As a consequence, hardness and chewiness were used as the main
responses in RSM for the texture of instant rice.
3.2. Effect on texture
Estimation of texture in terms of hardness and chewiness over
independent variables X
was shown in Figs. 1 and 2.
3.2.1. Effect on hardness
Fig. 1 showed that the hardness of rice decreased as pressure in-
creased at lower moisture content but the reverse result was ob-
tained for higher moisture content. But an increase in drying
temperature caused a harder texture. The statistical analysis in
Table 3 indicated that all variables had a signiﬁcant effect on hard-
ness, especially moisture content in linear, quadratic and interac-
tion terms. At higher moisture content, thinner case hardening
and bigger pore size were noticeable in the SEM images (Fig. 4).
High pressure induced high gelatinization, which also resulted in
a softer texture in the rehydrated rice.
The hardness of cooked instant rice had positive correlation with
density (Table 4). Density was related to the porosity of the struc-
ture of processed rice seen in the SEM images. The cracks and pores
in instant rice permitted rapid entry of water and heat transfer dur-
ing cooking, resulting in a softer texture rehydrated rice.
3.2.2. Effect on chewiness
Chewiness refers to number of chews required to masticate
cooked rice before it was suitable for swallowing or the amount
of work required to chew the sample for sensory evaluation. It
was observed that pressure and drying temperature had main ef-
fects on chewiness (Table 3). If pressure was ﬁxed at the same
moisture content, chewiness also increased with the increase in
drying temperature. Initial moisture content affected chewiness
in the interaction term and with pressure in the quadratic term
(P< 0.05) (Table 3). At low moisture content (mc < 48% wt), chew-
iness was decreased with increased pressure while at high mois-
ture content (mc > 48% wt) the result was contrast, chewiness
trend to increase when increased pressure (Fig. 2).
Variables and their levels employed in a central composite design.
Experiment Moisture content (X
) Pressure (X
) Drying temperature (X
Code value Real value (%) Code value Real value (lb/in
) Code value Real value (°C)
11.68 34.9 0 20.0 0 200.0
2 1.68 60.1 0 20.0 0 200.0
3 0 47.5 1.68 11.6 0 200.0
4 0 47.5 1.68 28.4 0 200.0
5 0 47.5 0 20.0 1.68 166.4
6 0 47.5 0 20.0 1.68 233.6
71 40.0 1 15.0 1 180.0
81 40.0 1 15.0 1 220.0
91 40.0 1 25.0 1 180.0
10 1 40.0 1 25.0 1 220.0
11 1 55.0 1 15.0 1 180.0
12 1 55.0 1 15.0 1 220.0
13 1 55.0 1 25.0 1 180.0
14 1 55.0 1 25.0 1 220.0
15–20 0 47.5 0 20.0 0 200.0
56 W. Prasert, P. Suwannaporn / Journal of Food Engineering 95 (2009) 54–61
3.3. Effect on the whiteness index (WI)
All factors inﬂuenced the whiteness index, especially pressure.
At high moisture content, the whiteness index decreased when
pressure increased (Fig. 3). This result is in accord with the study
on parboiled rice by Bhattacharya (1996), who found that pressure
and steaming time had marked effects on product’s Hunter color.
Yellowish color was prominent when paddy was parboiled for
longer times. Islam et al. (2002) showed that brightness of par-
boiled rice decreased with the increase in steaming temperature.
The deterioration of the whiteness of parboiled rice was more pro-
nounced at higher temperatures. Leelayuthsoontorn and Thipaya-
rat (2006) found that the WI of cooked rice decreased as the
cooking temperature increased. The cooking temperature was
clearly an important factor inﬂuencing WI. However, at low mois-
ture content, the whiteness index also increased with increased in
The instant rice preparation processes were carried out in tripli-
cate to conﬁrm the results. The predicted and observed values are
shown in Table 5. The observed values were not statistically differ-
ent at the 95% conﬁdence level. The results showed that the equa-
tion was able to predict the texture and color of rehydrated
3.5. Effect on rice grain structure
The SEM images showed that instant rice grains had a hollow
structure and were case hardened (Fig. 4). In the soaking step,
the increase in grain dimensions has been attributed to the swell-
ing of starch granules and subsequent widening of cracks in the
grain by water diffusion (Ahromrit et al., 2006). Puffed kernels, in
which could be seen larger hollow centers, occurred noticeably
Regression coefﬁcients of the polynomial function and the coefﬁcients of determination (R
Coefﬁcient Hardness Adhesive Springiness Cohesive Gumminess Chewiness WI
943.6 0.97 0.41 2,439.98 2,355.99
38.71 0.013 0.0219 32.55 61.67
30.8 0.005 0.0018 97.81 82.90
38.35 0.002 0.003 94.64 97.82
41.34 0.002 0.0024 155.07 157.06
53.95 0.004 0.0133 35.74 46.41
32.79 0.008 0.0029 11.29 7.86
52.59 0.001 0.005 108.86 106.56
9.1 0.0006 0.001 7.49 8.76
26.86 5E05 0.002 17.15 19.00
0.927 0.377 0.274 0.462 0.553 0.633 0.836
0.072 0.425 0.113 0.043 0.032 0.062 0.054
Fig. 1. Contour plot of hardness of rehydrated instant rice as a function of instant
rice process conditions.
Fig. 2. Contour plot of chewiness of rehydrated instant rice as a function of instant
rice process conditions.
W. Prasert, P. Suwannaporn / Journal of Food Engineering 95 (2009) 54–61 57
when both pressure and drying temperature increased altogether.
The pufﬁng phenomenon resulted from the sudden expansion of
water vapor in the granule. In the drying step, moisture was re-
moved from the surface of the rice grains faster than from the inte-
rior (Baz et al., 1992). The surface of rice became slightly harder
than the center during drying (Lin and Jacobs, 2002). This caused
case hardening, which blocked water vapor leaching during the
drying process and resulted in pufﬁng of the grain, giving it a lar-
ger, hollow structure in the center that allows easy rehydration.
3.6. Effect on density, rehydration ratio and volume increase
The results in Table 6 show that only pressure and moisture
content affected density, rehydration ratio and volume increase.
Leelayuthsoontorn and Thipayarat (2006) also found that high
pressure caused larger pore size and thickness with a sponge-like
texture. As a consequence, low density rice was obtained under
conditions of high moisture and high pressure. The rehydration ra-
tio had a negative correlation with density (r=0.546) and a posi-
tive correlation with volume increase (r= 0.542) (Table 4). Rice
was rehydrated more rapidly because of the increase in its surface
area as its volume increased.
3.7. Effect on pasting properties
The integrity of the starch granule and its hydration properties
can be investigated easily by measuring the pasting behavior of
rice ﬂour before and after treatment. All pasting properties of in-
stant rice were far lower than those of milled rice (Fig. 5), but in-
stant rice showed higher cold paste viscosity, which is the typical
hydration property of pregelatinized ﬂour (Lai, 2001).
The disruption of molecular order within starch granules during
steaming caused loss of starch granule integrity and the destruc-
tion of crystallinity, resulting in cold soluble starch (Lai, 2001)
and a decrease in the magnitude of all pasting properties. A similar
observation was previously reported (Hagenimana et al., 2006).
Unlike pregelatinized starch, peak viscosity was still observed in
instant rice ﬂour, which indicated the presence of partially ungel-
atinized starch polymers.
Increase in pressure caused degradation and gelatinization of
starch. The RVA proﬁle of instant rice indicated that instant rice
could absorb water more rapidly and required a shorter cooking
time than milled rice. It indicated granule rigidity and molecular
re-association, which were signiﬁcantly enhanced by the hydro-
thermal treatment. The change in pasting properties depended
mainly on the combined effects of moisture, pressure and drying
temperature. However, the formation of disulphide bonds in the
protein fraction and the complexation of lipid with amylose also
Correlation coefﬁcients between texture and physical properties of instant rice.
Texture Density (g/cm
) Rehydration ratio Volume increase (ml)
Hardness .564* .291 .127
Adhesiveness .123 .133 .020
Springiness .169 .284 .249
Cohesiveness .758** .756** .481
Gumminess .102 .194 .202
Chewiness .152 .125 .144
*, ** signiﬁcant at P< 0.05 and 0.01, respectively.
Fig. 3. Contour plot of whiteness index of rehydrated instant rice as a function of
instant rice process conditions.
Predicted and observed values for the response variables.
Response variables Predicted value Observed value
Hardness 5958.30 5929.77
Chewiness 2392.74 2348.53
Whiteness index (WI) 77.88 77.89
Not signiﬁcant for t-test at 95% level conﬁdence.
Analysis of variance showing the effect of variable as a linear terms and interaction
(cross product) on response parameters (P< 0.05).
Source Density (g/cm
) Rehydration ratio Volume Increase (ml)
0.444 1.457 2.707
0.022 2.322 11.185
2.319 0.160 0.953
3.641 0.017 1.509
3.572 0.906 1.124
Signiﬁcantly different at the 95% conﬁdence level.
58 W. Prasert, P. Suwannaporn / Journal of Food Engineering 95 (2009) 54–61
had effects. Derycke et al. (2005b) suggested that the formation of
disulphide bonds during the parboiling process restricted starch
granule swelling capacity. Moreover, the formation of starch lipid
complexes also restricted the swelling capacity, hence lowering
the pasting properties.
3.8. Amylose–lipid complexes
Amylose–lipid complex formation depended on both heat and
moisture applied during the instant rice preparation process. A-
type crystallinity was either greatly reduced or completely
Fig. 4. Scanning electron micrographs transverse of instant rice (dry grain) at different pressure and drying temperature treatment at (a) 40% moisture content (b) 55%
W. Prasert, P. Suwannaporn / Journal of Food Engineering 95 (2009) 54–61 59
destroyed. The effect of process conditions on the formation of
crystalline amylose–lipid complexes was investigated using XRD
as shown in Fig. 6.
The X-ray patterns of milled rice showed the A pattern which dis-
appeared in instant rice. Instant rice showed the V-type pattern, or
the intensity of the reﬂections at 2h= 13 and 20°. This result indi-
cated that the instant rice preparation process destroyed the crystal-
line structure of the starch granules. This result agrees well with
Miyoshi (2002) and Shih et al. (2007) found that the formation of
amylose–lipid complexes occurred during heat-moisture treatment
of starch, as indicated by the occurrence of the V-type pattern. Before
gelatinization, starch had limited binding capacity with lipid be-
cause most of the lipid in the system was unable to come into con-
tacted with starch (Pilli et al., 2008). After gelatinization, the
V-type pattern was detected either because more complexes were
formed during heating or the crystalline regions had increased in
size. The V-type crystal pattern was greatest when the highest pres-
sure was used in the process. A similar result was also observed in the
parboiling process, in which amylose–lipid complexes were created
during the heating step. The diffraction lines of amylose–lipid com-
plexes increased progressively with the degree of parboiling. (Priest-
ley, 1976; Biliaderis et al., 1993; Derycke et al., 2005a).
Processing conditions affected the physical and physicochemi-
cal properties of instant rice. The hardness and chewiness of rehy-
drated rice was decreased as moisture content and pressure
increased. At high moisture content, the whiteness index de-
creased when pressure increased. Only pressure and moisture con-
tent affected density, rehydration ratio, and volume increase
because they increased the kernel’s porosity, as demonstrated by
the SEM image. The cracking and pufﬁng in the structure of instant
rice were important to the texture of the rehydrated rice. Pressure
was the main factor inﬂuencing the pasting properties of instant
rice. All pasting properties of instant rice were far lower than those
of milled rice. But instant rice showed higher cold paste viscosity,
which is the typical hydration property of pregelatinized ﬂour. The
instant rice preparation process caused development of amylose–
lipid complexes observed as the V-type pattern in the X-ray
This research was funded by Kasetsart University Research and
Development Institute year 2007–2008 and partially student fund-
ing from the Graduate School Kasetsart University. Additional
thanks to the Thailand Rice Research Institute, Department of Agri-
culture for their information and support.
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