Effect of hydrothermal treatment of rice flour on various rice... annaporn *

Journal of Cereal Science 51 (2010) 284–291
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Journal of Cereal Science
journal homepage: www.elsevier.com/locate/jcs
Effect of hydrothermal treatment of rice flour on various rice noodles quality
Supawadee Cham, Prisana Suwannaporn*
Department of Food Science and Technology, Kasetsart University, 50 Paholyothin Rd., Jatuchak, Bangkok 10900, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 September 2009
Received in revised form
7 January 2010
Accepted 8 January 2010
The rice noodle industry in Thailand is facing problems regarding rice flour quality. This research aims to
study the effects of hydrothermally modified rice flour on improving rice noodle quality. High-amylose
rice flour (Chai Nat 1 variety) was modified using heat–moisture treatment (HMT) and annealing (ANN).
Response surface methodology (RSM) with face-centered central composite design (FCCD) was applied
to optimize the hydrothermal treatment condition. The effects of treatment conditions – moisture
content; heating temperature and heating time on pasting; rheology; and textural properties of rice flour
gel – were observed. A contour plot showed that all responses using HMT increased when moisture
content and heating temperature increased. But heating time had no significant effect on response
variables. ANN showed a lower response than HMT for all parameters. The optimum modified conditions
were then matched with those of commercial flour for fresh, semi-dry and dry rice noodles; this showed
no significant differences in texture or cooking quality (P0.05). The storage modulus (G0 ) after cooling of
HMT (19,100 Pa) was much higher than that of ANN (5490 Pa). The differences in rheological properties
of both treatments supported their proper uses to achieve various rice noodle qualities.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Hydrothermal treatment
Rheological properties
Rice noodle
Optimization
1. Introduction
The rice noodle industry in Thailand is facing problems with
overuse of additives to enhance eating quality (Thailand Research
Fund, 2009). Native rice flour has poor resistance to shear force,
and low elastic gel-forming ability. Most native starches have
a marked tendency to lose their viscosity and thickening power
during cooking. Rice gel quality is greatly dependent on grain variety,
amylose content and aging period (Pitiphunpong and Suwannaporn,
2009; Takahashi et al., 2005). Rice cultivars with high amylose
content, low gelatinization temperature and high gel consistency are
suitable for rice noodle making (Yoenyongbuddhagal and
Noomhorm, 2002). Rice of intermediate amylose content yields
a softer product with higher cooking loss, while flour from lowamylose rice has no potential for making usable rice noodles
(Jayakody and Hoover, 2008; Jobling, 2004). In commercial practice,
chemically modified starch (such as by cross-linking or acetylation)
Abbreviations: ANN, annealing; cP, centipoise; DCp, heat capacity; FCCD, face
centered central composite design; G0 , storage modulus; G00 , loss modulus; DH,
enthalpy; HMT, heat–moisture treatment; Mc, moisture content; Pa, Pascal; R2,
coefficient of determination; RSM, response surface methodology; Tc, conclusion
temperature; Tg, glass transition temperature; Tgel, gelatinization temperature; To,
onset temperature; Tp, peak temperature.
* Corresponding author. Tel.: þ66 2 562 5038; fax: þ66 2 562 5021.
E-mail address: prisana.s@ku.ac.th (P. Suwannaporn).
0733-5210/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jcs.2010.01.002
together with various hydrocolloids are widely used in the rice
noodle industry.
There is an increasing interest in more natural treatments of rice
flour. Two hydrothermal treatments which are commonly used in
modifying the physicochemical properties of starch are annealing
(ANN) and heat–moisture treatment (HMT). In general, treatment
of starch with excess water (>60% w/w) is referred to as ‘‘annealing,’’ whereas ‘‘heat–moisture treatment’’ is applied to starch with
low moisture content (<35% w/w). Both physical modifications
should apply at temperatures above the glass transition temperature (Tg) but below the gelatinization temperature (Tgel) (Jacobs and
Delcour, 1998; Stute, 1992; Tester and Debon, 2000). Hydrothermal
treatment is known as a process which increases starch crystallinity
and its perfection (Jacobasch et al., 2006; Tester and Morrison,
1990), granule rigidity (Jacobs et al., 1995), polymer chain realignment, partial crystallite melting (Stute, 1992), and starch chain
associations (amylose–amylopectin and or amylopectin–amylopectin) (Ozcan and Jackson, 2003). These hydrothermal treatments
could suppress granule swelling, retard gelatinization and increase
starch paste stability. Starch gel structure is altered and gel hardness increased (Chung et al., 2000; Jacobasch et al., 2006; Lii et al.,
1996; Lim et al., 2001; Liu et al., 2000). Yoenyongbuddhagal and
Noomhorm (2002) reported that HMT of rice flour could enhance
the cooking and textural qualities of rice noodles. Hormdok and
Noomhorm (2007) applied HMT and ANN of rice starch for
noodle making, and revealed the possibility of utilizing these
S. Cham, P. Suwannaporn / Journal of Cereal Science 51 (2010) 284–291
starches in composites with poor quality rice flour to produce
noodles of acceptable quality.
Different types of rice noodles exhibit different eating qualities,
and hence require different treatment conditions. In this study,
response surface methodology (RSM) was designed to optimize the
hydrothermal treatment conditions for fresh, semi-dried and dried
rice noodles in order to improve their qualities and reduce the use
of additives. Pasting, rheological and textural properties of rice flour
gel were studied and compared to those of commercial rice noodle
flours in order to match different rice noodle qualities.
2. Experimental
2.1. Raw materials
High amylose rice flour was used in this study on account of its
good gel forming property that is suitable for rice noodle making.
Moreover, it exhibited lower breakdown value, which reflected
higher stability of the starch granule and less soluble starch
(Suwannaporn et al., 2007). High amylose rice (Chai Nat 1 cultivar)
was obtained as paddy rice from the Thailand Rice Research Institute. The paddy rice was dehusked using a McGill sample sheller
(Rapsco, Brookshire, TX), and the rice bran removed using a McGill
No. 2 mill. Samples were milled to a constant degree of milling
(DOM¼90). The DOM was measured using a Satake Milling Meter
MM-1B (Satake Engineering Co., Ltd., Tokyo, Japan). Polished rice
grains were steeped in water for 4 h and then wet-milled using
a double-disk stone mill (locally made in Thailand). Rice slurry was
centrifuged using a basket centrifuge type H-130 G (Kokusan
Ensinki Co., Ltd., Tokyo, Japan). The rice cake was then dried in
a tray dryer at 455 C until the moisture content reached 10–12%.
Rice flour of particle size 100 mesh was obtained using a hammer
mill (Ultra-Centrifugal Mill, Type ZM 1; Retsch GmbH, Haan,
Germany). Samples were packed in polyethylene bags, sealed, and
kept at 4 C (Gullapanayutt, 2004).
2.2. Methods
2.2.1. Calorimetric profile of rice flour
A differential scanning calorimeter (DSC) (DSC StarÒ System;
Mettler Toledo AG, Greifensee, Switzerland) was used for calorimetric measurements. The instrument was calibrated with indium
and zinc standards before taking sample measurements. All
measurements were conducted under a nitrogen atmosphere.
For glass transition temperature (Tg) measurement, starch
samples (10–14% mc) were heated from 25 to 95 C at a rate of
increase of 1 C/min. The samples were immediately cooled with
liquid N2 to 5 C at a rate of 20 C/min, and then reheated at an
increase of 1 C/min. The Tg was determined as the peak point on
the first derivative curve of the heat-capacity thermogram. The
temperature at the midpoint of the change in slope of the DSC heatcapacity thermogram was taken as the glass transition temperature
(Tg) (Chung and Lim, 2004; Zhong and Sun, 2005).
For calorimetric profile measurement, rice flour slurry (70% mc)
was equilibrated for 1 h at room temperature. The samples were
then scanned in a temperature range of 25–95 C at a heating rate
of 5 C/min, using an empty pan as a reference. Onset temperature
(To), peak temperature (Tp), conclusion temperature (Tc) and
enthalpy of gelatinization (DH) were determined (Adebowale and
Lawal, 2003). Each experiment was done in triplicate.
2.2.2. Experimental design
2.2.2.1. Heat–moisture treatment. The face-centered central
composite design (FCCD) – with three levels of treatment
temperature, heating time and moisture content – was assigned.
285
The FCCD was applied to estimate the relationship between variables on pasting, rheological and textural properties of rice flour
gel. The FCCD consisted of eight factorial points, six axial points 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 extraneous
factors. In HMT, the moisture content of rice flour was adjusted
from 18% to 27% (X3; 1 to þ1 level). The mixture was incubated
overnight in sealed aluminum foil bags, and then heated in a tray
dryer at temperatures of 90–120 C (X1; 1 to þ1 level) for 1–3 h
(X2;1 to þ1 level). Variables, levels and code values are presented
in Table 1.
2.2.2.2. Annealing. The FCCD was selected using three levels of
heating temperature and time. It consisted of six factorial points,
two axial points and five center points, leading to 13 sets of
experiments. The variables, levels and code values applied for the
annealing process in this study are presented in Table 1. Rice flour
was annealed in excess water at a temperature slightly above the
glass transition temperature and below the gelatinization temperature: 60–70 C (X1; 1 to þ1 level) for 12–36 h (X2; 1 to þ1
level). After that, samples were centrifuged to remove excess water,
and then dried using a tray dryer at 455 C until its moisture
content reached 10–12%.
According to the statistical method, a second-order polynomial
function Eq. (1) was assumed to approximate the response under
consideration (Khuri and Cornell, 1987). Where Y was the response
variable, B0, Bi, Bii, Bij were the regression coefficients of variables
for intercept, linear, quadratic and interaction terms, respectively.
Xi and Xj were independent variables. The data reported in the
table were the average value of triplicate observations. Contour
Table 1
Variables, levels and code values employed in a central composite design for
hydrothermal treatment.
Experiment
HMT
Temperature
( C) (X1)
Heating time (h)
(X2)
Moisture
content (%) (X3)
Code
value
Real
value
Code
value
Real
value
Code
value
Real
value
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15–20
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
90
120
90
120
90
120
90
120
90
120
105
105
105
105
105
1
1
1
1
1
1
1
1
0
0
1
1
0
0
0
1
1
3
3
1
1
3
3
2
2
1
3
2
2
2
1
1
1
1
1
1
1
1
0
0
0
0
1
1
0
18
18
18
18
27
27
27
27
22.5
22.5
22.5
22.5
18
27
22.5
Experiment
ANN
1
2
3
4
5
6
7
8
9–13
Temperature ( C): X1
Tempering time (h): X2
Code value
Real value
Code value
Real value
1
1
1
1
1
1
0
0
0
60
70
60
70
60
70
65
65
65
1
1
1
1
0
0
1
1
0
12
12
36
36
24
24
12
36
24
286
S. Cham, P. Suwannaporn / Journal of Cereal Science 51 (2010) 284–291
plots were obtained by using Statistica (version 8) (StatSoft, Inc.,
USA). Predicted values of rice noodle qualities were obtained using
Eq. (1).
Y ¼ b0 þ
3
X
bi Xi þ
i¼1
3
X
i¼1
bii Xi2 þ
2
3
X
X
Bij Xi Xj þ e
(1)
i ¼ 1 j ¼ iþ1
The relationships between the responses (pasting, rheological
and textural properties) were judged by correlation coefficients of
determination (R2). The significance or P-value was decided at
a probability level of 0.05. The calculation of the optimum condition
was performed using a multiple response optimization method.
Condition variables were optimized compared to commercial rice
noodle flour (National Starch & Chemical Co., Ltd., Bangkok,
Thailand).
2.2.3. Pasting properties
Pasting properties of modified rice flour were determined by a
Rapid Visco Analyzer (model RVA3D; Newport Scientific,
Warriewood, NSW, Australia) according to AACC standard method
No. 61-02.01 (AACC, 2000). Rice flour samples were run in duplicate. First, 25 ml of distilled water was added directly to a metal
RVA canister. Then 3.000.01 g of rice flour was weighed, added to
the water, and immediately measured by RVA. These measurements were made using the standard Newport Scientific rice
profile. An initial 10 s of high-speed (960 rotations min1) stirring
was used to disperse the sample prior to the beginning of the
measuring phase at 160 rotations min1. Temperature was held at
50 C for 1 min; then raised to 95 C in 3.5 min; held for 2.5 min;
cooled to 50 C in 3.5 min; and held for 5 min. The RVA instrument
provided the following parameters: peak viscosity (PV), the highest
viscosity during heating; trough (T), the lowest viscosity; breakdown (BD), PV minus T; final viscosity (FV), the viscosity at the
completion of the cycle; and setback (SB), FV minus PV. All values
were reported in cP.
Water absorption ¼
Cooking loss ¼
profile analysis was done using a TA.XT2 texture analyzer (Stable
Micro Systems, Ltd., Godalming, Surrey, UK) using a stainless steel
punch probe (P/6, 6.0 mm diameter). Distance was set at 10 mm
and pretest speed at 1.0 mm/s. Hardness, adhesiveness and
springiness were determined.
2.2.6. Preparation of rice noodles
Rice noodles were prepared using optimized hydrothermal
treatment conditions obtained in Sections 2.2.2.1 and 2.2.2.2.
Control samples were prepared from commercial rice noodle
premix, flour codes SP1, SP2 and SP3 (National Starch & Chemical
Co., Ltd., Bangkok, Thailand), for fresh, semi-dried and dried
noodles, respectively. Rice flour was mixed with water to obtain
a concentration of 40% (w/w). The rice flour slurry was equilibrated
for 1 h; then 50 ml of the rice slurry was spread evenly on a stainless steel plate, steamed for 5 min to complete gelatinization, and
cooled down to room temperature. Fresh rice noodles were
prepared by scraping the noodle sheet into thicknesses of
approximately 1.0 mm, and then cutting into strips of 10 mm width.
Semi-dry noodles (or ‘‘pad Thai’’ style noodles) were prepared from
retrograded fresh noodle sheets by hot-air drying at 150 C for 10
min, and then cutting into small strips of 3 mm width. Dried rice
noodles were prepared by prolonged drying of noodle strips until
the moisture content reached 10–12%.
2.2.7. Determination of rice noodle qualities
2.2.7.1. Cooking qualities. Cooking qualities were determined
according to AACC standard methods No. 66-50.01 and No. 5450.01 (AACC, 1999a,b): 5 g of dried rice noodle was boiled in 150 ml
distilled water for 4 min, drained for 5 min, and then weighed.
Cooking water was evaporated and dried at 105 C to a constant
weight. Cooking loss was expressed as a percentage of dry matter
lost during cooking to dry sample weight. The water absorption was
the percentage of weight increase in cooked rice noodles compared
to dried samples.
ðWeight of cooked noodle Weight of dried noodleÞ
100
Weight of dried noodle
ðWeight of dry matterÞ
100
Weight of dried noodle
2.2.4. Rheological properties
Rice flour suspension was measured using a dynamic rheometer
(Physica MCR 300 Series; Anton Paar, Graz, Austria). Rice flour
suspension (20% w/w) was loaded on the parallel plate of the
rheometer, and a thin layer of low-density silicon oil was gently
applied to the edge of the exposed sample to prevent moisture loss.
Rheological properties of rice flour paste were measured using
a dynamic rheometer equipped with a parallel plate system (50 mm
diameter). The gap size was set at 1000 mm. The strain and frequency
were set at 0.5% and 1 Hz, respectively. Samples were heated from 25
to 95 C and cooled down at a rate of 2 C/min. Samples were run in
triplicate. Data related to dynamic viscoelastic properties during
gelatinization were used to obtain the storage modulus (G0 ).
2.2.5. Textural properties
Rice flour gel was prepared by suspending 20% (w/v) of rice flour
in distilled water and gelatinizing in a boiling water bath for 30
min. Rice gel was then placed in a cylinder tube, cooled and
equilibrated at room temperature for 4 h. The rice gel obtained had
a dimension of 20.0 mm in diameter and 20.0 mm height. Texture
2.2.7.2. Textural properties. Dry and semi-dry rice noodles were
soaked in water for 10 min, boiled in water for 3 min, and immediately cooled in tap water. Cooked noodles were drained on
a stainless steel screen and measured immediately. Fresh noodles
were measured immediately after cooling down to room temperature. Tensile strength of the noodles was measured following the
method of Stable Micro Systems and Bhattacharya et al. (1999). The
noodle strand was wound around the parallel rollers of a spaghetti/
noodle tensile grip analyzer (TA.XT; Stable Micro Systems, UK) to
anchor the sample ends and reduce any slippage. The maximum
force (g) referred to the resistance to breakdown of the noodle. The
distance (mm) at which the strand started to break indicated
extensibility.
A cooked noodle strand of 1 mm thickness was compressed by
a cylinder probe (35 mm diameter) until the deformation
reached 75% at a speed of 1 mm/s. The pause between the first
and second compressions was 0.5 s. From the force/time curve of
the texture profile, textural parameters including hardness and
adhesiveness were obtained. Ten measurements were made for
each sample.
S. Cham, P. Suwannaporn / Journal of Cereal Science 51 (2010) 284–291
287
storage modulus (G0 ) were selected for a further predictive model.
These responses were mainly affected by temperature, moisture
content and time (P0.05). Similarly, the work of Hoover and
Vasanthan (1994) found that a gradual increase in moisture
content and treatment temperature could allow polymer chain
motion, which consequently had a greater effect on enhancing
setback, gel hardness and storage modulus of flour. Therefore, the
contour plots for HMT were generated as a function of temperature
and moisture content, with time held as a constant factor.
Both ANN and HMT apparently increased setback, and storage
modulus of starch gel when temperature, moisture content and
time increased (Fig. 1). The increase in final viscosity and setback
after modification supported the fact that the modification process
tended to increase the region of crystallinity, as a result of the
reorientation of starch granules and their tendency to re-associate
to form a precipitate or gel (Adebowale and Lawal, 2003; BeMiller
and Whistler, 1996).
The highest gel hardness (160 g) was obtained from ANN of rice
flour at 65–70 C for 24 h. Even higher gel hardness (280 g) was
obtained when HMT rice flour was exposed at a temperature
between 105 and 115 C and a moisture content around 18–22.5%.
Temperature and moisture content were the dominant factors
affecting starch granules. Liu et al. (2000) reported that the higher
the temperature and moisture content, the more perfect the crystalline starch granules. These phenomena suggested that starch
granule swelling was restricted by hydrothermal treatment. Each
treatment caused a different alteration of the granular structure.
During HMT, the increase in rheological properties was attributed to the increase in cross-linking between starch chains. This
allowed the formation of more junction zones in the continuous
phase of the gel, resulting in an increase of gel hardness, final
viscosity, setback, and storage modulus (Eerlingen et al., 1996;
Hoover and Manuel, 1996; Shih et al., 2007; Takahashi et al.,
2005). In the case of ANN, temperature and time were the dominant factors affecting gel hardness. The increase in crystalline
perfection of granules by the ANN technique affected starch gel
properties. Crystalline perfection resulted from an increase in
mobility of the amorphous part which facilitated the ordering of
3. Results and discussion
3.1. Glass transition temperature of rice flour
According to the data obtained from the DSC, the glass transition
and gelatinization temperatures of rice flour were 58 C and 75 C
respectively. At low moisture content (14%), water was absorbed
and bound with starch in amorphous phase. Tg was observed as the
temperature at which the molecular mobility of the amorphous
compound became apparent (Blanshard and Lillford, 1993; Huang
et al., 1994; Roos, 1995a,b). It was revealed that the heat capacity
(DCp) of rice flour was distinctly different between the glassy and
gelatinized states. The glass transition of rice flour produced very
small DCp (0.03 J/g), whereas that of the corresponding gelatinized
starch exhibited a substantially higher value (15.07 J/g). The gelatinization endotherm is affected by the crystalline and amorphous
transition. On the other hand, the glass transition endotherm is
affected only the amorphous region which precedes the melting of
crystallites (Roos, 1995a,b).
The glass transition and gelatinization temperatures of rice flour
obtained in this study were applied to determine the ANN and HMT
process conditions applied in our experimental design. Both
physical modifications should apply at temperatures above the
glass transition temperature (Tg) but below the gelatinization
temperature (Tgel) (Jacobs and Delcour, 1998; Stute, 1992; Tester
and Debon, 2000).
3.2. Effect of hydrothermal treatment on rice flour gel
The lack-of-fit test measuring the fitness of the model showed
no significant lack-of-fit, together with a high R2 (Table 2). This
indicated that the model was sufficiently accurate for predicting the
responses of: final viscosity, setback, gel hardness, and storage
modulus of both HMT and ANN.
The equation coefficients of ANN and HMT conditions that affect
responses are shown in Table 2, together with their R2. The responses
studied were better explained by a second-order model. Responses
with high fitting (R20.85); final viscosity, setback, gel hardness and
Table 2
Regression equations and coefficients of determination of the effects of hydrothermal treatment conditions on rice flour gel qualities.
Hydrothermal treatment
condition
Coefficient
ANN
Constant
Linear
Quadratic
Interaction
Response
b0
b1
b2
b11
b22
b12
Lack of fit
HMT
2
Coefficients of determination
R
Constant
Linear
b0
b1
b2
b3
b11
b22
b33
b12
b13
b23
Quadratic
Interaction
Lack of fit
Coefficients of determination
*Significantly different at a 95% confidence level.
ns
Not significantly different at a 95% confidence level.
R2
Final viscosity (cp)
Setback (cp)
16,862.6*
492.7*
22.4*
3.0*
0.7*
0.01ns
10,565.3*
280.0*
4.9*
1.3*
0.5*
0.2ns
Gel hardness (g)
1709.32*
54.96*
6.83*
0.47*
0.03*
0.110ns
Storage modulus (Pa)
19,692.1*
480.0*
93.5*
3.1*
0.3*
1.2ns
0.385ns
0.269ns
0.955ns
0.249ns
0.913
0.901
0.856
0.871
8605.02*
55.71*
62.91ns
317.71*
0.21*
17.36ns
7.18*
0.54ns
0.88ns
3.30ns
2096.75*
78.63*
131.3ns
545.51*
0.52*
60.27ns
9.54*
2.10ns
1.69ns
9.13ns
3531.84*
72.09*
80.51ns
16.57*
0.33*
18.80ns
0.59*
0.40ns
0.09ns
0.88ns
63,010.2*
170.6*
771.8ns
3555.1*
2.4*
654.5ns
34.8*
20.8ns
14.3ns
58.3ns
0.494ns
0.605ns
0.079ns
0.317ns
0.927
0.899
0.849
0.826
288
S. Cham, P. Suwannaporn / Journal of Cereal Science 51 (2010) 284–291
Fig. 1. Contour plot of hydrothermal treatment conditions on physicochemical properties of rice flour and its overlaid of (a) ANN fresh noodle, (b) HMT semi dried rice noodle, (c)
HMT dried rice noodle.
S. Cham, P. Suwannaporn / Journal of Cereal Science 51 (2010) 284–291
289
Table 3
Comparison of the observed and predicted values of the response variables in various rice noodle qualities using a t-test.
Rice noodle
Response variables
Commercial rice flour
Hydrothermally treated rice flour
Fresh noodle
Final viscosity (cp)
Setback (cp)
Hardness (g)
Storage modulus (Pa)
2100–2600
1700–2200
130–160
12,000–14,500
2565.30
2145.80
153.93
14,204.00
2550.77105.11
2048.53171.34
150.19000.69
14,000.00120.24
Semi-dried noodle
Final viscosity (cp)
Setback (cp)
Hardness (g)
Storage modulus (Pa)
3500–4000
1400–1800
260–290
10,000–12,000
3702.76
1516.35
274.32
11,883.78
3650.14112.24
1418.54065.34
270.85019.15
11,500.15 184.05
Dried noodle
Final viscosity (cp)
Setback (cp)
Hardness (g)
Storage modulus (Pa)
4000–4500
1900–2500
260–290
13,000–15,000
4317.91
2067.28
280.73
13,644.40
4255.12000.55
2250.48011.34
275.15010.77
14,350.00124.19
Predicted valuens
Observed valuens
ns
Not significantly different at a 95% confidence level in the same row.
double helices and probably greater ordering primarily in the
amorphous regions (Lin et al., 2008). Rice gel became harder
because of the more ordered rearrangement of starch molecules
during the ANN process. Moreover, hydrothermal treatment
allowed the leached starch molecules to form a more continuous
gel (Biliaderis, 1998; Chung et al., 2000; Lii et al., 1996; Tester et al.,
1998).
proportional to the granule rigidity and inversely proportional to the
swelling ratio. Hydrothermal treatments are known to suppress
granule swelling and retard gelatinization (Takahashi et al., 2005).
At a constant shear-rate applied after peak, G0 of native rice flour
dropped at a higher rate than that of hydrothermally treated flour
due to shear-thinning phenomena. This indicates more rigid and
thermally stable starch granules in hydrothermally treated flour.
3.3. Optimization of hydrothermal treatment conditions
3.4.2. During cooling
On the contrary, G0 during cooling – which indicated gelling
behavior at a lower temperature – increased to a greater extent and
more rapidly in hydrothermally treated flour (Fig. 2). This demonstrated a strong interaction during cooling which caused aggregation between starch granules, and perfection formed due to starch
chain associations during gel storage (Rao and Tattiyakul, 1999). G0
at this point was an index for rice noodle quality similar to the set
back value obtained from a Rapid visco-analyzer (Suwannaporn
et al., 2007). It was used to verify that an elastic and extensible
sheet of rice flour gel could be obtained. At 25 C, G0 of HMT rice
flour gel (19,100 Pa) was much higher than that of ANN (5490 Pa).
ANN was the modification of flour slurry in excess water (>60%)
at temperatures below gelatinization (<70 C); as a consequence,
starch chains were free to move resulting in the mobility within the
amorphous region. The ANN process only improves the crystallinity
The optimization of HMT and ANN treatment conditions for
various rice noodles was determined using their texture and cooking
quality. Conditions that obtained pasting, textural and rheological
properties comparable to commercial rice noodle flour were selected
(Fig. 1). Data points within the optimized area of each noodle
product were selected and run in triplicate for validation purposes.
Results in Table 3 show that there was no significant difference
between predicted value and observed value (P0.05). Rice flour
gel prepared by hydrothermal modification gave similar qualities to
those of commercial rice flour gel.
3.4. Rheological properties of hydrothermally treated rice flour
The rheogram in Fig. 2 shows the temperature-dependent
storage modulus (G0 ) of hydrothermally treated and native rice
flour during heating from 25 to 95 C and cooling to 25 C. Storage
modulus (G0 ) and loss modulus (G00 ) were determined in dynamic
measurements related to the elastic behavior of the dual nature of
the polymer melt, which was partly elastic solid and partly viscous
fluid. Measurements of G0 and G00 provide information on polymer
structure and might be related to molecular weight distribution or
cross-linking (Rao and Tattiyakul, 1999).
3.4.1. During heating
G0 of native and hydrothermally treated rice flour had similar
patterns (Fig. 2). G0 started to increase dramatically at temperatures
around 65 C and reached its peak at 85 C, indicating that both
starch granules could swell and begin to gelatinize at the same
temperature. Hsu et al. (2000) reported that the initial increase in G0
was caused by starch granules swelling progressively and becoming
closely packed in the suspension. The increase in G0 was a result of
structural changes, an irreversible swelling of starch granules,
melting of crystallites, or a leaching of components of starch granules (Rao and Tattiyakul, 1999). However, G0 of native rice flour was
higher than that of hydrothermally treated flour because hydrothermal treatment could retard the swelling power of starch granules. Lii et al. (1996) found that the rigidity of the gel was
Fig. 2. Rheogram to show storage modulus (G0 ) of hydrothermal treatment rice flour
during heating and cooling.
290
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Table 4
Cooking and textural qualities of various rice noodles.
Noodle
sample
Tensile
strength (g)
Hardness
(g)
Adhesiveness
(g.s)
Cooking
yield (%)
Cooking
loss (%)
Fresh noodle
ANN
21.17a
SP1Ò
25.60a
134.35a
135.98a
3.40a
2.16b
98.78a
99.81a
7.85a
6.50a
Semi-dry noodle
HMT
39.20b
SP2Ò
43.82b
200.11b
205.85b
2.34b
1.09c
260.65b
265.70b
10.25c
9.85c
Dry noodle
HMT
51.19c
SP3Ò
54.31c
215.03c
216.90c
3.29a
1.94b
420.50c
435.65c
8.85b
8.45b
a-c
Means in the same column followed by the same lowercase superscript letter are
not different at P > 0.05.
SP1Ò, SP2Ò and SP3Ò are commercial rice flours (National Starch & Chemical Co.,
Ltd.).
of existing helices, thereby ‘‘perfecting’’ the starch crystallites
without formation of additional helices. HMT was the modification
of flour above the gelatinization temperature (100–120 C), and at
restricted moisture content (<30%). On the other hand, HMT
resulted in a partial melting of the crystallites, followed by
a realignment of the polymer chains (Eerlingen et al., 1996). It has
been reported that the number of double helices was increased
during HMT. Moreover, the formation of additional helices was also
promoted (Stute, 1992). Therefore, HMT improved the crystallinity
of starch granules to a greater extent than ANN.
The differences in rheological properties of such treatments
support better uses of hydrothermal treatment for different rice
noodle qualities. Strong and elastic gel forming, with low shear
thinning, were required characteristics for rice noodle flour. However
a softer texture was required for fresh noodles, whereas a stronger
and more elastic gel was suitable for semi-dry and dry noodles.
3.5. Cooking and textural quality of rice noodles
Cooking and textural qualities of rice noodles made by either
hydrothermal modification or from commercial rice noodle flour
were not significantly different (P0.05) (Table 4). Tensile testing,
which assessed the breaking strength of noodles, correlated well
with noodle quality (Bhattacharya et al., 1999). Hardness of rice
flour gel has been reported to be a dominant factor in rice noodle
quality (Bhattacharya et al., 1999; Ozcan and Jackson, 2003;
Surojanametakul et al., 2002; Yoenyongbuddhagal and Noomhorm,
2002; Yunt et al., 1996). Semi-dried and dried noodles had higher
tensile strength and gel hardness than fresh noodles because of the
more retrograded gel induced after the drying process. These types of
rice noodles were difficult to break down, and were more stretchable
after cooking. A more pronounced effect on tensile strength was
obtained by HMT-treated rice flour. On the contrary, fresh rice
noodles required a softer texture with comparatively low tensile
strength after cooking, which could be obtained by ANN. Moreover,
all rice noodles made from hydrothermally treated rice flour showed
lower adhesiveness, which indicated less surface stickiness of cooked
noodles. It gave a ‘‘clean and smooth’’ texture which was a required
quality in rice noodles. Since hydrothermal treatment created more
intact starch granules with a more orderly crystalline starch matrix,
this resulted in a relatively low degree of amylose leaching (Tester
and Debon, 2000).
4. Conclusion
Hydrothermal treatments using HMT and ANN could be offered
as an alternative for the rice noodle industry. Rice noodles prepared
by these methods resulted in similar eating quality to noodles made
from commercial rice flours, but with a safe and completely natural
product concept. Fresh rice noodles require a softer texture, which
could best be obtained by ANN. Semi-dried and dried noodles need
higher tensile strength and gel hardness, for which HMT was more
appropriate. The low amylose leaching in hydrothermally treated
rice flour gave a ‘‘clean and smooth’’ noodle surface, which is
instrumental in the quality of rice noodles.
Acknowledgements
This research was funded by the Kasetsart University Research
and Development Institute (year 2007). Partial student funding was
obtained from the Graduate School, Kasetsart University. The
authors would like to thank the Thailand Rice Research Institute,
the Department of Agriculture, and the National Starch & Chemical
Co., Ltd. (Thailand) for their information and support.
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