EXOTHERMIC TRANSITIONS ON COOLING OF GELATINIZED NATIVE RICE STARCH STUDIED BY DIFFERENTIAL SCANNING CALORIMETRY

Journal of Physical Science, Vol. 18(2), 37–47, 2007
37

EXOTHERMIC TRANSITIONS ON COOLING OF GELATINIZED
NATIVE RICE STARCH STUDIED BY DIFFERENTIAL
SCANNING CALORIMETRY

A.A. Karim1*, Y.P. Chang1, A. Fazilah1 and I.S.M. Zaidul2

1Food Biopolymer Research Group, Food Technology Division,
School of Industrial Technology, Universiti Sains Malaysia,
11800 USM Pulau Pinang, Malaysia
2Department of Food Science, Faculty of Food Science and Technology,
Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

*Corresponding author: akarim@usm.my

Abstract: Differential Scanning Calorimetric (DSC) experiments were designed to
investigate the exothermic events on programed cooling of gelatinized native rice starch-
water system. These exothermic transitions, with peak temperatures of 85oC–127oC, were
attributed to amylose-lipid complexes recrystallization. Starch concentration and cooling
rate showed significant effect on the manifestation of these transitions. The 1:1 starch to
water ratio system showed broader and flattens exothermic transitions at higher peak
temperatures (120oC–127oC) which were stable over different cooling rates. On the other
hand, the 1:2 and 1:3 starch to water ratio systems showed sharpen and narrow
exotherms at lower peak temperatures (85oC–100oC) which became smaller with lower
cooling rate. These observations suggest the presence of two types of amylose-lipid
complexes in native rice starch-water system.

Keywords: rice starch, thermal analysis, exothermic transitions, amylose-lipid complex

Abstrak: Eksperimen menggunakan Kalorimeteri Pengimbasan Pembezaan (DSC) telah
direkabentuk untuk mengkaji kejadian eksotermik apabila sistem kanji beras natif-air
disejukkan. Peralihan eksotermik yang diperhatikan antara suhu puncak 85oC–127oC
telah dicadangkan disebabkan oleh penghabluran kompleks amilosa-lipid. Kepekatan
kanji dan kadar penyejukan menunjukkan pengaruh yang signifikan terhadap manifestasi
peralihan ini. Sistem kanji-air dengan nisbah 1:1 menunjukkan peralihan eksotermik
yang lebar dan rata pada suhu yang lebih tinggi (120oC–127oC) dan ianya stabil pada
kadar penyejukan yang berbeza. Sebaliknya, sistem kanji-air 1:2 dan 1:3 menunjukkan
eksoterma pada suhu yang lebih rendah (85oC–100oC) yang menjadi semakin kecil pada
kadar penyejukan yang lebih rendah. Pemerhatian ini mencadangkan kehadiran dua
jenis kompleks amilosa-lipid dalam sistem kanji natif-air.

Kata kunci: kanji beras, analisis terma, peralihan eksotermik, kompleks amilosa-lipid

---

Thermal Transitions on Cooling of Rice Starch 38
1. INTRODUCTION

Differential scanning calorimetry (DSC) is a technique commonly
employed to probe thermal properties of starch based on the heat flow changes
associated with both first-order (melting) and second-order (glass transition)
transitions of polymeric materials.1,2 Normal cereal starches contain lipids (in
quantities around 1% on a dry weight basis.3–5 It has been shown that in rice
starches, the internal granular lipids are mainly free fatty acids and
lysophospholipids.6 The lipids exist as amylose-lipid inclusion complexes in
native starch granules5,7 or the amylose-lipid complexes were formed during
starch gelatinization.8 Amylose-lipid complexes have been shown to affect many
technologically important properties of starch-containing foods by changing the
granule swelling, solubilization, and crystallization of starch polymers.8 In the
case of native rice starch, other than the gelatinization peak (starch crystallite
melting), programed heating in a DSC produced melting and crystallization of
amylose-lipid complexes within the starch granule. An exotherm was recorded
between 110oC and 120oC, which Biliaderis et al.1 suggested was the result of
crystallization of starch-lipid complexes. Generally, amylose-lipid complexes
melt (endothermic transition) in the temperature range of 85oC–130oC. The
transition is reversible, as an observable exotherm appeared on the DSC cooling
curves,9,10 which is well-defined and more reproducible2 as compared to those
multiple melting thermal profiles on the DSC heating curves.

It is believed that native amylose-lipid complexes formed during cooking
or processing do affect the storage stability, for example retrogradation of starch-
containing food.2,11 With the increase in application of rice starch in various food
products, (e.g. as a fat replacer in food), the aim of the present work was to
investigate the effects of starch concentration, cooling rate and heating-end-
temperature (or cooling-start-temperature) on the manifestation of amylose-lipid
complexes transition present in native rice starch during cooling in DSC
experiment which may have practical significance on food quality and stability.


2.
MATERIALS AND METHOD

2.1
Materials

Rice starch was obtained from Sigma Chemical Company, St Paul, MO,
USA. Rice starch was defatted by Soxhlet extraction (7 h) with n-propanol/water
(3:1, v/v), based on the method of Vasanthan and Hoover.12 The initial moisture
content of the starch was determined from the loss in weight on drying triplicate
samples at 105oC to constant weight.

---

Journal of Physical Science, Vol. 18(2), 37–47, 2007 39
2.2
Determination of Amylose Content

Amylose content of the rice starch was determined using the
spectrophotometric method described by Jarvis and Walker.13 Amylose (Type III
from potato, amylopectin free, from Sigma Chemical Company, St Paul, MO,
USA) and amylopectin (from potato, Fluka Company, Switzerland) were used as
standards.

2.3 DSC
Measurement

A modulated DSC (Q100, TA Instruments Inc., New Castle, Del.,
U.S.A.) was used. The studies were carried out for rice starch : water ratio of 1:1,
1:2 and 1:3 (dry weight basis). Starch was accurately weighed to 0.01 mg in a
hermetic aluminium DSC pan, and distilled water was added directly to obtain
total weight of 10.00 ± 0.20 mg. The sample pans were hermetically sealed and
equilibrated at room temperature for at least one hour. The pan was then placed in
the DSC cell, heated from 20oC to 120oC or 140oC at 5oC min–1 and cooled at
1oC min–1, 5oC min–1 or 10oC min–1 with a constant purge of nitrogen gas at 50
ml/min. An empty aluminium pan was used as the reference to balance the heat
capacity of the sample pan. All measurements were performed in duplicate. Heat
flow and temperature were calibrated using pure indium. Data analysis was
carried out using the Thermal Advantage Q series Q100-0021 (TA Instruments
Inc., New Castle, Del., U.S.A.). The cooling curves were defined by the complete
recrystallization temperatures (Tcxo), peak transition temperatures (Tcx) and their
exothermic enthalpies (?H).

2.3
Statistical Analysis

The data was statistically analyzed by one-way ANOVA (for comparing
more than two means), using SPSS Version 12.0 For Windows (SPSS Inc.,
Chicago, Illinois). Duncan test was also carried out to perform comparison of
means at 95% probability level.


3.
RESULTS AND DISCUSSION

3.1
Effects of Defatting on Thermal Transitions

The amylose content of rice starch, determined by amylose-iodine blue
complex, was 19.7%. The gelatinization (heating) curves as well as cooling
curves of native and defatted rice starches are presented in Figures 1 and 2.
Defatting did not appear to alter the main gelatinization temperature of rice

---

Thermal Transitions on Cooling of Rice Starch 40
starch. Similar observation has been found in other cereal and cassava starches as
reported by Vasanthan and Hoover.12 However, the smaller endotherm peak at
about 100oC (curve a), has disappeared after lipid removal (curve b) on DSC
heating scan as shown in the heating curves (Fig. 1). After gelatinization, native
rice starch-water showed a distinct exothermic phase transition at 80oC to 85oC
when it was cooled from 120oC (curve a). However, lipid removal from the starch
seems effectively erased the above-mentioned transition (curve b), and no other
additional exothermic peak present at lower temperature (<70oC) as depicted in
Figure 2. This observation has eliminated the possibility of amylose chain
association which gives rise to exothermic transitions at <70oC as reported by
Sievert and Würsch.14 It confirms that the exotherms observed on DSC cooling
scan were due to the formation of amylose-lipid complexes present in native rice
starch. A similar observation has been reported on whole grain milled rice and
milled rice flour.11










0.1 W g–1









Figure 1: Heating curves of (a) native rice starch-water system and (b) defatted
rice starch-water system (starch to water ratio of 1:3).

---

Journal of Physical Science, Vol. 18(2), 37–47, 2007 41




0.05 W g–1
)

–1

(W g
ow

fl

h
e
at
c

ermi
th

do
En






Figure 2: Cooling curves of (a) native rice starch-water system and (b) defatted
rice starch- water system (starch to water ratio of 1:3).

3.2
Effects of Starch-water Ratio

Figure 3 shows the exothermic phase transitions during cooling at 5oC
min–1 on gelatinized native rice starch-water matrices of 1:1, 1:2 and 1:3 (w/w
dry starch basis) from 140oC. The exothermic event occurred right after the
cooling process was started from 140oC for 1:1 gelatinized starch-water system.
Therefore, no observable phase transition was shown for 1:1 gelatinized starch-
water system when it was cooled from 120oC (Table 1). The phase transitions
were broad and flatten with Tcxo of ~119oC for 1:1, sharpen prominently and
bigger for 1:2 but became slightly smaller and broader for 1:3, with Tcxo of ~89oC
and 85oC, respectively. There was an increase in both Tcxo and Tcx with increased
starch concentration, however 1:2 showed the highest enthalpy of crystallization
(~ 2.0 J/g). Table 2 gives the Tcxo, Tcx and ?H data obtained in which starch to
water ratio of 1:1 showed Tcx ranged from 120oC to 127oC whereas 1:2 and 1:3
systems showed Tcx ranged from 85oC to 100oC.

We suggest that the exothermic peaks showed for 1:1 starch-water
systems represent different types of crystalline form from those present in 1:2 and
1:3 starch to water ratio systems. It has been reported that two forms of amylose-
lipid complexes exist in many starch-water systems.15–17 In type I complexes, the
helical segments are randomly distributed, it has a lower Tp (melting peak during
DSC heat scan) and is assumed to be formed when rapid nucleation occurs, and
have little crystallinity which might not be detected using X-ray diffraction.

---

) –1


(W g

e
a
t
flow


i
c
h


erm

d
oth

En







Figure 3: Manifestation of the exothermic event during cooling of 1:1, 1:2 and
1:3 gelatinized native rice strch-water matrix from 140oC at 5oC min–1


Table 1: Tcxo, Tcx and ?H of the exotherm obtained on cooling at 1, 5 and 10oC
min–1 of gelatinized rice starch-water matrix from 120oC.

Starch to
Cooling rate oC
*Exothermic event on cooling
water ratio
min–1
Tcxo oC
Tcx oC
?H J/g
1:2 1
99.2 ± 0.53a
103.2 ± 0.52a
0.59 ± 0.06a
5
91.7 ± 0.31b
97.0 ± 0.19b
0.99 ± 0.02b
10
86.0 ± 0.19c
92.7 ± 0.86b
1.15 ± 0.10b
1:3 1
89.7 ± 1.58a
91.9 ± 1.46a
0.76 ± 0.08a
5
84.7 ± 0.77b
87.5 ± 0.69b
1.90 ± 0.14b
10
83.1 ± 0.27c
86.3 ± 0.67b
1.37 ± 0.15b

There were no observable transitions for 1:1 gelatinized rice starch-water systems within the temperature range
studied.
*Mean ± standard deviation (n = 2). Means within a column (compared within same starch to water ratio) with
the same letter are not significantly different at p < 0.05.

---

Journal of Physical Science, Vol. 18(2), 37–47, 2007 43
Table 2: Tcxo, Tcx and ?H of the exothermic obtained on cooling at 1, 5 and 10oC
min–1 of gelatinized rice starch-water matrix from 140oC.

Starch to
Cooling rate
*Exothermic event on cooling
water ratio
oC min–1
T cxo oC
Tcx oC
?H J/g
1:1 1
119.5 ± 2.06a
124.6 ± 0.94ab
0.92 ± 0.35a
5
118.7 ± 0.15a
127.1 ± 0.07a
0.96 ± 0.16a
10
108.5 ± 2.12b
120.2 ± 2.88b
0.80 ± 0.03a
1:2 1
94.4 ± 1.87a
99.3 ± 2.3a
0.81 ± 0.06a
5
89.3 ± 0.47b
91.9 ± 0.92b
2.00 ± 0.18b
10
88.1 ± 0.09b
90.6 ± 0.12b
1.88 ± 0.13b
1:3 1
89.5 ± 0.20a
91.6 ± 0.23a
0.72 ± 0.3a
5
85.1 ± 1.48b
87.9 ± 1.58b
1.40 ± 0.02b
10
83.3 ± 0.42b
86.7 ± 0.49b
1.65 ± 0.35b

*Mean ± standard deviation (n = 2). Means within a column (compared within same starch to water ratio) with
the same letter are not significantly different at p < 0.05.

Type II complexes are packed in a crystalline register which is believed to have a
lamellar-like organization of amylose complexes; i.e., the polysaccharide chains
are so folded as to have their chain axes perpendicular to the surface of the
lamella and exhibit a Vh-type crystallinity.18 Tufvesson and Eliasson16 have
reported that in potato starch-monoglyceride-water systems, type I complex
started to melt at 88.5oC and type II at 112.9oC during DSC measurements. It is
well-documented that the transition of amylose-lipid complex is heat-
reversible.9,10 Therefore, our observations on Tcxo ranged from 83oC to 94oC for
1:3 and 1:2 systems, and 109oC–120oC for 1:1 system conformed with the type I
and type II complexes behavior. It is anticipated that low moisture contents shift
the melting temperatures of inclusion complexes to higher temperatures,
crystalline amylose-lipid complexes (type II) are readily formed during cooling
of 1:1 starch-water system in which annealing and recrystallization are likely to
occur. For 1:2 and 1:3 systems, a decrease of Tcxo and Tcx with decrease in starch
to water ratio is conceivable if we considered the recrystallization as well as the
melting of amylose-lipid complex (type I) are solvent-facilitated processes, thus
the presence of more water promotes higher mobility of the whole system and
made the recrystallization process appeared at lower temperature. The enthalpy of
exotherm of 1:2 system was higher compared with the enthalpy of exotherm of
1:3 system, was most probably due to concentration effect, in which the
availability of amylose and lipid were higher in 1:2 system.

---

Journal of Physical Science, Vol. 18(2), 37–47, 2007 44

3.3
Effects of Cooling Rate

Figure 4 gives a comparison of the DSC thermograms obtained using
cooling rates of 1, 5 and 10oC min–1 on the 1:2 system. Generally, the transitions
occurred at lower temperatures and were larger when a faster cooling rate was
employed. Slower cooling rate of 1oC min–1 produced much broader and smaller
crystallization peak as compared to cooling rates of 5oC min–1 and 10oC min–1.
Similar observations were obtained for 1:3 systems (Table 2). However, there is
no specific trend observed for 1:1 systems in which we supposed it was due to
the crystalline formed (type II) were more stable and not affected by cooling rate
significantly. In contrast, the exothermic peak at ~85oC–100oC shown for the 1:2
and 1:3 samples cooled at 10oC min–1 were the largest, became smaller when it
was cooled at 5oC min–1, and nearly diminished when it was cooled at 1oC min–1.
This observation strengthen our assumption that the complexes formed at 1:2 and
1:3 systems are mainly type I crystalline which would not be stable during
prolonged heat treatment17 (cooling rate as low as 1oC min–1 from 140oC, in this
case, similar to prolonged heat treatment). Tufvesson and Eliasson16 have stated
that it was possible to transform amylose-lipid complex from type I into type II
when it was heat-treated, as type I was then partially or totally melted. In
addition, there was an increase in Tcxo (from 88oC to 94oC for 1:2 system, 83oC to
90oC for 1:3 systems) and Tcx (from 91oC to 99oC for 1:2 systems, 87oC to 92oC
for 1:3 systems) with decreasing cooling rate, again, this point to the fact that low
cooling rate induced annealing which has allowed reorganization of basic
structure segment and results in higher peak transition temperature.

)
100C min–1
–1 g
W

50C min–1

0.1 W g –1

mic heat flow (

10C min–1

Endother




Figure 4: Manifestation of the exothermic event during cooling of 1:2 gelatinized
native rice starch-water matrix from 140oC at 1, 5 and 10oC min–1.

---

Journal of Physical Science, Vol. 18(2), 37–47, 2007 45
3.4
Effects of Heating-end-temperature or Cooling-start-temperature

Table 1 shows the Tcxo, Tcx and ?H data obtained at heating-end-
temperature of 120oC. There is no significant influence of heating-end-
temperature (140oC of Table 1) observed for 1:3 gelatinized native rice starch-
water systems for all the phase transition parameters. However, the Tcxo and Tcx
for the 1:2 system became higher at heating-end-temperature of 120oC but the ?H
showed lower value as compared to heating-end-temperature of 140oC. This may
be attributed to the amount of amylose-lipid complex present at higher starch to
water ratio (1:2) was not completely melted at 120oC as compared to those of 1:3
system, therefore the recrystallization enthalpy showed lower value which
indicates less recrystallization (insufficient melting) if the system was heated
until 120oC, instead of 140oC. However, the system that has not been completely
melted provides less ‘raw material’ for reorganization during the cooling process,
thus higher thermal stability (shown by peak transition temperature) was
achieved.


4.
CONCLUSION

The DSC studies of native rice starch revealed the exothermic event
during cooling was due to amylose-lipid complexes recrystallization. Starch to
water ratio of 1:1 showed vastly different exothermic peak (in appearance and
temperature location in thermograms) as compared to starch to water ratio of 1:2
and 1:3. In addition, the exothermic peak of 1:1 system was not affected by
cooling rate as compared with those of 1:2 and 1:3 systems in which, higher
cooling rate produced larger exothermic peak. It is believed that the differences in
responses exhibited by the native rice starch system to these factors are due to the
presence of two types of amylose-lipid complexes.


5. ACKNOWLEDGEMENT

The authors would like to express their appreciations to the Ministry of
Science, Technology and Innovation, Malaysia (MOSTI) for the financial support
of this project through a Fundamental Research Grant Scheme (FRGS). One of
the authors (C.Y. Ping) gratefully acknowledged MOSTI for the post doctoral
fellowship.

---

Journal of Physical Science, Vol. 18(2), 37–47, 2007 46

6. REFERENCES

1.
Biliaderis, C.G., Page, C.M., Maurice, T.J. & Juliano, B.O. (1986).
Thermal characterization of rice starches: A polymeric approach phase
transitions of granular starch. J. Agric. Food Chem., 34, 6–14.
2.
Cie?la, K. & Eliasson, A-C. (2003). DSC studies of gamma irradiation
influence on gelatinisation and amylose-lipid complex transition
occurring in wheat starch. Radiat. Phys. Chem., 68, 933–940.
3.
Morrison, W.R., Scott, D.C. & Karkalas, J. (1986). Variation in the
composition and physical properties of barley starches. Starch/Stärke, 38,
374–379.
4.
Morrison, W.R. (1988). Lipids in cereal starches: A review. J. Cereal
Sci., 8, 1–15.
5.
Morrison, W.R., Law, R.V. & Snape, C.E. (1993). Evidence for inclusion
complexes of lipids with V-amylose in maize, rice and oat starches.
J. Cereal Sci., 18, 107–109.
6.
Juliano, B.O. (1983). Lipids in rice and rice processing. In P.J. Barnes
(Ed.). Lipids in cereal technology. New York: Academic Press, 305–330.
7.
Morrison, W.R., Tester, R.F., Snape, C.E., Law, R.V. & Gidley, M.J.
(1993). Swelling and gelatinization of cereal starches. IV. Some effects
of lipid-complexed amylose and free amylose in waxy and normal barley
starches. Cereal Chem., 70, 385–391.
8.
Hoover, R. (1998). Starch-lipid interactions. In R.H. Walter (Ed.).
Polysaccharide association structures in food. New York: Marcel
Dekker, 227–256.
9.
Biliaderis, C.G., Page, C.M., Slade, L. & Sirett, R.R. (1985). Thermal
behavior of amylose-lipid complexes. Carbohydr. Polym., 5, 367–389.
10.
Eliasson, A.C. & Krog, N. (1985). Physical properties of amylose-
monoglyceride complexes. J. Cereal Sci., 3, 239–248.
11.
Marshall, W.E. & Normand, F.L. (1991). Exothermic transitions in
whole grain milled rice and milled rice flour studied by differential
scanning calorimetry. Cereal Chem., 68, 606–609.
12.
Vasanthan, T. & Hoover, R. (1992). Effect of defatting on starch
structure and physicochemical properties. Food Chem., 45, 337–347.
13.
Jarvis, C.E. & Walker, J.R.L. (1993). Simultaneous, rapid,
spectrophotometric determination of total starch, amylose and
amylopectin. J. Sci. Food Agric., 63, 53–57.
14.
Sievert, D. & Würsch, P. (1993). Amylose chain association based on
differential scanning calorimetry. J. Food Sci., 58, 1332–1345.

---

Document Outline

• ??
• ??
• ??
• ??
• ??
• ??
• ??
• ??
• ??
• ??
  • ??
  • ??

---