Journal of Cereal Science 51 (2010) 57–65
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Journal of Cereal Science
journal homepage: www.elsevier.com/locate/jcs
Retrogradation of waxy and normal corn starch gels by temperature cycling
Xing Zhou a, Byung-Kee Baik b, Ren Wang c, Seung-Taik Lim a, *
a
Graduate School of Life Sciences and Biotechnology, Korea University, Seoul 136-701, Republic of Korea
Washington State University, Department of Crop & Soil Sciences, Pullman, WA 99164-6420, USA
c
Department of Food Science & Technology and Carbohydrate Bioproduct Research Center, Sejong University, 98 Gunja-Dong, Gwangjin-Gu, Seoul 143-747, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2009
Received in revised form
10 September 2009
Accepted 22 September 2009
Gelatinized waxy and normal corn starches at various concentrations (20–50%) in water were stored
under temperature cycles of 4 C and 30 C (each for 1 day) up to 7 cycles or at a constant temperature of
4 C for 14 days to investigate the effects of temperature cycling on the retrogradation of both starches.
Compared to starches stored only at 4 C, both starches stored under the 4/30 C temperature cycles
exhibited smaller melting enthalpy for retrogradation (DHr), higher onset temperature (To), and lower
melting temperature range (Tr) regardless of the starch concentration tested. Fewer crystallites might be
formed under the temperature cycles compared to the isothermal storage, but the crystallites formed
under temperature cycling appeared more homogeneous than those under the isothermal storage. The
effect of starch content on the retrogradation was greater when the starch gels were stored under cycled
temperatures. The reduction in DHr and the increase in conclusion temperature (Tc) by retrogradation
under 4/30 C temperature cycles became more apparent when the starch concentration was lower (20 or
30%). Degree of retrogradation based on melting enthalpy was greater in normal corn starch than in waxy
corn starch when starch content was low.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Temperature cycling
Retrogradation
Corn starch
1. Introduction
The starch granule, commonly composed of both amylose and
amylopectin, is semicrystalline. The crystalline regions in granules
appear in clusters of branched amylopectin chains. Amylose,
mainly linear starch chains, is largely amorphous and randomly
distributed between amylopectin clusters (Bemiller, 2007). When
the starch granule is heated in the presence of water, the semicrystalline structure in granules transforms to an amorphous form;
this process is termed gelatinization. Gelatinized starch, however,
tends to re-associate in an ordered crystalline structure during
storage, which is termed retrogradation (Yuan et al., 1993).
As the retrogradation of starch affects the acceptability and shelf
life of starchy food, its control in rate and degree has been
substantially studied by food scientists. Starch retrogradation
occurs in three phases: nucleation, i.e. formation of critical nuclei;
propagation, i.e. growth of crystals from the nuclei formed; and
maturation, i.e. crystal perfection or continuous slow growth. The
Abbreviations: DR, the degree of retrogradation; DSC, differential scanning
calorimeter; Tc, conclusion temperature; To, onset temperature; Tp, peak temperature; Tr, melting temperature range; DH, melting enthalpy; DHg, melting enthalpy
for gelatinization; DHr, melting enthalpy for retrogradation.
* Corresponding author. Tel.: þ82 2 3290 3435; fax: þ82 2 921 0557.
E-mail address: limst@korea.ac.kr (S.-T. Lim).
0733-5210/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jcs.2009.09.005
overall crystallization rate mainly depends on the nucleation and
propagation rate (Eerlingen et al., 1993). A temperature near the
glass transition temperature favors nucleation, whereas a higher
temperature up to the melting temperature favors propagation
(Baik et al., 1997; Durrani and Donald, 1995; Silverio et al., 2000).
When the storage temperature of gelatinized starch was cycled
between the temperature for nucleation and the temperature for
propagation, the rate of retrogradation could be accelerated
(Bemiller, 2007; Slade et al., 1987). This type of temperature cycling
process that induces a stepwise nucleation and propagation
promotes the growth of crystalline regions and perfection of crystallites (Silverio et al., 2000).
The degree of starch retrogradation and the property of the
starch crystallites formed are influenced not only by the storage
time and temperature, but also by starch concentration (Jang and
Pyun, 1997; Liu and Thompson, 1998; Longton and Legrys, 1981)
and the botanical origin of the starch, i.e. starch crystallinity,
molecular ratio of amylose to amylopectin and structures of
amylose and amylopectin molecules (Elfstrand et al., 2004;
Fredriksson et al., 1998; Jane et al., 1999; Klucinec and Thompson,
1999; Lai et al., 2000; Russell, 1987a; Sasaki et al., 2000; Vandeputte
et al., 2003; Varavinit et al., 2003). Several studies have investigated
starch gelatinization or retrogradation behavior as a function of
a wide range of moisture content. It was generally agreed that
maximum retrogradation enthalpy occurred at the so-called
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X. Zhou et al. / Journal of Cereal Science 51 (2010) 57–65
intermediate water level when the starch concentration was 40–
60%, but the precise relationship between starch concentration and
retrogradation enthalpy varied according to storage temperature
(Jang and Pyun, 1997) and starches of different origin (Liu and
Thompson, 1998). Silverio et al. (2000) have investigated the effect
of temperature cycling on the retrogradation of amylopectins isolated from 10 different starches. Park et al. (2009) found that 40%
waxy corn starch gel retrograded at the cycled temperatures had
a larger amount of resistant starch but remained softer than those
stored at 4 C. However, the influences of amylose content and
starch concentration together with temperature cycling on starch
retrogradation have not been reported.
Differential scanning calorimetry (DSC) has been widely used to
study the thermal behavior of starch gelatinization and retrogradation. The DSC melting endotherm of starch provides the enthalpy
as well as temperatures for melting the crystalline structure in
starch, which reflects the degree and perfection of the crystallinity
(Durrani and Donald, 1995). The melting enthalpy (DH) is often
positively related to the amount of crystals (double or single helical
structure) of the starch (Liu et al., 2006). The onset temperature (To)
represents the melting temperature of the least stable crystallites.
The peak temperature (Tp) suggests the melting temperature for
the majority of starch crystallites. The conclusion temperature (Tc)
indicates the melting temperature of the most perfect crystallites.
The melting temperature range (Tr), i.e. TcTo, indicates the degree
of heterogeneity of the crystallites (Biliaderis, 1992). The higher the
melting temperature (To, Tp, Tc) and the narrower the melting
temperature range (Tr), the more stable and uniform the crystallites
are (Durrani and Donald, 1995).
In this study, temperature cycles of 4 C for 1 day and 30 C for 1
day, together with various starch concentrations (20–50%) were
used to investigate the effects of temperature cycling and starch
concentration on the retrogradation of waxy and normal corn
starches. Crystallinity in the retrogradated starches was evaluated
in the terms of melting enthalpy and temperatures using DSC.
2. Experimental
heating at 105 C for 5 h. Both starches contained crude protein less
than 0.35%, and ash less than 0.15% (manufacturer’ data).
2.2. Gelatinization and retrogradation
The starch (dry solid) was weighed into an aluminum DSC pan
and distilled water was added until the starch was fully wet. The
excess moisture was allowed to evaporate in a balance until the
total weight of starch and water reached 10 mg to achieve the
desired starch concentration of 20, 30, 40 or 50%. The pan was
hermetically sealed, and equilibrated for 24 h at 4 C.
After equilibration the DSC pans were heated in a convection
oven for 15 min at 105 C to gelatinize the starch (Silverio et al.,
2000). After cooling to room temperature (30 min), the DSC pans of
gelatinized starch were stored for 14 days under two different
conditions: at constant 4 C (stored in the cold chamber), or under
the temperature cycles of 4 C for 1 day and 30 C for 1 day (4/30 C).
For temperature cycling, after storage in the cold chamber at 4 C for
1 day, the DSC pans were immediately taken out and stored in
a convection oven kept at 30 C for 1 day, and then this step was
repeated.
2.3. DSC analysis
A differential scanning calorimeter (DSC6100, Seiko Instruments
Inc., Chiba, Japan) was used to determine the thermal characteristics of starch gelatinization and retrogradation. To determine
gelatinization characteristics, the equilibrated DSC pans were
directly heated by DSC from 20 to 120 C at a rate of 5 C/min. For
retrogradation properties, the DSC pans of retrograded starches
were heated under the same conditions every 2 days or after each
temperature cycle. Indium and mercury were used for temperature
calibration, sapphire was used for heat capacity calibration, and an
empty pan was used as a reference. All measurements were performed in triplicate. The respective enthalpy (J/g) was expressed on
a dry starch weight basis. The degree of retrogradation (DR) was
calculated as the ratio of enthalpy of retrogradation to enthalpy of
gelatinization.
2.1. Materials
2.4. Statistical analysis
Waxy and normal corn starches were gifts from Samyang Genex
Company (Seoul, Korea). The moisture content of waxy corn starch
was 13.2% and that of normal corn starch was 12.6%, determined by
All numerical results are averages of at least three independent
replicates. Data were analyzed by one-way analysis of variance
Normal
Waxy
20%
(14.4J/g)
30%
(14.5J/g)
Endothermic Heat Flow
20%
(16.4J/g)
30%
(17.6J/g)
40%
(14.7J/g)
40%
(18.5J/g)
50%
(15.0J/g)
50%
(19.6J/g)
M2
M1
0.1m W
20
40
G
60
80
M1
100
Temperature (°C)
0.1m W
120
20
G
40
60
80
100
120
Temperature (°C)
Fig. 1. DSC gelatinization thermograms of waxy and normal corn starches at various concentrations in water. Data in brackets are values of melting enthalpy for gelatinization (DHg).
Each value is the mean of triplicate measurements.
X. Zhou et al. / Journal of Cereal Science 51 (2010) 57–65
A
59
Normal
Waxy
Endothermic Heat Flow
2d
2d
20%
14d
20%
14d
2d
2d
30%
14d
14d
2d
30%
2d
40%
14d
14d
40%
2d
50%
2d
14d
50%
14d
0 .1 m W
20
30
0 .1 m W
40
50
60
70
20
80
30
Temperature (°C)
B
40
50
60
70
80
Temperature (°C)
Waxy
Normal
1cycle
1cycle
20%
Endothermic Heat Flow
7cycles
1cycle
20%
7cycles
30%
1cycle
7cycles
30%
1cycle
7cycles
40%
1cycle
7cycles
40%
7cycles
1cycle
50%
1cycle
7cycles
50%
7cycles
0.1mW
20
30
0.1mW
40
50
60
70
80
Temperature (°C)
20
30
40
50
60
70
80
Temperature (°C)
Fig. 2. (A) DSC thermograms of waxy and normal corn starches retrograded at 4 C for 2 and 14 days at various starch concentrations; (B) DSC thermograms of waxy and normal
corn starches retrograded for 1 and 7 temperature cycles of 4 C for 1 day and 30 C for 1 day at various starch concentrations.
(ANOVA) using ORIGIN 8.0 (OriginLab Inc., USA). The statistical
significance were determined by Tukey’s test (p < 0.05).
3. Results and discussion
3.1. DSC thermograms for gelatinization
For both waxy and normal corn starches, notable changes in the
DSC thermograms of starch gelatinization occurred when starch
concentration increased (Fig. 1). Multiple endothermic peaks were
observed when the starch concentration was above 40%. While the
endothermic peak (G) around 70 C remained relatively unchanged,
the peak (M1) between 80 and 100 C and the peak (M2), which
appeared at above 100 C in normal corn starch, became more
evident and moved to higher temperatures when the starch
concentration increased. The G and M1 endotherms were associated with starch gelatinization, which occurred by the disruption of
amylopectin double-helices, whereas the M2 endotherm appeared
at above 100 C in normal corn starch was due to the melting of the
amylose–lipid complex (Donovan, 1979; Evans and Haisman, 1982;
Garcia et al., 1997; Jang and Pyun, 1996; Liu et al., 2007; Russell,
1987b). Amylopectins in waxy and normal corn starches have
similar structure profiles (Jane et al., 1999), however the melting
enthalpy for gelatinization (DHg, calculated by the G or G þ M1
endotherm size, depending on the starch concentration) of normal
corn starch was smaller than waxy corn starch at all starch
concentrations tested (Fig. 1), which was probably due to less
amylopectin content in normal corn starch than in waxy corn
starch. There has been much controversy in trying to explain the
presence of the two endotherms (G and M1) for starch gelatinization. Some authors (Donovan, 1979; Evans and Haisman, 1982;
Russell, 1987b) attribute the existence of the two transitions taking
place at different starch contents to heterogeneity in water distribution. Biliaderis et al. (1986) suggests a partial melting, followed
60
X. Zhou et al. / Journal of Cereal Science 51 (2010) 57–65
80
Waxy
Waxy
Normal
Normal
70
60
50
Melting Temperature (°C)
40
30
20
80
70
60
50
40
30
20
2
4
6
8
10
12
14
Storage Time (days)
1
2
3
4
5
6
7
Temperature Cycles
Fig. 3. Onset temperature (To) (open symbol) and the conclusion temperature (Tc) (solid symbol) of waxy (upper) and normal (lower) corn starches at various concentrations: 20%
(square), 30% (round), 40% (up triangle) and 50% (diamond). Starches were retrograded at 4 C for 14 days (left) or under the temperature cycles of 4 C for 1 day and 30 C for 1 day
up to 7 cycles (right).
by reorganization (crystallite perfection) and final melting of perfected crystallites in a DSC scan. Since starch crystallite perfection is
a slow process, it is less likely to occur in a short time, such as
during the DSC scan. Garcia et al. (1997) used SEM and TEM to
observe the structural changes of cassava starch granules during
gelatinization and illustrated that a competition of granules for
water during heating would take place when the starch concentration was high. In other words, the presence of the two endotherms (G and M1) indicates heterogeneity in water distribution
during starch gelatinization. Starch may solubilise in different
manners when its moisture content is changed. After gelatinization, the heterogeneous distribution of water as well as some water
limitation among the starch molecules might influence the rate and
degree of starch retrogradation too.
3.2. DSC thermogram for retrogradation
Waxy and normal corn starches retrograded at a constant
temperature of 4 C exhibited broad endotherms at all concentrations tested (Fig. 2A). The shapes of these retrogradation endotherms varied as a function of starch concentration. After storage
for 2 days, the main peak was observed at about 55 C for 20% waxy
corn starch, moving to a higher temperature as starch concentration increased. A lower-temperature shoulder was apparent for
waxy corn starch of 40% and 50% concentration. After further
storage to 14 days, the major peak after the first 2 days storage was
still evident, but the region of the former lower-temperature
shoulder was much enhanced, becoming the main peak for 40% and
50% waxy corn starch. For 20% and 30% waxy corn starch, only one
peak was apparent, shifting to lower temperatures as the storage
time increased. These observations are in agreement with the
results of Liu and Thompson (1998). The retrogradation thermogram for normal corn starch stored under 4 C was similar to that for
waxy corn starch. However, a main peak and a low temperature
shoulder were only apparent in 50% normal corn starch, and the
peak temperature was lower than that of waxy corn starch.
A broad distribution of the endotherm may indicate the
heterogeneity of retrograded starch crystallites. The development
of a lower-temperature shoulder or the shift of peak temperature to
lower temperature indicates that the increase of the endotherm of
retrograded starch during storage at 4 C largely results from the
growth of less perfect crystallites.
When the starch gels were stored under the 4/30 C temperature
cycles, the thermograms of the retrograded starches exhibited
narrower peaks than those for the starch isothermally retrograded
at 4 C, regardless of concentration (Fig. 2B). It indicates that the
crystallites formed under temperature cycles were more homogeneous than those formed under a constant 4 C. The endothermic
peak for melting of the retrograded starch became larger with the
increase in concentration. Contrary to the starch stored at
a constant 4 C, the peak melting temperature of retrograded starch
at the cycled temperatures increased during storage. Therefore it
X. Zhou et al. / Journal of Cereal Science 51 (2010) 57–65
61
45
Waxy
Waxy
Normal
Normal
40
35
30
Melting Temperature Range (°C)
25
20
15
10
5
45
40
35
30
25
20
15
10
5
2
4
6
8
10
12
14
Storage Time (days)
1
2
3
4
5
6
7
Temperature Cycles
Fig. 4. Melting temperature range (Tr) of retrograded waxy (upper) and normal (lower) corn starches at various concentrations: 20% (-), 30% (C), 40% (:) and 50% (A). Starches
were retrograded at 4 C for 14 days (left) or under the temperature cycles of 4 C for 1 day and 30 C for 1 day up to 7 cycles (right). Starch of 30% concentration retrograded at 4 C
(B) was included for comparison.
appears that more stable crystallites were formed under 4/30 C
temperature cycled storage.
3.3. Melting temperatures for retrogradation
The melting temperature of retrograded waxy and normal corn
starches exhibited similar trends under the two retrogradation
conditions studied (Fig. 3). When stored at a constant 4 C, starch of
low concentration showed lower conclusion temperatures (Tc) than
those of high concentration. After storage at 4 C for 14 days, the Tc
values were 59.6 C and 70.6 C for 20% and 50% waxy corn starch,
respectively, and those for 20% and 50% normal corn starch were
59.5 C and 69.6 C, respectively. The Tc values of both starches at
various concentrations are significantly (p < 0.05) different.
Compared to Tc, the onset temperatures for melting (To) exhibited
smaller changes with concentration. Only To values of 20% and 50%
starches are shown in Fig. 3. The To values were 33.4 C and 31.1 C
for 20% and 50% waxy corn starch, respectively, and 34.3 C and
31.5 C for 20% and 50% normal corn starch, respectively, after
storage at 4 C for 14 days. Consequently, the melting temperature
ranges (Tr) were 26.2 C and 39.5 C for 20% and 50% waxy corn
starch, and 25.2 C and 38.1 C for 20% and 50% normal corn starch,
respectively, after storage at 4 C for 14 days (Fig. 4).
Starch at lower concentration exhibited smaller Tr and lower Tc,
indicating that the crystallites formed at lower starch concentration
were more homogeneous, but less stable to thermal treatment. The
Tc values of both waxy and normal corn starches were little affected
(no difference at p < 0.05) by storage time, whereas the To
increased: from 28.3 to 33.2 C (p < 0.05) in 40% waxy corn starch
and from 26.1 to 32.6 C (p < 0.05) in 40% normal corn starch during
14 days storage at 4 C.
When both waxy and normal corn starches were stored under
the 4/30 C temperature cycling, To values were higher by 18–19 C
compared with To values at constant 4 C, regardless of the starch
concentration studied. Silverio et al. (2000) reported that To was
only controlled by propagation temperature, regardless of the type
of the starch. Storage at 30 C during the propagation step might
melt some unstable crystallites formed at 4 C, which accounted for
the To increase under 4/30 C temperature cycles (Baik et al., 1997;
Durrani and Donald, 1995; Elfstrand et al., 2004; Park et al., 2009;
Silverio et al., 2000). The remaining crystallites could be melted at
higher temperatures. Tc values of both waxy and normal starches at
20% and 30% concentration increased by about 4 C and 2 C after 14
days, respectively, whereas those at 40% and 50% concentration
were relatively unchanged compared with the Tc values at constant
4 C. The greater To but relatively similar Tc under temperature cycled
storage compared to those stored at 4 C resulted in much smaller Tr
values. The crystallites formed under the 4/30 C temperature cycled
storage were more uniform and heat stable than those formed at
a constant 4 C. The stability of the crystallites of both waxy and
normal corn starches was more improved at lower concentration, as
indicated by a large increase in Tc. However, the crystallites of both
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X. Zhou et al. / Journal of Cereal Science 51 (2010) 57–65
18
Waxy
Waxy
16
14
12
Melting Enthalpy for retrogradation (J/g)
10
8
6
4
2
0
18
Normal
Normal
16
14
12
10
8
6
4
2
0
0
2
4
6
8
10
12
14
Storage Time (days)
0
1
2
3
4
5
6
7
Temperature Cycles
Fig. 5. The melting enthalpy for retrogradation (DHr) of waxy (upper) and normal (lower) corn starches at various concentrations: 20% (-), 30% (C), 40% (:) and 50% (A). Starches
were retrograded at 4 C for 14 days (left) or under the temperature cycles of 4 C for 1 day and 30 C for 1 day up to 7 cycles (right). Starch of 30% concentration retrograded at 4 C
(B) was included for comparison.
starches became more homogeneous at high concentration during
storage, as indicated by the large decrease in Tr.
3.4. Melting enthalpy for retrogradation (DHr)
The DHr of waxy corn starch stored at a constant 4 C was
strongly influenced by starch concentration, and maximum retrogradation occurred at 50% starch concentration, whereas that of
normal corn starch was less affected by the studied starch
concentrations (Fig. 5, no significant difference at p < 0.05).
When stored under 4/30 C temperature cycles, the DHr values of
waxy and normal corn starches were relatively smaller compared
to those of starches stored at constant 4 C (Fig. 5). Again, the
propagation temperature might have melted some unstable crystallites formed at 4 C, accounting for the decrease in DHr under 4/
30 C temperature cycles (Baik et al., 1997; Durrani and Donald,
1995; Elfstrand et al., 2004; Silverio et al., 2000). This could also be
regarded as the annealing affect in which more stable crystallites
were developed under the propagation temperature, as indicated
by the higher To, at the expense of the less stable crystallites
(Durrani and Donald, 1995; Shi and Seib, 1995; Silverio et al., 2000).
The reduction in DHr became more pronounced when the starch
concentration was low (Fig. 5). After 14 days’ storage, the DHr of 20%
waxy corn starch was 10.1 J/g when stored at the constant 4 C, but
3.5 J/g when stored under 4/30 C temperature cycles. The DHr of
50% waxy corn starch was 16.3 J/g with a constant 4 C storage and
14.5 J/g with cycled temperature storage (p < 0.05). The DHr values
of 20% and 50% normal corn starches dropped from 9.0 J/g to 6.4 J/g
(p < 0.05) and from 9.4 J/g to 9.0 J/g (no significant difference at
p < 0.05), respectively, by storing starch under the cycled temperature rather than constant 4 C. As discussed previously, the crystallites formed at a lower starch concentration under 4 C were
more homogeneous but less stable. The stable crystallites formed at
lower concentration of starch after storage at 4 C could be due to
the melting of more unstable crystallites during the subsequent
storage at 30 C. This melting resulted in a significant decrease in
DHr, whereas the crystallites were further perfected under a propagation temperature of 30 C, as indicated by the higher To and Tc.
The crystallites formed in low concentrations revealed a greater
degree of annealing under 4/30 C temperature cycles. These results
were consistent with those reported by Ward et al. (1994), who
found that 25% amylopectin showed smaller DHr but higher To than
40% amylopectin when both were nucleated at 1 C and then
propagated at 23 C.
3.5. Degree of retrogradation (DR)
Degree of retrogradation (DR) is often expressed as the ratio of
DHr to DHg (Baik et al., 1997; Jane et al., 1999; Vandeputte et al.,
2003; Varavinit et al., 2003; Ward et al., 1994). When starch
gelatinized at a concentration greater than 30% was retrograded at
constant 4 C, DR of normal corn starch was lower than that of waxy
X. Zhou et al. / Journal of Cereal Science 51 (2010) 57–65
100
100
20%
Degree of Retrogradation (DR)
90
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
30%
90
80
0
2
4
6
8
10
12
14
100
2
4
6
8
10
12
14
100
40%
90
50%
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
2
4
6
8
10
12
14
2
4
Storage Time (days)
6
8
10
12
14
Storage Time (days)
100
100
20%
90
Degree of Retrogradation (DR)
63
30%
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
1
2
3
4
5
6
1
7
100
2
3
4
5
6
7
100
40%
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
50%
90
0
1
2
3
4
5
6
7
Temperature Cycles
1
2
3
4
5
6
7
Temperature Cycles
Fig. 6. The degree of retrogradation (DR) for waxy (blank) and normal (filled) corn starches at various concentrations. Starches were retrograded at 4 C for 14 days (upper) or under
the temperature cycles of 4 C for 1 day and 30 C for 1 day up to 7 cycles (lower).
corn starch, whereas DR of normal corn starch at 20% concentration
was higher than that of waxy corn starch (Fig. 6). These results were
roughly in agreement with those reported by Liu and Thompson
(1998). They claimed that the lower retrogradation enthalpy of 50%
normal corn starch was due to a smaller proportion of amylopectin
in normal corn starch than in waxy corn starch, since retrogradation of starch occurring below 100 C is primarily due to the
amylopectin (Fredriksson et al., 1998). This theory does not explain
why normal corn starch at 20% concentration retrograded to
a larger extent than its waxy corn starch counterpart. It is more
likely that at lower concentrations, amylose partially contributes to
the amylopectin crystalline formation in normal corn starch.
Similar to the case of the isothermal storage, normal corn
starches of high concentration (i.e. >30%) exhibited lower DR than
that of waxy corn starches when stored under 4/30 C temperature
cycles (Fig. 6), and showed high DR when starch content was 20% or
30%. Furthermore, the retrogradation endotherm was too small to
be calculated for 20% waxy corn starch after the first 4/30 C
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X. Zhou et al. / Journal of Cereal Science 51 (2010) 57–65
temperature cycle, whereas a relatively large retrogradation
endotherm was observed for 20% normal corn starch stored under
the same condition (Fig. 2B). Similar results were reported by Shi
and Seib (1995). They observed no retrogradation endotherm in
waxy corn starch or waxy rice starch at 25% concentration after
storage at 4 C for 1 day followed by storage at 23 C for 4 weeks. On
the other hand, normal wheat and corn starches at 25% concentration showed significant retrogradation under the same storage
conditions. These differences in starch retrogradation between
waxy and normal starches occurred because the rate of nucleation
at 4 C was slow for waxy starch at 25% concentration compared to
those starches containing amylose. The amylose molecules in the
normal corn starch at low concentration probably promoted the
retrogradation of amylopectin molecules of gelatinized normal
starch during storage.
To date, the exact influence of amylose on starch retrogradation
remains unclear (Vandeputte et al., 2003). Russell (1987a) studied
the retrogradation properties of high amylose corn, waxy corn,
potato and wheat starches at 43% concentration and reported that
a degree of cooperation appears to exist between amylose and
amylopectin. Gudmundsson and Eliasson (1990) showed that
synergistic interactions between amylose and amylopectin
occurred during retrogradation when the starch had very high
amylose content (75–90%), and the total starch concentration was
about 50%. They deduced that at low amylopectin content the
amylose component functions as nuclei and/or co-crystallizes with
the amylopectin to some degree. Fredriksson et al. (1998) also
found that high amylose barley starch retrograded to a higher
extent than waxy and normal barley starches when the starch
concentration was about 50%. At a starch concentration of 50%,
amylose may promote crystallization of amylopectin, especially
when amylose is present in a greater amount than amylopectin. In
our study, normal corn starch retrograded to a larger extent than
waxy corn starch especially at the early storage period, either at
constant 4 C or under 4/30 C temperature cycles when the starch
content was low (20% or 30%), and 20% waxy corn starch gave no
DH after 1 cycle. The amylose in normal corn starch may have
synergistic interactions with amylopectin for recrystallization at
low starch concentration.
It is well known that starch retrogradation occurs as two
kinetically distinct processes: rapid gelation of amylose via
formation of a double helical chain segment followed by helix–
helix aggregation; and slow recrystallization of the short amylopectin chains (Baik et al., 1997; Miles et al., 1985; Ring et al., 1987).
When waxy corn starch of a low concentration (i.e., less than 30%) is
gelatinized, the amylopectin clusters are relatively far apart,
making it difficult for them to re-associate. Amylose molecules in
normal corn starch of low concentration could freely leach out
during gelatinization, and gelate quickly (Ratnayake and Jackson,
2007). Some double helical chains of gelated amylose molecules
might act as nuclei, which facilitate recrystallization of amylopectin
molecules. It is also possible that the gelation of amylose could
make less water available for amylopectin molecules, thus causing
the amylopectin clusters to combine. As a result, DR of normal corn
starch stored at the constant 4 C could be higher than that of waxy
corn starch counterpart in this study. When starch was subjected to
4/30 C temperature cycling treatment, storage at 4 C for 1 day was
too short for waxy corn starch of 20% concentration to form
extensive nucleation, and the crystallites formed were relatively
weak. On subsequent storage at 30 C, those unstable crystallites
could be melted. On the other hand, amylose in 20% normal corn
starch might function as nuclei and also co-crystallize with
amylopectin at 4 C. The formed crystallites might readily proceed
a subsequent propagation during the storage at 30 C. At a higher
concentration, because of the heterogeneity in the water
distribution during starch gelatinization, some of the clusters were
close to each other after gelatinization and easily recrystallized, due
to the reduced mobility of amylopectin molecules. This might
reduce the amount of leached out amylose, subsequently lowering
its synergistic effect on retrogradation of amylopectin molecules.
It was also observed that after three temperature cycles, DR of
waxy corn starch at 30% concentration exceeded that of normal
corn starch. Retrogradation of amylopectin occurred faster in
normal than in waxy corn starch at the early stage of storage due to
the partial contribution of amylose to the amylopectin retrogradation in normal corn starch. Retrogradation of amylopectin leveled off in normal corn starch after three temperature cycles,
whereas it continued in waxy corn starch resulting in greater DR,
because of higher proportion of amylopectin in waxy than normal
corn starch.
4. Conclusion
Retrogradation characteristics of corn starch gels can be modified by using temperature cycling. The degree of recrystallization
was less under 4/30 C temperature cycling compared to isothermal
4 C storage, based on DH of the DSC endotherm. The crystallites
formed, however, appeared more homogeneous with higher
thermal stability by temperature cycling. Under temperature
cycling, annealing of starch was greater when the starch content
was low (20% vs. 50%), implying the significance of chain mobility.
Level of retrogradation was greater in normal corn starch than in
waxy corn starch at a low concentration (20 or 30%), indicating that
the amylose might have synergistic interactions with amylopectin
for recrystallization. Overall data show that the temperature
cycling induces different retrogradation behavior compared to
typical isothermal storage, and is applicable to control retrogradation properties of starchy foods.
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