16
DOI 10.1002/star.200400298
Francesca Lionettoa
Alfonso Maffezzolia
Marie-Astrid Ottenhofb
Imad A. Farhatb
John R. Mitchellb
The Retrogradation of Concentrated Wheat Starch
Systems
a
Department of Innovation
Engineering,
University of Lecce,
Lecce, Italy
b
Division of Food Sciences,
School of Biosciences,
University of Nottingham,
Loughborough, UK
Starch/Stärke 57 (2005) 16–24
The ageing of non-expanded wheat starch extrudates containing 37% and 51% water
on a dry solids basis (d.s.b.) at 257C was studied using Differential Scanning Calorimetry (DSC), Wide Angle X-ray Diffraction (XRD), proton Nuclear Magnetic Resonance
(NMR) relaxometry and Dynamic Mechanical Thermal Analysis (DMTA). The retrogradation rate increased with water content (,0.02 h21 at 37% water (d.s.b.) compared to ,0.06 h21 at 51%). While a good correlation was found between the DSC,
XRD and NMR data, the kinetics of retrogradation measured by DMTA was delayed.
The findings were interpreted in terms of the different molecular processes probed by
the different techniques. In addition to the kinetics, information on the physical structure of the partially crystalline retrograded materials were obtained. DSC suggested a
broad bimodal melting behaviour, which was attributed to the melting of the crystalline
structure followed by the dissociation of the double helices. XRD suggested that at
both water contents, the recrystallisation of amylopectin led principally to the A-polymorph. DMTA suggested a significant interaction between the amorphous and crystalline phases, with a requirement of a minimum relative crystallinity index of ,0.8 (e.g.
,80% of the crystallinity index of the fully retrograded material), before any increase in
the elastic modulus (at 257C) was measured.
Keywords: XRD; DSC; NMR; DMTA; Starch crystallisation
Research Paper
1 Introduction
The conversion of the partially crystalline native starch
granule was successfully described using a “side-chain
liquid-crystalline model” by Donald and co-workers [1].
This conversion often occurs through thermal processing
in the presence of varying amounts of water (e.g. baking,
pasting, etc.), which can be combined with mechanical
shear (e.g. in extrusion) or through chemical (e.g. the use
of solvents such as DMSO) and biochemical (e.g. enzymatic hydrolysis) processes, etc. [2].
On cooling and during early storage of converted/gelatinised starch, amylose gelation or retrogradation occurs
[3], while during longer storage (hours to weeks, depending on composition and storage conditions), amylopectin
gelation or retrogradation occurs [3], which essentially
leads to the partial recrystallisation of amylopectin. Retrogradation happens because gelatinised starch is stored
in the supercooled state i.e. below its melting temperature. It is therefore not at thermodynamic equilibrium and,
during storage, starch molecular packing and recrystallisation occurs.
Starch retrogradation is scientifically and technologically
important since it leads to significant changes in the
mechanical properties of starch-based products and thus
greatly affects their sensory (e.g. texture and flavour perception), nutritional (susceptibility to enzymic hydrolysis
[4]) and processing (shredding, cutting, etc.) characteristics. Consequently, numerous investigations have been
carried out on this phenomenon but the molecular level
mechanisms involved are not fully understood. Furthermore, the majority of the existing literature relates to high
water content systems, which are of limited relevance to
many food systems, such as baked goods.
Correspondence: Imad A. Farhat, Division of Food Sciences,
School of Biosciences, University of Nottingham, Loughborough
LE12 5RD, UK. Phone: 144-1159-516134, Fax 144-1159516142, e-mail: [email protected].
The aim of this research was to study the changes
occurring during storage of concentrated wheat starch
systems as part of a large research programme aimed at
gaining an enhanced molecular understanding of the
transformations occurring during the processing and
storage of starch materials (starches from various botanical sources and a range of starch-containing systems).
This was achieved by studying the same materials, with
comparable processing and storage histories using a
range of complementary techniques, which probe different physical properties over different distances and timescales, thus avoiding misleading comparisons often
made between different techniques performed in different
studies under different conditions.
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Starch/Stärke 57 (2005) 16–24
Retrogradation of Concentrated Wheat Starch Systems
2 Materials and Methods
Native wheat starch was obtained from Avebe (Veendam,
the Netherlands), lot no. 0394-1.
2.1 Sample preparation
Non-expanded wheat starch/water ribbons were prepared by extrusion through a 1630 mm slit die using a
Clextral BC-21 co-rotating, intermeshing twin-screw
extruder. The extrusion temperature profile was 40, 90,
120, 757C, the solid feed rate was 5 kg/h and the screw
speed was 300 rpm. Different amounts of distilled water
were introduced into the second zone of the extruder
barrel to obtain samples containing 37% or 51% water on
a dry weight basis (w/w d.s.b.) i.e. grams of water per 100
grams of dry solid. Water contents were determined directly after extrusion by drying the sample at 1057C for
24 h.
The samples were sealed in airtight aluminium bags (for
the XRD and DMTA studies) or in stainless steel DSC pans
(for the calorimetry study) or in NMR tubes (for the NMR
study) to prevent loss of water and stored in an incubator
at 25617C.
2.2 X-ray diffraction
Wide angle X-ray diffraction (XRD) measurements were
carried out to obtain information regarding long-range
crystalline order. An AXS D5005 diffractometer (Bruker,
Karlsruhe, Germany) was used. The X-ray generator was
equipped with a copper tube operating at 40 kV and
30 mA and irradiating the sample with monochromatic Cu
Ka radiation with a wavelength of ,0.154 nm. XRD diffractograms were acquired at room temperature over a 2y
range of 4–387 at 0.17 intervals with a measurement time
of 6 s per 2y interval. These were acquired at regular
storage time intervals for up to 10 days of storage on
disks (,25 mm of diameter) cut from the extruded ribbons.
17
rapid cooling of the samples did not show any thermal
events in the amylopectin-melting region. The thermograms were normalized to the dry matter weight of each
sample. The sample contained in the stainless steel pan
was weighed before and after heat treatment to check for
potential moisture loss.
2.4 1H relaxation NMR
Proton relaxation NMR measurements were carried out to
obtain information on the changes in molecular mobility
during storage. A Resonance Instruments MARAN spectrometer (Witney, UK) operating at a resonance frequency
of 23 MHz was used. The samples were sealed in 8 mm
diameter tubes and stored at 25617C as described
above.
The Free Induction Decay (FID) was acquired using the
solid-echo pulse sequence with a spacing of 10 ms between the two 907 pulses. The behaviour of the mobile
components was followed using the spin-echo decay
acquired using the Carr-Purcell-Meilboom-Gill (CPMG)
pulse sequence with a spacing of 200 ms between the 907
and 1807 pulses. Sixteen scans with a recycle delay of 3 s
were recorded. The probe temperature was set to 257C
using a flow of compressed air.
The CPMG decays were fitted to a continuous distribution
of exponentials with relaxation times spread logarithmically between the values of 2t (0.4 ms) and 2nt (410 ms;
n=1024, number of echoes acquired) using RI WinDXP
software (Resonance Instruments, Witney, UK). The
relaxation spectrum showed essentially one main relaxation distribution. In the interest of clarity, a single spin-spin
relaxation time (T2), corresponding to the maximum of this
main relaxation distribution, was used to monitor retrogradation.
2.5 Dynamic mechanical thermal analysis
2.3 Differential scanning calorimetry
Dynamical mechanical thermal analysis (DMTA) was used
to monitor the changes in the mechanical properties and
the macromolecular relaxation processes occurring during retrogradation.
The samples were sealed in high-pressure stainless steel
pans and stored at 257C as described above for different
times prior to analysis. The melting (melting of crystallites
and dissociation of double helical structures) behaviour of
the retrograded starch was studied using a power compensated Perkin-Elmer DSC7 (Norwalk, CT, USA) differential scanning calorimeter. An empty stainless steel pan
was used as reference. The samples were heated from
207C to 1607C at 107C/min. A second heating scan after
A Rheometric Scientific (Piscataway, NJ, USA) dynamic
mechanical thermal analyser DMTA IV operating in bending mode was used. Rectangular strips (approximately
7 mm x 14 mm62 mm) were cut from the extruded ribbons and clamped in the single cantilever geometry. The
analysis was performed during heating from 230 to 707C
at six frequencies between 0.3–30 Hz. A relatively low
heating rate of 17C/min was used to ensure adequate
thermal equilibrium across the sample. In order to alle-
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F. Lionetto et al.
Starch/Stärke 57 (2005) 16–24
viate potential water loss during the measurement, the
sample was covered with a thin film of silicone oil and no
data were acquired above 707C.
The results of dynamic mechanical analysis are valid only
if the material exhibits linear viscoelasticity, which implies
that E’, E’’ and tan d must be independent of the strain
amplitude. Therefore, particular attention was devoted to
the choice of the strain amplitude. A preliminary test at
fixed frequency and variable strain (strain sweep test) was
performed to determine the strain amplitude values that
did not affect the moduli values and, based on these
results, a strain amplitude of 0.1% was selected.
2.6 Retrogradation kinetics
The results obtained from the different techniques were
fitted, using a non-linear least-squares routine (MS Excel
Solver), to a stretched exponential based on the Avrami
kinetics equation [5] as described elsewhere [6].
UðtÞ ¼
Y 1 YðtÞ
¼ expðktÞn
Y1 Y0
(1)
where k is the kinetic constant and n is the Avrami exponent, related to the dimensionality of nucleation and
crystal growth. It must be noted that the above reported
form of the Avrami equation provides the correct physical
dimensions of k (s21) [6, 7].
3 Results and Discussion
3.1 Recrystallisation kinetics and
polymorphism
The XRD diffractograms of the wheat starch samples,
containing 51% water (w/w d.s.b.), stored at 257C for up
to ten days are shown in Fig. 1. The diffractogram
recorded shortly after extrusion showed predominantly a
diffuse pattern typical of amorphous systems with a sharp
peak centred at around 2y < 19.77. This peak did not
change significantly in intensity or width during storage
and was assigned to the presence of a crystalline V-type
amylose-lipid complex, which is likely to have formed
during the extrusion process [8]. Native wheat starch
granules have an A-type crystalline structure [9], which
was largely disrupted by the extrusion process as suggested by the absence of any significant peaks related to
the A-type crystallites in the diffractograms acquired
shortly after extrusion (Fig. 1).
Fig. 1. XRD diffractograms of wheat starch extrudates
containing 51% water (d.s.b.) stored at 257C for different
ageing times.
Significant variations in the diffractograms were observed
starting from ,15 hours of ageing. The sample with the
lower water content (37% d.s.b.) showed a similar behaviour but with a longer delay before significant crystallisation could be observed. After approximately 2 or 3
days of ageing, depending on the moisture content (51%
or 37% water d.s.b. respectively), only slight changes in
the diffractograms were observed indicating that the “full”
retrogradation was achieved.
The comparison of the diffractograms of the “fully” retrograded wheat starch extrudates to those of native wheat
(typical A-type) and potato (typical B-type) (Fig. 2) suggested that both samples had a predominantly A-type
pattern. The polymorph formed as a result of the recrystallization, depends largely on the water content and
storage temperature conditions and is not necessarily the
same as that of the original native starch [11, 12]. Marsh
[11] mapped the polymorphism of recrystallised wheat
starch as a function of water content and storage temperature. He found that the A-polymorph was favoured in
conditions of lower water contents and/or higher storage
temperatures, as opposed to the B-polymorph, which
was favoured at higher water contents and/or lower storage temperatures. Results obtained by Marsh [11] suggested that, at 257C, the samples studied would be near
the boundary between the mainly A- and mainly B-polymorphic domains and that a mixture of A- and B-type
crystals would be expected particularly for the higher
water content sample.
On storage, well-defined diffraction peaks emerged indicating an increasing crystalline fraction. This was
assigned to the recrystallisation of amylopectin [10, 11].
The crystallinity indices of the samples with different
extents of ageing were calculated from the XRD diffractograms using the Wakelin correlation method [13].
According to this method, the relative crystallinity index, w,
of a sample is calculated relative to a minimum and a
maximum value corresponding to an amorphous (w=0) and
a crystalline (w=1) reference standard respectively. For
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Starch/Stärke 57 (2005) 16–24
Retrogradation of Concentrated Wheat Starch Systems
19
3.2 Differential scanning calorimetry
Fig. 2. Comparison of the XRD diffractograms of the
‘fully’ retrograded (7 days storage, 257C) wheat starch
extrudates (51% and 37% d.s.b.) with the diffractograms
of native potato starch (B-type) and native wheat starch
(A-type).
each sample, the diffractograms acquired shortly after
extrusion and on the “fully” retrograded samples (i.e. at
the longest storage time) were taken as the amorphous
and crystalline standards, respectively. The correlation
method is easy to automate and adopt for routine analysis
but suffers from the assumption that the fresh sample is
totally amorphous and the “fully” retrograded sample is
maximally crystalline. However, it has the advantage of
excluding the contribution of the crystalline amylose-lipid
complex and hence only reflecting amylopectin recrystallisation. The rate of recrystallisation of the sample containing 51% water was significantly higher than that at
37% (Fig. 3). These results were consistent with those
found using the other experimental techniques discussed
below.
The thermograms acquired shortly after extrusion
showed no significant thermal events in the region between 257C and 1407C (Fig. 4) except for a minor endotherm centred at ,1277C, which is likely to be related to
the melting of the complex formed between amylose and
the endogenous starch lipids, as seen in the XRD data.
Typically, wheat starch contains ,0.8% lipids and ,28%
amylose [14]. As the storage time increased, a bimodal
endothermic event, with components centred at ,637C
and ,1007C for the sample containing 51% water,
became increasingly significant. These endotherms were
assigned to the complex melting behaviour of starch
crystallites in limited water content conditions. The sample containing 37% water showed similar behaviour with
the endotherms occurring at slightly higher temperatures
(,677C and ,1067C, respectively). A limiting melting enthalpy of 8.160.07 J/g starch (dry matter) was achieved
after full retrogradation for both water contents.
The temperature range over which melting occurred was
very broad (from ,50 to ,1157C) suggesting a wide range
of “melting” events. These could be attributed to a broad
distribution of crystals with varying stability. However, this
is not compatible with the well-defined bimodal nature of
the endotherm. It is tempting to interpret it in terms of the
melting of the two polymorphs of the mixed structure.
This is, however, not compatible with the XRD results
which suggested that the A-polymorph was principally
formed (Fig. 2). It has been suggested that recrystallisation or crystal perfection phenomena could take place
during the DSC heating scan owing to an increase in molecular mobility particularly after the onset of melting [15,
16], leading to a higher temperature melting event. However, such molecular rearrangements are unlikely given
the limited timescale available at the heating rates
employed (107C/min). Another more plausible interpretation for this bimodal melting pattern is that the lower
Fig. 3. Comparison of the extent of retrogradation calculated from XRD data through the crystallinity index (D) and
from DSC data through the relative change in melting enthalpy (s) on wheat starch extrudates with 37% (open
symbols) and 51% (filled symbols) water content (d.s.b.),
stored at 257C. The lines are the best fits to the experimental results using Equation (1).
Fig. 4. Melting endotherms of wheat starch extrudates
containing 51% water (d.s.b.) stored at 257C for different
times and heated from 20 to 1607C at 107C/min.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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20
F. Lionetto et al.
temperature endotherm is related to the disruption of the
packing of double-helices of the amylopectin A- and short
B-chains, while the higher temperature endotherm
reflects the subsequent dissociation of double helices.
Such an interpretation is compatible with the models
proposed by Donald and co-workers [17, 18] of the melting of native starch in limited water content conditions.
Also, that the appearance of the high temperature endotherm occurs first is compatible with the model proposed
by Goodfellow and Wilson [3], where the double helical
structures would form and subsequently pack into crystalline lattices.
The melting enthalpy of the retrograded amylopectin was
calculated from the area of the combined endotherm as
an indication of the extent of retrogradation and was
expressed on the basis of the dry matter weight of each
starch sample. The normalized DSC melting enthalpies
were compared to the XRD crystallinity indices in Fig. 3.
The progress of retrogradation monitored by the two different techniques was comparable with the sample of
Starch/Stärke 57 (2005) 16–24
lower water content (37% w/w d.s.b.) showing a slower
retrogradation rate than that with 51% water (w/w d.s.b.).
The retrogradation rates calculated using Equation (1)
were comparable for both techniques at ,0.06 h21 for the
sample containing 51% water (d.s.b.) and 0.02 h21 at
37% water (d.s.b.).
3.3 NMR relaxometry
The FID and CPMG NMR signals showed a systematic
dependence on the duration of storage, implying that the
NMR properties were affected by the extent of reordering
of the gelatinised amylopectin (Fig. 5). The rapidly
decaying component of the FID (Fig. 5a insert) reflects the
behaviour of the starch protons as this component is
believed to result principally from C-H and nonexchangeable O-H on the starch polysaccharide’s backbones and rigid chains. The liquid-like component was
recorded through the spin-echo decay (CPMG). The
CPMG decay acquired with a t spacing of 0.2 ms is
Fig. 5. Effect of ageing on the 1H-NMR
solid-echo (a) and spin-echo (b)
decays of wheat starch extrudates
containing 51% water (d.s.b.) aged for
different times at 257C.
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Starch/Stärke 57 (2005) 16–24
Retrogradation of Concentrated Wheat Starch Systems
believed to be mostly related to O-H protons (water and
exchangeable starch O-H) and highly mobile C-H protons
(end chains in non-retrograded, high water content systems) [10, 19]. As the storage time increased, the rate of
relaxation (1/T2) of the FID and CPMG decays increased,
suggesting an overall decrease of the molecular mobility
as a result of retrogradation (Fig. 6). The increase of the
rate of relaxation of the rigid component was paralleled by
an increase in its relative contribution to the total NMR
signal, reflecting the increased fraction of amylopectin
chains becoming restricted in their mobility as a result of
the re-ordering process. These observations are in
agreement with previous reports on similar systems [10,
20]. The decrease in the mobility of the liquid-like component can be explained by the combined effects of (i) the
increased rigidity of the matrix and its effects on water
relaxation behaviour through proton exchange and crossrelaxation processes and (ii) the “immobilization” of some
water molecules in the structure of the crystal unit cell
[19]. The latter being less likely since the water content of
the crystalline fraction is only ,4% [9] in the case of the Apolymorph and water in the crystalline lattice is unlikely to
contribute to the mobile CPMG signal.
Relaxation times for the solid-like component of the FID
were calculated by fitting the decay to three components
(2 exponentials and a slowly decaying Gaussian), however
the T2 values obtained were relatively shorter (typically
decreasing from ,17 ms to ,14 ms) than reported in other
studies [10]. This is likely to be due to distortions of the FID
recorded using the spin-echo sequence, since the shortest inter-P90 pulse spacing (t) of 10 ms (this is the lowest,
realistically achievable value considering the instrument
and probe dead-times) is not greatly shorter than the T2 of
the fastest decaying component of the FID, as would
be normally required for the solid-echo pulse sequence
21
[21]. These distortions may explain why the solid-like
component of the FID was best fitted with an exponential
line shape rather than the expected Gaussian one. As a
result of these issues, little emphasis was given to the
absolute T2 of the solid-like component and only those of
the liquid-like component were used to monitor the retrogradation kinetics (Fig. 6). As expected, the spin relaxation times (T2) in the fresh and retrograded materials
increased with increasing water content.
3.4 Dynamic mechanical thermal analysis
Like any semi-crystalline polymer, the amorphous regions
of starch undergo molecular relaxation motions, which
are constrained by the presence of the crystalline phase.
During starch retrogradation, the relative amounts of
amorphous and crystalline phases change as a result of
recrystallisation. As a consequence, the glass-rubber
transition would be expected to change with ageing time
in terms of magnitude (as this transition reports on the
amorphous fraction of the material) and possibly its temperature (as usually the glass transition temperature (Tg)
of partially crystalline polymers increases with the degree
of crystallinity). The changes in the macromolecular mobility occurring at the glass transition are therefore an indirect measure of the retrogradation process and can be
accurately followed by DMTA. This mechanical spectroscopy technique detects the amount of energy absorbed
by a material, particularly when the measurement frequency approaches that of a given molecular relaxation
process. In this case, the retrogradation was analysed
through the behaviour of the amorphous phase of starch
rather than that of the crystalline phase, as was the case
for XRD and DSC.
In the starch samples with the higher water content (51%
d.s.b.), the mechanical changes associated with the
glass-rubber transition overlapped with those associated
with the melting of the ice phase formed through the
freezing of the so-called “freezable” water when the
sample was cooled to 2307C. The formation of an ice
phase was confirmed by DSC. The sample with the lower
water content (37% d.s.b.) showed no significant ice formation in DSC experiments and thus was considered for
a detailed analysis of the DMTA data.
Fig. 6. Effect of ageing on the spin-echo relaxation rate of
the liquid component (1/T2 liquid) of the NMR signal for
extruded wheat starch with 37% (u) and 51% (n) water
content (d.s.b.). The lines are the best fits to the experimental results using Equation (1).
The DMTA thermograms acquired on the sample containing 37% water (d.s.b.) showed principally one main
relaxation over the temperature range between 230 and
607C (Figs. 7 and 8). This transition, the temperature of
which increased with frequency (Fig. 8, insert), was
assigned to the a-relaxation associated with the glass
transition of the amorphous starch-water phase. The
initial, unrelaxed values of the storage (E’) and loss (E’’)
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F. Lionetto et al.
Starch/Stärke 57 (2005) 16–24
crystalline fraction in the sample, suggested an interaction between the relaxation behaviour of the amorphous
phase and the degree of crystallinity. This suggestion was
supported by the systematic decrease in the activation
energy of relaxation on ageing from ,280 kJ/mol shortly
after extrusion to ,176 kJ/mol after ,3 days of storage.
Fig. 7. Viscoelastic behaviour during a DMTA scan at 1 Hz
of wheat starch extrudates (37% water d.s.b.) stored at
different times at 257C and heated at 17C/min. Open and
filled symbols represent storage (E’) and loss (E’’) moduli,
respectively.
moduli, i.e. in the glassy state (e.g. at 2307C), showed no
significant dependency on ageing time with values of
,1029 Pa and 1028 Pa, respectively. In contrast, the
relaxed value (in the rubbery state) depended strongly on
storage time (Fig. 7). This increase of the relaxed modulus
with time reflects the increase of the crystalline fraction in
the material that reduces the amorphous phase and consequently the entity of the glass transition. This can be
observed not only from the reduction of the storage
modulus but also from the decrease of the tan d peak with
time. The presence of a more significant crystalline phase
could hamper the relaxational motions of the amorphous
phase, leading to a decrease in the height of tan d peak
(Fig. 8). Furthermore, the increasing width of the tan d
peak with increasing storage time, i.e. with increasing
The storage modulus measured by DMTA in the rubbery
state can therefore be a useful small-deformation rheological approach to monitoring the progress of retrogradation. The plot of the extent of retrogradation in terms
of the relative change in E’ on storage showed a comparable pattern to the results obtained by DSC, XRD and
NMR (Fig. 9). It is interesting, however, to note that the
kinetics of the change was significantly slower for the
DMTA compared to the other techniques. This divergence
is considered in a more detailed analysis of the DMTA
data. The aged/retrograded starch samples can be
viewed as composite materials, in which the crystalline
phase acts like a rigid inclusion in the amorphous rubbery
matrix. Considering such a biphasic system, the development of the starch storage modulus as a function of the
crystalline content, during ageing at 257C, can be described by different mathematical models used in the
micro-mechanical theory of composite materials [22–24].
The upper and lower bounds for the dependence of the
composite modulus, E*, on the volume fraction, Xc, of the
crystalline component for a two-phase model are given
by an “in-parallel” and “in-series” model of the semicrystalline and amorphous phases. Assuming that both
crystalline and amorphous phases are subjected to the
same strain, the in-parallel model can be applied using
the mixture rule [23]:
E * = (12Xc)Ea* 1 XcEc*
(2)
where E*, Ea* and Ec* are the complex moduli of the sample, the amorphous and crystalline phases respectively,
and Xc is the crystallinity index calculated from XRD
Fig. 8. DMTA thermograms at 1 Hz acquired on wheat
starch extrudates (37% water d.s.b.) stored at different
times at 257C and heated at 17C/min. Insert: The multifrequency comparison of tan d peaks on samples stored
for 23 h at 257C and heated at 17C/min.
Fig. 9. Comparison of the relative extent of retrogradation at 257C of wheat starch extrudates (51% water
d.s.b.) as sensed by different techniques.
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Starch/Stärke 57 (2005) 16–24
Retrogradation of Concentrated Wheat Starch Systems
experiments. The real and imaginary components of
Equation (2) can be easily separated giving the following
expression of the storage modulus:
E ’ = (12Xc)E ’a 1 XcE ’c
(3)
On the other hand, assuming an in-series connection of
the amorphous and crystalline phases, in which they are
subjected to the same stress, the complex modulus can
be modelled according to Equation (4):
1
Xc
ð1 X c Þ
¼
þ
E E c Ea
(4)
from which the storage modulus can be calculated as
[23]:
2
E0 ¼
2
2
2
E 0c X c ðE 0a þ E 00a Þ þ E 0a ð1 X c ÞðE 0c þ E 00c Þ
½E 0c ð1
XcÞ þ
E 0a X c 2
þ
½E 00c ð1
XcÞ þ
E 00a X c 2
(5)
The change of the storage modulus, measured at the frequency of 1 Hz, as a function of the degree of crystallinity,
is reported in Fig. 10. The experimental results show a
behaviour very close to the in-series model up to a crystallinity index of 0.8. At higher crystallinity fractions, there
is probably a phase inversion with the crystalline phase
becoming the continuous phase. In such a case, both
crystalline and amorphous phases would be subjected to
the same strain, explaining the similarity with the in-parallel model.
4 Conclusions
The retrogradation of concentrated wheat starch extrudates (at two starch concentrations of 66% and 77%) was
studied through a combination of several experimental
techniques, which provided complementary results.
23
These techniques monitored different aspects of the retrogradation process and hence their sensitivity to the
“onset” of the reordering process was different. Therefore, comparisons of kinetics data from different techniques must be made with caution. From the multi-technique investigation on retrogradation, complementary
information rather than a pure similarity of results must be
searched.
DSC, which was believed to monitor the enthalpy of dissociation of the ordered structures (the melting of crystals
and the dissociation of double helices) correlated well with
the development of crystallinity measured by XRD and the
decrease in molecular mobility of both the solid-like and
liquid-like components probed by NMR relaxometry. This
correlation suggests that under the conditions of this
study (water content, storage temperature, type of starch,
etc.), it was not possible to distinguish reliably between
the recovery of short range molecular order (double helices), which would be detectable mainly by DSC and NMR
relaxometry, and the subsequent aggregation of these
double helices to form crystallites. The delayed beginning
of retrogradation, as detected by DMTA through the storage modulus (at 257C), can be attributed to the fact that
the development of significant modulus would lag behind
the development of crystallites as a critical volume fraction
of the crystalline phase would be required.
The results showed that at 257C, wheat starch had a lower
retrogradation rate at 37% water (d.s.b.) than at 51%
water. This can be explained in terms of the plasticising
effect of water, which reduces the glass transition temperature by adding free volume and hence enhances the
mobility of the amylopectin A-chains and their ability to
crystallise.
Acknowledgements
The present work was carried out with the support of the
Marie Curie Fellowship financed by the European Community (EC Contract Number: HPMT-CT-2001-00404).
The authors would like to thank Mrs. V. Street for assistance in the extrusion of the samples.
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Fig. 10. Development of the mechanical properties of
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 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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(Received: March 29, 2004)
(Revised: August 26, 2004)
(Accepted: September 13, 2004)
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