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ARTICLE IN PRESS
www.elsevier.com/locate/jfoodeng
Journal of Food Engineering xxx (2005) xxx–xxx
Ultrasonic investigation of wheat starch retrogradation
Francesca Lionetto a,*, Alfonso Maffezzoli a, Marie-Astrid Ottenhof b,Imad A. Farhat b, John R. Mitchell b
a Department of Innovation Engineering, University of Lecce, via Monteroni, 73100 Lecce, Italyb Division of Food Sciences, School of Biosciences, University of Nottingham, Loughborough LE12 5RD, UK
Received 1 November 2004; accepted 18 April 2005
Abstract
A recently developed technique, based on ultrasonic wave propagation, was applied to study the effect of storage time on the
retrogradation process of wheat starch extrudates during storage at room temperature. A specifically designed experimental set-
up enabled to propagate in situ ultrasonic waves continuously for several days at constant water content and storage temperature
(�34% water and 25 �C). The velocity and the attenuation of the ultrasonic waves changed as a result of the recrystallisation process
of amylopectin molecules of wheat starch samples during storage.
The propagation of ultrasonic waves, acting as a dynamic mechanical deformation at high frequency (10 MHz), gave access to
the complex longitudinal modulus that, compared to the complex Young modulus obtained from low frequency DMA, enabled a
better insight of the changes in viscoelastic behaviour of starch extrudates during retrogradation. The results of the ultrasonic mon-
itoring of starch retrogradation were correlated with those obtained from X-ray diffraction and differential scanning calorimetry.
The present study demonstrated the potential of the ultrasonic technique in detecting the changes in physical properties of con-
centrated starch systems.
� 2005 Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic wave propagation; Wheat starch; Retrogradation; DSC; XRD; DMA
1. Introduction
Ultrasonic techniques are finding an increasing num-
ber of applications in the food industry for both the
analysis and modification of foods (Knorr, Zenker,
Heinz, & Lee, 2004; McClements, 1995; Mulet, Bened-
ito, Bon, & Sanjuan, 1999; Povey & Mason, 1998).
Low intensity ultrasound, which involves low power le-
vel of ultrasonic waves, is a non-destructive technique
applied to monitor processes, e.g. thickness and liquidlevel measurements or detection of extraneous matter
in foods (Knorr et al., 2004; McClements, 1995; McCle-
0260-8774/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2005.04.015
* Corresponding author. Tel.: +39 0832 297387; fax: +39 0832
297525.
E-mail address: [email protected] (F. Lionetto).
ments, 1997; Mulet, Benedito, Golas, & Carcel, 2002) or
to evaluate the textural properties of fruits, cheese andfatty tissues (Benedito, Carcel, Clemente, & Mulet,
2000; Benedito, Carcel, Sanjuan, & Mulet, 1999; Miz-
rach, Flitsanov, Akerman, & Zauberman, 2000) and
the rheological properties of honey, oils and dairy prod-
ucts (Buckin & Kudryashov, 2001; Greenwood &
Bamberger, 2002; Kulmyrzaev & McClements, 2000;
Saggin & Coupland, 2001). Another field, where ultra-
sound has been shown to be a good alternative to tradi-tional analytical techniques, is the quantitative
assessment of food composition, such as the sugar con-
tent of some fruit juices and drinks (Contreras, Fairley,
McClements, & Povey, 1992), the solid fat content of
oils and adipose tissues (McClements & Povey, 1987),
the composition of fish and meat based products (Ghae-
2 F. Lionetto et al. / Journal of Food Engineering xxx (2005) xxx–xxx
ARTICLE IN PRESS
dian, Coupland, Decker, & McClements, 1998; Simal,
Benedito, Clemente, Femenia, & Rossello, 2003). Never-
theless, the application of ultrasonic analysis has been
successful only in the case of food products with two
phases, showing distinct physical properties (Benedito,
Mulet, Clemente, & Garcia-Perez, 2004).One of the most important advantages of the ultra-
sonic technique consists in its capacity to monitor
non-destructively the phase transitions occurring in a
material as the changes in material density and elastic
properties, associated with the transition, affect signifi-
cantly the material acoustic response. In polymer sci-
ence, for example, ultrasonic wave propagation has
been recently applied to study time dependent kineticprocesses, such as the polymerisation kinetic of thermo-
setting resins (Lionetto, Rizzo, Luprano, & Maffezzoli,
2004; Maffezzoli, Quarta, Luprano, Montagna, &
Nicolais, 1999a, Maffezzoli, Tarzia, Cannoletta, Monta-
gna, & Luprano, 1999b; Matsukawa & Nagai, 1996), the
water sorption in dry hydrogels (Maffezzoli, Luprano, &
Montagna, 1998; Maffezzoli et al., 1999a, 1999b) and
the crystallisation process, for example, in poly(ethyleneoxide) systems (Alig, Tadjbakhsch, Floudas, & Tsitsili-
anis, 1998).
The possibility to monitor non-destructively the
phase transitions of many food components during
manufacturing or storage is of crucial importance for
process control and optimisation. From this necessity,
widely present in the food engineering field, arose the
idea of applying low intensity ultrasound to follow theretrogradation process of starch during storage at room
temperature.
Starch, the storage polysaccharide of many plants, is
found in the form of partially crystalline water-insoluble
granules, the size and composition of which depends on
the botanical source. Native starch is a mixture of two
polysaccharides, amylose and amylopectin, and contains
also small amounts of non-carbohydrate constituentssuch as lipids, phosphates and proteins. Amylose is
essentially a linear polymer, while amylopectin is a
highly branched macromolecule. Amylopectin, whose
content varies from 70 to 85% depending on starch
botanical source, is assumed to be the main responsible
of the crystallinity of the native starch granules (Buleon,
Colonna, Planchot, & Ball, 1998a, 1998b; Jenkins,
Cameron, & Donald, 1993; Miles, Morris, Orford, &Ring, 1985).
During processing, starch undergoes a conversion
from its native partially crystalline granular structure
to a polymeric solution or melt, depending on the
amount of water and the type of process used. In the
most general case, when heated in presence of excess
water, starch granules undergo an order-disorder transi-
tion, called gelatinisation. During cooling and storage ofgelatinised starch at temperature lower than the gelatin-
isation temperature, amylose retrogradation rapidly
occurs (Goodfellow & Wilson, 1990) while, on longer
time scale storage (hours or weeks or even months,
depending on composition and storage conditions),
amylopectin retrogradation occurs.
Starch retrogradation, which essentially involves
molecular packing and recrystallisation phenomena,leads generally to significant changes in the mechanical
properties of starch-based products and thus greatly
affects their sensory (e.g. texture and flavour perception
(Buleon et al., 1998a, Buleon, Le Bail, Colonna, &
Bizot, 1998b)), nutritional (susceptibility to enzymic
hydrolysis (Farhat et al., 2001)) and processing (shred-
ding, cutting, etc.) characteristics. However, not always
the recrystallisation process of starch leads to texturefirming. Other factors are known to be involved such
as gluten emulsifiers, fat, cellular structures, etc.
(Hallberg & Chinachoti, 2002).
Retrogradation happens because gelatinised starch is
supercooled and stored below its melting temperature.
Therefore, the drive towards thermodynamic equilib-
rium during storage leads re-ordering and crystallisa-
tion. Since starch retrogradation is accompanied bychanges in its viscoelastic properties, it may be expected
that it is also paralleled by corresponding changes in its
acoustic behaviour.
Due to the great importance of starch retrogradation
from a scientific and technological point of view, a large
number of studies have been carried out on this area
using different analytical methods (Farhat et al., 2001;
Fredriksson, Silverio, Andersson, Eliasson, & Aman,1998; Miles et al., 1985; Ortega-Ojeda & Eliasson,
2001; Zeleznak & Hoseney, 1986). The few applications,
found in the literature, of the ultrasonic wave propaga-
tion on starch based products are devoted to the mea-
surement of the rheological properties of dough
(Letang, Piau, & Verdier, 1999; Letang, Piau, Verdier,
& Lefebvre, 2001; Ross, Pyrak-Nolte, & Campanella,
2004). To our knowledge, there are no reports of studieson the monitoring of starch retrogradation using ultra-
sonics. The lack on the market of ultrasonic instruments
suitable for this task has probably hindered the applica-
tion of ultrasound to starch based products. The re-
sponse of the material to small deformations applied
very rapidly could offer a better characterization of the
mechanical behaviour in starch based products, which
is of great interest for food scientists. Moreover, theuse of ultrasound to characterize starch retrogradation
could be very promising since ultrasonic technique is ra-
pid and non-destructive, can be used in optically opaque
systems and can be easily adapted as an on-line tool for
process monitoring.
The aim of this work was to apply ultrasonic wave
propagation to the study of the changes occurring in a
model concentrated wheat starch system during storageat room temperature, with the material passing from a
completely rubbery state to a biphasic one, in which
F. Lionetto et al. / Journal of Food Engineering xxx (2005) xxx–xxx 3
ARTICLE IN PRESS
the crystalline phase acts like a rigid inclusion in the
amorphous rubbery matrix. In order to carry out the
ultrasonic measurements, a purpose designed experi-
mental set-up was developed.
Fig. 1. Experimental set-up for ultrasonic measurements.
Fig. 2. Waveforms of the reflected signals from the interface between
transducer delay line and starch sample (echo A0) and from the
interface between the sample and the aluminium wall (echo A1).
2. Experimental techniques
2.1. Sample preparation
Non-expanded wheat starch/water gel ribbons were
prepared by extrusion through a 1 · 30 mm slit die using
a Clextral BC-21 co-rotating intermeshing twin extru-
der. The extrusion temperature profile in the four zonesextruder barrel was 40, 90, 120, 75 �C, the feed rate of
solids was 5 kg/h and the screw speed was 300 rpm. Dis-
tilled water was introduced into the second zone of the
extruder barrel. The extruded samples contained �34%
water (wet weight basis, w.w.b.). The water content
was determined directly after extrusion by drying the
samples at 105 �C for 24 h. The samples were collected
in the form of strips, sealed in airtight aluminium foilbags to prevent loss of water and stored in an incubator
at 25 ± 1 �C.
2.2. Ultrasonic analysis
The experimental set-up for ultrasonic measurements
consisted of a narrow-band longitudinal wave contact
transducer (V611-RB, Panametrics, USA) operating a10 MHz (13 mm crystal diameter), a pulser-receiver card
(model SFT 4001H PCI, Sofratest, France) with an A/D
flash converter at 60 MHz, a custom-made measurement
cell and a PC with a data analysis software (LABVIEW
6, National Instruments, USA). The starch sample (in a
form of a disc of 12 mm diameter and 1.8 mm thickness)
was wrapped by a thin PVC film to avoid moisture loss
during the test. The sample was then loaded in the mea-surement cell, which consisted of an aluminium box,
specifically developed to enable the continuous monitor-
ing of the changes in the starch acoustic properties with-
out sample dehydration over a storage time of �9 days.
Four bolts were fastened sufficiently to provide a good
contact pressure between the starch specimen and the
walls of the aluminium box. A schematic diagram of
the ultrasonic set-up is reported in Fig. 1.A thin film of silicon oil was used as a coupling agent
between the sample (wrapped by PVC) and the trans-
ducer, in order to ensure a good transmission of ultra-
sonic waves, which propagate very poorly in air.
Finally, to prevent water loss, which could significantly
modify (by affecting the retrogradation kinetics) the vis-
co-elastic properties of the sample, the measurement cell
was entirely sealed with silicon. The samples wereweighed before and after the ultrasonic measurement,
but no significant moisture loss was observed. All the
experiments were replicated three times. Ultrasonic data
were collected continuously each 20 s for the first 58 h of
storage at room temperature. Then, to prevent any pos-
sible overload of the measurement equipment, the exper-
imental data were continuously acquired for 12 h with
an interval of other 12 h. The overall storage period
for the starch samples was about 9 days.
The ultrasonic measurements were carried out using apulse-echo technique, in which a single transducer prop-
agated and received longitudinal waves, normally inci-
dent on the specimen surface. The typical signal
obtained is displayed in Fig. 2. In the pulse-echo mode,
an ultrasonic wave, generated by the transducer, under-
goes a first reflection at the interface between the trans-
ducer delay line and the sample, generating a first echo
of amplitude A0. If a good coupling between the trans-ducer delay line and the sample is assured by means of
fluid coupling agents, only a small part of the ultrasonic
wave is reflected back to the transducer. The remaining
part of the wave, instead, continues travelling through
the sample until it reaches the interface between the sam-
ple and the wall of the measurement cell, where a con-
siderable fraction of it returns back to the transducer.
4 F. Lionetto et al. / Journal of Food Engineering xxx (2005) xxx–xxx
ARTICLE IN PRESS
The received oscillations are converted by the transducer
into an electrical pulse and displayed as a second echo of
amplitude A1 (Fig. 2).
The two echoes are relatively spaced by a time t,
called ‘‘time of flight’’, which is the time necessary for
the ultrasonic wave to travel across the sample. Thewave time of flight relates to the elastic properties of
the sample, i.e. to its ability to transmit longitudinal
waves. Moreover, the two echoes displayed in Fig. 2
have also different amplitudes because the ultrasonic
wave, during its travel inside the sample, undergoes an
attenuation depending on the material damping behav-
iour. The temporal distance between two successive ech-
oes and their amplitude ratio are important parametersfor assessing the sample acoustic behaviour through the
calculation of the velocity and attenuation of the ultra-
sonic waves.
The longitudinal velocity, c, in the pulse-echo mode,
is defined as the ratio between two times the sample
thickness, d, and the time of flight, t, of the ultrasonic
wave through the sample, calculated as the difference be-
tween the times relative to both echoes displayed in Fig.2 (t = t1 � t0) (Krautkramer & Krautkramer, 1990):
c ¼ 2dt
ð1Þ
The attenuation, a, is defined from the decrease of the
amplitude, A, of a plane wave across a thickness, d:
A ¼ A0 expð�adÞ ð2Þ
where A0 is the amplitude of the incident wave. The
attenuation a is measured in dB/mm according to
(Krautkramer & Krautkramer, 1990):
a ¼ 1
2d20 log
A0
A1
� �ð3Þ
For each velocity and attenuation measurement, ten
signal acquisitions were made and averaged. The timeof flight and the signal amplitude were computed from
the averaged signal.
It should be noted that the sound velocity in a mate-
rial depends on its density and elastic properties,
whereas the sound attenuation depends on the viscous
behaviour and homogeneity of the material. Therefore,
ultrasonic measurements of c and a would detect the
phase transformations of the material that affect itsacoustic behaviour.
The interaction between an acoustic wave and a mate-
rial can provide information on its mechanical proper-
ties. These latter can be determined from the velocity at
which the wave propagates and from themanner in which
the acoustic wave is attenuated in the sample. When the
sample dimension normal to the propagation direction
of the acoustic wave is large compared to the wavelength,the wave propagation is governed by the complex bulk
longitudinal modulus L*, related to the bulk (K*) and
shear (G*) complex moduli as follows (Ferry, 1980; Pere-
pechko, 1975):
L� ¼ K� þ 4
3G� ð4Þ
The real and imaginary components of the complex
bulk longitudinal modulus (L* = L 0 + iL00) can be calcu-
lated from the measurement of ultrasonic longitudinal
velocity and attenuation according to the following
equations (Perepechko, 1975):
L0 ¼qc2 1� ak
2p
� �2h i
1þ ak2p
� �2h i2 and L00 ¼2qc2 ak
2p
� �1þ ak
2p
� �2h i2 ð5Þ
where L 0 is the elastic or storage component of the mod-
ulus, L00 is the viscous or loss component of the modu-
lus, c is the ultrasonic velocity, a the attenuation, q the
material density and k the wavelength of propagation,
calculated as the ratio between velocity and frequency
f(k = c/f).
For the studied case, the term ak/2p remains lowerthan 0.05 over then entire ageing period, therefore the
contribution of attenuation can be neglected. Therefore,
L 0 and L00 have been calculated from the simplified
equations:
L0 ¼ qc2; L00 ¼ 2qc3ax
ð6Þ
where x is the angular frequency (x = 2pf) and q the
density of the wheat starch extrudates, which, experi-
mentally determined at room temperature by the Archi-
medes� principle, results equal to 1323 kg/m3. The
storage longitudinal modulus L 0 corresponds to the stiff-
ness of a system that is deformed, changing its dimen-
sions in one direction while, in the other twodirections, the dimensions are constraint to remain con-
stant, as it occurs in samples where two dimensions are
much larger than the third (Ferry, 1980).
2.3. Dynamic mechanical thermal analysis
Dynamic mechanical analysis (DMA) at low fre-
quency was used to monitor the changes in the mechan-ical properties occurring during retrogradation. In a
dynamic mechanical experiment, a small sinusoidal
deformation, within the linear viscoelastic range, is ap-
plied to the sample, which responds with a sinusoidal
stress shifted in phase to the input deformation if the
material is viscoelastic. The ratio of the output stress
amplitude to the input deformation amplitude defines
the complex modulus E*, which consists of a real part,the storage modulus E 0, and an imaginary part, the loss
modulus E00.
A Rheometric Scientific dynamic mechanical thermal
analyser DMTA IV operating in bending mode was
used. Rectangular strips (approximately 7 mm · 14
Fig. 3. Evolution of the ultrasonic velocity (�) and attenuation (h),
measured at 10 MHz, versus storage time for wheat starch extrudates
with 34% water (w.w.b.).
F. Lionetto et al. / Journal of Food Engineering xxx (2005) xxx–xxx 5
ARTICLE IN PRESS
mm · 2 mm) were cut from the extruded ribbons, previ-
ously stored at 25 ± 1 �C in the incubator for different
ageing times, and clamped in the single cantilever geom-
etry. The analysis was performed during heating from
�30 to 70 �C at 1 Hz with a strain amplitude of 0.1%.
A relatively low heating rate of 1 �C/min was used to en-sure adequate thermal equilibrium across the sample. In
order to alleviate potential water loss during the mea-
surement, the samples were covered with a thin film of
silicon oil. The samples were weighed before and after
the DMA experiment and no significant moisture loss
was observed.
2.4. X-ray diffraction
Wide angle X-ray diffraction (XRD) measurements
were carried out to monitor the recrystallisation of amy-
lopectin during storage. A Bruker AXS D5005 diffrac-
tometer was used. The X-ray generator was equipped
with a copper tube operating at 40 kV and 30 mA and
irradiating the sample with a monochromatic Cu Ka
radiation with a wavelength of �0.154 nm. XRD spectrawere acquired at room temperature over the 2h range of
4–38� at 0.1� intervals with a measurement time of 6 s
per 2h intervals. The angular range encompassed the
main diffraction peaks of starch crystals.
XRD diffractograms were acquired at regular storage
time intervals up to �9 days on disks (�25 mm of diam-
eter) cut from the extruded ribbons, stored at 25 ± 1 �Cin an incubator in airtight aluminium foil bags.
2.5. Differential scanning calorimetry
Starch samples of 40mg weight were sealed in high-
pressure stainless steel pans soon after the extrusion
and stored at 25 ± 1 �C in an incubator for different
times prior to analysis. The melting behaviour of the ret-
rograded starch was studied using a power compensateddifferential scanning calorimeter (DSC 7, Perkin–Elmer,
USA). An empty stainless steel pan was used as a refer-
ence. The samples were heated from 20 �C to 160 �C at
10 �C/min. Three replicas were used for each measure-
ment. The thermograms were normalized to the dry
matter weight of each sample.
3. Results and discussion
The evolution of the ultrasonic velocity and attenua-
tion at 25 �C with storage time is reported in Fig. 3 for
wheat starch extrudates containing 34% water (wet
weight basis). The ultrasonic velocity is characterized
by a non-linear increase with storage time, whereas the
ultrasonic wave attenuation shows a bell shaped curve.A first rapid increase in the ultrasonic velocity is
accompanied by a steep increase in the wave attenua-
tion. Then, a few hours before the attenuation peak,the ultrasonic velocity slows down but continues to in-
crease for very long time at a reduced rate. After 9 days
of measurement at 25 �C, the velocity increase is small
but not yet negligible, whereas the attenuation value is
settled on a smaller value than the initial one, indicating
that the aged sample presents a reduced molecular
absorption compared to that of the gelatinised sample
at the beginning of the measurement.The increase in the longitudinal velocity, shown by
starch extrudates upon storage at room temperature,
indicates that a phase transformation, with a consequent
stiffening of gelatinised starch matrix, occurred. The
stiffening of starch extrudates does not originate from
water loss, since samples, weighed before and after ultra-
sonic measurements, do not show any significant weight
variation. Therefore, the slow increase in the longitudi-nal velocity can be assigned to the retrogradation pro-
cess, mainly relating to the partial recrystallisation of
the outer chains of amylopectin, as the retrogradation
of amylose would have mostly occurred during post-
extrusion cooling and sample handling (Fredriksson
et al., 1998; Goodfellow & Wilson, 1990; Miles et al.,
1985) and would not therefore be expected contribute
significantly to the long-term starch stiffening observedby the ultrasonic experiments.
The ultrasonic attenuation may be considered as the
equivalent of a damping factor in a dynamic mechanical
experiment (Maffezzoli et al., 1999a, 1999b), represent-
ing a measure of the energy loss as the wave travels
through the retrograding starch system. The ultrasonic
attenuation can be caused both by wave scattering and
absorption. In the case of the concentrated starch sys-tem analysed in this work, the scattering contribution
is negligible, because the dimensions of the amylopectin
crystals, typically in the order of 10–100s of nm (Buleon
et al., 1998a, 1998b; Jenkins et al., 1993), are far smaller
6 F. Lionetto et al. / Journal of Food Engineering xxx (2005) xxx–xxx
ARTICLE IN PRESS
than the wavelength of the ultrasonic waves used for the
experiment at 10 MHz, which varies between 183 and
195 lm depending on the ultrasonic velocity in the
starch samples. Therefore, only wave absorption is
responsible for the wave attenuation observed in the
wheat starch extrudates. This explains why the attenua-tion is lower in fully retrograded samples rather than in
gelatinised samples tested shortly after extrusion. In
fact, the reduction of the degree of mobility, as a conse-
quence of an increased crystalline fraction, inevitably
leads to a smaller viscous dissipation of the ultrasound
energy.
Since the ultrasonic wave propagation can be consid-
ered as a high frequency dynamic mechanical analysis,the longitudinal moduli (L 0, L00) obtained from ultra-
sonic experiments at 10 MHz were compared with the
Young moduli (E 0, E00) obtained from low frequency
(1 Hz) dynamic mechanical analysis by means of a rhe-
ometer operating in bending mode, as reported in Fig. 4.
It should be kept in mind that the great difference in the
frequency of the small oscillations applied with the two
techniques affects the sample response. In general, thefaster a material is deformed, the stiffer it behaves, and
the less able it is to dissipate stresses because the molec-
ular relaxation become less likely in the increasingly
shorter experimental time scale.
The storage moduli L 0 and E 0 show a similar trend,
characterized by a continuous increase with the ageing
time, reflecting the growth of the starch elastic proper-
ties arising from the increase of the crystallinity fractionat the expenses of the amorphous phase.
Fig. 4. Comparison of the storage (a) and loss (b) components of the
complex longitudinal modulus measured using ultrasound at 10 MHz,
with the complex Young modulus measured by DMA at 1 Hz.
A great difference in the magnitude orders of L 0 and
E 0 values is expected if one accounts that, for a rubbery
material such as the starch extrudates, G 0 � K 0 (Ferry,
1980) and hence, from Eq. (4), L 0 may be considered
equal to the bulk modulus K 0, which indicates the mate-
rial compressibility under a given amount of externalpressure. The reported values of L 0 for starch extrudates
(4.44 · 109 Pa for samples which are essentially amor-
phous shortly after extrusion) are consistent with the va-
lue of water bulk modulus at 20 �C (2.2 · 109 Pa)
(Halliday, Resnick, & Walker, 1997) if one accounts
for the high water content of the studied samples (34%
by weight).
The loss moduli L00 and E00 increase in the early stagesof ageing process as the crystallinity increases. The L00
curve presents a distinct peak. Although the relaxation
originating the loss is assumed to occur mainly in the
amorphous regions, the increase in the wave energy dis-
sipation may be related to the interaction between amor-
phous and crystalline regions. The reduction of the
entity of L00 with ageing can be attributed to the concur-
rent decrease of the amorphous regions and their mobil-ity in the starch sample and to a reorganization of water
molecules inside the sample with a consequent increase
in the amount of restricted water in starch structure,
as reported in a previous paper (Lionetto, Maffezzoli,
Ottenhof, Farhat, & Mitchell, 2005).
In contrast with ultrasonic experiments, it is very dif-
ficult to follow continuously the retrogradation process
using mechanical (DMA) measurements over severaldays. Conventional dynamic mechanical analysers, de-
signed for studying synthetic polymers, are not fully sui-
ted for biosystems such as starch gels because of risks of
significant loss of water (the most effective plasticizer for
this class of materials) during the test as it is not easy to
control the relative humidity surrounding the sample
while continuously increasing the temperature. Coating
the sample with an inorganic oil film can be adoptedas a solution, but the efficiency of such approach is
uncertain and, in addition, the oil may have a plasticiz-
ing effect on some biopolymers. Moreover, on ageing,
the surface of the sample does not remain perfectly flat,
leading to a slight error in the modulus calculation. The
experimental difficulties in using DMA experiments to
follow starch retrogradation, particularly in situ, explain
the relatively limited reports of experimental data of E 0
and E00 during retrogradation shown in Fig. 4.
The experimental set-up developed in this study for
the ultrasonic tests overcomes all these problems. The
retrogradation of the same starch sample can be moni-
tored continuously for many days in situ, without water
loss. Finally, a constant pressure is applied on the sam-
ple, assuring a flat surface absolutely necessary for a
good transmission of the ultrasonic waves from thetransducer to the sample and vice versa. This leads to
a strong reduction of the experimental errors enabling
F. Lionetto et al. / Journal of Food Engineering xxx (2005) xxx–xxx 7
ARTICLE IN PRESS
to obtain reliable measurements. Therefore, the ultra-
sonic dynamic mechanical analysis can be a valid alter-
native to the low frequency dynamic mechanical analysis
for studying the growth of the viscoelastic properties
during starch retrogradation.
The ultrasonic results have been also compared withthose of ‘‘more established’’ techniques in this research
area, wide-angle X-ray diffraction (XRD) and differen-
tial scanning calorimetry (DSC), which monitor starch
retrogradation through the growth of the diffraction
peaks of recrystallised starch and the increase of its
melting enthalpy, respectively. In Fig. 5 the extent of ret-
rogradation, as monitored by different techniques, has
been plotted as a function of the storage time. The stor-age longitudinal modulus (L 0), the loss modulus (L00),
the DSC melting enthalpy and the X-ray crystallinity in-
dex values reported in Fig. 5 were divided by their indi-
vidual maximum value. XRD and DSC data were
collected discretely at intervals of several hours.
The progress of retrogradation, monitored by the dif-
ferent techniques, is in good agreement. The loss modu-
lus L00 presents a peak slightly before the plateau valuereached by the crystalline index, calculated from XRD
data, and by melting enthalpy, measured by DSC. On
further storage, a strong decrease in L00 and a continu-
ous increase in the L 0 curve is observed from ultrasonic
experiments, whereas XRD and DSC data indicate a
sluggish growth of crystalline fraction inside the sample.
The more likely explanation of this behaviour is that,
after three days of storage at room temperature, a reor-ganization of crystallites inside starch sample occurs.
Such crystal perfection phenomena have been demon-
strated also by an increase of melting temperature ob-
served by DSC measurements reported in a previous
study (Lionetto et al., 2005).
Fig. 5. Comparison of the extent of retrogradation at 25 �C of wheat
starch extrudates (34% water w.w.b.) as sensed by different techniques.
The consequent more homogeneous distribution of
the crystallites leads to a reduction of L00 and a further
slight stiffening of the starch sample, monitored by the
continuous and gradual increase of L 0.
The close correlation among the acoustic properties
of starch extrudates and other physical properties, mea-surable by DSC and XRD, is an indication of the valid-
ity of the ultrasonic wave propagation in monitoring the
retrogradation process of starch based systems.
4. Conclusions
In this work, the propagation of ultrasonic longitudi-nal waves was applied to monitor non-destructively the
retrogradation process of extruded wheat starch. A spe-
cifically developed experimental set-up, consisting of an
ultrasonic probe in contact with the starch sample and a
purpose built sample-holder, enabled a continuous mea-
surement of ultrasonic velocity and attenuation during a
several days lasting experiment without sample
dehydration.Ultrasonic wave propagation was able to monitor the
recrystallisation of amylopectin molecules because the
acoustic behaviour of the material is greatly affected
by the changes in its viscoelastic properties during
retrogradation.
An increase in the ultrasonic velocity with storage
time was a good indication of the growth of the elastic
properties arising from the increased crystalline fractionin starch extrudates. The bell-shaped behaviour of the
ultrasonic wave attenuation suggested a high degree of
viscous dissipation of the ultrasonic energy in corre-
spondence of the attenuation peak and, consequently,
a more homogeneous reorganization of crystallites in
starch samples.
The potential of the ultrasonic wave propagation to
be used as a high frequency DMA technique for a con-tinuous monitoring of retrogradation was assessed by
comparing ultrasonic results with those obtained from
low frequency dynamic mechanical experiments.
The results of the ultrasonic monitoring of starch ret-
rogradation were in good agreement with those ob-
tained from X-ray diffraction and differential scanning
calorimetry. The close correlation among the acoustic
properties of starch extrudates and other physical prop-erties, measurable by DSC and XRD, was a further indi-
cation of the validity of the ultrasonic wave propagation
in monitoring the retrogradation process of starch based
systems.
Acknowledgements
The present work was carried out with the support of
the Marie Curie Fellowship financed by the European
8 F. Lionetto et al. / Journal of Food Engineering xxx (2005) xxx–xxx
ARTICLE IN PRESS
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.
References
Alig, I., Tadjbakhsch, S., Floudas, G., & Tsitsilianis, C. (1998).
Viscoelastic contrast and kinetic frustration during poly(ethylene
oxide) crystallization in a homopolymer and a triblock copolymer.
Comparison of ultrasonic and low-frequency rheology. Macromol-
ecules, 31, 6917–6925.
Benedito, J., Carcel, J., Clemente, G., & Mulet, A. (2000). Cheese
maturity assessment using ultrasonics. Journal of Dairy Science, 83,
248–254.
Benedito, J., Carcel, J., Sanjuan, N., & Mulet, A. (1999). Use of
ultrasound to assess Cheddar cheese characteristics. Ultrasonics,
38, 727–730.
Benedito, J., Mulet, A., Clemente, G., & Garcia-Perez, J. V. (2004).
Use of ultrasonics for the composition assessment of olive mill
wastewater (alpechin). Food Research International, 37, 595–601.
Buckin, V., & Kudryashov, E. (2001). Ultrasonic shear wave rheology
of weak particle gels. Advances in Colloid and Interface Science, 89-
90, 401–422.
Buleon, A., Colonna, P., Planchot, V., & Ball, S. (1998a). Starch
granules: structure and biosynthesis. International Journal of
Biological Macromolecules, 23, 85–112.
Buleon, A., Le Bail, P., Colonna, P., & Bizot, H. (1998b). Phase and
polymorphic transitions of starches at low and intermediate water
contents. In D. S. Reid (Ed.), The properties of water in foods
(pp. 160–178). London: Blackie Academic & Professional.
Contreras, N. I., Fairley, P., McClements, D. J., & Povey, M. J. W.
(1992). Analysis of the sugar content of fruit juices and drinks using
ultrasonic velocity-measurements. International Journal of Food
Science and Technology, 27, 515–529.
Farhat, I. A., Protzmann, J., Becker, A., Valles-Pamies, B., Neale, R.,
& Hill, S. E. (2001). Effect of the extent of conversion and
retrogradation on the digestibility of potato starch. Starch/Starke,
53, 431–436.
Ferry, J. D. (1980). Viscoelastic properties of polymers. New York:
John Wiley & Sons.
Fredriksson, H., Silverio, J., Andersson, R., Eliasson, A. C., & Aman,
P. (1998). The influence of amylose and amylopectin characteristics
on gelatinization and retrogradation properties of different
starches. Carbohydrate Polymers, 35, 119–134.
Ghaedian, R., Coupland, J. N., Decker, E. A., & McClements, D. J.
(1998). Ultrasonic determination of fish composition. Journal of
Food Engineering, 35, 323–337.
Goodfellow, B. J., & Wilson, R. H. (1990). A Fourier-transform IR
study of the gelation of amylose and amylopectin. Biopolymers, 30,
1183–1189.
Greenwood, M. S., & Bamberger, J. A. (2002). Measurement of
viscosity and shear wave velocity of a liquid or slurry for on-line
process control. Ultrasonics, 39, 623–630.
Hallberg, L. M., & Chinachoti, P. (2002). A fresh perspective on
staling: the significance of starch recystallization on the firming
bread. Journal of Food Science, 67, 1092–1096.
Halliday, D., Resnick, R., & Walker, J. (1997). Fundamentals of
physics (5th ed.). New York: John Wiley & Sons.
Jenkins, P. J., Cameron, R. E., & Donald, A. M. (1993). A universal
feature in the structure of starch granules from different botanical
sources. Starch/Starke, 45, 417–420.
Knorr, D., Zenker, M., Heinz, V., & Lee, D. (2004). Applications and
potential of ultrasonics in food processing. Trends in Foods Science
and Technology, 15, 261–266.
Krautkramer, J., & Krautkramer, H. (1990). Ultrasonic testing of
materials. Heidelberg: Springer-Verlag.
Kulmyrzaev, A., & McClements, D. J. (2000). High frequency dynamic
shear rheology of honey. Journal of Food Engineering, 45, 219–224.
Letang, C., Piau, M., & Verdier, C. (1999). Characterization of wheat-
flour-water doughs. Part I: Rheometry and microstructure. Journal
of Food Engineering, 41, 121–132.
Letang, C., Piau, M., Verdier, C., & Lefebvre, L. (2001). Character-
ization of wheat-flour-water doughs: a new method using ultra-
sound. Ultrasonics, 39, 133–141.
Lionetto, F., Maffezzoli, A., Ottenhof, M. A., Farhat, I. A., &
Mitchell, J. R. (2005). The retrogradation of concentrated wheat
starch systems. Starch/Starke, 57, 16–24.
Lionetto, F., Rizzo, R., Luprano, V. A. M., & Maffezzoli, A. (2004).
Phase transformations during the cure of unsaturated polyester
resins. Materials Science and Engineering A, 370, 284–287.
Maffezzoli, A., Luprano, V. A. M., & Montagna, G. (1998). Ultrasonic
characterization of water sorption in poly(2-hydroxyethyl methac-
rylate) hydrogels. Journal of Applied Polymer Science, 67, 823–831.
Maffezzoli, A., Quarta, E., Luprano, V. A. M., Montagna, G., &
Nicolais, L. (1999a). Cure monitoring of epoxy matrices for
composites by ultrasonic wave propagation. Journal of Applied
Polymer Science, 73, 1969–1977.
Maffezzoli, A., Tarzia, A., Cannoletta, D., Montagna, G., & Luprano,
V. A. M. (1999b). Ultrasonic characterization of the kinetics of
water sorption in hydrogels. Macromolecular Symposia, 138,
149–155.
Matsukawa, M., & Nagai, I. (1996). Ultrasonic characterization of a
polymerizing epoxy resin with imbalanced stoichiometry. Journal
of Acoustical Society of America, 99, 2110–2115.
McClements, D. J. (1995). Advances in the application of ultrasound
in food analysis and processing. Trends in Foods Science and
Technology, 6, 293–299.
McClements, D. J. (1997). Ultrasonic Characterization of food and
drinks: principles, methods and applications. Critical Reviews in
Food Science and Nutrition, 37, 1–46.
McClements, D. J., & Povey, M. J. W. (1987). Solid fat content
determination using ultrasonic velocity measurements. Interna-
tional Journal of Food Science and Technology, 59, 697–701.
Miles, M. J., Morris, V. J., Orford, P. D., & Ring, S. G. (1985). The
roles of amylose and amylopectin in the gelation and retrograda-
tion of starch. Carbohydrate Research, 135, 271–281.
Mizrach, A., Flitsanov, U., Akerman, M., & Zauberman, G. (2000).
Monitoring avocado softening in low-temperature storage using
ultrasonic measurements. Computers and Electronics in Agriculture,
26, 199–207.
Mulet, A., Benedito, J., Bon, J., & Sanjuan, N. (1999). Low intensity
ultrasonics in food technology. Food Science and Technology
International, 5, 285–297.
Mulet, A., Benedito, J., Golas, Y., & Carcel, J. A. (2002). Non invasive
ultrasonic measurements in the food industry. Food Reviews
International, 18, 123–133.
Ortega-Ojeda, F. E., & Eliasson, A. C. (2001). Gelatinisation and
retrogradation behaviour of some starch mixtures. Starch-Starke,
53, 520–529.
Perepechko, I. (1975). Acoustic methods of investigating polymers.
Moscow: Mir Publishers.
Povey, M. J. W., & Mason, T. J. (1998). Ultrasound in food processing.
London: Blackie Academic & Professional.
Ross, K. A., Pyrak-Nolte, L. J., & Campanella, O. H. (2004). The use
of ultrasound and shear oscillatory test to characterize the effect of
mixing time on the rheological properties of dough. Food Research
International, 37, 567–577.
Saggin, R., & Coupland, J. N. (2001). Oil viscosity measurement by
ultrasonic reflectance. Journal of the American Oil Chemists
Society, 78, 509–511.
F. Lionetto et al. / Journal of Food Engineering xxx (2005) xxx–xxx 9
ARTICLE IN PRESS
Simal, S., Benedito, J., Clemente, G., Femenia, A., & Rossello, C.
(2003). Ultrasonic determination of the composition of a meat-
based product. Journal of Food Engineering, 58, 253–257.
Zeleznak, K. J., & Hoseney, R. C. (1986). The role of water in the
retrogradation of wheat starch gels and bread crumb. Cereal
Chemistry, 63, 407–410.