Aspects of the Physical Chemistry of Starch

Journal of Cereal Science 34 (2001) 1–17
doi:10.1006/jcrs.2000.0402, available online at http://www.idealibrary.com on
MINI REVIEW
Aspects of the Physical Chemistry of Starch
R. Parker and S. G. Ring∗
Institute of Food Research, Norwich Research Park, Colney Lane, Norwich, NR4 7UA, U.K.
Received 20 November 2000
ABSTRACT
Starch makes an important contribution to food structure and hence quality. Often there is a need
to modify the behaviour of starch in food materials. One approach is to use knowledge of the physical
chemistry of starch to modify component interactions and hence change behaviour. In this review
we examine recent research on the phase behaviour and dynamics of starch relevant to this approach.
 2001 Academic Press
INTRODUCTION
Many cereal products have been developed empirically over very many years. Often there is
opportunity to develop new products to satisfy a
market requirement, or a need to modify product
performance for a particular application. Often
this development will be largely empirical—
different ingredients will be mixed together or
different processes tried until the required material
properties and consumer acceptability has been
achieved. Any approach which ‘guides’ empiricism, or enables products to be ‘engineered’
for a particular application, is to be welcomed as
development time will be reduced and improved
products may result. The ability to ‘engineer’ a
product is facilitated by prediction relating processing, to structure development, to material
properties. Structure over a range of lengthscales,
from the macroscopic to the molecular, has a
potential impact on these properties. For example,
the material properties of cereal foams are influenced by the density of the foam, whether it is
closed or open cell, and the mechanical properties
of the foam matrix. With a single cereal-based
matrix of fixed composition it would be possible
to prepared foamed structures with quite different
∗ Corresponding author: E-mail [email protected]
0733–5210/01/040001+17 $35.00/0
behaviour1. The foam matrix will be multiphase,
consisting of protein-rich and starch-rich domains,
within which low molecular weight components,
including water, will be distributed depending on
their affinity for the biopolymers. In this review
we will focus on the molecular properties of one
of those phases—the starch-rich phase. Our aim
is to discuss research on the molecular properties
of starch within the context of the physical chemistry of polymeric materials, more particularly
synthetic polymers. This physical chemistry can
help provide insight into how material properties
may be altered through changing molecular interactions or composition. There are many excellent
recent reviews on starch structure, biosynthesis
and granule organisation so these aspects will only
briefly be considered2,3.
THE MACROMOLECULES
Starch consists of two main polysaccharides, amylose and amylopectin. Both polysaccharides are
based on chains of 1→4 linked -D-glucose [Figure
1(a)] but whereas amylose is essentially linear,
amylopectin is highly branched containing on average one branch point which is 1→4→6 linked
for every 20–25 straight chain residues. Linear
regions of the amylose chain form a dark blue
complex with polyiodide ions in aqueous solution
at room temperature. This interaction is a basis
 2001 Academic Press
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R. Parker and S. G. Ring
CH2OH
O
O
Rotation angles (Φ , φ )
HO
O
CH2OH
HO
O
(a)
O
H
OH
(b)
A form
a, b plane
B form
a, b plane
2.12 nm
1.17 nm
1.85 nm
1.85 nm
(c)
Figure 1 The glycosidic linkage of amylose showing the
preferred chair conformations, and the possibility for conformational flexibility about the glycosidic linkage; (a) the 2dimensional cluster model of amylopectin with short chains
arranged in clusters on longer chains, (b) the arrangement of
double helices in the A and B crystalline forms of starch (c).
for defining amylose as that starch polysaccharide
which, under standardised conditions, binds 20%
of its weight of iodine while under the same conditions amylopectin generally binds <1% w/w.
This iodine binding allows a distinction to be made
between amylose and amylopectin and permits the
determination of the amylose content of native
starch. Most starches contain between 20 and 25%
w/w amylose although some waxy starches contain
very little, if any, amylose (<1%) and other
starches, such as amylomaize, which contain in
the region of 65% amylose. Although a fraction
of the amylose population is linear and is quantitatively hydrolysed by the exo-acting enzyme amylase, a fraction is lightly branched4–8. For example, purified wheat amyloses were hydrolysed
from 79 to 85% by -amylase. Isoamylolysis allowed quantitation of a short chain fraction representing between 3·2 and 7·6% w/w of the
amylose depending on the variety of wheat examined9. Typical molecular weights of extracted
amylose are in the region of 105 to 106 g mol−1.
In aqueous solution the amylose behaves as a
somewhat stiff flexible coil with a typical hydrodynamic radius of 7 to 22 nm2.
The branching within amylopectin is not random3,10. Debranching amylopectin with a debranching enzyme such as isoamylase followed by
size exclusion chromatography11, or high pressure
anion exchange chromatography12 reveals an essentially bimodal population of chains with two
main populations with peak DP of around 12–14
and >4511. The short chain fractions of many
amylopectins also have shoulders of around DP
18–2013,14. The short chain fraction is the most
abundant by weight and number. There is also
the possibility that amylopectin can contain some
longer chains, of sufficient uninterrupted length
to give rise to significant iodine binding behaviour15,16. Classification of the constituent chains
can also be based on how the chains are linked to
the rest of the molecule. The C chain carries the
sole reducing end of the molecule, the A chain is
only linked to the rest of the molecule through its
potential reducing end while the B chain is linked
in this way and also carries one or more A chains.
Typical estimates of the ratio of A : B chains is in
the region of 1:1 to 1·5:110. The fine structure of
amylopectin is dependent on botanical origin, with
variations reported in the A : B chain ratio, the
length and abundance of short and long chains,
and the form of distribution of the constituent
chains. The structure of different amylopectins
is generally characteristic of a particular species
although small varietal differences are also observed9,11,17. Current models2 of amylopectin structure depict short linear chains, 10 to 20 units long,
arranged in clusters on longer chains, with the
longer chains spanning more than one cluster
[Figure 1(b)]. Examination of the products of a
limited -amylolysis of amylopectin reveals the
Aspects of the physical chemistry of starch
presence of branched clusters of short chains lending support to the model18,19. Typically, the cluster
model is a two dimensional representation of a
structure which must pack in the starch granule
to account for the measured density of
>1500 kg m−3, and the known crystallographic
structure which imposes dimensional constraints
on how the crystalline chains can be arranged.
Amylopectin is one of the largest biopolymers
known with typical molecular weights being in the
region of 108 g mol−1 and a hydrodynamic radius
of 21–75 nm. A consequence of the branching
is that for its molecular weight the amylopectin
molecule is relatively compact. A further consequence is that if the domains of amylopectin
molecules interpenetrate the branching and clustering of chains will hinder their separation.
GRANULE ORGANISATION
Starch occurs naturally as water-insoluble granules
whose form is characteristic of its botanical origin.
When viewed under polarised light the granules
are birefringent. As the radial refractive index
is larger than the tangential refractive index a
preferred radial distribution of chains is indicated2.
The starch granule is also partially crystalline with
crystallinities in the region of 30% being reported.
A number of crystalline forms are known [Figure
1(c)], the A form20 which is found in most cereal
starches, including wheat, consists of starch double
helices packed into a monoclinic array. The B
form21,22, which is found in some tuber starches
and high amylose cereal starches, is a more highly
hydrated and open structure, consisting of double
helices packed in a hexagonal array. As waxy
starches, containing only amylopectin, still have
crystalline granules the participation of amylopectin chains within the crystalline domains is
indicated. Acid etching of the granules, or lintnerisation as it is known, leads to a preferential
erosion of the amorphous fraction. Examination
of the product has shown that the length of chain
participating in the crystalline domains is comparable to the short chain fraction of amylopectin
hence the suggestion that it is the short chains of
amylopectin which form the double helices23,24.
Using high intensity X-ray sources it is now possible to map the crystallinity of different regions
of the starch granule25,26. This has given information on the orientation of crystalline domains
within individual granules. For example, for potato
3
starch the B-type domains showed a preferred
radial orientation while in wheat starch no such
orientation was observed of the A-type crystallites.
The approach has also been used to probe the
distribution of A and B crystalline domains. For
legume starch granules27 it was found that the
A crystalline domains were concentrated in the
granule periphery28. Other types of order can be
probed by other techniques. Solid state NMR
experiments have been used to complement Xray measurements29,30. The spectra of the A and
the B forms of starch are characteristically different, and different to amorphous starch.
X-ray and neutron scattering experiments31 on
starch granules have revealed an intriguing peak
in the scattering profile representing a periodicity
in the range 8·9 to 10·9 nm32–35. Through analysis
of the whole profile, in terms of a model of repeating amorphous and crystalline layers, it was
found that, for a range of starches of different
botanical origin, within experimental error, this
periodicity had a constant value of 9 nm. The
constancy of this spacing is surprising given that
the starches examined had different crystalline
forms and different amylopectin structures. A recent interpretation of this proposed lamellar structure is that the amylopectin forms a side chain
liquid crystal structure36–38. It is further proposed
that the lamellae are tilted as in a proposed superhelical structure36,39.
Another aspect of granule structure is the shells2
observed directly by light microscopy and by scanning electron microscopy of granules which have
been etched by acid and enzyme treatment. These
structures have a radial repeat length of between
120 and 400 nm and are considered to consist of
repeating amorphous and semi-crystalline layers.
Other structures40 observed on the surface of starch
granules by atomic force and other microscopies
are ‘blocklets’ of about 30 nm in diameter41. One
interpretation is that these blocklets represent the
amylopectin cluster, another is that the ‘blocklets’
are some surface contamination. If all this information is considered as a whole then clearly the
starch granule is a very complex macromolecular
assembly, and it is perhaps useful to distinguish
between that which has received widespread acceptance and that which is still debated. Areas
which remain a focus of discussion, largely because
they represent recent research, are the blocklet
concept and proposed models relating to the lamellar structures having a 9 nm spacing. Models of
granule assembly should recognise the diversity of
4
R. Parker and S. G. Ring
chemical structures which make up the granule,
for example how is the amylose arranged, what is
the significance of the variability of amylopectin
structure, and the lengths of the constituent chains
of amylopectin, to its organisation in the granule.
GELATINISATION, MELTING AND
DISSOLUTION
Starch is usually processed by heating in the presence of water which disrupts the native crystalline
structure, a phenomenon known as gelatinisation.
In an excess of water (>90% w/w), above a characteristic temperature known as the gelatinisation
temperature, the starch granule loses its native
crystalline order and swells irreversibly to many
times its original size. At the same time the starch
polysaccharide amylose (if present) is preferentially
solubilised. Substantial solubilisation of high molecular weight amylopectin is not observed although there are reports for some starches of the
solubilisation of a low molecular weight amylopectin42. Although the gelatinisation temperature
is relatively easy to determine experimentally it is
important to bear in mind that the gelatinisation
process occurs over a limited temperature range
for a single granule and a somewhat wider temperature range for a population of granules. As
heating is continued, past the gelatinisation temperature, the granule continues to swell. At temperatures less than 100 °C and in the absence of
mechanical shear, the swollen granules, enriched
in amylopectin, maintain their integrity. The increase in volume fraction of starch granules in the
suspension leads to an increase in viscosity of the
paste. If the temperature is such that the solubilised
amylose is slow to aggregate, the viscosity of these
pastes, in common with other particulate suspensions, is dominated by the volume fraction
occupied by the starch granules43. At concentrations greater than about 6% w/w for cereal
starches, the gelatinised granules fill the available
volume producing a viscoelastic material, the properties of which are additionally influenced by the
deformability of the swollen starch granule and
hence the amylopectin concentration in the granule. The swelling of the granule on gelatinisation
clearly has a major impact on the rheology of
starch pastes. What molecular features affect swelling? One approach for gaining insight into the
swelling properties of starch might be to consider
it as a network structure44. For neutral, chemically
crosslinked, polymer networks the extent of equilibrium swelling observed is dependent on the
extent of crosslinking and the affinity of the network for the solvent45. The network will swell until
the osmotic pressure generated by the network, as
a result of its affinity for the solvent, is balanced
by the restorative stiffness of the network resisting
swelling. Clearly, neutral amylopectins will largely
have similar affinities for water and it might be
proposed that it is the effective crosslinking of the
entangled amylopectin network which might limit
swelling. Changing the solvent characteristics, for
example by increasing temperature, or by mixing
with other compounds which have a high affinity
for the starch chain, would lead to increased
swelling. For polyelectrolyte networks there is an
additional contribution to the osmotic pressure
driving swelling, coming from the mobile ions
which are required to neutralise the charge on the
polymer45. The pressure generated by this Donnan
effect is greatest at low ionic strength. Some amylopectins, for example potato amylopectin, are
weakly phosphorylated and therefore behave as
polyelectrolytes. Potato starch is also a high swelling starch whose swelling is reduced if ionic
strength is increased. Clearly there is a polyelectrolyte effect in operation and one potential
way of modifying the swelling properties of these
starches would be to alter their polyelectrolyte
characteristics. While the consideration of the
gelatinised granule as a network structure might
be useful, it is important to bear in mind that the
foregoing discussion is based on the equilibrium
swelling of polymer networks i.e., there is a reversibility in swelling if the conditions are changed.
The swelling of the starch granule after gelatinisation is profoundly non-equilibrium as illustrated by the observation that starches such as
wheat starch which have been subjected to different thermal histories, such as slow versus rapid
heating, swell to different extents and as a consequence the starch pastes have different rheologies43,46.
As the starch granule is a complex structure it
is perhaps not surprising that the description of
the processes occurring during gelatinisation are
similarly complex. One approach has been to
study the loss of various types of order with a range
of techniques during the gelatinisation process. As
the different techniques probe different aspects of
the gelatinisation process, it should not be surprising that they do not exactly map onto each
other. Ideally experiments should be carried out
Aspects of the physical chemistry of starch
during the gelatinisation process, often, for practical necessity, they are carried out on starch
materials which have been quenched to room
temperature, allowing a potential reordering of
structure. One such study used solid state NMR,
X-ray diffraction and differential scanning calorimetry47. Although these techniques probe
different aspects of the loss of order of the granule,
it was found that the extent of loss of order as a
function of temperature was broadly similar. This
conclusion was consistent with a more recent
study48 which followed the loss of crystallinity of
maize starch, as a function of temperature, using
a synchrotron source of X-rays. In this case, in
addition to the disappearance of the native crystalline structure, it was also possible to monitor
the crystallisation of starch as a lipid complex at
higher temperatures. In addition to the loss of
crystalline order, the loss of lamellar order has
also been followed as a function of temperature32,38. In the initial stages of gelatinisation there
is no marked shift in the peak in the scattering
profile, and therefore no marked change in lamellar spacing. The feature does, however, become weaker. Finally, another study examined
order loss by birefringence measurements, differential scanning calorimetry and X-ray diffraction.
It was found that the loss of birefringence occurred
over a narrower temperature range than the loss
of crystalline order49,50. In conclusion, there is a
broad consensus that, for a population of granules,
all the forms of order (crystalline, lamellar, orientational) are lost or modified over a broadly
similar temperature range. To further assess order
loss as a function of temperature it is necessary
to consider heterogeneity in structure over the
population studied. For example, in pea starches
which give a C-type diffraction pattern (a mixture
of the A and B forms) it was found that the B
form was present in the interior of the granule and
the A form in the periphery27. On gelatinisation,
birefringence is initially lost from the interior of
the granule followed by the periphery. On examination of the loss in crystallinity it is found that
the B-type crystallinity is lost first. A population
of granules might be expected to show further
heterogeneity. For example, in wheat starch the
distribution in granule sizes is bimodal and the
different sizes might display somewhat different
gelatinisation behaviour. A further source of
heterogeneity during gelatinisation occurs as a
result of concentration gradients. As the population of starch granules gelatinises over a limited
5
range of temperature, those that gelatinise first
have access to the available water. On gelatinisation the granules which swell reduce the
amount of water available for the swelling of other
granules51. At intermediate starch concentrations,
of around 40–50% w/w, gradients in starch concentration are observed in a macroscopic sample.
As reducing the water content elevates the temperature of ‘gelatinisation’, one effect is to broaden
the transition. Double peaks are observed at these
concentrations in plots of order loss, as assessed
by X-ray diffraction or differential scanning calorimetry, as a function of temperature52. This is
at least in part due to the heterogeneous nature
of the sample. As the physical descriptions of the
gelatinisation process are becomingly increasingly
sophisticated, it is worthwhile posing the question—how might the gelatinisation characteristics
of a starch be changed through modifying aspects
of granule organisation? To attempt to answer this
question some physical insight is needed into the
underlying physical processes. During gelatinisation crystalline order is lost. From comparison with synthetic polymer systems it can be
proposed that the temperature dependence of this
loss in crystallinity is dependent on the crystalline
polymorph present, its degree of perfection, the
length of chain involved in the crystalline unit and
the diluent content (with increasing diluent content
reducing the temperature of the transition)53. As an
individual amylopectin molecule can participate in
a number of crystalline domains or lamellae it is
the local diluent content which is important. There
are a number of ‘models’ of gelatinisation32 which
give somewhat difference emphasis to the relative
importance of these factors. For example, it has
been proposed that the hydration of the amorphous regions of the granule is strongly temperature
dependent, and that at a certain temperature there
is a large change in the water content of these
regions, with a result that the local diluent content
is sufficiently high that gelatinisation can occur. It
has been suggested that this hydration is sufficiently
strong to overcome the forces holding the crystalline lattice together. The physical origin of this
marked change in hydration has been attributed
to exceeding a glass transition54 (see later), or
simply that the affinity of the amorphous regions
of starch for water shows a strong temperature
dependence. Estimates of the average water content of a hydrated starch granule are in the region
of 27–33%51. For this average composition the
glass transition occurs below ambient tem-
6
R. Parker and S. G. Ring
Transition temperature (°C)
300
250
200
150
100
DP14
DP12
DP10
50
0
10
20
30
40
50
Water content (% w/w)
60
Figure 2 The effect of water content on the melting and
glass transitions of starch. Key: solid line, estimates of the
melting temperature of A-type short-chain amylose crystals
with degree of polymerisation (DP) 10, 12, 1461,63; Β, glass
transition of pregelatinised starch55; •, melting transition of
granular wheat starch64.
peratures55. Furthermore, solid state NMR studies
on granular starch have shown that the amorphous
fraction is plasticised into a rubbery state at ambient temperatures38. Generally, water is a poor
solvent for the starch polysaccharides and experiments which have measured the affinity of the
starch chain for water show a weak temperature
dependence56. These observations suggest that the
amorphous fraction of starch may not have a
dominant role in controlling gelatinisation.
One of the essential elements of the gelatinisation process is the loss of crystalline order52.
To gain insight into the factors affecting this loss
of order, one approach is to examine the behaviour
of highly crystalline materials, preferably single
crystals of macroscopic size. Short linear chains of
amylose crystallise readily and under appropriate
conditions highly crystalline materials of the
different polymorphic forms of starch are obtained57–60. The effects of diluent content, and
polymorphic form, on the temperature dependence of dissolution have been examined61. It
is also possible to examine the effect of chain
length on the process, with the expectation that
the longer the length of chain involved in the
crystalline unit the higher the melting temperature62—an expectation which was confirmed
experimentally63. A schematic diagram of the expected dependence of the melting of A-type crystallites on water content is shown in Figure 2. Also
included is an estimate of how the dissolution
temperature varies with increasing chain length.
The melting temperature of the low water content
materials is experimentally inaccessible due to
thermal degradation. In common with polymeric
crystalline materials45,53, the addition of a diluent,
in this case water, depresses the observed melting
temperature, Tm. For an A-type starch crystallite
formed from chains 12 units in length, the expected
peak melting temperature at high water contents
is about 75 °C. As a comparison, the predicted
high molecular weight limit of Tm in excess water
is 150 °C. With decreasing water content, the
melting temperature rises, reaching a predicted
150 °C for chains 12 units in length at a water
content of 16% w/w. The polymorphic form of
starch also affects the dissolution behaviour. At a
fixed water content, the melting and dissolution of
the B-form occurs at >20 °C lower temperature61.
For the crystallites in wheat starch the gelatinisation/dissolution temperature in excess
water is in the region of 60 °C rather than the
75 °C of the highly crystalline material, probably
indicating an effect of crystal perfection/size on
the observed dissolution. The form of the increase
with decreasing water content in the range 50 to
5% w/w water is remarkably similar64 (Figure 2),
suggesting that dissolution experiments on very
crystalline materials provide useful insight into
the very much more complex phenomenon of
gelatinisation. A further indication of usefulness is
gained from experiments on smooth-seeded pea
starches28 the granules of which contain a mixture
of the A and B crystalline forms of starch. The
prediction that the dissolution of the B domains
should occur at a lower temperature than the
A domains was confirmed by X-ray diffraction
experiments on the gelatinisation process.
Although the spherulites formed from the short
amylosic chains are somewhat analogous to the
starch granule, particularly for the B spherulites
which are formed from the stacking of crystalline
lamellae65, they differ in at least one important
respect. In the case of the spherulite, disruption
of the crystalline lattice leads to a solubilisation of
the amylosic chain. In the granular case, disruption
of a crystalline domain will only lead to a partial
solubilisation because the amylopectin molecule
spans different domains or lamellae. The initial
partial solubilisation of a crystalline domain increases the local water content of the granule
increases facilitating the subsequent dissolution of
crystalline material.
For native starches, the length of chain of the
short chain fraction and the crystalline form of
Aspects of the physical chemistry of starch
the granule are associated11,66. It is found that
starches which have a relatively short short-chain
fraction e.g. wheat and maize starch, have an A
crystalline form while starches which have a longer
short-chain fraction, e.g. potato starch, have a
B-crystalline form. In vitro experiments on short
amylosic chains reveal the same type of behaviour
with the shorter chains crystallising in the A form
and longer chains in the B form57,59. Crystallisation
at small undercoolings, below the equilibrium dissolution temperature of the crystallites, favours the
production of A form while a large undercooling
favours the production of the B form. This suggests
that the B form is a kinetic product. The associations between amylopectin chain length and
crystalline form suggest that these considerations
remain relevant to granule assembly. Both crystal
type, and chain length involved in the crystal,
affect dissolution. One strategy to ‘design’ a cereal
starch (A-type crystalline form) with a reduced
gelatinisation temperature, might be to reduce
the chain length of the short chain fraction of
amylopectin. If a higher gelatinisation temperature
was required, then increasing the chain length of
the short chain fraction might initially lead to a
depression in gelatinisation temperature, as the
crystalline form changed from A to B, followed by
a subsequent increase as the chain length effect
started to dominate.
INTERACTIONS WITH WATER AND LOW
MOLECULAR WEIGHT SOLUTES
To start to understand the role of water as a
diluent, it is useful to examine predictive relationships describing the composition dependence
of melting45,52. The classical description of the
compositional dependence of polymer melting in
the presence of a diluent is given by
1/Tm=1/Tm0+(R/Hu). (Vu/V1). [v1−v12]
(1)
where Tm0 is the melting temperature of the pure
polymer, Vu and V1 are the molar volumes of
polymer repeating unit and diluent, respectively,
and v1 is the diluent volume fraction. Hu is the
enthalpy of fusion per repeating unit and ; is the
Flory-Huggins interaction parameter67 characterising the interaction energy per solvent
molecule.
This relationship predicts that the smaller the
diluent size relative to that of the polymer repeating unit, in this case an anhydroglucose unit,
7
and the stronger the favourable interaction between the diluent and polymer, the stronger the
depression in Tm. It is well known that the starch
polysaccharides precipitate from aqueous solution
at room temperature, indicating that water interacts weakly with the starch chain and at room
temperature it can be considered a poor solvent.
Physicochemical measurements on water sorption
behaviour can provide more quantitative estimates
of solvent quality. This requires equilibrium behaviour which can be achieved if the behaviour
of oligosaccharides is examined63. For polysaccharides non-equilibrium effects generally dominate68, particularly for compositions which are
glassy at the temperature of measurement. The
values of the parameter obtained for concentrated aqueous mixtures of maltooligomers
were in the range 0·7 to 0·8 at room temperature63,
confirming that water is a poor solvent under
these conditions. To refine this approach more
information is needed on the temperature and
composition dependence of . The relatively small
molecular size of water is predicted to lead to a
strong depression in Tm. It has been found that
the Flory-Huggins approach fits the available experimental data for starch crystallite dissolution
reasonably well. The prediction that replacement
of water by other larger water-soluble solutes that
interact weakly with the starch chain, such as low
molecular weight carbohydrates, would lead to
an elevation of Tm is also confirmed experimentally69,70.
BIOPOLYMER/BIOPOLYMER INTERACTIONS
A further aspect of phase behaviour that should
be considered is the miscibility of biopolymer
mixtures. The immiscibility of concentrated solutions of chemically different synthetic polymers
is a well described phenomenon, and can result
from either an unfavourable energetic interaction
between the polymers45, or the solvent having very
different interactions with the different polymers71.
Biopolymers show the same general type of behaviour with aqueous mixtures of proteins and
polysaccharides72, and different polysaccharides73,74 exhibiting immiscibility. At equilibrium,
macroscopic phase separation is observed with the
formation of separate layers enriched in each of
the biopolymers. This macroscopic phase separation can be arrested, for example by gelation
of one of the components, and as a result useful
8
R. Parker and S. G. Ring
textures can be produced. Even the two chemically
similar starch biopolymers amylose and amylopectin, differing primarily in their extent of
branching, phase separate from concentrated solution73. This is an example of quite a small difference in molecular structure producing a potentially
large effect on the microstructure of the mixed
biopolymeric material. The way that solvent distributes itself between ‘phases’ as a result of their
relative affinity for water can have an important
impact on material properties. For example, if the
same amounts of two glucan polymers amylose
and dextran are mixed in concentrated solution,
at a temperature where crystallisation is not observed on practical timescales, phase separation is
observed with the formation of a dextran-rich and
amylose-rich phases. The phase volume of the
dextran phase is much larger than that of the
amylose phase, i.e., water has a preferred interaction with the dextran. The material properties
of this biphasic system will depend on the phase
volumes and polymer concentrations in each
phase. There will also be an effect on stability.
For example, consider a concentrated solution
containing 40% w/w water with 30% w/w each
of dextran and amylopectin at room temperature.
If the water was distributed evenly then storage
at room temperature for this composition would
represent a quench of 80 °C for chains 12 units
in length (Tm>100 °C), and the amylopectin in the
amylopectin-rich phase would tend to crystallise. If
the amylopectin phase was phase-concentrated,
by the presence of dextran, then the resulting
increase in amylopectin concentration would increase the driving force for crystallisation.
GLASS TRANSITION BEHAVIOUR
Included in the schematic of Figure 2 is another
transition which has an important influence on
material properties75,76—the glass transition which
is characterised by the glass transition temperature
Tg. Crystalline anhydrous -D-glucose melts at
150 °C, if this melt is cooled at a rate, which is
rapid compared to the rate of crystallisation, then
the viscosity of the liquid will progressively increase
until at 7 °C the viscosity reaches about 1012 Pa s77.
At these enormous viscosities the material behaves
as a brittle solid. Glasses may be stable to crystallisation for many years, the enormous viscosity
of the glass arresting crystal nucleation and growth.
At the calorimetric Tg a sharp change in heat
capacity is observed, indicative of a change from
solid-like to liquid-like behaviour within the timescale of the calorimetric experiment. This provides
an experimentally convenient method for the determination of Tg as the midpoint of the step
change. The calorimetric approach is useful for
the determination of the glass transition behaviour
of simple amorphous biopolymer mixtures. As
the degree of polymerisation of the glucan chain
increases there is an increase in glass transition
temperature, reaching 173 °C for maltohexaose78.
Addition of water to the carbohydrates depresses
the glass transition, as illustrated in Figure 2 which
shows the composition dependence of the Tg of an
starch/water mixture55. The Tg of the dry polymer
is experimentally inaccessible for familiar
reasons—thermal degradation intervenes. The addition of water has a strong plasticising effect,
causing a marked depression in Tg, until at 20%
w/w water, Tg reaches room temperature. More
recently a more detailed study on the glass transition behaviour of different glucan polymers including amylose, amylopectin, pullulan and
dextran has been reported79. While all the glucans
behave in the same general way, in that water is
a very effective plasticising agent, there are subtle
differences which lead to a glass transition range
approaching 30 °C at a water content of 10%
w/w. The branching of polymeric materials is
thought to depress Tg as a result of an internal
plasticisation from the short chain branches. Although water is an ubiquitous plasticiser of starchy
materials, it is volatile, and small changes in its
content can lead to large changes in mechanical
behaviour. For this reason, the use of other, nonvolatile, plasticisers has been examined. Most studies have been carried out with low molecular
weight hydroxy compounds such as glycerol80–83
or sorbitol84, often in combination with water.
Mixing 29% w/w glycerol with amorphised barley
starch containing a minimal quantity of water (1%)
depressed the glass transition to 70 °C85. Glycerol
is therefore a less effective plasticiser than water.
Initial additions of glycerol to starch resulted in a
depression of a single glass transition. As water
was added this glass transition was further depressed and an additional lower glass transition
appeared, suggesting that the ‘ternary’ starch/
glycerol/water mixture phase separated. The
lower Tg was close to the glass transition of pure
glycerol. There are a number of relationships
which describe the way that the Tg of mixed binary
systems should vary as a function of composition,
Aspects of the physical chemistry of starch
Tgm=(w1Cp1Tg1+w2Cp2Tg2)/(w1Cp1+w2Cp2)
(2)
The glass transition temperature of the mixture,
Tgm, is related to the glass transition temperatures
of the individual components and the heat capacity
increment at the glass transition, Cpi, where wi is
the mass fraction of the i th component. Replacing
water which has a relatively low Tg (−139 °C)88,89
with glycerol (Tg −80 °C)90 would lead to an
elevation in the Tg of the mixture. Recently, experiments were reported where the development
of the X-ray diffraction pattern of granular
starches, on solvation in the presence of ethylene
glycol and glycerol, was measured as a function
of time91. It was found that the development of
crystalline order took several hours at room temperature for ethylene glycol, and several days for
glycerol. As the relative ability of the diluents to
depress Tg would be in the order water > ethylene
glycol > glycerol, the observations are consistent
with the view that replacement of water by glycerol
would elevate the Tg of the mixture, and as a
consequence, at a fixed temperature slow the rate
of development of crystalline order.
At low water contents, the Tg of the starch/
water mixture is very sensitive to small changes in
water content and it follows that some of the
material properties are equally sensitive. At Tg the
structural relaxation time is of the order of 100 s.
This structural relaxation time is relevant to mechanical behaviour—how quickly a material should
relax after a mechanical perturbation—and potentially transport properties within the material92.
As a result there is interest in knowing not only
Tg but also how the associated structural relaxation
time will depend on temperature and composition.
Fortunately for most amorphous organic
materials—low molecular weight liquids, synthetic
polymers, and biopolymers (including flexible proteins such as gluten and polysaccharides)—the
expected dependence of structural relaxation time
on temperature is broadly similar. An expression
which has been shown to be widely applicable
in describing this temperature dependence for
synthetic polymers is the Williams-Landel-Ferry
relationship93:
log aT=−c1,0(T−T0)/(c2,0+T−T0)
(3)
where aT is the ratio of relaxation times at the
200
Temperature (°C)
and these have found use in describing the behaviour of starch mixtures. One such is due to
Couchman86,87 and is of the form
9
Rubbery region
(beyond WLF region)
100
Loaf crust
Glass
0
–100
0
10
τ , relaxation
time
centre
20
30
40
50
Water content (% w/w)
0.1 ns
1 ns
0.1 µ s
0.1 ms
100 s
60
Figure 3 Effect of temperature and water content upon the
relaxation time calculated from the glass transition curve55
and assuming WLF behaviour93. Water content of loaf crust
and centre shown at 20 °C.
temperature, T, and a reference temperature T0.
If Tg becomes the reference temperature then,
log aT=−c1g(T−Tg)/(c2g+T−Tg)
(4)
with values of the coefficients, c1g and c2g, obtained
from fitting data on a range of synthetic polymers,
being 17·44 K−1 and 51·6 K, respectively. At the
calorimetric glass transition the shear viscosity, ,
is of the order of 1012 Pa s and the shear stress
relaxation time, , is of the order of 100 s, and is
given by =/Gx, where Gx is the high frequency
limit of the shear modulus which is 1010 Pa for
many materials. The temperature and composition
dependence of the relative relaxation time, aT, is
shown schematically in Figure 3 for an amorphous,
high molecular weight, biopolymer/water mixture
and is based on the observed glass transition behaviour of starch/water mixtures. As the glass
transition is approached, either through reducing
the temperature or the water content, there is
predicted to be a very marked change in relaxation
behaviour. Structural relaxation and molecular
mobility slows and, as a result, there is a very
marked change in material properties for a relatively small change in either water content or
temperature. Although the effect is most marked
as the glass transition is approached, and is particularly relevant to the behaviour of low moisture
cereal products, or frozen products where crystallisation has freeze-concentrated the biopolymer,
even for intermediate water content materials such
as bread there is a very marked change in relaxation behaviour (i.e. orders of magnitude) for
relatively small changes in water content. This
10
R. Parker and S. G. Ring
change in molecular mobility will have an impact
both on stability to crystallisation and local viscosity. The above discussion was concerned with
a hypothetical biopolymer/water mixture. There
is a need for more experimental data on the
composition and temperature dependence of the
relaxation behaviour of cereal biopolymers. When
sufficient data have been gained there is potential
for a ‘dynamic synthesis’ where the relaxation
behaviour of cereal biopolymers over a wide temperature and composition range has been compared with the known behaviour of other
polymeric materials. For a more detailed discussion on the use of equation 3 to describe the
behaviour of synthetic polymers the reader is referred to the work of Ferry93. The use of this
approach to predict temperature-dependent mechanical behaviour is useful. It could also be extended
to make some prediction of how transport properties should vary with temperature but care is
needed in the application of this approach. For
example, there is an expectation that the increase
in viscosity/structural relaxation time as an undercooled liquid approaches the glass transition
would slow diffusion. While this is the case for a
macroscopic particle embedded in a highly viscous
matrix, for smaller molecules such as water there
can be a marked uncoupling of its motion from
the shear viscosity with diffusion being much more
rapid than expected94–96. The examination of the
dynamics of these mixtures reveals other relaxations as well as the main structural relaxation97,98. Their effect on material properties of
starchy materials remains to be established.
STRUCTURAL RELAXATION
If the structural relaxation is sufficiently slow, it can
lead to time dependent changes in the properties of
glassy materials over practical timescales of hours
to weeks99–107, and is potentially relevant to the
observed ageing of low water content carbohydrate
products108–110. The ‘equilibrium’ structure of an
undercooled liquid is temperature dependent. For
example it is expected that reducing temperature
would increase the density of a material. As a
liquid is undercooled towards the glass transition its
viscosity increases as does the associated structural
relaxation time. If an amorphous material is rapidly quenched into the glass state, the structural
relaxation time may be so high that the amorphous
structure, and resulting density, is effectively
‘frozen’. The structure will gradually evolve and
at very long times it will have a fully relaxed,
‘equilibrium’ structure. If it is cooled again, further
structural relaxations and rearrangements within
the undercooled liquid will occur until, given
sufficient time, a new ‘equilibrium’ structure is
obtained. The structure of the undercooled
amorphous polymer liquid can therefore show a
dependence on time and thermal history. This
relaxation behaviour has been extensively studied,
particularly for polymers and inorganic glasses.
The temperature-dependent changes in bonding
between molecules and their configuration are
associated with changes in volume, enthalpy, heat
capacity and material properties, including mechanical behaviour and diffusivity. The observed
changes in mechanical behaviour of amorphous
glasses with time is often described as embrittlement, as the material becomes stiffer and
less compliant. At Tg there is a sharp change in
behaviour as the liquid structure becomes arrested
over the timescale of the cooling. If the cooling is
slower, there is more time available for the structure to relax and the appearance of Tg will occur
at a lower temperature. Liquid structure can be
characterised on a temperature scale through the
notion of a fictive temperature, Tf, the temperature
at which a particular structure would be fully
relaxed.
Although the increase in density of a liquid
with time is one way of probing this structural
relaxation, its use is rather restricted. As densification also affects the energetics of interaction
between molecules, and the accessibility of liquid
configurations, it can be probed in a calorimetric
experiment where structural relaxation is observed
as a peak in heat capacity preceding Tg or an
overshoot at Tg. There is often a requirement to
be able to predict the change in the material
properties of a glassy product with time. Common
questions might be ‘at what temperature do I
need to store the product to minimise this timedependent change?’, ‘how might fluctuations in
water content affect the observed behaviour?’.
Fortunately there are various phenomenological
approaches for describing the observed time-dependent behaviour, a widely applied one, which
has a useful predictive capability, is the ToolNarayanaswamy method104, which has been applied to polymeric systems. The dependence of
structural relaxation on time, t, can be described
by an empirical relaxation function, , of the form
(5)
(t)=exp[−(t/0) ]
Aspects of the physical chemistry of starch
(6)
∗
where A, x (0<xΖ1) and h are constants. h
can be determined from the dependence of the
calorimetric Tg on scanning rate. These relationships can be used to calculate the time dependence of Tf following a temperature step and
from this the heat capacity change with temperature can be predicted. By appropriate selection of the constants A, x and the
experimentally observed behaviour may be modelled. For a simple carbohydrate, such as maltose111, it was found that a single set of constants
described the dependence of ageing on time and
temperature and had a useful predictive utility.
More recently it was found that the above approach was useful in describing the ageing of a
plasticised starchy material. Figure 4(a) shows the
predicted change in relaxation behaviour as a
hypothetical starchy material is heated through
the glass transition. On heating there is a marked
decrease in relaxation time. The associated change
in heat capacity is shown in Figure 4(b); a sharp
change in heat capacity is observed in the vicinity
of 315 K indicative of a glass transition, there is
also an overshoot in heat capacity as a consequence
of the structural relaxation. The way that the Tf
evolves with time, on storage at 300 K is shown
in Figure 4(c). At this temperature, although liquid
structure slowly changes over the course of 100 h,
it does not equilibrate over this timescale. The
change in Tf gives an indication of how material
properties might change with time on storage of
this hypothetical product. If the effects of variations
in water content can be usefully incorporated in
the model, then this approach could become more
generally useful for describing the effect of ageing
on the material properties of low moisture cereal
products.
RETROGRADATION OF STARCH
If a gelatinised starch/water mixture is cooled to
room temperature there is a strong driving force
favouring crystallisation. The processes which
occur on cooling are generally known as retrogradation112–117. In the case of the high molecular
weight linear polymer, amylose, the driving force
3
Relaxation time (10 s)
∗
(a)
300
200
100
0
300
310
320
Temperature (K)
330
340
1.2
(b)
Normalised heat capacity
0=A exp [xh∗/RT+(1−x)h∗/RTf]
400
1.0
0.8
0.6
0.4
0.2
0.0
300
320
340
360
Temperature (K)
380
400
324
(c)
Fictive temperature (K)
and (0<Ζ1) is a measure of its non-exponentiality. 0 is a characteristic time which is
dependent on both temperature, T, and, to an
extent, liquid structure (characterised by Tf ) and
has been successfully obtained using the expression
11
322
320
318
316
314
312
0.1
1
10
100
Time (h)
Figure 4 Variation in structural relaxation time with temperature for a starch product (a) and its predicted effect
on the observed calorimetric behaviour (b) and the timedependent evolution of fictive temperature Tf (c).
is strong, with an effective quench of at least
120 °C if the material is held at room temperature.
12
R. Parker and S. G. Ring
Such a strong quench is not conducive to the
formation of very crystalline materials. In fact, if
a concentrated aqueous amylose solution is cooled
to room temperature there is a very rapid precipitation/phase separation process. The clear
amylose solution rapidly becomes opaque indicating the formation of polymer aggregates. To
produce the opacity the aggregates are at least the
order of the wavelength of light in size. At the
same time, for sufficiently concentrated solutions
(typically greater than 1–2% w/w), network formation is observed through the formation of an
elastic gel. Network formation, as assessed by the
development of stiffness, is generally complete
within a few hours at room temperature. Electron
microscopic examination of the network reveals
coarse network strands which consist of assemblies
of many individual polymer strands118. Subsequent
to this precipitation, a slow crystallisation of the
amylose is observed by X-ray diffraction over the
course of a few days. This crystallisation does not
have a marked effect on material properties, at
the end of this time, these materials are still poorly
crystalline. One proposal for the structure of the
initial aggregate is that it is formed from assemblies
of double helices117 with a typical dimension, orthogonal to the helix axis, of from 10–20 nm119.
These structures further aggregate to form a fractal
structure. The aggregation process has also been
likened to a phase separation in which polymerrich and polymer-deficient domains are formed
and if the amylose is sufficiently concentrated the
polymer-rich domains will form an interconnected
network114. The amylose gel is far from equilibrium
and its structure will be very dependent on the
condition of its preparation.
The effective quench for the formation of crystallites from short linear chains is much more
modest. If concentrated amylopectin/water mixtures are quenched to room temperature or below,
a slow crystallisation, as assessed by X-ray diffraction, of the amylopectin is observed. In this case
the crystallisation is associated with the development of stiffness of the material and typically,
at high water contents, takes days or weeks to
approach a plateau value112,120. At the end of this
time the extent of crystallinity of the amylopectin
is comparable to that found in the native starch
granule, i.e., in the region of 30%. The length of
chain involved in the crystalline unit is relatively
short. On examination of the behaviour of amylopectins from different botanical sources it was
observed that the longer the length and abundance
of the short chain fraction of amylopectin the
greater the tendency to retrograde and crystallise
from aqueous solution120–122. Wheat amylopectin,
which has a relatively short, short chain fraction,
generally shows a reduced tendency to retrograde,
even so it can lead to very significant timedependent changes in material properties. In a
detailed examination of the retrogradation of
maize amylopectins it was found that retrogradation was proportional to the amount of
short chains having a DP of 16–30 and inversely
proportional to the level of short chains with a
DP of 6–11122. Treatment of starch with -amylase
(an exo-acting enzyme) shortens the short chain
fraction and reduces the rate of retrogradation123.
The rate of retrogradation is expected to be at
a maximum approximately mid way between the
melting temperature of the crystallites, Tm and the
glass transition temperature Tg. This reflects a
balance between the effect of temperature on the
driving force favouring crystallisation, and the
slowing of mobility as the glass transition is approached. Bell-shaped curves of extent of retrogradation versus temperature are observed
supporting this suggestion124.
For materials prepared by gelatinising starch
and then cooling to room temperature the precipitation of amylose is a very rapid process. The
crystallisation of amylopectin leads to an increase
in stiffness of the gelatinised granule and a slow
firming of the material. Amylopectin crystallisation
and the associated firming can be abolished by
reheating to 60 °C.
If amylose in hot aqueous solution is cooled to
room temperature, the amylose precipitates and
retrogrades with the formation of a partially crystalline precipitate or gel. The precipitate needs to
be heated to temperatures <120 °C to initiate
dissolution. If the aqueous solution is saturated
with 1-butanol prior to cooling, a precipitate is
still formed which will dissolve on heating to
100 °C. This precipitate consists of amylose in a
single helical form and can contain the 1-butanol
as a complexed guest molecule. On the basis of
X-ray diffraction experiments, the complex may be
either crystalline or amorphous125. The amorphous
material is formed on rapid cooling while slow
cooling favours the formation of crystalline material. The important point is that the addition of
a hydrophobic compound prevents the formation
of the B-crystalline form—the single helical complex is the preferred product126–128. A further indication of this preference is the observation that
Aspects of the physical chemistry of starch
the amylose complex forms immediately after gelatinisation48. As the B form of amylose is thermally
more stable this might again indicate the role of
kinetics in influencing crystallisation. Depending
on the hydrophobic compound added, (e.g., if it
is charged) it is also possible to induce a conformational change to a helical form which does
not readily crystallise or precipitate. From the
foregoing it can be seen that the addition of
hydrophobic surfactants to starch-rich products
would modify the retrogradation behaviour
through favouring the formation of single helical
conformations. Depending on the conditions, this
could be single helices in solution or an amorphous/crystalline complex. While the interaction
with amylose is relatively strong, favouring the
formation of crystalline materials, the interaction
with the shorter chains of amylopectin would be
expected to be weaker. It is known that after
addition of surfactants to bread products, the crystalline single helical Vh form of amylose is observed
and the development of the B form on ageing of
the bread is retarded.
CONCLUSIONS
Although starch has long been studied from a
polymeric perspective, recent research has added
to this knowledge more particularly in terms of its
phase behaviour and dynamics, and how they are
modified by the presence of other compounds.
This information is relevant to the practical usage
of processed starch materials which are generally
metastable. To devise strategies for the control
of material properties, and their stability, it is
practically useful to know ‘where the system is
going’ (its underlying phase behaviour) and ‘how
quickly its going to get there’ (influenced by its
dynamics). Important advances are also being
made on the levels of organisation of the granule.
During the same period our understanding of
starch biosynthesis has also increased markedly.
The processability of starch is strongly influenced
by granule organisation. An important problem
for the future is the biochemical control of granule
assembly which will involve research from both
biological and physical perspectives.
Acknowledgements
The authors acknowledge the support of the core strategic grant of the BBSRC.
13
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