[158]. Treatment of amylopectin wit

[158]. Treatment of amylopectin with β-amylase to produce β-limit dextrins
destroyed the gel formation ability [158]. The outer branches of the clusters
in the amylopectin molecule thus seem to be necessary for gel formation. The
amylopectin gels are thermoreversible; they melt when heated to 40 to 60°C
[91,158]. A stable enthalpy value has been obtained after about the same time
as a stable modulus.
10.5.3 AMYLOSE ANDAMYLOPECTIN
The incompatibility between amylose and amylopectin, discussed previously,
certainly influences the type of gel formed. Measurements of the elastic modulus (E) for amylose–amylopectin gels after different aging times have shown
a complicated dependence on the amylopectin concentration [57]. Gels formed
with amylose and amylopectin at different amylose/amylopectin ratios (r) were
studied at a total polysaccharide concentration of 8% [186]. Gels formed at r
> 0.25 (corresponding to 1.6% amylose) for these mixtures. Below r= 0.43
(corresponding to 2.4% amylose), the mixed gel behaved like an amylopectin
gel, and above r = 0.43 it behaved like an amylose gel. In both studies, it was
suggested that the structure of the gel was a continuous phase with a dispersed
phase, and at a certain concentration (e.g., r= 0.43 [186]) an inversion point
can be obtained. Below this value, amylopectin is continuous, and above it
amylose is. A local cocrystallization was also suggested to occur at the interface of microdomains. Different properties could thus be expected to be
obtained for the starch gel depending on the amount and type of material
solubilized during gelatinization. High soluble amylose levels and swelling
powers have been found to increase elasticity, whereas high levels of soluble
amylopectin are detrimental to gel formation and reduce elasticity [187].
FIGURE 10.8Changes in shear modulus with time for potato starch amylopectin.
(Data from Kalichevsky et al. [173].)
15
10
Shear modulus (kPa)
5
0
01020
Time (days)
30 40
© 2006 by Taylor & Francis Group, LLC
426 Carbohydrates in Food
10.5.4 STARCHGRANULES
A gelatinized starch suspension forms a more rigid gel than can be expected
from the rigidity of an amylose gel formed from the amount of amylose leached
[146,157]. Moreover, the solution centrifuged from such a gelatinized starch
suspension shows very low viscosity [188]. The granules thus are important
for the rheological properties of the starch gel. Starch granules remain more
or less fragmented after most heat treatments [59,60,154,189]. The function
of the granules could be to act as filters in the amylose–amylopectin matrix
[153]. The starch granules influence the rheological properties of the starch
gel due to their phase volume and their deformability, but also due to adhesion
between the filler phase and the matrix. These parameters usually influence
each other, so it is difficult to isolate the influence of one single parameter. It
has been suggested that in dilute suspensions viscosity is governed by the
volume fraction of swollen granules, whereas in concentrated systems it is
governed by particle rigidity [155]. Dynamic mechanical thermal analysis
(DMTA) on wheat starch preparations at intermediate moisture (25 to 60%,
w/w) indicated an increase in G′due to swelling, followed by a decrease due
to melting and softening [189a].
10.5.4.1 Phase Volume
The weight percent of starch to reach close-packing differs among starches;
for example, it is 2.8% for corn and 0.25% for potato [188]. It is thus evident
that in most applications the starch granules will be close-packed in the gel.
The shape and size distribution of the granules will affect the packing behavior
and influence the level of starch required for close-packing. When the granules
are close-packed, less soluble amylose will be present.
The viscosity (η) of a starch suspension — expressed as η/CQ, where C
is the concentration (g dry starch per g suspension) and Qis a quantity
describing the swelling of the particle in dilute suspension (g swollen starch
per g dry starch) — has been plotted against CQ[154]. The values obtained
for different starches and different cooking conditions could be superimposed,
but the fit was not always perfect; however, the results still illustrate the
importance of the phase volume (ϕ) of the swollen granules.
Attempts have been made to account for the solubilization of material
from the starch granules when ϕis calculated [28]. The ϕfor a single starch
is very much related to the cooking conditions, and a reason for the different
values in viscosity obtained from different methods is that different ϕvalues
are obtained depending on the heat regimens and mechanical treatments used.
10.5.4.2 Deformability
Because the starch granules are close packed in most food gels, their deformability has to be taken into consideration when analyzing the rheological
© 2006 by Taylor & Francis Group, LLC
Starch: Physicochemical and Functional Aspects 427
behavior. The resistance against deformation has been measured for individual
gelatinized starch granules, and considerable variation was found between
individual granules [190]. The variability was not related to granule size.
It is possible to change the deformability of the starch granules, and
softening during gelatinization is one obvious example [153]. For a heated
maize starch suspension two types of flow behavior have been observed: shear
thinning in combination with a Newtonian region or a dilatant behavior [191].
The dilatant flow was suggested to occur when the granules are close packed
but too rigid to deform. A shear-thinning behavior can be imposed either by
an increase in temperature (to make the granules softer) or an increase in
concentration (to increase the stresses applied). For the shear-thinning behavior
to occur, the granules must have lost their birefringence.
The rigidity of starch granules will depend on the degree of gelatinization.
When concentrated wheat starch suspensions were heated to different temperatures, corresponding to the double DSC endotherm obtained at limited water
levels, an increase in relaxation modulus (G) was first obtained (i.e., during
the first peak in the thermogram). With further heating (i.e., when the second
peak in the thermogram was reached), Gdecreased [153]. This was interpreted
as being due to the starch granules becoming softer when more of their
crystallinity was melted. Similar results were obtained when pea starch granules were added to an amylose solution at ϕ= 0.8 [157]. The reinforcement
of gel rigidity decreased when the temperature to which the granules had been
heated increased.
When gelatinized starch was treated with enzymes (salivary amylase), the
resistance against deformation decreased considerably, although the diameter
did not change [190]. At the same time, the shear stress of the gelatinized starch
suspension decreased. The reason was not that the phase volume changed but
that the deformability of the individual granules in the dispersed phase
decreased. With time, crystallinity develops in starch gels, and the B-pattern
emerges [146]. The development of crystalline domains will cause the deformability of the granules to decrease, and more rigid gels are thus obtained.
It is not always possible to separate the effects of phase volume and
deformability. The contribution of phase volume and deformability can also
change with concentration. For several starches, a crossover in plots of viscosity vs. concentration has been observed [155]. At low concentrations, the
high-swelling starches have a higher viscosity than low-swelling starches, and
at high concentrations the reverse is true.
10.5.4.3 Adhesion
The rheological behavior of a gelatinized starch suspension might also depend
on adhesion between the dispersed phase and the matrix. The influence of
lipids on the rheological properties of a starch suspension has been attributed
partly to changes in adhesion between the filler and matrix [192]. Interactions
© 2006 by Taylor & Francis Group, LLC
428 Carbohydrates in Food
between starch and other polysaccharides have also been interpreted in terms
of changes in adhesion [193].
10.5.5 STARCHGELS
The rheological properties of a starch suspension will change during at least
three processes of relevance to food processing: gelatinization, retrogradation,
and during freezing and thawing; thus, it is necessary to be able to measure
the rheological properties of starch gels during and after these processes have
taken place. As is described in this section, the rheological properties are
influenced by starch source, starch concentration, temperature, heating rate,
and mechanical treatment. Storage temperatures also have an influence. Furthermore, the addition or presence of other components (e.g., lipids, proteins,
other polysaccharides, sugars, salts) influences the rheological properties.
10.5.5.1 Gelatinization
To follow the changes in rheological properties during the gelatinization of
increase in viscosity is not measured until swelling and leaking have proceeded
to some extent. The increase in viscosity is due to the swelling of starch
granules and can be rationalized by the increase in phase volume, as has been
investigated in, for example, maize starch [194]. The situation is more complicated, however, than the mere phase volume. The shape of the granules
influences their packing behavior, and the deformability of the granules then
comes into play. The viscogram reveals another effect that contributes to the
development of a peak in viscosity — shearing causes fragmentation of the
starch granules and perhaps even complete dissolution of the granules.
The mechanical degradation influences the leaking of amylose and amylopectin from the granules. As already discussed, amylose and some amylopectin leach out from the granules during heating without any stirring. With
shearing, this leaking is enhanced, and the composition of the leached material
is certainly affected. This,