Biomedical Engineering Reference
In-Depth Information
Mixed protein gel rheology has been carried out by a number of workers, including
the work mentioned above. Kavanagh and co-workers (Kavanagh et al., 2000 ) studied
β
-La mixtures at two different pHs (3 and 7), keeping the total protein
concentration at 15% w/w. Their results tended to con
-Lg and
α
rmtheideathatthetwoprotein
species, when partially unfolded into the molten globule state, gave results consistent
with non-speci
c mixed network formation between protein species, rather than a
synergistic interaction between the individual protein gel systems. However, without
immuno-staining it was not possible to establish whether this mixed network was
formed by cross-interaction of
fibrils of the two segregated species or by formation of
mixed
fibrils from the two unfolded species in the molten globule state. The modulus
G 0 of 15%
-Lg. Results for G 0 for
the mixed gels lay between the two bounds ( 10.7 )and( 10.8 ),andmuchclosertothe
upper bound for the pH 3 systems. Perhaps as expected from the structural viewpoint,
the bicontinuous model ( 10.9 ) did not produce any improvement. Large-deformation
measurements for mixed pure protein systems are scarce, re
α
-La was greater than for the same concentration of
β
ecting the need for (and
high cost of) large amounts of sample.
More recently, Loveday and co-workers (Loveday et al., 2009 ) have reviewed exper-
imental and theoretical work, particularly on the
fibrillar mixed gels formed at low pH.
α
Their overall conclusion was that
fibrillar gels from
-La seem to be of higher modulus
β
than those from
-Lg, most probably because of the higher density of
fibrils and the
different
fibril properties. They concluded that more data on the rheological properties of
the gels is needed, using a wide range of protein concentrations, and that systematic
studies should include strain sweep data to determine whether strain-stiffening or strain-
softening behaviour is exhibited, and the persistence and contour lengths of any
fibrils.
The same was true for
fibril content) data. In addition, they asserted that
more studies would be desirable on the stress
fibril density (or
-
strain behaviour of the
fibrils and on
fibril-
network models to predict modulus
-
strain relationships.
10.4.4
Polysaccharide mixtures
10.4.4.1
Gellan and
-carrageenan
Mixtures of gellan and
κ
-carrageenan have been examined by Nishinari et al.( 1996 ). At
low polymer concentrations, a cooperative interaction between their ordered conforma-
tions was demonstrated from viscosity measurements. At higher polymer concentrations,
a two-phase system is formed, as shown by differential scanning calorimetry and
rheological measurements.
Figure 10.9 shows heating differential scanning calorimetry (DSC) curves for
mixed gels of gellan and
κ
-carrageenan with mixing ratios 3/1 and 1/1, for various
concentrations. All the DSC curves clearly show separated endothermic peaks from
helix
κ
-
κ
-carrageenan (lower temperature peak) and for gellan
(higher temperature peak), with peaks shifted to higher temperatures with increasing
concentra t ion. These DSC peaks should not be attributed to a gel
coil transitions for
sol transition
because the gels still show quite a large elastic modulus at these temperatures, as
shown in Figure 10.10 .
-
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