Structure of Self-Assembled Globular Proteins - Food Colloids: Self-Assembly and Material Science - page 53

depends on the strength of the individual bonds. Consequently, different
structures can have similar absolute values of the shear modulus. Also different
moduli can be found for systems with the same structure. For instance, simply
reducing the temperature of globular protein gels from 801Cto201C increases
the shear modulus reversibly by more than a factor of 3 without a significant
change in the structure. 33,68
The question is whether this relatively simple model can usefully be applied to
globular protein gels. One diculty is that oscillatory shear measurements show
that the gels are not fully elastic. 69 The storage modulus G 0 has a weak power-
law dependence on the frequency, and G 00 is almost frequency-independent.
Another diculty is that the shear modulus continues to evolve logarithmically
with time after all proteins have aggregated. 33,70 Even if we assume that the G 0
determined experimentally can be approximated as the elastic modulus of the
gel, other conditions still have to be fulfilled before the fractal gel model may be
applied. In the first place, the structure needs to be self-similar over a significant
length-scale, implying that R a has to be significantly larger than the elementary
unit of the fractal structure. This means that the homogeneous (transparent)
globular protein gels cannot be interpreted using the fractal gel model because
R a is close to the elementary unit of the aggregates. In fact, these gels are not
self-similar on any length-scale. Secondly, Equation (8) is only valid if all blobs
are connected to each other through their elastic backbone. This means that the
protein concentration should not be close to C g , so that the sol fraction is
negligible. Thirdly, the gels do have some heterogeneity: they are characterized
by a distribution of blob sizes. In the derivation of Equation (8) an average blob
size is used, which is only valid if the polydispersity of the blobs is independent
of the protein concentration. This is doubtful for very heterogeneous gels
formed at high ionic strength or close to pI.
Clearly, one should not simply apply this model to globular protein gels in
order to derive the fractal dimension of the blobs. The mere observation that G 0
increases with C following a power law generally over a limited concentration
range is not sucient proof that the structure of the gels is fractal over any
significant length-scale. The concentration dependence of the blob size should
be determined independently using scattering techniques. This has so far been
done for one system: b-LG at pH ¼ 7 and 0.1 M NaCl. 28 The structure and the
mechanical properties of the extensively heated systems were found to be
consistent with the fractal gel model for C 4 2C g and C o 90 g L 1 . In this
concentration range, the correlation length was significantly larger than the
elementary unit of the gels, and at most less than a micrometre. Similar detailed
comparison needs to be made for more homogeneous or heterogeneous gels in
order to test whether the fractal gel model can be applied more generally.
3.5 Conclusions
It appears that globular proteins may form rigid rods when they are strongly
charged and little salt is present. When salt is added or the charge density is