Physical Chemistry of Foods

It is also seen that theproperties of the primary particlesplay a large
role. This is borne out by results of dynamic measurements obtained for
casein gels. The value of tandranges between 0.1 and 0.6, depending on
several conditions, and the value ofG^0 increases with increasing frequency
o. For a fractal network of rigid particles tandwould almost equal zero and
G^0 would be independent ofo, since at the very small deformations applied,
bonds between particles would not break (and if they break they would not
reform). Hence these observations on casein gels must be explained by
deformation of the casein particles.
This also plays a part in large deformation and fracture. Before
fracture occurs, the stress-carrying strands have become stretched. That
means that Eq. (17.18) can also be used for the fracture stress witha¼2,
albeit with a very different value for K^0. The strain at fractureefr will
strongly depend on the deformability of the primary particles; it will also be
larger for a smaller value ofj.
The most important effect of the particle material on the mechanical
properties of the gel may be that it determines thestrength of the junctions.
The junctions inside a cluster and between clusters are in principle identical.
Each junction can involve a great number of bonds, and the number can
vary with time and with physicochemical conditions. It is observed that
casein gels increase in modulus during aging after the network has fully
developed. The rate of increase depends on several conditions, but it is
always faster for rennet gels (e.g., by a factor 5 in 20 min) than for acid gels
(e.g., by a factor 5 in 20 h). The main cause of the increase appears to be that
the junction zone between two particles increases in area with aging. This
must also be the explanation for the value of the syneresis pressurepsyn
decreasing after the gel has fully formed, as illustrated in Figure 17.16b.
After all, junctions must break for substantial syneresis to occur.
In Table 17.4 some properties of three types of casein gels are
compared. Two have stretched strands (a¼2) and one hinged strands (a¼
3), but otherwise the spatial structure is much the same (values ofBandD).
The two acid gels are built of virtually the same primary particles. It is seen
that the stretching of the strands (type 2) leads to a much higher modulus,
but not to a higher fracture stress; the fracture strain is much higher for the
hinged strands. All these results are in qualitative agreement with the
structure models (can you explain them?). The rennet gel has a relatively low
modulus, despite having stretched strands. This is probably due to the
primary particles being much more deformable (having a smaller Young
modulus) than those in the acid gels. In agreement with this, the fracture
strain is very high. The fracture stress is much smaller than for the acid gels.
This would agree with the rennet gels being far more prone to syneresis,
since syneresis involves the breaking of strands.