the stress, whereas the sintered bonds that are broken do not reform, at least
not within a short time.
- Stress overshoot. Here, much of the network structure is broken
down, and the system showsyielding. It should be realized that the inherent
inhomogeneity of the structure, with ‘‘weak’’ spots or planes, causes the
structure breakdown to be very uneven; in several small regions the original
structure may almost remain. The magnitude of the stress overshoot tends
to be larger for a higher strain rate and for a more brittle fat (smaller linear
range). - Plastic flow. After yielding and the accompanying stress over-
shoot, the material flows, having a very high Binghamviscosity, often
between 5 and 50 kPa?s; it is somewhat strain rate thinning and exhibits
some elasticity. These phenomena are probably explained by the irregular,
spiky shape of the network fragments present, causing strong frictional
forces, and possibly to colloidal interaction forces between structural
elements (cf. Section 17.4).
Other plastic fats show similar behavior, but the scales of the curve
may vary significantly.
Yield Stress. Because the spreading, cutting, and shaping of a
plastic fat all involve permanent deformation, the yield stress, as illustrated
in Figure 17.24, is an essential parameter. It correlates well with the values
obtained by some practical tests and with the sensory perceived firmness.
Figure 17.25a gives a few examples ofsyas a function of the fraction
solid. The latter is an essential variable, but whereas the modulus scales with
jto a power of 4 to 7 for nearly all plastic fats, the scaling exponent is
generally about 2 for the yield stress. This agrees with the network structure
having undergone extensive change before yielding occurs. Like the
modulus, the yield stress tends to be higher if the fat has crystallized at a
higher value of lnb 0. When the temperature is lowered,sytends to increase
strongly (often by a factor of about 10 for a 10 K decrease), not only because
jincreases but also because of increased sintering. The latter effect is
especially strong for a multicomponent fat, and if the cooling rate is slow
(can you explain this?). Also temperature fluctuations tend to increase the
firmness of the fat, as well as its brittleness: the fat that melts on temperature
increase tends to cause extensive sintering upon cooling, increasing stiffness
as well as firmness of the fat.
Figure 17.25b illustrates the phenomenon ofwork softening. When a
fat is strongly worked (kneaded) at constant temperature, its yield stress will
markedly decrease (generally by 30–70%). The firmness starts to immedi-
ately increase again upon storage: at first crystals and network remnants will
aggregate, being kept together by van der Waals forces, and then sintering