nucleation and growth occur, and the crystals rapidly aggregate because of
van der Waals attraction, forming a fractal network of low dimensionality
(about 1.7). If the initial supersaturation is high, nucleation, growth, and
aggregation go on, and second-generation fractal aggregates form that fill
the pores of the initial network. In this way the network becomes much
more even, its permeability greatly decreases, and its stiffness greatly
increases. The fractal structure is lost, and the scaling exponents with crystal
concentration for permeability and modulus are quite high, generally 4–7.
Fats of a wider compositional range show more secondary nucleation
and hence smaller crystals and a higher modulus. These fats are also further
removed from equilibrium, more recrystallization occurs and hence more
sintering; the latter is enhanced by temperature fluctuations. However, ifb-
crystals can be formed, slow (re)crystallization can lead to the formation of
quite large crystals and a weak network.
In practice, large deformations are generally applied. The linear region
tends to be very small, up to a strain of 10^4 –10^3. With increasing strain,
the system becomes more and more viscoelastic, and its structure gradually
breaks down; then yielding occurs, accompanied by a stress overshoot, and
the system finally shows plastic flow. The yield stress, or firmness, is the
most important quantity in practice, and it approximately scales with solid
fat content squared. Other variables affecting the modulus have a
comparable effect on the yield stress.
Working of a fat causes work softening, i.e., it considerably decreases
the yield stress. Upon standing, the yield stress increases again, since at first
aggregation and then (much more slowly) sintering occurs, but the original
value is not reached. During the making of a plastic fat, cooling generally
goes along with vigorous agitation, to allow removal of the heat of fusion. If
the product now leaves the agitator after fat crystallization is more or less
complete, it will have undergone strong work softening, and it keeps
increasing in firmness for some time. On the other hand, if the fat leaves the
agitator at a stage where crystallization is not yet complete, extensive
sintering occurs within a relatively short time, causing the fat to obtain a
yield stress that is significantly higher than is obtained by the other
procedure.
It should be stressed that quantitative relations are quite complex and
depend strongly on fat composition and on the temperature history.
Closely Packed Systems. These are very concentrated dispersions
of soft solid particles in a liquid. The particles are closely packed and deform
each other. The modulus of the particles primarily determines the modulus
of the system. The coherence of the system is due to the restricted volume.
Several systems can be distinguished.