a 5. Hence the surface of the crystals will be in local equilibrium with the
liquid phase. There is no equilibrium at the interface betweena 4 anda 5 , but
it would need solid state diffusion (which is quite slow) to achieve it.
Other temperature regimes can be analyzed in a similar manner, and
predictions can be made about the influence on the solids content. Although
the values obtained cannot be precise—the form of the phase diagram can
only be guessed—the predicted trends are well observed, at least for milk fat,
the only fat that has been extensively studied.
Consequences. An example of experimental results is given in
Figure 15.25. Here the fat had been cooled in two steps, i.e., about the
regime just discussed. It is seen that the differential melting curve has two
maxima, a few kelvins above the crystallizing temperatures applied. It is also
seen that these two maxima disappeared upon storage of the fat. This
cannot—or more precisely, cannot only—have been due to polymorphic
FIGURE15.24 Hypothetical phase diagram of a binary mixture forming a solid
solution over the whole compositional range. The X-axis gives the mass fraction of
the higher melting component. For situations between the liquidus and the solidus
curve, a mixture of liquid and solid (compound crystal) material is present.