other triglyceride crystals, and presumably also for crystals in another
polymorph.
The hysteresis at lower temperatures has quite a different explanation:
as discussed in Section 15.4.3, precooling leads to an increased solid fat
content, provided that compound crystals form.
In Section 14.2.3secondary nucleationis discussed, with particular
reference to triglycerides. It is obvious that copious secondary nucleation
can occur in multicomponent fats, at least at high supersaturation. The
condition seems to be very slow crystal growth, while nevertheless the
growth regime is ‘‘kinetic roughening’’; the latter implies a high super-
saturation. This situation can arise in multicomponent fats owing to the
fierce competition between different molecules for incorporation into the
crystal. Secondary nucleation appears to be abundant in milk fat, occurs to
a lesser degree in a typical margarine fat, and is presumably absent in cocoa
butter. The phenomenon is quite important in practice, since it greatly
reduces averagecrystal sizeformed.
Crystal Growth. Triglyceride crystal growth tends to bevery slow:
see, e.g., Figures 15.9 and 15.12b. To obtain reasonably fast crystallization,
the supersaturation needs to be high. Fortunately, this can often be realized
(see, e.g., Figure 15.22), because of the high values of the molar heat of
fusion and of the molar mass of the solvent. (Can you explain the relation?)
It may be realized that lnb¼4 corresponds to a supersaturation ratio of 55.
At high lnbvalues, the crystals formed tend to be small and of platelet
shape (e.g., length : width : thickness ¼10 : 3 : 1, or even more slender).
Although crystal growth tends to be slower for a wider range of crystallizing
triglycerides, the extent of secondary nucleation tends to be higher as well,
still causing the crystals formed to be small. At very high lnbvalues, crystal
growth can be in the regime of kinetic roughening for some of the crystal
faces, which then will become curved.
At small values of lnb, larger and more isometric crystals tend to
form, especially for crystallization in the b-form. At very low super-
saturation, a special kind of (large) spherulites often form; they consist of
narrow platelets growing out from one point, and they bifurcate when
growing longer. A kind of spherical, prickly sponge is formed, partly
consisting of liquid fat.
Recrystallization. Equilibrium is rarely reached in partially solid
fats. It is quite common that changes occur after the crystallization
apparently is finished (i.e., no measurable change in fraction solid within,
say, an hour). Such recrystallization tends to be slower for a smaller fraction