of liquid fat, which often means a lower temperature. Recrystallization can
involve change in polymorph, change in the composition of compound
crystals, or both. Two extremes on a scale of situations will be discussed.
In a fat of verysimple composition, like cocoa butter (only a few
crystallizing triglycerides of very similar molecular structure), crystallization
phenomena closely resemble those of a pure triglyceride in oil. Recrystalli-
zation primarily involves a change in polymorph(s), as indicated in Figures
15.20 and 21. Generally, the a-form is very short lived, but further
transitions tend to be slower. Theb-form is often reached. Presumably, the
changes are solid-state transitions, which implies that crystal size remains as
it was. In some fats, especially if the triglyceride has chains of different
lengths, theb^0 -form is persistent, except—of course—when the temperature
comes close to theb^0 clear point or above.
Multicomponent fats, like milk fat, have a wide melting range, and
secondary nucleation is common, as discussed. Moreover, theapolymorph
may at low storage temperature persist for a long time. Thebpolymorph
forms hardly, if at all. All this means that most of the crystalline fat is
generally in compoundb^0 crystals. As discussed, equilibrium (within this
polymorph) will rarely exist, and even if it is reached (which may take weeks
or even longer), it is disturbed when the temperature is changed. It is
commonly observed that temperature fluctuations lead to significant
changes in the melting curve.
Many fats, e.g., shortening and margarine fats, behave in an
intermediateway. Both polymorphic and compound crystal changes occur,
often concomitantly. The recrystallization has important practical con-
sequences. The transition fromb^0 tob, especially when it occurs slowly,
causes the formation of large crystals, like the spherulites discussed above.
This gives the fat an undesirable texture: too soft, grainy, and subject to oil
separation. Some fats are strongly ‘‘b-tending,’’ especially those with little
spread in fatty acid length (like partly hydrogenated soybean oil). The
presence of diglycerides then counteracts the transitionb^0 !b; presumably,
the diglyceride acts as a strong growth inhibitor forb-crystals. Changes in
compound crystal composition tend to increase the firmness of the fat. This
is discussed in Section 17.3.
Estimation of the Fraction Solid. We have seen that the
proportion of the fat that is solid, cS, can greatly depend on the
temperature history of the fat. It can also markedly depend on the
method of estimation applied. A classical method iscalorimetry, where the
heat of melting per unit mass (i.e.,DHf) is measured. But this value varies
significantly among triglycerides, as shown in Table 15.2; it roughly parallels
the melting point. Since a constant (average) value is taken, this means that