(generally by differential scanning calorimetry). Results are shown in Figure
15.22 (curves marked lnb¼0) for theaandb^0 polymorphs; curves for
various supersaturations are also given. It is seen that conditions can be
chosen such that nucleation and crystallization occur either in thea-orin
theb^0 -form. Theb^0 crystals of the present system are quite stable, and it
takes a very long time before the transition to theb-form will occur. Thea
crystals, however, are quite unstable and soon transform intob. The latter
tend to be large crystals that form a very weak network, whereas the much
smallerb^0 crystals form a firm fat, even at low concentrations. This can be of
great practical importance, for instance in margarine.
Another example is cocoa butter. It consists for 80–85% of the
triglycerides POP, POS, and SOS, which are very similar molecules that
form compound crystals, even in theb-form; the molecules will then be
packed as depicted in Figure 15.19b, SOS. Figure 15.18 gives an example of
the melting curve, and it is seen that the curve is very steep above 25 8 C, as is
to be expected if most of the crystals have the same composition. The shape
of the curve can, however, markedly depend on the temperature history.
Moreover, the composition of the fat can vary.
FIGURE15.22 Solutions of fully hydrogenated palm oil in sunflower oil, mass
fractioncS. Calculated curves for various supersaturations (lnb) as a function of
temperatureTfor theaand theb^0 polymorph. Parameters used fora,DHf¼ 98
kJ?mol^1 andTm¼315 K; forb^0 ,DHf¼160 kJ?mol^1 andTm¼330.5 K. (After
W. Kloek. Ph.D. thesis, Wageningen University, 1998.)