the resultingcSis significantly biased: at the low temperature end of the
melting rangecSis overestimated, at the high end underestimated. The
value ofDHfalso depends on the polymorph: see Table 15.3. It may thus be
that a transitionb^0 !boccurs without change incS, whereas calorimetry
shows an increased heat of melting. Moreover, compound crystals have a
lower value ofDHfthan the average value of the components.
Another classical method isdilatometry, based upon measurement of
the melting dilatation. The magnitude of the latter is subject to a similar
variation as the melting heat, and a similar bias as in calorimetry results.
Moreover, the method is time-consuming. The most convenient and rapid
method is calorimetry in a differential scanning mode (DSC), but this gives
the additional problem that the base line (hence the values of the specific
heat to be subtracted) is often quite uncertain, especially for a wide melting
range.
A newer method is pulsed wide-lineproton NMR. It can be executed in
various modes, but the best one usually is direct estimation of the fraction
liquid, which appears to be virtually unbiased. Incidentally, the trends
observed for changes incSdue to changes in compound crystal composition
have been confirmed by using this method.
Question 1
As mentioned, cocoa butter contains a very small fraction of trisaturated triglyceride.
Can this substance nevertheless play a role in chocolate manufacture? You may, for
the sake of simplicity, assume that the fat contains 75%SOS and 3%SSS as the only
crystallizable substances.
Answer
Yes. Assume that nucleation occurs in thea-form. From Table 15.3 we obtain fora
tristearateTm¼328 K andDHf¼110 kJ?mol^1. By use of Eq. (15.8) this gives, for
x¼0.03, anaclear point of 302 K, or 29 8 C. According to Table 15.2 theamelting
point of SOS is 23 8 C and, since it makes up only 0.75 of the triglyceride mixture, itsa
clear point will be somewhat lower, say by 3 K. This then means that SSS nucleation
will occur at a clearly higher temperature than that of SOS. The presence of SSS thus
allows cooling to be less deep to obtain a sufficient number of crystals.
Note A similar situation occurs in quite a number of (partly) hydro-
genated oils.