solute, andTmits melting temperature. Figure 15.18 gives a calculated
example for a mixture of equal parts of tristearin (SSS) and triolein (OOO).
The final melting point is not the 73 8 C of the SSS, but 69 8 C. At 45 8 C the
solubility of SSS has decreased to virtually zero. (Cooling to 5 8 C would lead
to crystallization from the melt of OOO, assuming equilibrium to be
reached.) Experimental results agree well with the calculations, indicating
that triglyceride mixtures can show virtually ideal solution behavior.
Assume that we have a natural fat that has a final melting point—
often calledclear point—of 40 8 C. Further assuming that PPP is the highest
melting component present, we may calculate its molar proportion in the
mixture. Table 15.2 yields, after some recalculation, Tm ¼ 339 K and
DHf¼165 kJ?mol^1. From these data we calculate thatxs¼0.008 at 313 K;
in other words, the mixture would contain less than 1%PPP. In practice, the
situation is more complex, since we do not have a binary mixture. For
FIGURE15.18 Melting curves. Mass fraction solidcSas a function of temperature
for a number of fats and for an equimolar mixture of tristearin and triolein. The
curves are observed when warming up slowly after the fats have been cooled to a low
temperature.