A typical example of the changes occurring upon cooling and heating
is given in Figure 15.21 for PPP. Upon cooling (curve 1), only thea-form
results (but if the cooling is done slowly, the main form obtained will beb^0 ).
On directly heating again (curve 2), theacrystals melt, subsequentlyb
crystals form (þa littleb^0 ), and finallybmelts. If the heating is halted at
468 C for 15 min, thea-form disappears, presumably changed intob^0 and
someb. Heating then (curve 3) causesb^0 to melt (note the double peak,
hence presumably twob^0 -forms), morebis formed, and finallybmelts. It
will thus depend on the triglyceride(s) present and on the temperature–time
regime applied what precisely will happen, but the same principles apply.
Multicomponent fats tend to form compound crystals, and this
significantly affects polymorphic forms and transitions. In some cases,
however, a group of closely related triglycerides can almost behave like a
single triglyceride, especially in thea-andb^0 -forms.
An example is given byfully hydrogenated palm oil.97%of its fatty
acid residues are either stearic or palmitic acid. When mixing it with a fully
liquid oil, its solubility nicely follows the Hildebrand equation (15.8),
provided that the parametersDHfandTmare experimentally determined
FIGURE15.21 Differential scanning calorimetry of tripalmitate. The vertical axis
gives the heat flow (e.g., in ðJ?kg^1 ?s^1 Þ, þ indicating exothermic and –
endothermic flow. Curve 1: cooling from 80 to 30 8 C. Curve 2: heating from 30 to
808 C. Curve 3: after heating from 30 to 46 8 C, the sample is kept for 15 min at 46,
then heated to 80 8 C. C: crystallization peak. M: melting peak. (After results by K.
Sato et al. Progr. Lipid Res. 38 (1999) 91.)