Investigation of the crystallization and melting of the tripalmitin/triolein system via hot stage microscopy, differential scanning calorimetry, and pulsed NMR

The isothermal crystallization kinetics and melting behavior of mixtures of tripalmitin (PPP) and triolein (OOO) were investigated using hot stage microscopy (HSM), with supporting differential scanning calorimetry (DSC) and NMR measurements. Some melting points determined via HSM and DSC were higher than predicted by the Hildebrand equation, but solid fat content melting profiles from NMR were close to ideal predictions. The observed deviations from ideal behavior with HSM/DSC could be attributed to concentration gradients in the liquid phase, rather than from nonideality. Sufficient time should therefore be allowed in melting experiments for diffusional equilibration to occurthe remelting time should generally be longer than the crystallization time. When crystallizing mixed PPP/OOO samples, two distinct polymorphs could be seen in the same “crystal”, with an outer ring of β′ surrounding an inner circle of β. Polymorphic transformation from β′ to β could be seen to occur via dissolution through a narrow liquid interface region between the two crystals. Growth rate data for β′ were proportional to the reduced saturation (or supercooling) calculated assuming ideal behavior. Growth rates of β via the solvent/melt mediated transformation of the β′ form could be related to driving forces based on the difference in solubility of the β′ and β forms.