The Crystallisation of Tripalmitin in Triolein as Observed using 
Hot Stage Microscopy 
David I. Stewart1, Andrew G. F. Stapley1, Zoltan K. Nagy1, and Peng S. Chong2
1Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, U.K., 
and 2Nestlé Product Technology Centre, York, YO31 8FZ, U.K. 

The isothermal crystallisation and subsequent melting behaviour of the tripalmitin (PPP)/triolein (OOO) system was investigated using hot stage microscopy (HSM). In blended samples, two distinct polymorphs could be seen as crystals grew, an outer ring of β′ polymorph surrounding an inner circle of β. Between the two a thin layer of liquid was observed into which the more soluble β′ form dissolved and from which the less soluble β form crystallised. Thus a direct observation of the mechanism of a melt-mediated transformation from β′ to β forms was seen. This transformation to β was not seen with pure PPP samples, which demonstrates that liquid oil content is an important variable governing polymorphic transformation. Melting points for β′ and β were determined both by HSM and DSC. Both could show a significant (positive) deviation from ideal behaviour as predicted by the Hildebrand equation. However, Solid Fat Content profiles obtained by NMR were consistent with ideal behaviour. The discrepancy may be due to mass transfer limitations within the microscope slides. Thus whilst HSM is shown to be an excellent technique for visualising crystal growth and polymorphic transformation it is not yet a reliable method for providing precise melting point determinations.