The melting point of triacylglycerides (TAGs) under atmospheric pressure depends on both the fatty acid composition and crystalline structure of the polymorphic state, which are influenced by the temperature treatment history of the TAG. In this contribution, the additional effect of high hydrostatic pressure is described. Samples were placed in a temperature-controlled cell and pressurized up to 450 MPa. The phase transition was investigated either by perpendicular light scattering and transmission or with a polarized-light microscope. The high-pressure polarized-light microscope allows a precise determination of the melting point. The investigated TAGs showed a significant nonlinear increase of the melting point with pressure. Light scattering and transmission were used to observe the phase change in the high-pressure cell. Similar to supercooling in temperature-induced phase transition, we found a dramatic increase of the delay time in our pressure-induced solidification. Even the dependency of this induction time on the control parameter pressure was similar to that in temperature-driven crystallization. We propose that different crystalline structures may be obtained by superpressuring instead of supercooling.

Phase change reactions of food ingredients are essential for food technology. We present an optical in situ-technique to examine high pressure induced freezing processes in edible fats up to 450 MPa. The results indicate that the thermodynamic parameters not only influence the solidification kinetics, but also influence the structure of the resulting solid fat.

High-pressure treatment is a promising option for improving mechanical properties and processing parameters of fat-containing products. To identify optimum processing windows, melting curves, crystallization kinetics, and pathways for transferring the optimized structures to atmospheric pressure need to be known. Here, we provide melting curves of different polymorphic forms of triolein in the industrially relevant pressure range. The melting points of different polymorphic forms are detected optically in thin samples during stepwise changes of pressure or temperature. For cross-nucleated spherulites, this method allows determining the respective melting points of nuclei and overgrown structures. Tracing the melting curves to atmospheric pressure confirms previous identification of the polymorphic forms at high pressure and enables identifying a previously reported but undefined structure as the β 2-form. Employing Raman spectroscopy, it is confirmed that the polymorph remained unaltered during the pressure release. With increasing pressure, the melting curves of the different polymorphic forms approach each other until they successively merge at the highest pressure levels studied.