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.
The effect of pressure on the thermal conductivity and density of olive, safflower, linseed, and castor oils in the temperature range of (283 to 333) K and pressures up to 400 MPa was studied. The thermal conductivity measurements were carried out using a transient hot-wire method with an estimated uncertainty of 2.7 mW·m−1·K−1. The density of olive oil was determined within an uncertainty of 0.3 % by a Jamin interferometer. Results reveal an increase in the thermal conductivity and density with pressure. The pressure dependency of the thermal conductivity of these plant oils correlates with the coefficient of isothermal compressibility. The temperature dependency of the thermal conductivity is linked to the isobaric thermal expansion coefficient. This agrees well with the vibrational theory of thermal conductivity due to Horrocks and McLaughlin. From this model, the relation between thermal conductivity and density λ/λ0 = (ρ/ρ0)
g
can be obtained. The application of our data to this relation leads to g ≈ 3, which is typical for organic liquids.
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