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This article presents results of the high pressure Raman spectroscopy of triolein. Triolein, a triacylglyceride (TAG) of oleic acid, is an unsaturated fat, present in natural oils such as olive oil. As a basic food component and an energy storage molecule, it has considerable importance for food and fuel industries. To generate pressure in the experiment, we used a high-pressure cylindrical chamber with sapphire windows, presented in (R.M. Siegoczyński, R. Kościesza, D.B. Tefelski, and A. Kos, Molecular collapse -modification of the liquid structure induced by pressure in oleic acid, High Press. Res. 29 (2009), pp. 61-66). Pressure up to 750 MPa was applied. A Raman spectrometer in "macro"-configuration was employed. Raman spectroscopy provides information on changes of vibrational modes related to structural changes of triolein under pressure. Interesting changes in the triglyceride C−H stretching region at 2650-3100 cm −1 were observed under high-pressures. Changes were also observed in the ester carbonyl (C=O) stretching region 1700-1780 cm −1 and the C−C stretching region at 1050-1150 cm −1 . The overall luminescence of the sample decreased under pressure, making it possible to set longer spectrum acquisition time and obtain more details of the spectrum. The registered changes suggest that the high-pressure solid phase of triolein is organized as β-polymorphic, as was reported in (C. Akita, T. Kawaguchi, and F. Kaneko, Structural study on polymorphism of cis-unsaturated triacylglycerol: Triolein, J. Phys. Chem. B 110 (2006), pp. 4346-4353; E. Da Silva and D. Rousseau, Molecular order and thermodynamics of the solid-liquid transition in triglycerides via Raman spectroscopy, Phys. Chem. Chem. Phys. 10 (2008), pp. 4606-4613) (with temperature-induced phase transitions). The research has shown that Raman spectroscopy in TAGs under pressure reveals useful information about its structural changes.

The article discusses further research of the new high-pressure phase in triolein and in triolein mixed with methyl alcohol. The transformation from the initial isotropic state of liquid to the high-pressure phase in case of the mixture of triolein-alcohol was observed by us for the first time. In this study, we compare the phase transformation under pressure, both for oil and for oil-alcohol mixture. Spectra of Raman and UV-VIS absorption show that the application of pressure to triolein leads to partial irreversibility of the observed processes. That partial irreversibility is important for the food conservation methods based on high-pressure technology since triolein is a good model of the most of vegetable oils components.

A high-pressure research on rapeseed oil and mixtures of rapeseed oil with methyl alcohol has been made. In all cases, at least one occurrence of the first order phase transition similar to that in castor oil was noticed [R.M. Siegoczyński, Reports of the Institute of Physics. No. 46, Publishing House of the Warsaw University of Technology (1998)]. Experiments were performed in a cylindrical chamber with a simple piston system, working together with 20 ton hydraulic laboratory press. The pressure in those experiments was approaching 1 GPa. Changes of pressure, volume, temperature and capacitance were recorded using an advanced data acquisition system. In this article, the changes of the relative electric permittivity ε r as function of pressure and also the compressibility of investigated liquids have been presented. Abnormal rise of the dielectric constant in the mixture of rapeseed oil with methyl alcohol was observed. These results suggest the occurrence of a new phase transition apart from the one observed earlier in rapeseed oil [A.J. Rostocki, R. Wiśniewski and T. Wilczyńska, J. Mol. Liq., in press].