Dielectric Spectroscopy of Vegetable Oils

The dielectric properties of flaxseed, sunflower, mustard, rapeseed, and olive oils were studied in the temperature range 25–70˚C and in the frequency range of the electric field 25 Hz–1 MHz. A mechanism for the hardening of vegetable oils is proposed according to temperature-dielectric spectroscopy. The hardening of oils is interpreted as a combination of crystallization processes of triglycerides of saturated fatty acids and glass transition processes of triglycerides of unsaturated fatty acids. The glass transition temperature should be considered the true pour point, which allows setting the low temperature limit of the operability of vegetable oils as insulating liquids. Linear correlations between physicochemical, dielectric properties, and fatty acid composition of oils were established. The data presented can be used to assess the quality of vegetable oils, identify falsifications and to obtain oils with a balanced composition of fatty acids. The data derived can also be utilized in the selection of oils as oleochemical raw materials and for the development of compositions of electrical insulating liquids.