Solidification of Precirol® by the expansion of a supercritical fluid saturated melt: From the thermodynamic balance towards the crystallization aspect

The following article presents the thermodynamic and crystallization aspects of the solidification through expansion of a melted fat, Precirol®, previously saturated with supercritical CO2. A simplified macroscopic energy balance is used to understand the mixture behaviour during expansion. It allows the quantification of the CO2 to be added to the mixture in order to control the state and the temperature of the compound at the outlet of the expansion device. The required physico-chemical parameters were determined experimentally. The variation of the operating parameters appearing in the energy balance revealed three regions/areas of possible states for the expanded lipid: solid, molten or solid–liquid equilibrium. This, together with the outlet temperature could be tuned by varying not just the inlet or saturation conditions, but also the ratio of supercritical CO2 to be expanded with the saturated fat. As for the crystallization aspect, the variation of chemical potential, which is the driving force, was calculated; the critical radius of nuclei and the nucleation rates were estimated according to the classic melt media theory. The main assumptions were that solidification occurred mainly at atmospheric pressure and that only homogeneous nucleation took place. The orders of magnitude obtained put in light the necessity to determine precisely the viscosity of the melt saturated fat and the solid/melt interfacial tension.