Heterogeneous Nucleation of 2-Oleodistearin on Tristearin Surfaces
This thesis is an investigation of the phenomenon of heterogeneous nucleation during the crystallization of multicomponent fats, specifically a fat (a fraction of shea stearin) rich in 2-oleodistearin, a major constituent of cocoa butter. An hypothesis generated by a computer simulation showed that certain triglycerides exhibited an affinity for the surface of the tristearin crystal. Experimental work showed that the addition of small amounts (> 1 % wt/wt) of tristearin to the shea stearin fraction resulted in the fractional crystallization of tristearin. The presence of crystalline SSS during the crystallization of SOS resulted in an increase in the nucleation rate, relative to the nucleation rate of the homogeneous case. This is indicative of heterogeneous nucleation. Furthermore, the crystallization of the tristearin can be suppressed by raising the crystallization temperature such that the added tristearin does not crystallize. When the crystallization of tristearin was suppressed in this manner, the increase in the nucleation rate indicative of heterogeneous nucleation was not observed. Using the obtained nucleation kinetics data as well as the use of an epitaxial crystallization model, estimates for the surface free energy and the putative size of the nuclei under heterogeneous and homogeneous nucleation was obtained. The second portion of this thesis deals with the development of a conceptual tool for the analysis of the crystallization trajectory of fats, with an aim to utilizing this tool to study heterogeneous nucleation in future work (not included in this thesis). Patterned after a concept taken from the study of dynamical systems, this tool is called the “phase space of crystallization”. Traditional models that describe the crystallization trajectory of a fat, such as Rousset’s FEM-TTT model, almost always focus on the thermodynamic (i.e. supersaturation) aspects of crystallization in time, while neglecting the temporal variation of kinetic (i.e. mass transfer) aspects. In this work, a second dynamic variable, the diffusivity, was introduced to characterize the mass transfer conditions encountered by crystallizing triglycerides. Utilizing this tool, it was shown the mass transfer conditions experienced by the system at the moment when crystallization began, had a significant impact on the crystal size of crystallizing triglyceride.