Phd Defence - Edmund Co "Heterogeneous Nucleation of 2-Oleodistearin On Tristearin Surfaces"

Final Examination for the Degree of PhD Food Science - EDMUND CO 
 
Examining Committee

Dr. Keith Warriner, Chair 
Dr. Alejandro Marangoni, Advisor 
Dr. Michael Rogers, Advisory Committee Member  
Dr. Maria Corradini, Department Member 
Dr. Dérick Rousseau, Ryerson University, External Examiner  

TITLE: HETEROGENEOUS NUCLEATION OF 2-OLEODISTEARIN ON TRISTEARIN SURFACES 
 
ABSTRACT:  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 atomic-scale molecular dynamics (ASMD) computer simulation of the interaction between a planar surface of a tristearin crystal and a number of triglycerides of interest showed that certain triglycerides (SOS being one of them) exhibited an affinity for the surface of the tristearin crystal. An affinity by a triglyceride for a surface should result in heterogeneous nucleation. Using the ASMD results as a hypothesis, the phenomenon was tested experimentally. 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 separate from the main crystallization event, i.e. crystallization of the SOS. In turn, 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, while compositionally present, is not supersaturated enough to crystallize and form the surfaces necessary for heterogeneous nucleation. 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. Charting the course of crystallization by treating the process as a trajectory in a 2dimensional space.