Modeling phase transitions during the crystallization of a multicomponent fat under shear.

The crystallization of multicomponent systems involves several competing physicochemical processes that depend on composition, temperature profiles, and shear rates applied. Research on these mechanisms is necessary in order to understand how natural materials form crystalline structures. Palm oil was crystallized in a Couette cell at 17 and 22 degrees C under shear rates ranging from 0 to 2880 s(-1) at a synchrotron beamline. Two-dimensional x-ray diffraction patterns were captured at short time intervals during the crystallization process. Radial analysis of these patterns showed shear-induced acceleration of the phase transition from alpha to beta(') . This effect can be explained by a simple model where the alpha phase nucleates from the melt, a process which occurs independently of shear rate. The alpha phase grows according to an Avrami growth model. The beta(') phase nucleates on the alpha crystallites, with the amount of beta(') crystal formation dependent on the rate of transformation of alpha to beta(') as well as the growth rate of the beta(') phase from the melt. The shear induced alpha- beta(') phase transition acceleration occurs because under shear, the alpha nuclei form many distinct small crystallites which can easily transform to the beta(') form, while at lower shear rates, the alpha nuclei tend to aggregate, thus retarding the nucleation of the beta(') crystals. The displacement of the diffraction peak positions revealed that increased shear rate promotes the crystallization of the higher melting fraction, affecting the composition of the crystallites. Crystalline orientation was observed only at shear rates above 180 s(-1) at 17 degrees C and 720 s(-1) at 22 degrees C .