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 deg. 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 α to β'. This effect can be explained by a simple model where the α phase nucleates from the melt, a process which occurs independently of shear rate. The α phase grows according to an Avrami growth model. The β' phase nucleates on the α crystallites, with the amount of β' crystal formation dependent on the rate of transformation of α to β' as well as the growth rate of the β' phase from the melt. The shear induced α-β' phase transition acceleration occurs because under shear, the α nuclei form many distinct small crystallites which can easily transform to the β' form, while at lower shear rates, the α nuclei tend to aggregate, thus retarding the nucleation of the β' 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 deg. C and 720 s-1 at 22 deg. C