Model fat systems consisting of fully-hydrogenated canola oil (FHCO) and canola oil (CO) were shear-crystallized using a rheometer with a parallel plate geometry at various cooling rates (0.2 to 5.0 ºC/min) and shear rates (0, 500, 1000, and 2000 s-1) to produce spheroidal fat crystals. These spheroids were characterized via rheology, polarized light microscopy (PLM), differential scanning calorimetry (DSC), and x-ray diffraction (XRD). Crystal spheroid formation was optimal at 1.0 ºC/min and viscosity profiles followed a three phase sigmoidal shape. PLM analysis revealed that spheroid size decreased with increased shear rate while sphericity increased. A multi-step mechanism was proposed for the formation of these crystal spheroids.
Subsequently, different emulsifiers were used to modify the structure of these crystal spheroids and it was found that the type and concentration of emulsifier had significant effects on spheroid microstructure. Below a critical concentration, emulsifier could be incorporated into the crystal matrix of FHCO while above they would crystallize independently. DSC analysis revealed additional melting fractions compared to the control that were attributed to emulsifier incorporation and co-crystallization with FHCO. XRD showed that the crystallized spheroids were mainly of the βʹ polymorph regardless of the presence or type of emulsifier.
A water phase was then introduced within these systems to study their encapsulation potential. Emulsifier type significantly affected crystal shell morphology and encapsulation efficacy. The liquid-state emulsifiers (GMO and PGPR) showed limited interaction with FHCO, with GMO delaying the interfacial crystallization of FHCO while PGPR excluded FHCO from the droplet interface completely. Of the solid-state emulsifiers (GMS, GMP, SMS, and STS), the MAGs produced smooth-surfaced crystal shells around the emulsion droplets while the sorbitan-based emulsifiers produced irregularly-shaped shells and droplet cores (SMS) or incomplete crystal shell formation (STS).
The shear-crystallization of our model fat blend also resulted in the formation of cylindrical crystalline assemblies. The average diameter size of these crystal cylinders decreased with increased shear rate. A “log-rolling” mechanism was proposed for their formation. These results demonstrate that laminar shear may be used to modify fat crystal microstructure and induce the formation of spheroidal and cylindrical crystalline assemblies.