Relationships Between Fat Crystal Characteristics and Emulsion Stability in Water in Oil Systems

Investigations were made into the stabilization of water in oil emulsions using crystalline particles of paraffin wax and fully hydrogenated canola and cottonseed oils. A model system was studied to develop a methodology of study and provide a benchmark for a subsequent study of a real-world system. The model system involved the use of light mineral oil, purified water, paraffin wax and glycerol monooleate emulsifier. The wax was crystallized prior to and following emulsification. Prepared emulsion samples were monitored for sedimentation and flocculation behaviour. Measurements of coalescence were obtained by pulsed field gradient NMR. Formation of the solid crystalline wax phase following emulsification resulted in emulsions more stable to flocculation and coalescence than samples containing the same amount of wax crystallized prior to emulsification. Analysis of emulsion samples with polarized light microscopy showed the wax crystals were associated with the water droplet interfaces rather than dispersed freely within the continuous oil phase. Another investigation employed similar experimental protocols but incorporated food-grade materials. Two different solid fats were used, chosen for their differing polymorphic (crystal habit) behaviour. Solid crystals of canola stearine (ß-tending) and cottonseed stearine (ß-tending) were compared in their abilities to stabilize emulsions at levels of addition between 0 and 2%. Each type of fat was incorporated into the emulsion in a pre-crystallized state, or while melted and crystallized following emulsification. Cottonseed stearine was found to be in the ß polymorph when quickly crystallized following emulsification from 45⁰ to 5⁰C over 6 minutes. Further calorimetric and X-ray diffraction investigations revealed this crystallization behaviour was a result of a solid-state transformation via an imperfectly formed ß̕ intermediate. With respect to the post-crystallized emulsions, where the polymorphism of the two fats were both in the ß-form, the canola stearine provided better stabilization against coalescence than the cottonseed stearine. This observation coincided with a stronger energy of interfacial attachment for crystallized canola than for cottonseed as calculated from measurements of contact angle and interfacial tension. With the pre-crystallized system, incorporation of cottonseed stearine resulted in reduced sedimentation and coalescence compared to samples containing pre-crystallized canola stearine. This difference was attributed to the presence of fine shards of ß̕-form crystals. The system that imparted the highest degree of overall stability incorporated the use of canola stearine srystallized following emulsification.