Introduction
The crystallized form of hydrogenated castor oil (HCO), a triacylglycerol (TAG) [1], plays a crucial role as a rheology modifier in many household products and cosmetics. These TAG crystals are appealing because they provide a pseudoplastic behavior, increase the low shear viscosity and act as a stabilizer, thereby preventing physical separation, at relatively low concentrations. The ability of fats to structure and stabilize is also frequently applied in the food industry [2]. When a TAG or any other material is used as a structurant, parameters such as the aspect ratio of the molecules, crystal shape, molecular forces, volume fraction, size- and shape distribution, and the bulk flow field will play a crucial role in obtaining the best structuring characteristics. An example of this can be found in the literature for 12-hydroxystearic acid where the crystalline network significantly impacts the oil binding capacity and elasticity [3], [4], [5]. The present paper will focus on identifying the different crystal shapes and the optimal process conditions when crystallizing HCO in oil in water (O/W) emulsion, the polymorphism of each shape is determined and the low shear viscosity increase at low concentrations is presented for each shape.
Triacylglycerols tend to crystallize in typically three different configurations or polymorphs, depending on the positioning of the three acyl groups. The three most common polymorphs are α, β′ and β, although more polymorphisms are known to exist [6], [7], [8]. Each polymorph crystallizes in a preferred morphology (or shape) and these shapes are determined by the relative growth rate of the different crystal surfaces. In most cases, TAG’s crystallize as spherulites which consist of a sphere from where needle like crystals form from the center [9]. Other known morphologies are needle shaped in the β configuration and an amorphous mass of very tiny crystals in the α configuration. According to [10], [11], HCO has typically one of the 3 morphologies: fibers, rosettes and irregularly shaped crystals. A rosette, similarly to a spherulite, consists of a sphere where fiber crystals form from one central point or interface. A combination of rosettes and fibers was obtained when the O/W emulsion was cooled slowly or at high isothermal conditions. More irregularly shaped crystals were formed when the cooling rate was increased or at relatively low isothermal temperatures. These crystal shapes were only obtained when a high amount of surfactant was added to the aqueous phase. The reason for this is not yet clear but numerous effects of surfactant on the crystallization of TAG’s have already been reported extensively [12], [13].
In the present work the emulsion crystallization was carried out in a micro- and meso-scaled continuous process enabling small emulsion sizes, a fast temperature response, and uniform temperature profiles. A micromixer was used to emulsify 4 wt% HCO with an aqueous solution containing an ionic surfactant. Coils provided the necessary residence time for the crystallization. This allowed for the identification of more precise temperature ranges in which the different morphologies are favored. The crystal shapes obtained after each crystallization protocol were characterized by light microscopy and atomic force microscopy (AFM). The crystal polymorphs were studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Last the low shear-viscosity increase obtained for a typical example of these morphologies was compared.
