Volume 10, Issue 3

To develop cosmetic products quickly and effectively, developers need reliable test methods for evaluating product stability. Phenomena that determine the stability of cosmetic emulsions include phase transitions taking place in the relevant application and storage temperature range. The tendency of ingredients to crystallize influences the stability of emulsions especially at low temperatures. In this article a Differential Scanning Calorimetry (DSC) was used to determine the temperature of the water/ice phase transition of the continuous water phase for oil in water emulsions. A rheological, oscillating test method was developed to measure the phase transition temperature and the results were compared with those of the DSC and standard cold storage tests. There was a correlation of the crystallization temperature of the external water phase with the storage stability at –10°C. The accompanying increase in viscosity with cold storage due to structural alteration of the emulsions could be confirmed by TEM micrographs.

Distributive fluid mixing in laminar flows is studied using the concept of concentration distribution mapping matrices, which is based on the original ideas of Spencer & Wiley [1], describing the evolution of the composition of two fluids of identical viscosity with no interfacial tension. The flow domain is divided into cells, and large-scale variations in composition are tracked by following the cell-average concentrations of one fluid using the mapping method of Kruijt et al. [2]. An overview of recent results is presented here where prototype two- and three-dimensional time-periodic mixing flows are considered. Efficiency of different mixing protocols are compared and for a particular example the (possible) influence of fluid rheology on mixing is studied. Moreover, an extension of the current method including the microstructure of the mixture is illustrated. Although here the method is illustrated making use of these simple flows, more practical, industrial mixers like twin screw extruders can be studied using the same approach.

The role of fluid immobilization (FIM) is not only of great importance for solid food systems with respect to their stability, but is also a crucial factor for the rheological behaviour of suspension systems [1]. Besides the fluid binding mechanisms known for porous solids an additional hydro-dynamically induced fluid immobilization (HIFI) is of importance for suspensions in flow. The contribution of the HIFI-mechanism is taken into account by a particle or aggregate shape factor, which for non-spherical or non-cube-like shapes depends additionally on the orientation in the flow field. In many food suspension systems sphere- or cube-like particles with hydrophilic or hydrophobic surface properties build aggregates in the watery and/or fat continuous phases. Such particle aggregates are characterised by different surface and volume FIM-properties. The fluid fraction which is immobilized at the surface f sif depends on surface characteristics like fluid adhesion, interfacial tension and roughness. The volume related immobilized fluid fraction f vif depends on porosity, pore size distribution or packing density.