Microstructures and conformational arrangement in emulsions caused by concentration ratios of pectin-based microgels and oil

Abstract

Ternary systems of two immiscible liquids and a solid form different microstructures depending on the concentration of each component. Pickering emulsions, capillary suspensions, agglomerates, granulates, emulsion gels or bijels are found in these systems. However, in contrast to solid particles, microgels are deformable, can undergo volume changes, do not have clear contours and might interact with each other in a different manner as solid particles do. We investigated in this study, if previously found conformations and relations for solid particles also apply for pectin-based microgel particles in oil-water-systems. We investigated the microstructures that are formed in oil-in-water-emulsions containing pectin-based microgels and the textures that arise from these microstructures. By varying pectin-based microgel concentration (from 0.5–10 wt%) and oil concentration (from 5 to 20 vol%), we assessed the microscopic conformation of all components in the samples optically, measured the viscosity of the samples and their oil droplet size distributions. With the discussed data, we found concentration regimes for a microgel-stabilised, free-flowing emulsion (from 0.5 to 2.5 wt% microgel concentration) or a micro-structured ternary system (from 5 wt% microgel concentration upwards). Whilst an increase in the oil concentration did not affect the structure of the investigated formulation, an increase in microgel concentration led to the formation of agglomerates. A further increase in this concentration favoured the formation of a continuous particle-oil network throughout the aqueous phase. The change of state due to rearrangement of the components in other structures was visible in the viscosity of the samples. We also showed the influence of the microgel and oil concentration on the oil droplet size in the emulsion state. We obtained mean droplet sizes ranging from around 2 μm–6 μm. The obtained information was summarised in a ternary diagram, where all found microstructure states were qualitatively drawn. With this information, we can show what possibilities pectin-based microgel particles offer in order to adjust the texture of food applications and stabilise oil at the same time.