Several food products consist of highly concentrated suspensions of particles in oil or fat media, such as peanut butter and other types of spreads. The addition of an immiscible secondary liquid to such suspensions, even in small quantities, leads to a transition from a liquid-like to a solid-like structure due to the agglomeration of the solid particles. Information about the agglomeration of solid particles can be derived from the determination of the interactions between particles and the parameters that affect them. Consequently, this agglomeration and spanning filling network formation could be used to create food products with a tailored structure, and as a result, assist in food product design.
The aim of this project is to explore the agglomeration of particles in fat-continuous suspensions on both macro and micro scales. For this purpose, model food systems were established as highly concentrated suspensions of crystalline sucrose or glass particles in vegetable oil. The effects of adding different secondary immiscible liquids (water, saturated sugar solution and glycerol) were evaluated. The particle network formed upon the addition of such secondary liquids was assessed by monitoring particle and agglomerate size using laser diffraction and also by means of analysing the flow behaviour of the samples. In addition, interaction forces between a particle and a surface mediated by a secondary immiscible liquid inside oil medium were measured using an atomic force micro- scope (AFM). The use of an AFM in viscous media such as vegetable oil and the creation of bridges composed of different liquids are both challenging tasks. Therefore, in this project, two methods for the formation of liquid bridges between particle and surface were developed. One was based in the creation of water bridges by saturating the oil medium through an increase in relative humidity, while the other consisted of using an inverted microscope to locate individual small droplets of secondary liquid dispersed in oil continuous phase.
Macroscale observation showed that there is an increase in apparent viscosity upon addition of a secondary liquid for the sugar and glass suspensions. This could be supported by the agglomeration of the fine solid particles, which are preferentially wetted by the hydrophilic secondary liquid, forming liquid bridges. Analogously, on a microscale, strong and long-ranged adhesion forces were measured between a glass particle and a glass and sugar surfaces in oil in the presence of water or glycerol as secondary liquids; which confirms the formation of liquid bridges. The measured forces upon separation were dominated by capillary attraction when a liquid of high interfacial tension and low viscosity such as water formed the liquid bridges. When a highly viscous liquid formed the bridge (glycerol), a dynamic viscous interaction contributed to the adhesion leading to a higher force, which was less dependent on the volume of the bridge.
Furthermore, the effect of material parameters, such as particle size, shape, concentration, interfacial tension and viscosity and wetting properties of the secondary liquid, on the network formation in suspensions and on the interaction forces of particles and surfaces in oil upon the addition of secondary liquids was evaluated. The apparent viscosity of the suspensions with added secondary liquid showed to be inversely proportional to the particle size and dependent on particle shape – irregular particles form stronger networks. The viscosity of the secondary liquid had a significant effect in dynamic situations. An increase in viscosity for suspensions with added viscous secondary liquid was only observed at high shear rates. Similarly, on a microscale, a steeper increase in adhesion force was observed for more viscous liquid bridges upon an increase in the separation speed of particle and surface. Moreover, the addition of lecithin showed very comparable results both on macro and micro scales. The stabilization of secondary liquid droplets by the surfactant led to decreased adhesion energies, explaining the lower viscosities in suspensions of particles with added glycerol containing surfactant.
Finally, this work and the information about the agglomeration process may be further applied in an industrial context in order to adjust the properties of a particle network, enabling the creation of tailored solid particle networks in food products.
