Elena Blanco of the School's Soft Matter Physics research group is investigating particle dispersion in chocolate making.
My background is in Soft Matter and I have previously worked on emulsion and foam stabilisation using surface-functionalised particles. The ability of colloidal particles to adhere to liquid-liquid or liquid-air interfaces is known as Pickering stabilisation. Pickering foams and emulsions can be remarkably stable to coalescence because of the high energies of attachment for particles held at fluid-fluid interfaces.
Complex colloids with chemically heterogeneous surface properties, unconventional shapes and morphologies play a key role in the stabilisation of foams and emulsions. Existing studies confirm that the particle layer at the fluid-fluid interface enhances Pickering stabilisation. For this reason, these colloids may be engineered to give new functionality to structured assemblies.
The science of chocolate
The project at Edinburgh is focused on understanding the properties of dense, complex dispersions of heterogeneous particles in heterogeneous fluids such as occur in the processing of chocolate.
Chocolate is a semi-solid suspension of solid particles (cocoa solids, sugar and milk proteins) dispersed in a fat continuous phase. Several factors may influence the physical and sensory properties of the final product. Chocolate has a continuous fat phase in which sugar cannot disperse, so natural surfactants like lecithin are used as emulsifiers.
Lecithin, a by-product of soya oil, is a mixture of phospholipids; their amphiphilic nature allows them to act as dispersants at the interface between hydrophilic components (such as sugar) and hydrophobic components (such as fat) to facilitate their mixing. Once the surfaces of the solid particles become coated with lecithin, the chocolate becomes more liquid-like.
Conching and rheology
Chocolate-making involves a process of mechanical agitation called conching that was developed by Rodolphe Lindt in 1879. Conching breaks agglomerates, coats the solid particles with fat and reduces the viscosity, so creating the desirable flow (rheological) properties of chocolate.
A well-defined texture is dependent upon these flow properties. Chocolate behaves as a non-Newtonian fluid presenting a yield stress (the minimum amount of stress that must be applied for flow to occur) and plastic viscosity. It is not well understood how the rheology of chocolate is affected by the interactions between the different components of chocolate and the process of conching.
Chocolate is a luxury food that, during consumption, evokes a range of stimuli which activate the pleasure centres of the human brain. The average consumption in many European countries is around 8kg per person per annum. This project, which is sponsored by Mars Chocolate, aims to use world-class scientific expertise to deliver top quality, pleasurable products.