Mixtures of proteins and protein-stabilised droplets: rheology of emulsions and emulsion gels

Abstract

The soft materials formed from emulsions stabilised by proteins, like yogurt, are referred to as emulsion gels. This designation is however not precise enough to reflect the variety of composition of these materials. Indeed, during emulsification not all the proteins in solution adsorb at the interface and the emulsion is thus a mixture of protein-stabilised droplets and unadsorbed proteins. The composition of this mixture affects the viscosity of the emulsions and the texture of the emulsion gels. The objective of this thesis is to study the rheological properties of protein-stabilised emulsions and the gels they form considering the full range of their composition. A first step has been to characterise separately the purified suspensions of protein-stabilised droplets, and of suspensions of pure proteins and their gelation. These components have then been combined, resulting in emulsions and emulsion gels of well-characterised compositions, thus allowing a rigorous approach to the rheology of these systems. The viscosity of purified suspensions of proteins and of protein-stabilised droplets has been studied. It was found that these systems are conveniently studied in the framework developed for soft colloidal suspensions, for which the viscosity scales with the volume fraction. The properties of the droplet and protein suspensions have then been used to model the behaviour of their mixtures. The viscosity models for the two types of pure suspensions have facilitated the development of a semi-empirical model that relates the viscosity of protein-stabilised emulsions to their composition. The gelation of the pure suspensions has then been characterised. Indeed, at low pH, proteins can aggregate and also form gels, either of protein molecules in solution or of protein-covered droplets. The rheological properties of these fractal networks were found to depend on their volume fraction, in good correspondence with previous studies on colloidal gels. Protein gels and droplet gels display very similar mechanical properties when the scaling by the volume fraction is used to describe their concentration. These results have then been used to characterise the rheological properties of emulsion gels over a wide range of compositions. The choice of parameters is important and it was shown that using the total volume fraction and the ratio of volume fractions of the components, rather than the individual volume fractions, makes it possible to change paradigm for these systems, from droplet- filled protein gels to composite gels. Using this approach it was demonstrated that the rheological properties of pure protein gels, emulsion gels and pure droplet gels vary continuously with their composition. Finally, the influence of the size of droplets has been briefly studied. Larger droplets were produced and the rheological properties of the droplet suspensions and droplet gels were compared with the results for smaller droplets and for proteins. It appeared that the increase in size only causes minor changes in the rheological behaviour of the emulsion and emulsion gel, and the variation with the volume fraction is consistent with the other types of samples.