LINKING FILLER PARTICLE PROPERTIES TO THE RHEOLOGY OF PARTICULATE GELS

The aim of this study was to link particle properties to the rheological characteristics of particulate gels. Particles of different composition were matched for size, surface character, and modulus, and added to acidified micellar casein gels. Two sets of spherical particles were prepared. In one case, spherical microgelled particles of whey protein were prepared according to a procedure developed by Saglam et. al. (2011). In the second case, oil-in-water emulsions were prepared with whey protein aggregates on the surface. The particles were matched for a similar distribution of particles sizes with an average between 3-5 µm. Particle modulus was characterized by AFM, and the fat interior of the emulsion selected to be as close as possible to the measured modulus of the whey protein microgel particles (0.1-0.3 MPa). In order to match the surface reactivity, the heat history of the whey proteins in microgelled particles and on the emulsion droplet surface was kept constant. The particles were introduced into 9% micellar casein suspensions at volume fractions of 0.05, 0.10, 0.15, and 0.20 and acidifed using GDL. Both sets of particles were found to be active fillers in the casein network. The properties of the gels were evaluated by performing large deformation experiments. Despite a slightly lower particle modulus of the emulsion droplets compared to microgel particles, the modulus of the emulsion-filled gels was higher than that of microgel-filled gels. The fracture stress of microgel-filled gels was higher than that of emulsion-filled gels, and at high volume fractions of added microgel particles the microgel-filled gels showed a lower fracture strain. Thus, making a controlled comparison between whey protein microgels and whey protein-emulsified oil droplets demonstrates that protein microgel-filled gels are: less stiff (lower modulus), more firm (higher fracture stress), and more brittle (lower fracture strain).