Casein micelles as sticky colloidal microgel particles
Résumé
Casein micelles, which make up to 80% of the protein content of milk, are one of the most common "association colloids". They are highly hydrated (~85% w/w of water) and roughly spherical particles of ~100 nm in diameter. In a variety of dairy processing operations (membrane filtration, spray-drying...), milk is concentrated leading to dense, and ultimately, solid dispersions of casein micelles. We recently performed osmotic pressure and rheological measurements in order to explore how the micelles behave and interact in such conditions and to understand the resulting phase (or "dynamical arrest") behavior.Our results indicate that strong analogies exist between casein micelles and some "model" colloidal systems. In conditions where the micelles are separated from each other, the dispersions have the exact properties of the polydisperse hard-sphere fluid; with viscosities and osmotic pressures that increase significantly approaching close packing (C » 180 g/L). As concentration is further increased, the micelles are pushed against each other and stick one to another. The dispersions turn into soft solids that strongly resist to osmotic compression. Remarkably, the variation in the elastic properties with C is then very similar to that observed with deformable colloids like nano-emulsions droplets or microgel particles.