Depletion flocculation of caseinate-stabilised emulsions: what is the optimum size of the non-adsorbed protein nano-particles?

We address the issue of the size of the protein aggregates (nano-particles) required to produce depletion flocculation in sodium caseinate-stabilised emulsions of neutral pH. We predict that a casein nano-particle radius of around 20 nm causes the strongest depletion flocculation of fine emulsion droplets (mean diameter ∼0.4 μm), i.e., corresponding to a size ratio of about 10:1. This optimum size is the result of a compromise between (i) it being small enough to generate a high number density of non-adsorbed nano-particles, and hence a substantial osmotic pressure, and (ii) it being large enough to give a substantial depletion layer thickness. It turns out that the estimated optimum size of casein nano-particles for inducing depletion flocculation is similar to the size of small casein aggregates actually found in sodium caseinate dispersions at low ionic strength.