Kinetics of Cream Formation by the Mechanism of Consolidation in Flocculating Emulsions

In this work we propose a theoretical description of the process of creaming of batch emulsions when a sharp boundary exists between a clear serum phase and the sedimenting drops. The creaming is represented as a continuous consolidation of partially aggregated network. The treatment reproduces correctly the trend for gradually decreasing rate of sedimentation as time goes on. The theoretical results are compared quantitatively with experimental measurements of the creaming rate. Oil-in-water systems, stabilized by proteins (β-lactoglobulin (BLG), bovine serum albumin, and mixtures BLG+β-casein) were investigated. Faster creaming is attributed to larger size of the sedimenting objects (flocs of emulsion droplets). In systems obeying the creaming mechanism with sharp boundary (SB) the flocs are smaller when the protein concentration is higher. This supports the hypothesis for the stabilizing role of the excess amount of protein (forming lumps and multilayers on the interface). Theoretical analysis demonstrates that the formation of flocs by gravitational coagulation is a much faster process than the consolidation of the cream. Hence, the dispersions first flocculate and then cream. With increasing β-casein content in mixtures BLG+β-casein the emulsions depart from the SB-type behavior and are characterized by the presence of small nonflocculated droplets, which do not sediment (the serum is turbid and the boundary with the concentrated dispersion is diffuse, DB behavior). This is connected with hindered flocculation, perhaps due to β-casein's augmented ability to prevent droplet aggregation.