Coalescence in β-Lactoglobulin-Stabilized Emulsions:  Effects of Protein Adsorption and Drop Size

The effects of two factors on the coalescence stability of protein containing, oil-in-water emulsions are studied: (1) protein adsorption on the drop surface and (2) drop size. The experiments are performed with β-lactoglobulin (BLG) solutions, with protein concentration spanning the range between 2 × 10-4 and 0.5 wt %. A combination of several experimental methods is applied: centrifugation for quantitative assessment of the emulsion stability, the Bradford method to determine protein adsorption, and the film trapping technique (FTT) to quantify the effect of drop size. An analysis of the conditions for coalescence in the cream during centrifugation is performed to interpret properly the experimental data. The obtained results convincingly show the presence of a well-defined threshold value of the BLG adsorption, Γ* ≈ 1.55 mg/m2, which is required for obtaining stable emulsions: the emulsions are very unstable at Γ < Γ*, a stepwise increase of stability is observed at Γ ≈ Γ*, and a further, much slower increase is observed at Γ > Γ*. The value of Γ* is slightly lower than the one for a dense monolayer of adsorbed BLG molecules (ΓM ≈ 1.65 mg/m2). The experiments show that the emulsion stability strongly decreases with the increase of the drop size (at constant protein adsorption). A simple relation between the value of the critical pressure, P CR, which characterizes the coalescence barrier (measured by FTT or centrifugation) and the drop size, is found experimentally: 1/P CR is a linear function of the drop radius. The same relation has been found applicable to two other systems, as well (emulsions stabilized by whey protein concentrate and single oil drops stabilized by an anionic surfactant of low molecular mass). Thus, we have been able to separate the effects of the protein adsorption and the drop size from each other.