Structure and stability of sodium-caseinate-stabilized oil-in-water emulsions as influenced by heat treatment

Abstract

The emulsion stability and rheological properties of heated (120 °C, 0–60 min) 30% w/w oil-in-water emulsions (droplet diameter ∼ 0.73 μm, pH 6.8) containing sodium caseinate (Na-CN), 2–6% w/w, were investigated. The creaming kinetics, determined by multiple light scattering (Turbiscan), showed that the phase separation of Na-CN-stabilized emulsions was markedly dependent on the duration of the heat treatment. The differences between unheated and heated emulsions were attributed to heat-induced physicochemical changes in the aqueous Na-CN nanoparticles. The heat treatment caused protein degradation and changed the intact Na-CN concentration in the continuous phase. The emulsion structures, with varied extents of depletion flocculation, were well reflected by small and large deformation rheology. In the recombined Na-CN emulsions, the depletion attraction was weakened at low and moderate Na-CN concentrations (2% and 4% w/w) but was strengthened at high Na-CN concentration (6% w/w). The former structural change was predominantly due to reduced depletion attraction, whereas reduced depletion attraction and decreased continuous phase viscosity influenced the latter structural change. The intact Na-CN concentration in the continuous phase is determined by the heat-induced physicochemical changes of Na-CN nanoparticles, which played a significant role in the physical stability of the emulsions. The insights from this study can be used to create novel droplet sizes and protein particle sizes to manipulate the droplet/protein size ratio, and therefore the extent of droplet−droplet interactions.