Manipulating Sodium Caseinate Behaviour at the Interface: Applications for Concentrated Emulsion Formulation
The effect of ionic strength, pH and droplet size distribution on the stability and rheological properties of concentrated emulsions formed using sodium caseinate was investigated. The emulsions were formulated with soybean oil concentration between 50 and 70 wt% and 1 wt% protein. In order to understand the role and response of the sodium caseinate interfacial thin film to physicochemical changes to the continuous phase the behaviour of sodium caseinate at the air-water and oil-water interfaces, as a function of pH and ionic strength, was studied using Langmuir trough, surface potential and pendant drop methods. Changes in measured system response can be explained by considering changes to protein conformation. Upon increasing ionic strength the data fit with the protein conformation changing from those states where the protein extends into the aqueous phase to those where it essentially lies flat on the interface. Aggregation and dispersion of the protein at the interfaces were detected at different pH values. Also, the buffer capacity of sodium caseinate was evaluated by preparing protein solutions at different pH and ionic strengths. Bridging flocculation and creaming occurred in the emulsions investigated, evaluated via static light scattering and Cryo-SEM. Emulsions with the appearance and texture of liquid-like through to gel-like were formulated by seemingly small changes to the ionic strength and pH of the aqueous phase. Shear-thinning was the flow behaviour of the emulsions with a shear dependent flow response that was function of the parameters evaluated. Time-dependent flow behaviour was detected for the emulsions at a low shear rate and they showed rheopexy behaviour. Viscoelastic properties of the emulsions and the interaction between the droplets were evaluated by strain sweep and creep-recovery tests.