Stability and rheology of emulsions containing sodium caseinate: combined effects of ionic calcium and non-ionic surfactant

We have investigated the effect of the combination of ionic calcium and non-ionic surfactant (Tween 20) on the visual creaming behaviour and rheology of n-tetradecane-in-water emulsions (4 wt% protein, 30 vol% oil, mean droplet diameter 0.4 μm) prepared at pH 6.8 with sodium caseinate. Varying concentrations of ionic calcium, expressed as the calcium/caseinate molar ratio R, were incorporated before homogenization, and varying concentrations of Tween 20 were added soon after homogenization. A stability diagram was constructed based primarily on whether the phase separation occurred after 48 h. Two distinct regions of enhanced emulsion stability could be identified. Shear rheology measurements indicated that most of the samples have a shear-thinning power-law-type behaviour. Addition of CaCl2 and/or Tween 20 resulted in a reduction in viscosity and a closer approach to Newtonian flow. Confirmation of the states of aggregation of the different emulsion compositions was obtained through confocal laser scanning microscopy and particle-size distributions from light scattering. Depletion flocculation may be induced by excess caseinate, excess surfactant, or a combination of both, and is influenced by interaction of the ionic calcium with the protein. Global stability behaviour can be understood in terms of the effect of ionic calcium on the nature of the adsorbed caseinate layer around the droplets and the state of aggregation of the non-adsorbed caseinate, and on the ability of Tween 20 to displace caseinate from the emulsion droplet surface. At low R, displacement from the interface of calcium-bound protein provides an extra source of (mainly protein-bound) ionic calcium in the aqueous phase, leading to fewer and larger caseinate aggregates that are then incapable of inducing depletion flocculation. At high R, displacement of protein from the oil–water interface results in emulsion restabilization through the disruption of calcium-induced caseinate bridges between the droplets.