CALCIUM INDUCED FLOCCULATION OF EMULSIONS CONTAINING ADSORBED BETA-CASEIN OR PHOSVITIN

The influence of calcium ions on the state of aggregation of beta-casein- and phosvitin-stabilised oil-in-water emulsions (0.5 wt% protein, 20 wt% n-tetradecane, pH 7) has been investigated. The extent and reversibility of flocculation was inferred from changes in apparent droplet-size distribution measured by the Malvern Mastersizer, and from direct observations under the light microscope. Complementary measurements are reported for the calcium binding isotherms for beta-casein and phosvitin in solution and for the effect of calcium ions on the electrophoretic mobilities of protein-coated droplets. With calcium ions present prior to homogenisation, the extent of droplet flocculation is greater for beta-casein-stabilised emulsions than for phosvitin-stabilised emulsions. This can be explained by the greater tendency of beta-casein to be precipitated by calcium. Conversely, with calcium ions added after homogenisation, it is the phosvitin-stabilised emulsions which are more susceptible to flocculation. This can be explained in terms of the greater binding affinity of phosvitin for calcium ions. Under conditions where protein solubility is not affected, we find that calcium-induced aggregation of these protein-coated droplets is reversible both towards dilution with buffer solution and towards dilution with calcium-free emulsion.