Kinetics of the calcium-induced instability of oil-in-water emulsions: Studies under quiescent and shearing conditions

The kinetics of calcium-induced aggregation of diluted emulsions (200 mL/L soya oil-in-water) stabilized by 5 and IO g/L beta-lactoglobulin or sodium caseinate were studied using light scattering Under shear conditions, the emulsions generally aggregated, showing an initial slow phase after which the reaction was very rapid. Under quiescent conditions, however, caseinate-stabilized emulsions rapidly formed initial aggregates in the presence of calcium ions after which there was no further aggregation. In contrast, beta-lactoglobulin-stabilized emulsions showed a gradual increase in particle size with time at a given destabilizing calcium concentration. Emulsions containing 5 g/L caseinate were more stable to calcium than those containing IO g/L caseinate, but emulsions stabilized by different amounts of beta-lactoglobulin showed little difference in calcium sensitivity. The presence of 50 mmol/L sodium chloride increased the concentration of calcium required to produce aggregation in emulsions containing either protein. The concentration (extent of dilution) of the emulsions was significant for caseinate, but not for beta-lactoglobulin, in determining the kinetics of the aggregation reactions The aggregation in both systems results from hydrophobic interactions and cross-linking by calcium ions, and the effect of even moderate shear is critical when the emulsions are on the verge of being destabilized by calcium.