On the effect of calcium ions on the sticking behaviour of casein-coated particles in shear flow

Abstract

The use of the particle scattering apparatus has been extended to investigate the effects of calcium ion addition on the aggregation of individual pairs of particles in a laminar shear field at a wall. In this case the particles are oil droplets in the size range 5–6 μm, stabilized by sodium caseinate in a buffers of ionic strength 0.05 mol dm −3 and pH 5–7, with Ca 2+ concentrations varied between 0 and 20 mM. The aggregation, or sticking, of mobile droplets with one already irreversibly stuck to the wall was investigated over shear rates ranging from 0.25 to 12 s −1. The percentage of particle collisions that lead to sticking is reproducibly seen to increase as the pH is reduced from 7 to 5, in accordance with decreasing net charge on the molecules of caseinate, which when adsorbed presumably leads to reduced net electrostatic repulsion between droplets. Surprisingly, beyond Ca 2+ concentrations of 5mM, the percentage of sticking does not markedly increase further (but even decreased, in some instances), in the pH range studied. However, this behaviour can be rationalized in terms of the competing effects of Ca 2+ and pH on electrostatic and steric mechanisms of colloid stabilization, via changes in the aggregation of the caseinate and the probable changes in the thickness and stickiness of the adsorbed protein film.