The fortification of food systems with calcium remains difficult; one challenge is to maintain the colloidal stability of insoluble calcium salts during processing and shelf life. Particle size reduction of insoluble salts may result in improved product stability. In this study, insoluble calcium citrate in two different particle sizes, conventional calcium citrate and micronised calcium citrate, were first evaluated in terms of physical and bulk handling properties, followed by protein adsorption and colloidal stability when dispersed in two dairy-based nutritional beverages differing in composition, i.e. infant milk formula stage 1 and 3. Particle size distribution analysis showed micronised calcium citrate (volume-weighted diameter = 5.10 μm) to have significantly smaller (p < 0.05) particle size than conventional calcium citrate (volume-weighted diameter = 88.2 μm). The adsorption of dairy proteins onto particles of calcium citrate resulted in caseins having greater affinity for both salts, followed by β-lactoglobulin. The smaller particle size of the micronised citrate resulted in higher affinity for casein and greater colloidal stability when dispersed in both infant milk formula solutions compared to conventional calcium salts. The results of this study provide knowledge on the application of micronised insoluble calcium salts in the fortification of nutritional dairy-based products.