Skim milk was acidified slowly using glucono-delta-lactone and HC1. On lowering the pH the steric stabilization of the casein micelles diminishes and as a result the micelles exhibit attractive interactions. These interactions can be quantitatively described using an adhesive hard sphere (AHS) model. The strength of the attraction is found to be proportional to 1/(pH - pC); it is suggested that pC may be related to an effective dissociation constant of the carboxylic acid groups along the kappa-casein backbone. If pH approaches pC, the polyelectrolyte-like kappa-casein brush collapses onto the surface of the micelles. The AHS model is quantitatively tested using viscosity and dynamic light scattering measurements.