The changes in processing functionality of concentrated milk are caused by a number of factors, amongst
the most important, the ionic equilibrium and the increase in the interactions between the casein micelles
because of their increased volume fraction. The objective of this work was to characterize the
physico-chemical properties of casein micelles as a function of their volume fraction, by using osmotic
stressing as a non-invasive method to obtain concentrated milk, in the attempt to preserve the ionic
balance during concentration. Osmotic concentration was carried out for 18 h at 4 C, using different
concentrations of polyethylene glycol dissolved in permeate as the stressing polymer. The viscosity of the
concentrated milk could be predicted using established rheological models, when the changes occurring
to the viscosity of the serum phase were taken into account. Both Eilers and Mendoza equations predicted
a maximum packing volume fraction of 0.8 for the casein micelles. After concentration up to 20%
protein, the casein micelles did not show a change in their size upon redilution. Light scattering measurements
carried out using diffusing wave spectroscopy without dilution suggested that casein micelles
behave as hard spheres with the characteristic of free diffusing Brownian particles up to a volume
fraction of 0.3, and restricted motion at higher concentrations. Results of total and soluble calcium
suggested release of colloidal calcium phosphate from the micelles at volume fractions >0.35. This
research brings new insights on the changes occurring in skim milk during concentration.