Physico-chemical properties of casein micelles in unheated skim milk concentrated by osmotic stressing: Interactions and changes in the composition of the serum phase

Abstract

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.