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The changes in processing functionality of concentrated milk are caused by a number of factors, amongstthe most important, the ionic equilibrium and the increase in the interactions between the casein micellesbecause of their increased volume fraction. The objective of this work was to characterize thephysico-chemical properties of casein micelles as a function of their volume fraction, by using osmoticstressing as a non-invasive method to obtain concentrated milk, in the attempt to preserve the ionicbalance during concentration. Osmotic concentration was carried out for 18 h at 4 C, using differentconcentrations of polyethylene glycol dissolved in permeate as the stressing polymer. The viscosity of theconcentrated milk could be predicted using established rheological models, when the changes occurringto the viscosity of the serum phase were taken into account. Both Eilers and Mendoza equations predicteda 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 measurementscarried out using diffusing wave spectroscopy without dilution suggested that casein micellesbehave as hard spheres with the characteristic of free diffusing Brownian particles up to a volumefraction of 0.3, and restricted motion at higher concentrations. Results of total and soluble calciumsuggested release of colloidal calcium phosphate from the micelles at volume fractions >0.35. Thisresearch brings new insights on the changes occurring in skim milk during concentration.
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