Novel details on the dissociation of casein micelle suspensions as a function of pH and temperature

Membrane filtration is a widespread process for fractionation and recombination of milk components. Although the dissociation of micellar caseins has been studied in detail in skim milk, it is important to better understand the dissociation dynamics occurring between the colloidal and noncolloidal fractions in systems of modified composition. This research aimed at understanding the dissociation of casein proteins in micellar fractions depleted of whey proteins. Casein micelle dispersions were tested at neutral pH and pH 6 (using glucono-δ-lactone as acidulant), after incubation at 4°C or 22°C, and compared with skim milk. The ionic composition of the serum phase was measured using inductively coupled plasma-mass spectrometry, and the protein distribution analyzed using reversed phase-HPLC coupled with mass spectrometry. When incubated at 22°C, there were no differences in casein micelle dissociation between skim milk and whey protein-depleted micelles (∼2.6% dissociated casein). No additional dissociation occurred by lowering the pH from 6.8 to 6 at 22°C, albeit there were more soluble ions at low pH (71% Ca and 65% P). At 4°C, there was an increased amount of β-casein found in the serum phase (23-33% of total β-casein). In addition, there was an uneven dissociation behavior of the various genetic β-casein variants, whereof A2 was more readily released with cooling. In skim milk, approximately 22%, 18%, and 14% of κ-, α_S2, and α_S1-caseins, respectively, were dissociated from the micellar phase upon cooling and acidification to pH 6.0. This was in contrast to whey protein-depleted casein suspensions, in which only 6%, 5%, and 3% of κ-, α_S2, and α_S1-caseins, respectively, had dissociated. The results suggested that the whey proteins in the serum phase play a role in the equilibrium between colloidal and soluble caseins in milk. This is of great relevance in processes such as cold membrane fractionation, where more attention should be given to the protein composition in the serum phase, especially when concentration is combined with fractionation of the serum proteins.