Solvent-Mediated Disruption of Bovine Casein Micelles at Alkaline pH

Abstract

The disruption of casein micelles at alkaline pH was investigated using turbidity measurements. The rate and extent of disruption of casein micelles at alkaline pH (8.0-11.0) increased with pH. Furthermore, the extent of alkaline disruption increased with increasing temperature (5-40 degrees C). Preheating milk for 10 min at 90 degrees C did not influence the extent of alkaline disruption of casein micelles, suggesting that whey proteins do not influence the alkaline disruption process. Levels of ionic calcium and serum calcium and phosphate decreased in a logarithmic fashion with increasing pH, indicating precipitation of calcium phosphate onto the casein micelles. A mechanism for alkaline disruption of casein micelles is proposed, in which increasing the milk pH improves the solvent quality for the caseins, thereby leading to the disruption of casein micelles into their constituent nanoclusters; increases in the net-negative charge on the caseins on increasing pH may contribute to micellar dissociation.

Physico-chemical changes in casein micelles of buffalo and cow milks as a function of alkalinisation

“…Indeed, we observed between pH 6.7 and 10.8, a significant and similar increase in the negative charge of the caseins from −20 to −24 mV for both milks. This change of charge was also reported and discussed by Vaia et al [20]. The consequences of these charge changes could have resulted in some modifications in the protein-protein interactions in the casein micelles leading towards disruption.…”

“…This delay in the physico-chemical changes can be related to the concentrations of proteins especially caseins (about 35 and 25 g·kg −1 in cow and buffalo milks, respectively) and minerals especially Ca (65.2 and 33.1 mmol·L −1 in buffalo and cow milks, respectively) and Pi (32.7 and 20.8 mmol·L −1 in buffalo and cow milks, respectively) [1]. The first part of this interpretation concerning the dependence of alkalinisation process on protein contents is supported by the results of Vaia et al [20] who indicated that the disruption of casein micelles induced by alkalinisation was slower when the concentration of proteins increased.…”

“…These findings were also in agreement with the increase in water Figure 10. Proposed mechanism explaining the changes in the ionisation of proteins between pH 6.7 and 10.8. pK values are reported by Vaia et al [20].…”

“…Rose [19] found an increase in casein concentration in the serum above pH 7. Vaia et al [20] determined decreases in concentrations of calcium (Ca) (particularly ionic Ca) and inorganic phosphate (Pi) in the aqueous phase with 388 S. Ahmad et al…”

“…According to the same authors, this dissociation is due to diminished cohesive interaction between the hydrophobic regions of caseins resulting in disrupted casein micelles. In addition to mineral changes in the aqueous phase, Huppertz et al [6] and Vaia et al [20] explain that the changes in the ionisation of proteins can also contribute to this disruption. The objective of this work was to study the evolution of physico-chemical properties of casein micelles and to propose a mechanism of the disruption of casein micelles induced by alkalinisation with a special attention paid to the mineral fraction.…”