Addition of calcium and magnesium chlorides as simple means of varying bound and precipitated minerals in casein micelle: Effect on enzymatic coagulation

Abstract

In casein micelle (CM), Ca is either precipitated in the colloidal calcium phosphate (CCP) stabilized by clusters of phosphoserine (SEP) residues, or is directly bound to SEP (or glutamic and aspartic acids) of caseins without inorganic phosphate. However, it is currently not possible to titrate separately the different micellar Ca forms, making it difficult to assess their respective importance for CM properties and behavior. Both Ca²⁺ and Mg²⁺ have the same binding constants with SEP. Moreover, MgHPO₄ is more soluble than CaHPO₄, and its natural concentration in milk is lower. Thus, upon addition of MgCl₂, Mg is mainly exchanged with CM in the bound form, whereas upon addition of CaCl₂, Ca is mainly exchanged in the precipitated form. Our objective was to assess the role of the 2 forms of micellar cations (bound and precipitated) during the enzymatic coagulation of cow milk. Magnesium chloride, CaCl₂, or KCl (10 mM) were added to milk and pH was adjusted to 6.6 after overnight equilibration. The KCl-supplemented milk was a positive control to assess the effect of the increased ionic strength after MgCl₂ and CaCl₂ addition. Mineral partition between soluble and colloidal phases after salt addition was assessed both experimentally and by using computer simulation. Enzymatic coagulation was proceeded at 30°C. Hydrolysis of κ-casein was followed by the quantitative determination of caseinomacropeptide released by RP-HPLC, aggregation of para-κ-casein micelles was measured through the evolution of backscattering properties of milk and the gel time and gel firming kinetics were determined using a Chymograph (Chr. Hansen, Horsholm, Denmark). The KCl addition did not affect mineral partition. As expected, CaCl₂ addition mainly increased the CCP content, whereas the addition of MgCl₂ mainly increased the bound divalent cations content. The kinetics of κ-casein hydrolysis was slowed down after CaCl₂ and MgCl₂ addition, and was negatively correlated with the concentration of divalent cations in the soluble phase of milk. Aggregation and gel firming curves plotted versus the progress of κ-casein hydrolysis were similar for both CaCl₂- and MgCl₂-supplemented milk. In view of the dual-binding model for CM assembly, this means that both Ca forms reduce electronegative repulsions between para-micelles by specific charge shielding. Inclusion of 2 Ca forms in structural models for CM allows a more detailed comprehension of how mineral equilibria affect CM properties.

Keywords: calcium; casein micelle; enzymatic coagulation; magnesium; mineral partition.