The role of calcium, phosphate and citrate ions in the stabilization of casein micelles.

To obtain greater insight into the interaction of Ca, citrate and phosphate ions with casein, 31PNMR measurements were performed on combinations of these ions with alphas- and kappa-caseins. It was found that addition of alphas-casein to a Ca phosphate solution in D2O at 27 degrees C and pD 6.4 resulted in a downfield shift of the 31P singlet. An almost identical shift was observed with kappa-casein, but no shift was found when only phosphate ions were present or when Ca2+ were added to phosphate ions in the absence of casein. Separate experiments with poly-L-lysine, mol. wt approx. 35,000, resulted in similar downfield 31P chemical shifts of Ca phosphate as with both caseins, whereas no shift was observed when poly-glycine was added. From these results it can be concluded that Ca and phosphate ions associate with casein in a co-operative manner, probably in the way described by ter Horst (1963) as a complex with the NH3+-groups of lysine or arginine in a structure such as: (casein--NH3+)--PO43---Ca2+. The formation of this complex may be enhanced by citrate ions, since preliminary results have shown that addition of Na citrate to a solution of alphas-casein with added Ca phosphate produces a broadening of the 31P signal as well as a chemical shift.

ESR study of aqueous dispersions of beta-lactoglobulin and spin-labelled glyceryl monostearate.

From the ESR spectra of aqueous dispersions of synthetic glyceryl monostearate (spin labelled at C-12) a critical micelle concentration of 30 mumol/l at room temperature was obtained, which agrees with that deduced from surface tension measurements. At monoglyceride concentrations smaller or larger than the critical micelle concentration, the monomers show increased motional restriction with increasing molar ratio of beta-lactoglobulin to monoglyceride up to a value of 10, as determined from calculated rotational correlation times; A similar progressive interaction was deduced from spectral changes observed on equimolar dispersions of beta-lactoglobulin and monoglyceride on raising the temperature to 55 degrees C at which the protein and monoglyceride coprecipitate. The relevance of these finding for non-labelled monoglyceride dispersions is indicated by the similarity of the pH-dependent flocculation behaviour of labelled and non-labelled monoglycerides, both in the absence and presence of beta-lactoglobulin; In addition, proton magnetic resonance and mechanical stability measurements suggest that spin-labelled glyceryl monosterate behaves analogously to non-labelled glyceryl monooleate.