A ¹H-NMR study of bovine casein micelles; influence of pH, temperature and calcium ions on micellar structure

Abstract

SummaryThe influence of pH, temperature and Ca depletion on bovine casein micelle suspensions in D2O containing simulated milk ultrafiltrate was studied by 1H-NMR spectroscopy.In the pH range of 5·8–7·5 the spectrum of the micelles showed very little pH dependence, indicating that no changes occurred in the dynamic behaviour of the proteins constituting the micelle.The NMR spectrum of casein micelles was strongly temperature dependent, particularly in the temperature range of 60–98 °C. Increase in temperature resulted in a strong increase in spectral intensity concomitant with changes in the spectral characteristics. In micelle suspensions these changes were reversible, and indicated that at clevated temperatures the rigid structure of the casein micelle started to melt, leading to an increased mobility of appreciable parts of the proteins in the micelle.Ca depletion of the casein micelles by addition of EDTA resulted in an increase in spectral intensity, which arose from the presence of casein components in the serum phase, The spectrum of these serum phase particlcs resembled closely the spectrum of a solution of total casein in simulated milk ultrafiltrate and was quite different from the spectrum of casein micelles.The implications of these results with respect to models of the structure of bovine casein micelles are discussed.

κ‐Casein micelles: structure, interaction and gelling studied by small‐angle neutron scattering

“…This indicates that the outer layer of the micellar particles contains very little scattering material. NMR measurements [30,311 confirm the high mobility of the glycomacropeptide part and the condensed nature of the para-u-casein. A further theoretical explanation for the observed scattering pattern would be that there is an adsorption of, for example, KCl on the particles.…”

“…In the Vreeman model the distribution of amino acid segments in the K-casein between core and outer layer is 105:64. Recent NMR measurements of Rollema et al [30] indicate that this ratio may be somewhat smaller, about 91 : 78. Maintaining the ratio 105: 64, we tried to fit the data by compacting the core and expanding the layer accordingly.…”

The multimeric structure and disulfide‐bonding pattern of bovine κ‐casein

“…Several studies including 'H-NMR spectroscopy (Griffin and Roberts, 1985;Rollema et al, 1988) and electron microscopy (Carroll and Farrell, 1983) indicate that k--casein is located mainly at the micelle surface. Early observations suggest that monomeric Ic-casein is able to interact with a,-casein and j3-casein although it has decreased stabilizing properties.…”

“…In contrast, Ic-casein is insensitive to calcium and plays a key role in maintaining the stability and solubility of the micelle structure (Linderstrerm-Lang, 1929;Waugh, 1971). Several studies, including 'H-NMR spectroscopy, indicate that Ic-casein is located mainly at the micelle surface (Griffin and Roberts, 1985;Rollema et al, 1988). It is believed that the micelles are composed of submicelles held together by calcium phosphate, although the exact quaternary structure of the casein micelle has remained controversial (Farrell, 1988).…”

Rheological behavior of concentrated skim milk dispersions as affected by physicochemical conditions: change in pH and CaCl2 addition

“…This condition was chosen as a pH lower than the physiological one but to avoid micellar flocculation (Koutina et al 2015). Also, although at pH 5.8 colloidal calcium phosphate is partially solubilized and the net charge decreases, CM maintain their global organization (Karlsson et al 2007;Rollema and Brinkhuis 1989;Uricanu et al 2004). Therefore, CM can be considered as rheological units in the analysis proposed in this study.…”