The effect of environmental conditions on the steric stabilization of casein micelles

Abstract

In an attempt to characterize the steric stabilizing sheath around the casein micelles of bovine milk, photon correlation spectroscopy techniques have been used to measure the micellar radius on exposure to ethanolic buffers of varying pH, ionic strength and calcium concentration. It is shown that on exposure to alcohol, the stabilizing protein sheath undergoes dimensional collapse and that immediately prior to aggregation, a minimum or core radius is reached, characteristic of the diluting buffer conditions. Defining barrier thickness as the difference between the micellar radius in alcoholfree buffer and this minimum radius, the same linear relationship is observed between barrier thickness and the critical ethanol concentration required to reach the core radius and induce subsequent aggregation, whether those variations in barrier thickness were achieved by altering the pH, ionic strength or calcium level of the buffer. Considering the initial rate of response to added ethanol as a measure of barrier strength, it is observed that thicker barriers are weaker whereas thinner barriers are more resistant to collapse and hence intrinsically stronger. This paradox is qualitatively resolved by considering the stabilizing sheath to possess some of the characteristics of a weak or soft gel, whose rigidity or extent of cross-linking is influenced by the variations in buffer conditions.

Multiscale Characterization of Casein Micelles Under NaCl Range Conditions

“…When higher amount of NaCl are considered, more chloride ions could bind to the protein surface so that the preferential hydration by water is overcome, leading to the observed salting-out effect. Another study 54 showed that NaCl addition to casein micelles in skim milk stabilized them against aggregation by an increase of the hairy layer but with no modification of the casein hydrodynamic radius.…”

Flocculation of protein-stabilized oil-in-water emulsions

“…Variations in emulsion stability obtained with different batches of sodium caseinate can be mainly attributed to differences in their mineral contents (especially ionic calcium) [102]. In commercial practice, the production of a cream liqueur with a long shelf-life requires that the 'free' calcium ion concentration be lowered by incorporation of a chelating agent (sodium citrate) [103].…”

Liquid droplet-like behaviour of whole casein aggregates adsorbed on graphite studied by nanoindentation with AFM

“…The pH of the concentrate was 6.7, Na-azide (0.2 g kg À1 ) was added to avoid bacterial growth. Following Horne and Davidson, (1986), the casein concentrate was diluted in a Ca-imidazole buffer (50 mM NaCl, 5 mM CaCl Á 6H 2 O (Prolabo), 20 mM imidazole, C 3 H 4 N 2 (Merk Darmstadt, Germany)). The pH of the buffer was adjusted to 7.0 using HCl diluted in MilliQ-water.…”