Citrate effect on the swelling behaviour and stability of casein microparticles

Abstract

Casein microparticles obtained from casein micelles by volume exclusion of added pectin and subsequent film drying remain stable in the acidic and neutral pH range, but swell strongly in the basic range. Calcium significantly impacts on the stability and water-binding behavior of phosphorylated caseins and the aggregates and gels formed from them. For a future effective and controlled use as a carrier for bioactive substances, e.g. via the gastrointestinal tract, we therefore investigated how the addition of the calcium chelating agent citrate affects the swelling and stability of the microparticles. Citrate concentrations of 2 mM and above cause a stronger swelling of the microparticles at pH 8, while above 4 mM the second characteristic swelling step starts earlier and thus can also be investigated within the observation time of 120 min. All swelling kinetics can be simulated using seven parameters of a dynamic model, which reproduces the individual swelling steps via volume inflows and outflows into a reservoir. While the rate coefficient for swelling step 1 increases linearly with citrate concentration, no such dependence could be found for swelling step 2. The more citrate is used, the faster the microparticles decompose in turbidity experiments after the addition of sodium dodecyl sulfate, which can be related to a weakening of the hydrophobic interactions.

Nearly Reversible Expansion and Shrinkage of Casein Microparticles Triggered by Extreme pH Changes

“…This observation is consistent with previous swelling experiments on casein microparticles [ 22 , 23 ]. Studies with the calcium chelator citrate have shown that calcium influences the first swelling step, while no such dependence exists for the second swelling step [ 25 ]. After reaching the maximum swelling value, an even faster de-swelling takes place in all cases, as a result of the pH jump by aqueous HCl solution.…”

“…The particle area can be observed microscopically and increases almost linearly with time during both swelling steps, with swelling step 2 always proceeding at a larger rate than swelling step 1 [ 23 ]. The expansion process can be suppressed or increased by increasing the free calcium concentration in the medium or by removing colloidal calcium phosphate by pre-treatment with the calcium chelator citrate [ 24 , 25 ]. A temperature post-treatment or an enzymatic cross-linking with the enzyme transglutaminase stabilizes the microparticles, so that they swell to an equilibrium final value, instead of disintegrating [ 26 , 27 ].…”

Soft Matter Engineering for the Fabrication of Functionalized Casein Microparticles

“…The decay curves can be fitted with a simple exponential function and the effect of certain manufacturing conditions or additives can be analyzed by looking at the decay rate. If, for example, the calcium chelator citrate is added during preparation, CMPs disintegrate the faster the more of it is present [25]. In the case of casein micelles, citrate is known to indirectly dissolve the colloidal calcium phosphate inside [26,27].…”

“…A calcium deficit can be achieved either by using pectin with a low degree of esterification for the preparation and drying, which removes calcium from the casein [28], or by adding a calcium chelator such as citrate to the casein dispersion in the first preparation step (see Fig. 1) [25].…”

“…Interestingly, the addition of citrate at the beginning of the preparation only accelerates the first swelling step while the second step does not change. This suggests that citrate destabilizes the interior of the microstructure by calcium removal but not the intermolecular contacts between them, which stabilize the overall structure of the CMPs [25]. Although such molecular conclusions cannot be made with the centroid method, however, a wide variety of influences on swelling can be evaluated uniformly with it, since the analysis is performed by averaging in a model-independent manner.…”