Citrate effect on the swelling behaviour and stability of casein microparticles

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.