Influence of pH on the stability and structure of single casein microparticles

Abstract

The aim of the study is to investigate the influence of pH on the structure and stability of casein microparticles prepared by depletion flocculation of casein micelles in solution followed by drying into a solid pellet. We have developed a microfluidic sample environment to perform microscopic single particle analyses. The setup allows a detailed investigation of the particle size and the internal structure after changes of milieu conditions. While the particles remain stable in HCl and ultrapure water, the addition of NaOH leads to a two-step swelling process, which finally results in degradation of the microparticle at pH values ≥ 11. At moderate pH values, the structural changes take several hours at pH 8, a few minutes at pH 11 and a few seconds at pH 14. The pH-induced size changes of the microparticle are described with a dynamic model assuming two independent solvent inflows occurring at different times and depending on a constant rate coefficient and the current particle volume. The model analysis shows that from pH 8 to pH 14 the characteristic times of the swelling processes decrease by three orders of magnitude while the rate coefficients increase by four orders of magnitude. Microparticles consist of building blocks, which are microscopically detectable as a microstructure with a characteristic size of a few μm. These building blocks are stretched in the first phase of the swelling process. This leads to the assumption that at basic pH values the microstructure swells in a first step and then in a second step the swollen microparticle disintegrates due to the weakening of the stabilizing contacts between the building blocks.