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

Abstract

Solvent flows in the fL/s range across the total surface of a casein microparticle cause its expansion and shrinkage. Microparticles prepared from the milk protein casein have a porous and flexible inner structure with water-filled channels and cavities. Solvent uptake occurs in two phases and results in disintegration if de-swelling is not triggered by acidification. So far, nothing is known about the reversibility of the swelling/de-swelling steps. We performed pH jump experiments between pH 11 and pH 1 on a single micro-particle and analyzed the swelling-induced size changes with system dynamics modeling. Both the swelling steps and the subsequent de-swelling process proceed reversibly and at an unchanged rate over a sequence of at least three pH exchange cycles. We observed that the duration of the first swelling step increased during the sequence, while the second step became shorter. Both of the time intervals are negatively correlated, while a statistical evaluation of only one swelling cycle for an ensemble of microparticles with different stabilities did not reveal any significant correlation between the two parameters. Our results indicate that the pH-induced swelling/shrinkage of casein microparticles is, to a large extent, reversible and only slightly influenced by the acid-induced decomposition of colloidal calcium phosphate.

Regenerated Fibers from Rennet-Treated Casein Micelles during Acidification

“…The loss of calcium-mediated stabilizing contacts leads to the increasing dissociation of the casein network. We have recently shown that the dissolution of calcium contacts in casein gel networks, more precisely in casein micro-particles, does not occur immediately but requires at least three medium exchange cycles of about 10 min from pH 11 to pH 1 and back [23]. However, chelating agents such as the citric acid or EDTA used here significantly increase the proportion of calcium and phosphate in the diffusible phase [24] without forming cross-links or interfering with gelation [25].…”