Kinetic description of changes in the size of casein microparticles under simulated gastric conditions.

Casein microparticles, which are gently produced by depletion flocculation and film drying, are suitable as a sustainable, biodegradable encapsulation material due to their pH-dependent swelling behaviour. However, the food protein casein is highly susceptible to degradation in the stomach due to its open, flexible conformation. In simulated gastric juice, the particles undergo different phases of shrinkage and swelling. Control experiments show that the initial two-stage shrinkage process and the subsequent swelling are due to the acidification of the particles. When pH 2 is reached, an almost exponential decay induced by pepsin takes place until a highly degraded final state is reached. The kinetic analysis of the results is based on the pH profile of the casein solubility as well as on pH- and pepsin-induced structural changes in the casein matrix, which are combined in a system-dynamic model. Data and model simulations show that microparticles can pass through the gastric environment.