Dataset Fits and Simulation

Casein microparticles produced from casein micelles by depletion flocculation and film
drying under pH-neutral ambient conditions are spherically round and have a spongelike
microstructure. Heat treatment at T = 50 °C and 80 °C creates new physical or
covalent bonds between the caseins, which influence the expansion and stability
behaviour under gastric conditions. For both thermally stabilised and untreated
microparticles, we observe similar complex changes in overall size in optically
transparent swelling cells after replacement of the medium by simulated gastric fluid.
Control experiments show that pepsin becomes active later in the process and causes
a final exponential decrease in particle area to a stable final value through proteolytic
degradation. The previous more or less pronounced expansion, the subsequent
shrinking process and the renewed swelling can be attributed to structural changes in
the casein phase due to the acidification of the particles. Accompanying the changes in
overall size, we resolved microstructural details using confocal fluorescence
microscopy. Before treatment with simulated gastric fluid, particles heat-treated at 80
°C show a broadened tubular microstructure at the surface and a depletion of the
casein phase in the centre, in contrast to the other preparations. After 2 hours of
gastric acid treatment, the tubular microstructure on the surface of all particle
preparations is retained despite drastic shrinkage. Due to accelerated enzymatic
degradation, we now observe a depletion of casein in the interior of the untreated
particles, whereas a homogeneously distributed casein phase is observed in the
thermally stabilised samples.