It considers that milk protein powder produced by atomization in a spray-dryer usually consists of primary particles and these particles contain internal vacuoles based on the spray dry process.
Thus, the dispersion process of milk protein isolate is mainly contributed by the disappearance of the aggregates of primary particles and also the release of micellar casein materials.
The SEM images in Fig. 6 clearly demonstrate the erosion of the particle surface under the effect of water transfer during the rehydration process.
Before wetting by water, MPI particles have smooth surfaces, which can be seen from .
After wetting and dispersing for a period of time,
the most significant difference is the increase in the roughness of the surface of the rehydrating particles.
This is due to water beginning to penetrate the external layer of micellar casein(Fig. 6B). For the longer rehydration periods,
a much rougher surface was observed and large breaches appeared to indicate the further erosion of the outer skin by water,
but the particle still did not completely break down due to the strong interactions between the micellar structures.
Finally, sufficient materials were solubilised and the insides of micelles were exposed leading to the eventual collapse of the structures.
Therefore, it is believed that the compactness skin outside of micellar casein particles is responsible for restraining the individual micelles into the surrounding liquid phase.
The agglomerated MPI were found to undergo a very similar solubilisation process as the non-agglomerated MPI with the only difference being at the beginning of the dispersion.
The size of the agglomerates decreased sharply to about 75 mm,
which is almost the same as the non-agglomerated particles after swelling.
It is due to the lactose solid bridges linking the particles being easily dissolved
and the agglomerates being quickly dispersed into the non-agglomerated particles.
This is exhibited by the schematic of agglomerated MPI rehydration in Fig.
After that, there was no significant difference between agglomerates
and non-agglomerates during the solubilisation process.
Consequently, agglomerated MPI is believed to have one additional step at first,
which is the dissolution of the solid bridges.