Whey protein particles (prepared either at pH 5.5 or pH 6.8)
were dispersed in 1% (w/w) WPI solution (pH w 7) at increasing
particle concentrations. Due to buffering effect of the particles, the
final pH of the dispersions could not be easily adjusted to the same
pH value. As a result, for the dispersions containing protein particles
prepared at pH 5.5 the final pH was w6.8e6.9, while for the
dispersion containing protein particles prepared at pH 6.8 the pH
was slightly above 7.
The heat stability of the dispersions was assessed after a heat
treatment at 90 C for 30 min. The microstructure of the particles
before and after heat treatment was analyzed with CLSM after the
samples were stained with Rhodamine B (Fig. 2). Similar to
observations by SEM (Fig. 1), we also see a clear difference in the
morphology of the particles prepared at different pH in CLSM
micrographs. The shape of the particles prepared at pH 6.8
is spherical whereas the particles are irregular when prepared at
pH 5.5.
Protein was homogeneously distributed inside the particles
when particles were prepared at 6.8 (Fig. 2, depicted as green areas
in the micrographs (in web version)). Some darker areas present
inside the particles prepared at pH 5.5 are due to larger and inhomogeneously
distributed pores present in these particles (Fig. 2,
first row). As can be observed, there is no aggregation or significant
change in the microstructure of the particles prepared at either pH
5.5 (Fig. 2, first row) or pH 6.8 (Fig. 2, second row) after heat
treatment. In Fig. 3 viscosities of the dispersions containing whey
protein particles (prepared at pH 5.5) and native whey proteins are
compared at a shear rate of 20 s1. Due to increased concentration of