3.3.1. Particle size and microstructure
Incubation of the MWP suspensions in different pH solutions (pH 2
to 8) led to some changes in their particle size distributions (data not
shown). For the non-homogenized systems, most MWP suspensions
had a single population of large particles, but a population of smaller
particles was observed at certain pH values, suggesting some breakdown
of the larger particles (see later). For the homogenized systems,
broad multimodal distributions were observed at all pH values with
particles ranging in diameter from around 30 nm to 30 μm. The mean
particle diameter (d32 or d43) for the homogenized MWP suspensions
had some changes over the pH range (Fig. 6). This type of behavior is
found to be quite different to that of protein-coated lipid droplets
(Demetriades & McClements, 1998), where extensive particle aggregation
occurs around the isoelectric point of the proteins (pI ≈ 5). This
difference could be due to the fact that theMWP ingredient had undergone
pre-treatment steps that would have been expected to denature
the globular proteins within them (e.g., heating, shearing, and extrusion),
whereas theWPI used to prepare the emulsions had not. Confocal
microscopy images showed that large aggregates were present in the
non-homogenized systems at all pH values, while an even distribution
of small protein particles was present in the homogenized systems at
all pH values (Fig. 7). At pH 4 (data not shown) and pH 5, the continuous
phase surrounding the protein particles was dark in color suggesting
that it was devoid of protein. On the other hand, at pH values
below 3 and above 6, the continuous phase became greenish in both
systems suggesting that some protein molecules may have dissociated
from the protein particles at pH extremes (i.e., pH 2 and 8). This effect
can be attributed to reduced electrostatic attraction and increased electrostatic
repulsion between protein molecules at pH values far fromthe
isoelectric point.