Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 299

Unheated Heat-treated

no XG

0.015
wt% XG

0.025
wt% XG

50 µ m

Figure 7 Semi-thin sections of gels investigated by PCLM after chemical fixation and
embedding. Gels were prepared by acidification of unheated and pre-heated skim
milk containing XG at three concentrations (0, 0.015 and 0.025 wt%). Protein
network microstructure appears as dark grey in the cross-sections of the gels.
(Copyright Nestec Ltd (2006))

series shown in Figure 7, the presence of XG causes a concentration of the
protein particles into protein-rich domains, resulting in more clearly delimited
and massive network-constituting entities following acid-induced aggregation.
The extent to which this occurs depends strongly on the polysaccharide
content. To describe this in a more quantitative manner, image analysis was
performed on the PCLM pictures. The proportion of the two-dimensional
image surface area covered by the aggregates, A aggregates /A image , was determined
to estimate the three-dimensional solution volume fraction effectively occupied
by the particle aggregates constituting the protein network. This surface area
ratio, ultimately representing a measure of the extent of local particle crowding,
was found to decrease with increasing XG concentration, as shown in Figure 8.
Furthermore, this decrease reflects well the phase separation observed at the
natural pH of the mixtures, quantitatively described by the estimated volume
fraction of the XG-rich phase (F PS ) in Figure 5. In unheated skim milk, the
inclusion of 0.015 wt.% XG resulted in a slight decrease in surface area ratio,
indicating rather weak phase separation (low F PS ), whereas incorporation of
0.025 wt.% XG resulted in a pronounced decrease in surface area ratio,
reflecting strong phase separation (high F PS ). In pre-heated skim milk the