Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 293

1 2 3

4 5 6

7 8 9

50 µ m

Figure 1 Phase separation observed by CLSM at 401C in unheated skim milk (14 wt.%
SMP) containing 0.02 wt.% XG (pH 6.5). Image 1 was recorded at 3.5 min
after the end of mixing. The set of images 2-9 depicts the evolution of the
microstructure over intervals of 5 min. Dark spots represent areas rich in
polysaccharide and deficient in protein.
(Copyright Nestec Ltd (2006))

than in the bulk, resulting in an osmotic pressure gradient. For a single sphere
this pressure is isotropic. As the particles approach due to Brownian motion, the
depletion layers overlap, with the result that there is a volume of solution
between the particles from which polymer molecules are excluded. As a conse-
quence, the osmotic pressure becomes anisotropic, resulting in a net osmotic
force pushing the particles together, as indicated by the arrows in Figure 2.
Therefore, addition of a non-adsorbing polymer to a dispersion of (spherical)
colloidal particles induces an effective attraction between the particles, resulting
in phase separation at sufficiently high polymer concentration. 9-11
At later stages of the phase separation process, macroscopic demixing into two
liquid layers became visible. After incubation for two days, we observed a slightly
turbid top layer (rich in XG, deficient in protein) and a very turbid bottom layer
(rich in protein, deficient in XG), with a sharp boundary between them.

19.3.2 Acid-Induced Aggregation and Gel Formation
Slow acidification of skimmilk is known to result in gel formation, 12, 13 involving
aggregation of the destabilized casein micelles into a three-dimensional network