Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 300

50

40

30

20

10

0

0.000 0.005 0.010 0.015 0.020 0.025

conc. Xanthan [%, w/w]

Figure 8 Ratio of the image surface area taken by the aggregates (A aggregates ) and the
total image surface area (A image ) determined from PCLM micrographs of gels
prepared from unheated skim milk ( J ) and pre-heated skim milk ( K )
containing XG at various levels (0, 0.015 and 0.025 wt.%). Points represent
mean values calculated from the 20 to 30 images that were analysed for each gel
sample. Error bars represent the least significant difference (LSD)

surface area ratio was found to decrease almost linearly with increasing XG
level, with phase separation (and a linear increase in F PS ) observed over almost
the entire range of polysaccharide contents investigated.
The observation that, at 0.025 wt.% XG, phase separation appears stronger
in pre-heated skim milk (higher F PS , Figure 5), while the extent of crowding of
protein particles is lower (higher A aggregates /A image , Figure 8) and the connec-
tivity between the regions of gel is higher, seems contradictory at first. How-
ever, the change in particle interactions due to skim milk pre-heating makes the
onset of gelation occur at much higher pH, trapping the evolving microstruc-
ture at an earlier stage of the phase separation process. For the given acidifi-
cation conditions (401C, 3 wt.% GDL), the onset of gelation in the unheated
skim milk took place at pH 4.94-5.06, while it occurred significantly earlier in
the pre-heated skim milk, namely when the pH had just dropped into the range
5.33-5.38. Hence, although phase separation at a given polysaccharide content
is stronger in pre-heated skim milk, the acid-induced gel formation results in
fixation of the phase-separating mixture at a stage at which the resulting gel
strands are still well connected to each other and thereby integrated into the
force-transmitting network through linkage to their neighbours.

19.5 Conclusions
We have demonstrated the importance of phase separation to the process of gel
formation in acidified dairy products with XG added as a supposed 'thickener/
stabilizer'. The gel microstructure can be designed by triggered arrest of the
phase-separating structure through acid-induced particle aggregation and