Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 296

5

4

3

2

pH 4.95

pH 5.35

1

0

0 5 10 15 20 25 30 35 40

Time [min]

Figure 4 Onset of gelation in unheated skim milk ( J ) and in pre-heated skim milk ( K )
based on rise of storage modulus G 0 from its baseline

milk pre-heating was also found to have a great impact on the phase separation
of skim milk + XG mixtures. In pre-heated skim milk, macroscopic phase
separation into two liquid layers first became visible at lower polysaccharide
concentration (0.005 wt.%) than in unheated skim milk (0.015 wt.% XG).
Moreover, in the polysaccharide content range investigated, the XG-rich upper
layer was found to be thicker in pre-heated skim milk than in unheated skim
milk. To describe the latter observation in a more quantitative way, the volume
fraction of the XG-rich phase (F PS ) was estimated (see Figure 5).
The effects of skim milk pre-heating on the phase separation behaviour can
be attributed to the change in particle interactions and to the increased volume
fraction of colloidal protein particles. The latter is due to both the increase in
casein micelle size on coating with denatured whey protein 22 and the presence
of soluble whey protein aggregates as new particle species of colloidal dimen-
sions. 23,24 The diameter of the soluble whey protein aggregates formed during
heat treatment of skim milk has been estimated to vary between 410 nm 23 and
B 60 nm, 24 i.e., of the order of the size of the smaller casein micelles.

19.4 Tailoring Gels from Particle Dispersions
Acid-induced gels were prepared with systematic XG level variation in un-
heated and pre-heated skim milk, acidified in the same manner at 401C with 3
wt.% GDL. To compare the gel strength developed, the values of the storage
modulus G 0 were determined from the gelation profiles at the point where the
samples had reached pH 4.60. The data are presented in Figure 6. Some PCLM
images of gels selected according to the rheological behaviour observed in
unheated skim milk samples are shown in Figure 7; the images are for the
reference sample (no added XG), the strongest gel (0.015 wt.% XG) and the
weakest gel (0.025 wt.% XG). Images of the corresponding gel samples made
from pre-heated skim milk, formed at the same XG levels, are presented for
comparison.