Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 295

(a)

6.5

6.0

5.5

5.0

4.5

4.0

0 20 40 60 80 100 120 140

Time (min)

(b)

70

60

50

40

30

20

10

0

0 20 40 60 80 100 120 140

Time (min)

Figure 3 Acid-induced gel formation in unheated skim milk (14 wt.% SMP) at 401C
after addition of 3 wt.% GDL (at time ¼ 0): (a) sample pH; (b) storage
modulus G 0 as determined by oscillatory rheometry (frequency ¼ 0.1 Hz, strain
¼ 0.04%)

formation of aggregates of heat-denatured whey proteins, either remaining in
the serum phase (soluble aggregates) or associated with casein micelles (micelle-
bound aggregates). It has been established 19,20 that both the soluble and the
micelle-bound denatured whey proteins, generally known to become insoluble
on lowering the pH (e.g., to 4.6), play an active role in acid-induced gel
formation, while the native whey proteins present in unheated skim milk
remain soluble during acidification and therefore play no such role.
Pre-heating of skim milk results in a shift of the onset of gelation to
significantly higher pH, as shown in Figure 4. This phenomenon, demonstrated
previously using diffusing-wave spectroscopy, 21 strongly suggests a significant
change in particle interactions due to skim milk pre-heating. Interestingly, skim