Computer Simulation of the Pre-Heating, Gelation, and Rheology of Acid Skim Milk Systems - Food Colloids: Self-Assembly and Material Science - page 285

70

60

low pH

50

medium pH

40

30

20

reference
sample

high pH

10

0

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

Strain

Figure 14 Large-deformation stress-strain behaviour of the samples under slow steady
shearing deformation. At a strain of 0.25 all the systems start to show

rupture of the structure, whether it is percolating or not. Also large clusters do
not relax, but break up into smaller ones. At large strains the stresses are
similar for all the systems

B

present one, but maybe the previous conclusions cannot be extended to these
bidisperse systems. For all the simulations considered here, at the end only
B

20% of all bonds have been broken, with the small particles having around
four bonds with neighbours, and still rather large clusters remaining.

18.6 Comparison with Experiment
Heat treatment is one of the main processing factors influencing the texture of
acid-induced milk gels, 9,10 and it leads to much stiffer gels than without the
treatment. 11 We cannot observe this distinction directly with the present
simulation. This is because the reference sample we use is not a system in
which the aggregation of casein micelles is compared with that of a whey
protein + casein mixture, but rather a system in which no complexation
whatever takes place between the whey proteins and the casein. So, in our
model, the whey proteins are still denaturated, and a whey protein gel will form.
The denaturation of the b-lactoglobulin during pre-heating is the most
important aspect of the pre-heating influencing the gel formation. 5,12 Artificial
mixtures of casein + b-lactoglobulin behave like skim milk. 13 The amount of
whey protein forming complexes with casein during pre-heating decreases with
increasing pH during the pre-heating. Heating above pH ¼ 6.6 leads to partial
coverage. Below pH ¼ 6.6 all the denaturated whey protein is attached to the
casein. At low pH the coating is inhomogeneous. 6,14 We have been able to
reproduce this generic effect in our study by changing only two different
parameters describing the interaction between the whey protein and the casein.