Computer Simulation of the Pre-Heating, Gelation, and Rheology of Acid Skim Milk Systems - Food Colloids: Self-Assembly and Material Science - page 279

1200

1000 medium pH

800

600

low pH

400

200

high pH

0

0 2 4 6 8 10 12 14

Time

Figure 7 Number of whey proteins bound to casein micelles as a function of time during
the heating process at different pH values. Of the 5400 small particles, only 2%
get bound at high pH, 20% at medium pH, and 7% at low pH

pre-heating phase are in unit binding probabilities for all the particles (p sl ¼ p ss
¼ p ll ¼ 1) and all the physical interactions being set to zero (A sl ¼ A ss ¼ A ll ¼ 0).
When irreversible bonds are formed readily, the clusters start growing.
Because of the high value of the bonding probability, the aggregation process
is largely diffusion dominated, leading to very open structures, with low fractal
dimensionalities. In the case of a binary gel, however, such a structural
parameter is of much less interest than in a monodisperse system. Locally the
structure of the whey protein gel strands may be of importance, since the gel in
all cases is formed by such strands forming connections between the casein
micelles. In that sense we have a hybrid gel, with the whey proteins forming
percolating clusters in the gaps between the large casein micelles, which act
essentially as spacer particles. In terms of the mass, the casein micelles dom-
inate the system, and hence also the mass autocorrelation function, which is the
key ingredient from which the fractal dimensionality is derived.
The monitored structural parameters include the number of bonds between
the particles, the number of particles in the largest cluster, and the average
number of particles per cluster. The number of bonds between the small and
large particles is plotted in Figure 8. There appears to be a distinct difference
between the systems at low and medium pH on the one hand, and the high pH
and untreated systems on the other. For the first two the average number of
whey proteins directly bound to a casein micelle is around 60, whereas for the
last two it is around 40; these values represent surface coverages of B 20% and

B 12%, respectively.

The situation for the number of bonds involving only whey proteins is just
the opposite: the average number of bonds per whey protein shows the same
quantitative dynamics for all four systems. Whey proteins bound to micelles are
immobilized and do not form bonds with other particles unless they are