Computer Simulation of the Pre-Heating, Gelation, and Rheology of Acid Skim Milk Systems - Food Colloids: Self-Assembly and Material Science - page 277

Figure 6 Snapshot of a system configuration at the end of the simulated heat treatment.
About 20% of the whey proteins form a patchy covering of the casein micelle
surface. Light small particles are towards the front of the box, and dark ones
towards the back
actual surface coverage depends on the parameters, but even for the most
extreme case considered only 25% is covered, with 65% of the small particles
forming free clusters or monomers in solution. When a very low binding
probability is combined with a moderate or high surface attraction, small
particles form aggregates which are bound to the surface by only a few bonds.
They form patches at the micelle surface; and if they would not be held close to
that surface by the reversible attraction, only a very small fraction of the
surface would be covered. We conclude that, by changing two different
parameters describing the complexation process, corresponding to the physical
and chemical binding (in an opposite way with pH), it is possible to have the
same type of complexing as hypothesized on the basis of experimental obser-
vations. 6 Table 1 shows that indeed both the parameters are needed, but that
just two can suffice. We note that a small change in pH seems to be related to a
large change in the parameter values, a situation which is not unexpected. In
reality, of course, many different kinds of interactions within the system will
change with acidity to varying extents.
In the full simulations discussed below, there are 27 large particles of radius
R l ¼ 8 and 5400 small particles of size R s ¼ 1, the same size ratio as for the pre-
processing. Rather than just making 27 identical copies of the small system, a
single large system is initialized. The only difference is that the large particles
may diffuse, but in practice they move only a little. The system is contained in a
cubic box of size 90 90 90 units with periodic boundary conditions. This
corresponds to volume fractions of 7.9% and 3.1% for the casein and whey