Computer Simulation of the Pre-Heating, Gelation, and Rheology of Acid Skim Milk Systems - Food Colloids: Self-Assembly and Material Science - page 273

Figure 1 Illustration of different kinds of complexes of casein micelles and whey proteins

formed by heat treatment at different pH values: (a) patchy coating with small
clusters at low pH (6.35), (b) homogeneous coating at intermediate pH (6.55),
and (c) small free clusters in solution at high pH (6.9)

With these calculations we can try to compare with the experimental results
of Vasbinder et al., 6 who hypothesized on the basis of both acidic and renneted
gelation that (i) at low pH (6.35) the whey proteins produce a patchy coating of
the casein micelles, (ii) at intermediate pH (6.55) a homogeneous coating is
formed, and (iii) at high pH (6.9) no coating is formed. These three cases are
illustrated in Figure 1.

18.3.2 Results and Discussion
To keep track of the aggregation dynamics we monitor four descriptive
parameters: the fraction of free monomers, the average cluster size, the fraction
of small particles having a bond with the large particle, and the fraction of
small particles in clusters bound to the large. At the beginning of the heat
treatment procedure some monomers ( 20%) happen to be within the binding

range, and so they immediately form dimers. After this rapid initial drop in free
monomers (Figure 2), there is a slow but steady decrease of free monomers. At
the end still

B

30% are free in solution.
Four curves are shown in Figure 2. The low pH curve corresponds to
parameter values A sl ¼ 25 and p sl ¼ 10 5 . Small particles are trapped within
a potential well of 25k B T when they come close to the surface, but only have a
small probability of actually forming a bond with that surface. The medium pH
curve has parameter values A sl ¼ 25 and p sl ¼ 10 3 , and the high pH system has
A sl ¼ 5 and p sl ¼ 0.1. Also a reference sample is considered with A sl ¼ 0 and
p sl ¼ 0, i.e., with only the repulsive core interaction. These are just the data of a
single set of simulation runs, but they are typical with respect to the slow
decrease of the monomer fraction. There is little significant discrimination
between the pH curves, the differences being within error of reproducibility.
Note that the pH level indicated is actually just a name given to the system
conditions. It remains still to be shown that for the given parameter values the
expected behaviour is obtained.

B