Computer Simulation of the Pre-Heating, Gelation, and Rheology of Acid Skim Milk Systems - Food Colloids: Self-Assembly and Material Science - page 276

70

60

medium pH

50

high pH

40

30

low pH

20

10

0

0 2.5 5 7.5 10 12.5 15

Time

Figure 5 Number of small particles bound in clusters to the large particle surface.
Comparison with Figure 4 shows that, at high pH and medium pH, most
particles bind as monomers. There are few bonds at low pH, but many particles
are part of clusters bound to the surface with one or two links

somewhat vague notion, whereas in the simulation it can easily be checked
whether particles are within the binding distance. At medium pH the binding
probability is two orders of magnitude smaller, while still a higher surface
coverage is observed. The low pH system shows qualitatively different behav-
iour. The rate at which bonds are formed is very much lower than that for
either of the other cases, but the rate at which particles collect near the surface
is more or less the same, as governed by the diffusion towards the casein micelle
surface. Once they are trapped - now because of the deep potential well rather
than the binding - a depletion zone develops and the rate slows down. 7 After a
while the particles trapped near the surface start forming clusters, which show
up in the low pH curve of Figure 5. Eventually for this system the surface
coverage obtained is similar, but the small particles are mainly in clusters of
average size 4. The coverage is therefore much less homogeneous. In all cases

the major fraction of the whey protein is not part of this coating layer, but is
free in solution, mainly in the form of dimers. Figure 6 shows a snapshot of the
simulated low pH system at the end of the heat-treatment phase.
We have performed some simulations for sets of systems with parameter
values in the ranges 5 r A sl r 25 and 0 r p sl r 1, and we have determined
what type of complex was established at the end of the heat-treatment process.
The results are collected in Table 1. For a binding probability of zero, obviously
no whey proteins can bind to the casein micelle surface. An almost bare surface,
with free whey protein clusters in solution, is also found for a slightly repulsive
or neutral surface and not too high binding probability. For high binding
probabilities many small particles are bound to the surface, most of them
directly, forming a more-or-less homogeneous coating of the large particle. The

B