Computer Simulation of the Pre-Heating, Gelation, and Rheology of Acid Skim Milk Systems - Food Colloids: Self-Assembly and Material Science - page 274

Figure 2 Fraction of free monomers as a function of time during the heating process. Due
to sample preparation some dimers are very rapidly formed. The irreversible
binding process results in a slow but steady decrease in number of the repulsive
monomers. Curves for different parameter values are identical within reproduc-
ibility
Over the duration of the simulated heat-treatment procedure, the denatu-
rated whey proteins by themselves form small (free) clusters, as can be seen
from the reference sample in Figure 3. Mainly dimers are formed, and a few
larger clusters of five or six particles. The average cluster size in the high pH
sample develops similarly, as can be expected. The repulsive surface prevents
bonds from occurring, and the whey proteins behave as if the casein micelle is
not interacting. At medium pH they do come close to the casein surface, and
when they do they have a high probability of binding. Once they bind, they are
immobilized and cannot form further bonds with other whey proteins at the
surface. This can explain the lower average cluster size found. At low pH the
whey proteins get trapped at the casein surface, but do not bind to it. Instead
they diffuse over the surface, where they have a higher probability of meeting
with other whey proteins than in solution. Hence larger clusters form, mainly at
the surface. At the surface the diffusion process is more efficient in bringing
monomers into proximity than in solution; this is an example of a surface-
enhanced reaction.
Figure 4 shows the number of bonds as a function of time. At medium pH a
sizeable fraction of the whey protein binds to the casein micelle surface. If also
particles not bound directly are taken into account (Figure 5), the fraction
associated with the casein micelle is even slightly larger, but most particles bind
as monomers. The coverage at high pH values is somewhat lower than that at
medium pH, but the difference is quantitative rather than qualitative. Many