Computer Simulation of the Pre-Heating, Gelation, and Rheology of Acid Skim Milk Systems - Food Colloids: Self-Assembly and Material Science - page 284

70

low pH

60

medium pH

50

40

30

high pH

reference
sample

20

10

0

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

Strain

Figure 13 Small-deformation stress-strain behaviour of the simulated systems under slow
steady shearing deformation. The low pH and medium pH systems have a
somewhat higher modulus than the high pH system and the untreated sample.
The linear regime is very short, with weakening apparent above a strain of 0.005

Figure 13 indicates that small-deformation behaviour of all the systems is
more-or-less the same. The linear deformation regime is very short, and at a
strain of 0.005 all the systems show weakening. The explanation for the weak-
ening is that, even at the low temperatures and following the equilibration
procedure, some bonds already break at the very small overall deformation, and
the bonds keep breaking due to the deformation, be it rather slowly. The low and
medium pH systems appear to have a somewhat higher modulus than the high
pH and reference systems, and they remain stiffer in the non-linear regime.
The stress reaches a maximum at a strain of about 0.25 (Figure 14). Whether
rupture indeed occurs and actual yielding takes place is not observed directly.
Only two of the systems are percolating, and so in principle could show a
yielding transition. No qualitative difference between the samples is observed;
the quantitative differences are small but significant. It appears that the number
of whey particles bonded to the casein micelles correlates positively with the
observed stiffness of the samples. There is no clear immediate explanation for
that. Maybe the affine deformation we use results in added stresses in the
coating of the casein micelles, which does not relax as quickly as that in clusters
of whey proteins because of the large size of the micelle. If so, that would be a
simulation artefact. In a real system the stress generated by external deforma-
tion forces is transmitted to the sample through either the particle network or
the interstitial fluid. Neither would be expected to result in added stresses at the
surfaces of the casein micelles. In our earlier studies of large-deformation shear
rheology, 8 we investigated the difference between methods with affine defor-
mation, which is mainly a simulation technique, and deformation through
stresses in the network, which is what is expected in these type of particle gels at
low shear-rate. There were no large differences found for rates similar to the