Interactions during the Acidification of Native and Heated Milks Studied by Diffusing Wave Spectroscopy - Food Colloids: Self-Assembly and Material Science - page 262

structures in the milk. It is noticeable that the beginning of the increase in
particle size occurs considerably earlier than the 'structure formation' point,
obtained by extrapolation to zero of the linear portion of the apparent radius
versus pH curve. At the same time as the apparent radius of the particles
decreases, the value of 1/l* increases slightly, and it comes to a plateau at about
the point where the radius has flattened off.
These results can be generally explained by changes in the casein micellar
structure and properties. The decrease in apparent radius is assumed to result
from the collapse of the hairy layer on the casein micellar surface; this has been
described from both theory and experiment. 19,20 The extent of the decrease in
radius (12 nm) is, however, somewhat larger than might be expected from other
estimates of the thickness of the hairy layer. We believe that the concomitant
small increase in the value of 1/l* is a result of the collapse of the hairy layer on
to the particle surface. This slightly increases the refractive index of the micellar
core and changes the value of F(q). It can be calculated that only a very small
change in refractive index is needed to change the value of l* by the observed
amount. Unfortunately, more detailed calculation of the optical properties of
the micelles is not possible without knowing some details of the structure of the
hairy layer. Removal of the hairy layer by the action of chymosin does not
cause a change in the value of 1/l*, 21 and so it is evident that the core of the
casein micelle does the bulk of the scattering of light, and the hairy layer, being
diffuse, does not play a significant part in the particle scattering, although it is
important in the hydrodynamic behaviour of the casein micelles.
During the next stage of the reaction as the pH is lowered below 5.3, the
apparent radius of the casein micelles increases slowly (compared with the later
structure-forming stage). At the same time the value of 1/l* increases consid-
erably. It is possible to interpret the change in apparent radius in two ways:
either there is genuine aggregation of particles, but to a low level, or the changes
in 1/l* and apparent radius arise from a progressive change in the relative
positions of the casein micelles as the inter-particle steric/electrostatic repulsive
forces diminish, attractive forces begin to become important, and the motions of
the particles and their orientations with respect to one another change (i.e., the
S(q) factor in Equation (3) begins to increase). At the end of the increase in 1/l*,
the apparent radius of the particles is approximately doubled. During this
period, the slope of the MSD remains close to unity, indicating that the particles
are freely diffusing. It is possible that this stage in the reaction may represent the
establishment of equilibrium between aggregated and non-aggregated micelles. 6
At a pH between 5.0 and 4.9, the apparent radius of the particles begins to
increase very rapidly, because the particles become highly aggregated. At this
stage the Stokes Einstein relation (Equation (2)) cannot be regarded as valid;
the change in apparent radius simply demonstrates the very rapid change in the
aggregation state of the milk. The slope of the MSD at around this time begins to
decrease, although it seems that aggregation can proceed for some time before
the change in MSD is apparent. However, at pH o 4.9, the slope of the MSD
decreases rapidly. These DWS measurements suggest that the milk is forming a
gel, and this is confirmed by the rheological measurements. At pH ¼ 4.9 we have