Interactions during the Acidification of Native and Heated Milks Studied by Diffusing Wave Spectroscopy - Food Colloids: Self-Assembly and Material Science - page 258

system, although some reasons why this may not be so have also been
proposed. 9
The acidification of milk becomes much more difficult to describe if the
milk is heated before the acidification step. Here, the acid gelation is known
to occur at a higher pH than in unheated milk and the gel strength is
greatly increased. 7 These effects have been attributed to the incorporation of
denatured whey proteins (WP) into the acid gel, but the increase in strength is
much greater than would be accounted for by simply incorporating about 20%
more protein. The heating of milk at temperatures in excess of about 751C
causes the denaturation of the WP, which then form complexes with the
k -casein ( k -cas) of the casein micelles. 10,11 These WP- k -cas complexes are
distributed between the serum of the milk and the surfaces of the casein
micelles, and the distribution depends on the pH at which the milk is heated.
At the normal pH of milk, the particles are partly on the micellar surface and
mainly free in the serum; the free particles have diameters in the region of 50
nm, 12 and so they are much smaller than the approximately 200 nm diameter of
the casein micelles. The result of these changes in the particulate structure of the
milk is that the apparently well-behaved system (in colloid chemistry terms) of
unheated milk becomes considerably more complicated. The casein micelles
now have modified surfaces, partly because they have lost some of their
k -casein to form the soluble WP- k -cas particles, and partly because there are
some regions of denatured WP on their surfaces. In addition, the serum
contains a new population of colloidal particles - the WP- k -cas complexes.
The detailed behaviour and interactions of these two kinds of particles during
acidification are essentially unknown, although it has been demonstrated that
the extent of solubilization of the complexes affects the gelation. 13-15 A
simplistic view is that the small complexes begin to aggregate into chains at
pH

5.3 because they have a higher isoelectric point than the caseins. These
aggregates then interact with the modified casein micelles to give the acid gel,
and they may be imagined as providing stronger inter-micellar links than those
formed in unheated milk.
The research described in this paper was designed to try to elucidate some of
the changes that occur in unheated and heated milks during acidification. For
this study, we have used mainly the technique of diffusing wave spectroscopy
(DWS), which is capable of measuring the scattering of light in turbid suspen-
sions such as milk. This technique was combined with measures of the bulk
rheological behaviour of the different milks.

B

17.2 Diffusing Wave Spectroscopy
The DWS technique has been extensively described previously, 16 and only basic
details will be given here. When light is passed into a turbid suspension, which
is sufficiently concentrated that every photon is extensively multiply scattered,
the paths of the photons may be considered to be random walks, and it is
possible to measure a correlation function from the scattered transmitted light.