Interactions during the Acidification of Native and Heated Milks Studied by Diffusing Wave Spectroscopy - Food Colloids: Self-Assembly and Material Science - page 266

complexes themselves, only their effect. In this view, the WP- k -cas complexes
serve at the same time to hold the micelles together and also to force them to
remain sufficiently far apart so that there is no direct interaction between the
micelles leading to gelation, and so S(q) will change only a little. None of this
can happen if the milk is not heat-treated, because in unheated milk there are
no intermediary particles to form an initial gel network.
As the pH of the heated milk continues to fall below the point where this
proposed network is formed, the charges on the casein micelles decrease to
a point where there begins aggregation of the micelles themselves and with it
the formation of a more rigid gel network. It is at this stage that the gela-
tion process begins to be detected by the rheometer. This implies that the
original weak gel would be kept in its state not simply by the interactions
between caseins and WP particles, but possibly by some steric or charge
repulsions on the micelles that diminish as pH is lowered. That is, the chains
of WP may no longer be able to keep the casein micelles apart, and therefore
there occurs more general aggregation, and the formation of a network of
casein micelles, aided by the WP complexes on their surfaces. Basically, the
original 'gel' is presumably formed between specific sites on the casein micellar
surface (where there are bound WP- k -cas complexes) and the WP- k -cas
complexes in the serum. The subsequently formed gel contains these sites,
but in addition it has a general interaction between the surfaces of the casein
micelles. That is, the final strong gel is produced by the partial collapse of the
first weak one.
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