Casein micelles are soft objects that respond to changes in
their environment. In particular, their structure is deformed
by compressive forces such as osmotic stress or the forces
exerted in any concentration or filtration process. We have
shown through SAXS that this deformation is nonaffine,
i.e., some parts of the micelle collapse, whereas other parts
resist deformation. None of the structural models previously
proposed for the casein micelle can account for the deformations
observed through SAXS. However, we show that
a model made of random cells, in which some cells are filled
and incompressible, whereas others are empty and collapse
under pressure, can reproduce this type of deformation. We
call this model the sponge model and we show that the
spongelike micelle has a triple hierarchical structure
(Fig. 6). The lowest level of the structure consists of the
CaP nanoclusters that serve as anchors for the casein molecules.
The intermediate level consists of hard regions
ranging in size from 10 to 40 nm that occupy about half
of the micelle volume at native concentration C ¼ 25 g/L.
Those regions are connected and/or partially merged with
each other, thus forming a continuous and porous material.
Each hard region contains an average of around seven nanoclusters,
with considerable variation as the width of the
distribution of volumes is >50%. The third level of structure
is the casein micelle, which contains ~30 of hard regions,
again with a considerable polydispersity.
In future studies, it would be interesting to examine how
the structure of the micelle is perturbed when both osmotic
pressure and composition of the aqueous phase are changed.
Such experiments would help in identifying the nature of the
interactions that stabilize the micellar edifice and that are
responsible for its peculiar structure.