proteins as the surfactant, generallyyis in the range 120 8 to 150 8. The effect
of adding small-molecule surfactants on the aggregation rate, as illustrated
in frame (f), may in part be due to a decrease iny.
- Colloidal repulsion. Strong repulsion between globules will
stabilize them against partial coalescence, although a large shear stress
may overcome the repulsion barrier, as discussed in point 2. Proteins can
provide strong repulsion, although it depends, of course, on the surface load
(illustrated in frame e). Moreover, repulsion will depend on such factors as
pH, ionic strength, and solvent quality. Small-molecule surfactants may
displace proteins from the globules (Section 10.3.3), which can decrease
repulsion. This must be an important part of the explanation for the relation
depicted in frame (f). - Permanence of junctions. Junctions just formed can be broken
again by the shear stress acting on the doublet, thereby effectively decreasing
the capture efficiency. Presumably, the strength of a junction depends on its
diameter, hence on the amount of oil contributing to it. If no oil is present, a
junction cannot be formed, hence almost fully solidified fat globules will not
show partial coalescence. But even at a far smaller fraction solid, it may be
difficult to get oil out of the crystal network. This will largely depend on the
size of the poresin the network, which depends, in turn, on original crystal
size. Considering a fat globule for the moment as a rigid sponge filled with
oil, local removal of oil from the sponge will cause the oil–water interface in
the pores at the outside of the sponge to become curved. This causes a
negative Laplace pressure that will resist the removal of oil, the more so for
smaller pores.
This mechanism may explain the shape of curve 4 in frame (d), which
refers to milk fat, a fat with notoriously small crystals (length often< 1 mm).
At higher fractions solid, the pores will then be smaller and the network
more rigid, preventing sufficient oil from becoming available to make a
strong enough junction. Curve 3 refers to a fat with far larger crystals, and
then the aggregation rate keeps increasing up to high values ofjS.
Kinetics. We have seen now that many variables affect the rate of
the process, but the actual situation is even more complex. In some systems,
subjecting the liquid to a well-defined shear field leads to a gradual increase
of the average particle size (determined after heating to melt the clumps). In
this way, a partial coalescence rate can be unequivocally established. Other
systems may show a very different pattern. The largest globules are
especially prone to partial coalescence, and the resulting aggregates even
more. Large clumps are formed that rapidly cream, and heating the liquid
leads to the formation of an oil layer, whereas the liquid beneath has a
decreased fat content and a decreased average globule size. Moreover, all