Physical Chemistry of Foods

important one is that the liquid shows Newtonian behavior. Many liquids
(e.g., most polymer solutions) are markedly strain rate–thinning, and the
shear stress on a sedimenting particle then is (far) smaller than the stress
prevailing in most viscometers. The sedimentation rate as predicted from the
determined apparent viscosity then is greatly overestimated. Sedimentation
in a whole dispersion is more complex. At high volume fractions,
sedimentation is greatly slowed down. Polydispersity causes the sedimenta-
tion profile (particle concentration as a function of height) to evolve in an
intricate manner, especially when aggregation of particles occurs; sedimen-
tation rate then is strongly enhanced.
The critical event incoalescenceis the rupture of the film between close
fluid particles. A hole can spontaneously form in a film, but in the presence
of some surfactant this can only happen if the film is very thin (a few nm).
Local thinning of a film may occur because of the development of capillary
waves on the film surfaces. These waves are readily damped if the interfacial
tension is large, the repulsion between the particles extends over a large
distance, and the film is small. This then implies that protein is a very good
stabilizer against film rupture, far more so than many small-molecule
amphiphiles.
For coalescence ofemulsion droplets, an important variable is whether
a flattened film between the droplets is formed. This is governed by the ratio
of the external stress over the Laplace pressure. The external stress can be
due to colloidal attraction (e.g., van der Waals forces), a shear stress, or
gravitational forces in a sediment layer. Small protein-stabilized droplets
will not deform, except in a sediment layer in a centrifuge, and they are very
stable to coalescence. If the drops are large, the interfacial tension is low,
and the external stress is high, droplets will deform and coalescence can
readily occur. Water-in-oil emulsions cannot be made with protein as the
surfactant, and it is often difficult to stabilize them against coalescence,
except by a layer of small hydrophobic particles (Pickering stabilization).
Mostfoamsare far less stable to coalescence than most emulsions.
This is because foam bubbles are large. This means that (a) the number of
bubbles is small, implying that relatively few coalescence events are needed
to produce a very coarse foam; and (b) the films are large and flat and
thereby relatively unstable to rupture. However, in most foods it takes a
while for films to drain to a thickness allowing rupture. Fairly thick films
can also rupture, but that is due to the presence of (extraneous) small
hydrophobic particles; some mechanisms have been proposed to explain the
film rupture.
Fat globules, i.e., oil droplets that contain fat crystals, can be subject
topartial coalescenceor clumping. A crystal may protrude somewhat from
the globule surface and can pierce the film between two approaching