Physical Chemistry of Foods

changes can reinforce other instabilities: an increase in particle size leads to
enhanced sedimentation, and coalescence of fluid particles is greatly
enhanced if they are for a long time close together—as in an aggregate or
a sediment. If a remedy has to be found for a change in dispersity, the type
of instability has to be established first; microscopy will often be useful, and
particulars like those given in Table 13.2 will also give clues. A way to
prevent coalescence, aggregation, and sedimentation is arresting particle
motion; this can generally be achieved by giving the continuous phase a
small yield stress. Except for Ostwald ripening and growth from solution, a
change in dispersity nearly always proceeds faster for larger particles.
Aggregationof particles can only occur after they encounter each
other, and this can be due to Brownian motion (perikineticaggregation) or
by a velocity gradient (orthokinetic aggregation). The rates of these
processes can be predicted, especially if the capture efficiency upon
encounter is unity; the process is then called fast aggregation. In practice,
slow aggregation often prevails because of colloidal repulsion and
hydrodynamic interaction. In some cases, the capture efficiency can be
predicted from theory. If the particles are large, say over 1mm, orthokinetic
aggregation tends to be faster than perikinetic, even for the small velocity
gradients that are induced by (small) temperature fluctuations.
The aggregates have an irregular shape (unless the particles
immediately coalesce), andfractal clustersmostly emerge when aggregation
is not disturbed by stirring or sedimentation. Fractal structures are scale-
invariant and are more tenuous if larger. This implies that the joint clusters
will take up an ever greater part of the volume as they increase in size, until a
network spanning the whole volume is formed, resulting in aparticle gel.
The essential parameter governing the properties of the aggregates is the
fractal dimensionality, which is the (average) proportionality factor between
log(number of particles) and log(radius) of the clusters. Its magnitude
ranges between 1.7 and 2.4, depending on several conditions.
Aggregation time, i.e., the time needed for a visible change to occur,
can vary greatly, even for the same aggregation rate. A visible change may
be the emergence of large particles, the formation of a sediment, or gelation.
It is usually one of the latter two, and gelation then tends to give the shorter
time. This is because fractal aggregation needs relatively few aggregation
steps before a gel is formed, as compared to the formation of dense
aggregates. Calculation of aggregation time is difficult, but some trends can
be well predicted.
Sedimentationcan be settling or creaming, depending on the sign of
the density difference between particles and surrounding liquid. Theory for
sedimentation of a single sphere is well developed. The simple Stokes
equation can be used if a number of conditions is fulfilled. The most