Physical Chemistry of Foods

1. Decreasing of particle size. This is often applied, for instance, to
   emulsions by use of a high-pressure homogenizer. In many cases, however, it
   is impossible or insufficient.


2. Lowering the density difference. This is rarely practicable.


3. Increasing the viscosity. This is often done. A problem may be that
   the liquid becomes too viscous for handling. This may be overcome by
   giving the continuous liquid a strongly strain rate–thinning character, as
   illustrated in Figure 13.10, curves 1 and 3. At very small shear stress, as
   relevant for sedimenting small particles (e.g., 0.01 Pa), the apparent viscosity
   then may be high, whereas at higher shear stresses, as prevail during pouring
   of the liquid (e.g., 100 Pa), the viscosity is small. Addition of a little
   polysaccharide of high molar mass is a suitable option. However, it should
   not induce depletion aggregation of the particles; see Section 12.3.3,
   especially the Question at the end.


4. Giving the liquid a yield stresswill immobilize the particles. This is
   discussed below.


5. Mildly agitating the liquid. This is generally applied in storage
   tanks by stirring. A suitable method may be occasional bubbling through of
   large air bubbles, which is often applied in huge milk tanks.


6. Preventing aggregation of the particles. See Chapter 12 for causes
   of aggregation, hence for measures to prevent it. Also coalescence of
   emulsion droplets should be prevented; see Section 13.4.

Immobilization of Particles. If the particles involved are
immobilized,sedimentationcannot occur. Neither can they encounter each
other, whereby also aggregation is prevented. Since sedimentation or
aggregation has generally to occur before fluid particles can coalesce,
immobilization will also preventcoalescence.
Proceeding with the discussion in point 3, above, one may try to give
the liquid not just a strongly strain rate–thinning character, but a yield stress
sy, as depicted in Figure 13.10, curve 2. Some combinations of
polysaccharides give a weak network that imparts a small yield stress at
very low concentration. The stress that a particle causes onto such a
network is given by the gravitational force [Eq. (13.21)] over the cross-
sectional area of the particle, hence

ssed¼

jFBj

ð 1 = 4 Þpd^2

&gjDrjd ð 13 : 28 Þ

To prevent sedimentation,syshould be larger thanssed. To give an example,
ford¼ 10 mm andjDrj¼100 kg?m
^3 ,ssed¼0.01 Pa results. Such a yield
stress would thus be needed to prevent sedimentation. It would be wise to