Physical Chemistry of Foods

crystals, may give a kind of Pickering stabilization. A third reason is
discussed below.

Compound Droplets. If the droplets contain various components
that differ significantly in solubility, Ostwald ripening is slowed down. The
reason is that a less soluble compound will leave a small droplet at a slower
rate than a more soluble one, which implies that the concentration, and
hence the chemical potential, of the less soluble compound in the drop
increases. This produces a driving force for the more soluble compound to
diffuse back to the small droplet. This would play a part in slowing down
Ostwald ripening in the essential oil emulsions mentioned.
Ostwald ripening will even stop if the droplet contains a solute that is
fully insoluble in the continuous phase, provided that its concentration
exceeds a critical magnitude. This can occur for droplets of an NaCl
solution in oil. It is simplest to consider the osmotic pressure of the salt
solution, given by

Posm¼mRT¼m 0

a 0

a

 3

RT ð 13 : 36 Þ

assuming ideal behavior. Heremis the osmolarity of the solution in the
drop, and the subscript 0 refers to the initial situation. The drop will not
shrink if the change in Laplace pressure due to a change in radius is equal to
that in osmotic pressure (not opposite and equal, since osmotic pressure is in
fact a negative pressure). After differentiation ofpLandPosmwith respect to
a, the following condition results;

m>

2 g

3 aRT

ð 13 : 37 Þ

Taking as an example a droplet of radius 0.1mm andg¼0.01 N?m
^1 , the
minimum osmolarity should be at least 27 mol?m
^3. For the almost fully
dissociated NaCl this corresponds to less than 0.1% NaCl. The water
droplets in butter and margarine always contain sufficient salt fully to
prevent Ostwald ripening.
Altogether, Ostwald ripening is rarely a problem in food emulsions,
apart from soft drinks containing essential oils.

13.7 RECAPITULATION

Various types of instability can cause changes in dispersity, i.e., in size or in
arrangement of the particles and often in both. These types are illustrated in
Figure 13.1, and some particulars are summarized in Table 13.2. Some