determine full size distributions. The largest drops in an emulsion are the
most susceptible to coalescence, as concluded above. This implies that
coalescence tends to increase the width of the size distribution; it may even
become bimodal.
Note A change in particle size of an emulsion may also be due to
aggregation of the droplets, and this should, of course, be checked,
e.g., by microscopic observation.
Finally, a word aboutextreme conditions. Although O–W emulsions
made with protein tend to be very stable, they may show marked
coalescence at conditions where the droplets are forced close to each other.
This can happen in a centrifuge, especially at high acceleration; by agitating
an emulsion of highjvalue; by freezing an emulsion, where ice crystals
press droplets together; or by drying an emulsion.
Water-in-Oil Emulsions. It is difficult to stabilize these emulsions
against coalescence, since suitable food grade surfactants are not available.
Suitable means sufficiently hydrophobic and providing strong repulsion
across an oil film; highly unsaturated monoglycerides provide some stability.
In fact, W–O emulsions are very rare in foods. Butter and margarine are not
simple emulsions, since the continuous oil phase contains triglyceride
crystals. These can providePickering stabilizationas depicted in Figure
13.18. The requirements are (a) that the solid particles have such a contact
angle with water and oil that they adhere to the aqueous drops but stick out
in the oil (see Section 10.6); (b) that the particles are not very small, say
larger than 20 nm [see Eq. (10.14) for the reason]; and (c) that the particles
are densely packed at the droplet surface, since otherwise bridging of drops
can occur, or even coalescence.
FIGURE13.18 Pickering stabilization. Small solid particles, preferably wetted by
the continuous phase, adsorbed onto emulsion drops.