can lead to the formation of bubbles or drops—and from the continuous
phase to these particles—whereby they can be disrupted into smaller ones. It
is by far the most common method for making food emulsions and foams.
Since all forces have to be transferred via the continuous phase, it is
inevitable that a large proportion of the mechanical energy applied is
dissipated, i.e., converted into heat.
A special type of agitation is due tocavitation. This is the formation of
vapor cavities in a liquid by local negative pressures, and the subsequent
collapse of these cavities. The latter phenomenon generates shock waves
that can disrupt nearby particles. Cavitation can be induced by ultrasonic
waves, and ultrasound generators are useful for making emulsions in small
quantities.
- Chemical energy.Some surfactant mixtures, during gentle mixing
of the emulsion ingredients, can produce instability of an oil–water
interface. Instability means that capillary waves form spontaneously, which
can lead to droplet formation by a kind of ‘‘budding.’’ To the author’s
knowledge this is not applied in food manufacture: the surfactant
concentrations needed are too high to be acceptable in a food.
For the technologist, the prime question when making a foam or an
emulsion concerns the physicochemical properties of the product made. This
mainly concerns
Volume fraction of dispersed phase j, since it determines some
essential properties, such as rheological ones. For an emulsion, the
value desired can mostly be predetermined by the proportions of
oil and water in the recipe. This is often different for foams,
especially when made by beating: the value ofjobtained then
depends on several conditions (see the next section). In foams
one often speaks of the overrun, i.e., the percentage increase in
volume due to incorporation of gas. The relation is percentage
overrun¼ 100 j/(1j).
Particle size distribution,f(d); see Section 9.3. It is of considerable
importance for the physical stability of the system. Generally, the
smaller the droplets or bubbles, the more stable the system.
Surface layerof the particles, i.e., thickness and composition. Again,
this greatly affects the stability.
Emulsion type, i.e., O–W or W–O. This determines several properties:
see Section 9.1.
Moreover, theeconomyof the process is of importance; thus may
include the amount of foam obtained (hence the overrun), the amount of
surfactant needed (related to the specific surface areaA¼ 6 j=d 32 ), and the
amount of energy needed.