What do chocolate, mayonnaise, salad dressing, milk, butter and
ice cream have in common? All of them are emulsions!

The naked eye can only see a homogeneous product. However, this
is not the case under the microscope, where thousands of small
droplets dispersed in a second liquid substance can be seen. In
each case, two substances that are normally immiscible, oil and
water, have been mixed using an emulsifying agent.

An emulsion can also present the above-mentioned products in a
different way. For example, a liquid can trap air bubbles and turn it
into a foam. In molecular gastronomy, emulsification is the technique
used to incorporate and stabilize air bubbles in a liquid mixture.
It is possible to incorporate air bubble into a liquid simply by
whisking vigorously. However, this phase is highly unstable and
the air escapes in a relatively short time. To avoid this instability,
an emulsifier can be incorporated into the solution.

Egg and milk protein, bread starch, gelatin and cream fat are
common emulsifiers that have used in traditional cuisine for a
long time. However, in recent decades, the food industry intensified
its research in this field and discovered new emulsifiers such as
soy lecithin and methylcellulose. These products are also called
surfactants, a word derived from “surface active agents,” since
their molecules act as a barrier (interface) between water and air.

These additives bring great pleasure to molecular gastronomy
enthusiasts by reducing the tension between the water and air
surface, which stabilizes the air and foam. To better understand
the forces at work, let’s take a closer look at what happens inside
air that is stabilized using soy lecithin.



The secret of a successful foam is the amount of air bubbles
dispersed in the liquid. In fact, a foam containing a larger number
of small air bubbles is generally more stable than one formed using
a small number of large bubbles.

So foam made with a whisk does not last as long as one made
using a hand blender, since this powerful machine’s cuts and recuts
more air bubbles. A large amount of bubbles dispersed in the
liquid also increases the viscosity of solutions, which gives foams
their creaminess.

Although their viscosity increases the stability, foam and air
remain relatively unstable; air bubbles gradually escape the liquid
in which they were incorporated. Three main causes accelerate
this phenomenon. First, the air can easily dissolve in liquids and
evaporate. Next, the internal pressure of very tiny bubbles increases
as their size decreases, eventually causing their membranes to
burst. Finally, as there is a significant difference between the
density of fluid and air, the two phases tend to separate, and liquid
will gradually migrate to the bottom of the dish.

Finally, even when made using translucent ingredients, foams are
always opaque. This unusual fact is explained by the many angles of
light reflected on the walls of the air bubble. The light that is diffused
in all directions thus creates the opacity and a certain brightness,
depending on the color of the initial ingredients.