(“coalesce”) and slowly rise due to their lower density, causing the emulsion to separate out.
In order to form an emulsion that will be stable over a longer time period, molecules that are
tensioactive need to be present in the mixture.
Tensioactive molecules contain both hydrophobic (it does not dissolve in water) and hydrophilic (water
loving) parts, so in an emulsion they tend to locate themselves so that their hydrophobic parts insert
themselves into, or surround, the oil droplets while their hydrophilic parts contact with the water phase,
keep the fat droplets incorporated.
Many different foods contain tensioactive molecules. There are two main sorts of molecules that are
tensioactive and can act as emulsion stabilizers.
Proteins are of two main kinds. Some of them are globular, and others make pu fibrous material. Both
kinds are chains whose subunits are aminoacids. Some of these aminoacids are hydrophilic, and
others are hydrophobic. Denatured proteins can also act as emulsifiers – if a protein is denatured
either by heating, acid or mechanical actions, it will unfold. In its unfolded form, it will expose both its
hydrophilic and hydrophobic groups, which will allow it to stabilize emulsions. An example of such a
protein that is used to stabilize emulsions in the kitchen is gelatine, which is used to stabilize a number
of sauces, most commonly the gravy sauce that most commonly accompanies roast meats.
Phospholipids are molecules that are similar to fat triglyceride molecules, except that one of their
hydrophobic fatty acids is replaced by a charged phosphate group, giving it a hydrophilic region.
Phospholipids are usually found in the form of membranes, and it is these phospholipid membranes
that stabilize the fat droplets in milk and cream. Phospholipids are also found in very high abundance
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in egg yolks, where they are organized into circular structures called micelles. Egg yolks are the most
common source of tensioactive molecules that are used to stabilize kitchen emulsions.