Sauces Thickened with Droplets: The principles of Emulsions

Emulsion under the microscope
On a previous post on the principles of sauce consistency, I gave a general description of the different thickening agents. In my next few posts I am going to look into the different thickening methods: what they are, how they work and give you the general guide lines for a successful realisation of sauces using this particular thickening method.
So to start lets have a look at sauces thickened by droplets of oil or water. One very particular method to thickened sauces is to fill the water based liquid with droplets of oil. This droplets of fat are much bigger and slow moving than molecules of water. The result of this dispersion of one liquid in another, is a thick and creamy mixture called an emulsion*.
An emulsion can only be made with two liquids that don't dissolve in each other, like the oil and the vinegar in a vinaigrette. These two liquids can be seen as the container and the contained. The container is called: a continuous phase and the contained one is called the dispersed phase. So an emulsion is the dispersion of a dispersed phase in a continuous phase of water (mayonnaise) or droplets of water in a continuous of oil (vinaigrette).
As you all know it is hard work to make an emulsion. The reason that justifies these frustrating minutes of whisking, is: when two liquids, that don't mix for chemical reason, are disturbed (whisking) and let to stand, they spontaneously take a configuration that minimize their contact with each other: they coalesce. They form two separate large masses, to expose less surface area to each other (see Picture). This behaviour is the expression of the force called: surface tension.
So, to make a emulsioned sauce the cook must pour a lot of energy to break this surface tension and create the droplets. The size of the droplets has a big influence on the stability of the emulsion: the smaller the droplet the more stable the sauce will be.
There are two main factors that make it easier for the cook to generate small droplets. The first one, is the thickness of the continuous phase, which drag harder on the droplets and transfers more shearing force to them from the whisk.

There is a little experiment that demonstrates this principle:
In a bottle put some water (about 1/2 pt) and some oil (about 2 tablespoons) (fig. 1), give it a good shake (fig. 2) the oil droplets are coarse and quickly coalesce (fig. 3). In another bottle put some oil but in the reverse proportions, 1/2 pt of oil and 2 tablespoons of water (fig. 4) and shake a little (fig. 5). The water breaks into a persistent cloud of droplets (fig. 6).

Fig. 1 Fig. 2 Fig. 3
Fig. 4 Fig. 5 Fig. 6
In conclusion it helps to start with as viscous a part of the continuous phase as possible and dilute in it, any of the other ingredients required in the recipe, after the emulsion has been established.

The second factor that helps the chef to form small droplets is the presence of an emulsifying agent.

An emulsifier is a molecule that lowers the surface tension of a dispersed liquid in an another. The way that emulsifiers work is that they create a coating on the droplets that shields them from the continuous phase. Such molecules are partly soluble in each of the different antagonists in an emulsion: one part of the molecule is water soluble and the other part soluble in fats. It is said that they have a hydrophobe (afraid of water) side and a hydrophile (that likes water) side.
They are two kinds of emulsifying molecules:
  • The first ones are, like the phospholipid lecithin in the egg, are small molecule that have an hydrophobe side that attach itself in the fat phase and an hydrophile head that is electrically attracted to the water molecules of the emulsion.
  • The second one are proteins which are much larger chains of amino-acids that have a number of hydrophobe and hydrophyle regions. The yolk proteins in the eggs and the casein in dairies are the best protein emulsifying agent.
Another type of molecules that can help in the stabilization of an emulsion are: the stabilizers. They are starches and plant particles. There are many kinds of particles and molecules that can help to stabilize an emulsioned sauce once its formed. They all have in common the property of getting in the way, so that the droplets that are getting towards each others meet the stabilizer instead. Large molecules like proteins or starches are very efficient in this role, pectins, gums and particles of pulverized plants too. Ground, white mustard seeds are a very efficient stabilizer thanks to its particles and gums that are released when wetted. Tomato paste too, is a really interesting stabilizer and emulsifier with its large protein content (3% !) as well as its cell particles. There is a but, though, once the emulsion has formed, the droplets may be so crowded that they bump against each other, then the force of the surface tension may pull them together and cause them to coalesce again.

Now, after the theory the practical stuff. Always keep in mind that emulsion sauces are very fickle ones. There is always a risk that the sauce splits. Three reasons for that:
the cook has added the liquid too quickly to the continuous phase, he might have added to much of the dispersed phase or he has allowed the sauce to get either too hot or too cold.
There are the basic rules that will ensure a perfect result, without the risk of the sauce separating along the way:
  • The first ingredients in the bowl must be the continuous phase and at least one emulsifying or stabilizing element. The dispersed phase must, ALWAYS, be added to the continuous phase. Otherwise it cannot be dispersed.
  • The dispersed phase must be added to the continuous phase very gradually, to begin with. Then, it requires an energetic and continuous whisking all the way through the making of the sauce. ONLY, when the sauce has started to thicken and become more viscous that the oil be added more rapidly.
  • Finally, the proportions of the two phases must be kept in balance. The right proportion is that the dispersed phase should not exceed three times the volume of the continuous phase. If the emulsion stats looking stiff it is the sign that it is time to add more continuous phase to the sauce.

When the sauce is finished, it should not be stored at a temperature that is either too hot nor too cold. It should not be kept at a temperature exceeding 60 degrees Celsius. They should not be stored at a low temperature either, under 15 degrees Celsius the surface tension increases making it more likely to coalesce. Butterfat and some oils, solidify at room temperature or in the fridge. This results in sharp-edged fat crystals rupturing the layer of emulsifier on the droplets. Then the sauce will coalesce and separate when stirred or warmed.

In the event of the sauce separating, there are two ways to reemulsify it.

  • The first one is to put the sauce in a blender to break the dispersed phase apart again. This has its limitation. It will only work if the sauce still has enough emulsifier left intact. It won't work in the event of the sauce being overheated. Especially, sauces like bearnaise or hollandaise that contains eggs, their proteins may have been cooked thus destroying their emulsifying properties.
  • The second one and more reliable one, is to start with a small amount of the continuous phase, adding in an egg yolk (optional), and carefully beat the broken sauce back into it. If the proteins in the sauce have coagulated (cooked) they must be strained out of the sauce first, then it is a good idea to add an egg yolk at the beginning of the process.
*Emulsion: is a word that comes from the Latin for "milk out" and refers to the milky liquid that can be squeezed out of some nuts or plant tissues. Milk, cream and egg yolks are natural emulsions.

Relative proportion of fat in water in common food emulsion**
Food ..............Parts fat to 100 parts water
Fat-in-Water emulsion

Whole milk ..................5
Semi-skimmed milk ..................15
Light cream ................25
Double cream ................70
Double cream reduce by a 1/3 ................160
Egg yolk ..................65
Mayonnaise .................400

Water-in-Fat emulsion

Butter ...................550
Vinaigrette ..................300

**Source: McGee on food and cooking